Thunderstorm and lightning warning System in the World

It is well known that thunderstorms are accompanied by lightning. However, the damaging cloud to ground lightning occurs at the beginning phase of a thunderstorm lifetime, that too for a short duration. This may occur for 30 minutes to 1 hour duration over a place. The thunderstorm and lightning warning system is an emerging science in the world. While there has been remarkable progress in recent years in the developing countries like USA, UK, Japan and Korea. India is one such developing country which has state of art thunderstorm and associated weather like wind, rainfall and lightning detection and warning system in recent years. India provides thunderstorm and associated weather forecast over a potential zone like meteorological sub-division and districts one to five days ahead. It further provides thunderstorm and lightning detection information at a place using lightning detection network blended with satellite and Doppler weather Radar observations every 10 minutes. It provides lightning warning at different locations (878 places) as well as districts (739 districts) every three hourly valid for next three hours using all the above observations and numerical high resolution models. Considering the short life of lightning, the lightning warning is communicated with fastest communication means like SMS, Mobile App, Whatsapp, Social media like Twitter and public media like TV and Radio.

The developed countries like USA provides forecast for thunderstorm and associated precipitation, high winds and tornadoes forecast 4-8 days ahead, 3 days ahead, 2 days ahead, 1 day ahead and upto 12 hours ahead in 4 hour slots. It does not provide lightning warning. Japan provides city specific thunderstorm forecasts for one week ahead and pictorial forecasts for thunderstorms upto 24 hours ahead at three hour intervals. It also provides nowcasts up to 60 minutes ahead based on Composite weather radar echoes. Korea also has the system to detect lightning with accuracy and efficiency. The detection data such as time of occurrence, location, intensity, and polarity of lightning strokes are processed into images to be distributed to the public on websites and mobile apps. The UK produces a routine delivered service upto six days, which blends observations and Atmospheric High Resolution model to provide thunderstorm forecast. It generates “Number of flashes of lightning seen per second at a point” at 15 minute intervals upto six hours.

Thus, While the National Meteorological Service of most countries do not issue lightning warnings, on account of short lifetimes, there are a few countries, who provide lightning strikes and their short range nowcast (about half an hour to 3 to 6 hrs ahead.

FAQs on NWP model charts

FAQ on NWP model charts

  • What is Numerical Weather Prediction (NWP)?

Numerical weather prediction (NWP) is a method of weather forecasting that employs a set of Hydro-dynamical equations that describe the motion in the atmosphere and oceans to predict the future weather condition based on current weather conditions.

Current weather observations serve as input (initial condition) to the NWP models through a process known as data assimilation to produce weather forecast of temperature, wind, precipitation, and other meteorological parameters.

  • What are the major steps involved in the NWP modelling system?

  There are 3 steps involved in the NWP modelling system,

  1. Pre-processing(initial condition)
  2. NWP model processing
  3. Post-processing

In the pre-processing, observed data, available at unevenly spaced observing station points, are subjected to different quality control checks, followed by interpolation techniques to prepare initial condition through a process known as data assimilation. Data assimilation is the science of combining different sources of information to estimate possible states of a system. 

With the above initial conditions, the dynamical equations are numerically integrated forward with time, in the model. After integration, future values of model variables at different grid points are generated, known as direct/raw model output.

These above raw model output may be of very little use for the weather forecasters rather, forecasters may like to have specific weather information, like, rainfall, visibility, divergence, vorticity, precipitable water content , different convective indices, etc, which are prepared by post processing of raw model output.

  • What is the reliability of NWP models forecasts in short to medium range time scale?

Due to the advances in observational technology, telecommunication system, computing powers and understanding of different physical processes, there is much needed progress in the field of NWP and the reliability of numerical model forecasts is much higher compared to that of a few decades earlier.

  • What are the numerical weather prediction (NWP) models run in IMD?

IMD runs a global model, viz., Global Forecasting System (GFS) model, a regional model, viz., weather research and forecasting (WRF) model and an ensemble forecasting system, viz., Global Ensemble Forecasting System (GEFS).In addition to these dynamical models, IMD also runs a cyclone prediction model, viz., Hurricane weather research and forecasting (HWRF) during Cyclone periods.

  • How many times are the models run in IMD?

The GFS and WRF models are run four times a day based on observation of 0000, 0600, 1200 and 1800 UTC respectively. The GEFS model is run two times a day based on 0000 and 1200 UTC observations.

What is UTC?

  The UTC stands for Universal Time Coordinate, previously known as Greenwich Mean Time (GMT). Indian Standard Time (IST) is UTC (GMT) plus 05 hours 30 minutes.

  • At what time the forecast products from these models available in IMD website based on above runs?

GFS model output is available approximately 4 hours after model run time. For 00 UTC run, products will be available at 0400 UTC. The GEFS model output of 00UTC run is available at 0630 UTC and 12UTC run is at 1830 UTC. WRF model output is available approximately at 0500 UTC for 00 UTC and at 1700 UTC for 12UTC.

  • Why is it delayed by about 4 hours 30 minutes in case of GFS model 10 day forecast; 5 hours 30 minutes in case of WRF model and GEFS model?

The observational data cutoff time for the generation of initial condition to GFS is 2 hours 45 minutes and the global data assimilation system (GDAS) takes around 45 minutes to generate initial condition. Finally GFS model takes around 7 minutes for one day forecasts. As such it takes 4 hours 30 minutes to complete 10 days forecast. In the GEFS model, there are 21 members, so it needs more nodes & computing time to complete the forecast. After completion of 3 days forecasts of GFS model, WRF analysis and 3 days forecast together takes around 1.30 minutes. Hence WRF model output is available after five hours 30 minutes of model run at 0000 UTC or 1200 UTC.

  • What are the weather forecast parameters generated from these models and available in IMD website?

The forecasts of surface wind, surface pressure, visibility, cloud cover, precipitation and wind, temperature, relative humidity (RH) at different heights of the atmosphere are generated and presented in map form (called as weather charts) in the website. In addition to these basic parameters, IMD also generates different diagnostic parameters and location specific forecast called Meteogram.

  • Why are the values in the maps/charts equally spaced? Is spacing between any two values remain same in any chart? Whether this spacing is same for all the models chart?

All the model charts are plotted with using square grid. The square grid is the most commonly used grid in the NWP models/charts over tropical regions. The resolution of the model is defined as the horizontal distance between the two adjacent grid points. The grid spacing is not same for all the model charts. The value of grid spacing is mentioned in the model charts for reference based on the resolution of the model.

  • What T1534 is as mentioned in GFS model?

GFS is a spectral global model. In spectral models, the horizontal resolution is designated by a “T” number. The ` T1534 ` indicates 1534 number of waves used by a spectral model. The T1534 mentioned in the GFS model indicates the horizontal resolution of GFS model. The approximate grid spacing can then be represented as

∆X=360°/ (3T+1) = 12 km in case of GFS model.

(“T “stands for triangular truncation ); for example: T254, T382, T574 and T1534, etc.

  • What the meanings of “based on — and valid for —-“in the NWP products.

     In general, the “based on “in each NWP model charts  indicates the time of model run i.e. initial condition of either 00 ,06,12 or 18 UTC  and   “ valid for”  indicates the forecast validity time which is  mentioned in the charts. For daily rainfall (24 hour accumulated), the validity period is always from 03 UTC (8.30 IST) of previous day to 03 UTC (8.30 IST) of that validity day.

In India, the rainfall accumulation is from 8.30 IST of the previous day to 8.30 IST of the validity day.

For Example, 72 hour rainfall (Day-3) forecast based on 00UTC of 04-07-2020 valid for 03UTC of 07-07-2020 means the 72 hour forecast is generated based on model run at 5.30 IST of 04-07-2020 and valid from 8.30 IST of 06-07-2020 to 8.30 IST of 07-07-2020.

  • Is there any difference between GFS/WRF Rainfall and Rainfall-India products quantitatively?

  No, both are displaying the 24 hour accumulated rainfall in mm except at different domains and with metrological sub-divisions map in the latter case.

  • What do the symbols on the NWP model wind forecast charts indicate?

These symbols are called “wind barbs”, and they indicate both the wind speed and the wind direction (at different heights starting from at a height of 10m above sea level to 100 hPa (~at 15 km) in pressure levels. The wind direction is indicated by the angle of the barb, and the wind speed is represented by the number of lines on the tail. Each full line equals 10 knots (nautical mile per hour) a smaller half-line is 5 knots. A solid black triangle is equal to 50 knots. Every item should be added together to determine the speed of the wind.

These wind barbs show average wind speeds over the grid. Wind gusts may be up to 40% stronger.

  • What is kt?  How does it differ from kmph?

KT means Knot. Knot is a unit of speed equal to one nautical mile per hour, exactly 1 knot equals to 1.852 km/h (approximately 0.514 m/s).

  • What is wind gust?

Gust is defined as, a sudden increase in wind speed above the average wind speed. More specifically, wind speed must temporarily peak above 16 knots (about 30 km per hour) after accelerating by at least 9–10 knots (about 17–19 km per hour) to qualify as a gust.

A gust is briefer than a squall and usually lasts 20 seconds or less. Air turbulence around an obstacle causes gusts; they occur frequently over buildings and irregular ground and are less frequent over water.

  • How do you estimate gustiness of wind from the model?

The gustiness is calculated by comparing the model output wind at each time step. The time step is the time duration for each computation on the model. It is uniform and fixed for a model run. For example, in case of GFS model, it is 450 seconds. Thus the gustiness reported in model output is the maximum 7- 8 minute average during forecast period. The forecasted values correspond to the maximum wind gusts at the given time.

  • What is average wind speed in the model? What is the averaging period?

The wind speed represented in the model output is average wind speed computed for a period of a few minutes. Hence while comparing the NWP model wind with the actual wind reported by a station, we should keep in mind that model wind is averaged over the period of a few minutes  and over an area equal to size of the grid (12 x12 km in case of GFS model).

Hence it is not truly the gusty wind as defined in Q FAQ No. 13.

  • What is hPa? Why are the charts generated in hPa levels? How does it relate with the actual height of the atmosphere?

Here “hPa” stands for hecta-pascal. The pascal (symbol: Pa) is the SI unit of pressure. Common multiple units of the Pascal is the hecta-pascal (1 hPa =100 Pa), which was previously called as millibar.

Meteorological forecasts and observations report atmospheric pressure in hectopascal (hPa) as per the recommendation of the World Meteorological Organization (WMO).so, model analysis and forecast charts are generated in constant pressure level. The most common are the 1000 hPa, 925 hPa, 850 hPa, 700 hPa, 600 hPa,500 hPa, 400 hPa,300 hPa, 200 hPa and 100 hPa levels. These are called as standard isobaric levels of atmosphere. Every location on the charts has the same pressure, however, heights will vary.

The actual height of the atmosphere is calculated by using the hydrostatic equation. I.e. P = 𝝆 x g x h; where, P=Pressure; 𝝆 =density of fluid; g=acceleration due to gravity; h=height from main sea level (MSL).

As we know that the density of air is heavier near earth’s surface. It becomes lighter as we go to higher altitudes. Thus, at lower altitudes, density being higher, atmospheric pressure is high. At higher altitudes, density being lower, atmospheric pressure is low. Thus, Atmospheric pressure decreases with altitude.It must be noted that atmospheric pressure is measured at the station level and converted in to mean sea level (MSL). This is due to the fact that, sea is at constant level throughout the earth’s surface and taken as a reference point.

  • What is total forecast period of different models? Why are they different?

The total forecast period of GFS/GEFS model is 10 days, while WRF model is only 3 days. The different total forecast period is due to its application in the forecast. GFS/GEFS is a hydrostatic global model meant for forecasting synoptic scale weather systems i.e. Western Disturbance, Monsoon Low Pressure system, etc.; whereas WRF is a meso-scale non-hydrostatic model meant for forecasting convective meso scale systems i.e. Thunderstorm, cloud burst, etc.

  • What is the time interval in forecast charts for a given run of the model?

The time interval in forecast charts of models are given below,

GFS model output is hourly up to 36 hours for some products used in aviation & power sector, 3 hourly up to 5 days for all the products and 6 hourly up to 10 days for main forecast (wind, rainfall) products.

 GEFS model forecast products are available at 6 hourly intervals up to 10 days for probabilistic forecast (wind, rainfall, temperature, etc).

WRF model forecast  is available at hourly up to 72 hours for some products used in aviation & power sectors, 3 hourly up to 3 days for all the products and 6 hourly up to 3 days for main forecasts (wind, rainfall, etc) products.

  • What is GPM height? How can it be interpreted?

The unit of the Geopotential height is the Geopotential meter (GPM). Geopotential height approximates the actual height of a pressure surface above mean sea-level.

With the analysis of Geopotential height chart, forecasters identifies the regions of high and low GPM height .High GPM region normally corresponds to fair weather and low GPM regions corresponds to adverse weather.

  • What are diagnostic products from GFS model, WRF model and GEFS model?

  There are two kinds of NWP model products namely; (a) direct model products and (b) diagnostic/derived model products. Directly available as model outputs are called direct model products, they are wind, temperature, pressure, Geopotential-height and humidity. The outputs such as vorticity, divergence, vertical wind shear, moisture flux; CAPE, CINE, Thunderstorm indices, Lifted Index, etc. are called diagnostic/derived model products. These derived products from GFS and WRF models are used by the forecasters for diagnosing the weather systems predicted by the model.

  • What do you mean by ensemble prediction system?

 As the atmosphere is a chaotic system, very small errors in its initial state can lead to large errors in the forecast. This means that we can never create a perfect forecast system because we can never observe every detail of the atmosphere’s initial state. Tiny errors in the initial state will be amplified, so there is always a limit to how far ahead we can predict any detail. To test how these small differences in the initial conditions may affect the outcome of the forecast, an ensemble system can be used to produce many forecasts.

Instead of running just a single model forecast, the computer model is run a number of times from slightly different initial conditions. The complete set of forecasts is referred to as the ensemble, and individual forecasts within it as ensemble members.

  • How many members are there in GEFS ensemble model?

The Global Ensemble Forecast System (GEFS) is a weather forecast model made up of 21 ensemble members (separate forecasts) to address the nature of uncertainty in weather observations.

  • What do you mean by control, ensemble mean, ensemble spread?

The mean forecast of 21 ensemble members is known as ensemble mean. The ensemble spread is a measure of the difference among the ensemble members and is represented by the standard deviation with respect to the ensemble mean (EM). The control is same as the deterministic model output. If the spread is more, the uncertainty in forecast is more and vice versa. If the difference between control and ensemble mean is more, the spread is more and hence uncertainty is more and vice versa.

  • How is probability rainfall calculated in GEFS model?

The probability of rainfall amount is typically derived by using forecast of 21 ensemble members. GEFS does not directly produce probability distribution of any variables for all possible events and it is calculated by using the simple probability of occurrence of the event at a grid point based on 21 forecasts from 21 ensemble members at that grid point.

  • What is the difference between rainfall forecast from GFS and GEFS?

The GEFS is a weather forecast model made up of 21 separate forecasts, or ensemble members. GEFS provide Ensemble mean (average of 21 members) and probability forecasts for an event from GEFS’s 21 ensemble members, whereas GFS provide only deterministic rainfall forecast (single value forecast)

  • What is the difference between GPM height based on GFS and GEFS?

GEFS provide Ensemble mean (average of 21 members) values of GPM height from GEFS’s 21 ensemble members along with ensemble spread, whereas GFS provide only single value deterministic GPM height forecast.

  • What is the difference in wind forecast by GFS and GEFS in different heights of atmosphere?

GEFS provide Ensemble mean (average of 21 members) values of wind along with ensemble spread (magnitude difference of wind speed among the ensemble members (21) with respect to and ensemble mean). GFS provide only single value deterministic wind forecast.

  • What does the shading indicates in different forecast products of GFS, WRF and GEFS?

The information about shading in all the forecast products of GFS, WRF and GEFS is mentioned in the legend of the respective charts. In the wind charts of GFS/WRF models, the shading indicates the magnitude of wind speed, whereas in GEFS model, the shading indicates the ensemble spread from ensemble mean position.

  • What do you mean by Isobars, Isotach, streamlines and contours?

Isobar is a line of constant pressure. Isobars are found only on surface charts. They connect lines of equal pressure in the units of hPa.

Isotach is a line of equal wind speed.

Streamlines are Lines of equal wind direction. Streamlines are used primarily in tropical regions since the pressure gradient is weak. They show areas of convergence, divergence and pressure circulation. The Geopotential height is normally displayed as contour a line of constant GPM values.

  • Whether forecast is available for any given location?

GFS Model based 3 hourly forecasts up to 10 days is available for around 1500 cities of India: http://nwp.imd.gov.in/gfs_meteograms.php.

Similarly, WRF model based forecast up to 3 days is also available for around 700 stations. http://nwp.imd.gov.in/wrf_meteograms.php

  • What is Meteogram?

Meteogram is a graphical presentation of one or more meteorological variables with respect to time, whether observed or forecast, for a particular location. Where forecast data is used, the Meteogram will typically be generated directly from a weather forecasting model based on the longitude, latitude and elevation of the location, but it can also be corrected by a meteorologist.

In the Meteogram, time is plotted along the X axis, while the values of the different weather parameters are plotted along the Y axis. The most common weather parameters in a Meteogram are precipitation, temperature, air pressure, cloud cover, wind speed and wind direction. Meteogram based on GFS &WRF is available in the link.

GFS model at: http://nwp.imd.gov.in/gfs_meteograms.php

WRF model at:  http://nwp.imd.gov.in/wrf_meteograms.php

  • How is cloud represented in Meteogram?

This panel has a blue background to show the cloud-free areas. The panel is divided into three horizontal layers for the display of low, middle, and high cloud cover, which are drawn as white bars. If the white bar covers the full height of its layer, that is 100% cloudiness. The white bars have no gap between them to better simulate the appearance of cloudiness in the panel.

  • How can I get forecast for my city and town from IMD GFS model?

The GFS model based weather forecasts are available in the web site link http://nwp.imd.gov.in/gfs_meteograms.phpfor most of the city/town. If you don’t find your city/town in the above link provided and want to get forecast from GFS model, then you can contact IMD/GFS at. nwp.imd@gmail.com

Frequently asked Questions (FAQs) on Monsoon

  • What is the all India monthly and seasonal rainfall?

All India monthly rainfall is the amount of accumulated rainfall received over India for a particular month. For example, All India monthly rainfall of June 2018 is 155.7mm. Similarly All India seasonal rainfall is the amount of accumulated rainfall received over India for a particular season. e.g. All India seasonal rainfall of South-West monsoon (JJAS) of 2018 is 804.1mm. These quantities are not constant; they vary from year to year.

  • What do we mean by long period average (LPA) of rainfall?

LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season)average over a long period like 30years, 50-years etc. It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. For example,LPA of south west monsoon rainfall over Kerala for the months June, July, August and September are 556mm, 659mm, 427mm and 252mm respectively. Current LPA of all India south west monsoon rainfallbased on the average rainfall over the period 1961 -2010 is 880.6mm.

  • What islarge excess, excess, normal, deficient, large deficient rainfall?

These are categories of rainfall used to describe realised rainfall averaged over various temporal scales like daily, weekly, monthly etcfor spatial scales like districts, states operationally.  Accordingly, when the realised rainfall is ≥60%, 20% to 59%, -19% to +19%, -59% to -20%, -99% to -60% of long period average (LPA), the rainfall is categorized as large excess, excess, normal, deficient, large deficient respectively.

  • What is below normal, normal and above normal rainfall for the country as a whole?

If ‘m” is the mean and “d” is the standard deviation of a long time series of any climate variable like rainfall, temperature etc.Assuming the time series is normally distributed, 68% of the observations fall within +/- 1 standard deviation (d) from the mean (d).  Therefore, when a realised value of the variable falls between m-d to m+d, it is categories as normal.  When the realised value is <(m-d), it is categorised as below normal and when the realised value is > (m+d), it is categorised as above normal. 

In case of monsoon season (June to September) rainfall over India as a whole, the long period average (LPA) is 88 cm and standard deviation is 9cm (about 10% of mean value).Therefore, when the rainfall averaged over the country as a whole is within ±10% from its long period average (LPA) or  90% to 110% of LPA, the rainfall is said to be “normal” and when the rainfall is <90% (>110%) of LPA, the rainfall is said to be “below (above) normal”.

  • What is the role of monsoon trough?

Monsoon Trough is an elongated low-pressure area which extends from heat low over Pakistan to Head Bay of Bengal. This is one of semi-permanent feature of monsoon circulation. Monsoon trough may be a characteristic of east west orientation of Himalayan ranges and north south orientation of Khasi-Jaintia Hills. Generally eastern side of monsoon trough oscillates, sometimes southwards and sometimes northwards.

Southward migration results in active/vigorous monsoon over major part of India. In contrast, the northward migration of this trough leads to break monsoon condition over major part of India and heavy rains along foothills of Himalayas and sometimes floods in Brahmaputra river.

  • What is heat low? What are its impacts on monsoon rainfall?

During the northward march of sun in northern hemisphere, the continent surrounding the Arabian Sea begin to receive large amounts of heat; not only in the form of radiationfrom sun, but also flux of heat from the earth’s surface into atmosphere (160 Watts/m2 for month of June over the arid zones of northwest India, Pakistan and middle eastern countries). As a result of this large input of power, trough of low pressure forms over this region. It is a semi-permanent feature of monsoon over India. The heat low is very shallow (extending up to 850 hPa(1.5 KM) leveland there exist a well-marked ridge above heat low. In spite of occurrence of cloudiness, the precipitation is very small. Intense heat low (pressure departure is below normal) acts as suction devise for moist air along the monsoon trough and to some extend related to good monsoon over India.During weak heat low (pressure departure is above normal) monsoon rainfall over India is greatly affected and results in deficient or scanty rainfall over vast area of country (eg. In 1987, central pressure over heat low area was mostly above normal, which proved to be drought year). Satellite measured estimates of longwave radiation indicates that tropical /subtropical deserts are heat sinks.

  • What is monsoon low, how does it influence monsoon?

An area with pressure at the centre lowest one, closed in shape with winds blowing around in anti-clockwise direction in Northern Hemisphere is Low Pressure Area (LPA).  The LPA is associated with a whirling motion of air, convergence and upward motion of air. In the low usually clouds and rainfall are present. LPA which seen during monsoon is monsoon low.

The monsoon lows may be intensified into monsoon depressions. The monsoon lows and depressions are the principal rain bearing systems of the south west monsoon period over India.Substantial amounts of rainfall are generated by the westward passage of monsoon depressions forming in the Bay of Bengal. These are low pressure areas having wind speeds between 17 and 33 knots in their circulation.

On the average, 2 depressions form in each of the monsoon months (June-September). However, year to year variation in their number is quite large. Those that form in early June are responsible for the advance of the southwest monsoon, and are not strictly monsoon depressions. In July and August they usually form north of 18°N in the northwest Bay, and the site of genesis shifts in September southward in the Central Bay.

  • What is Tibetan High? How is it related with monsoon rainfall?

Tibetan High is a warm anticyclone (in this wind are changing in a clock-wise direction in the Northern Hemisphere and it will have always outflow of winds) located over Tibetan Plateau (centre latitude at 28ºN, longitude 98ºE) in the middle/upper troposphere during monsoon period. It is marked at 300 hPalevel with centre 30ºN, 90ºE and extends 70ºE-110ºE.The outflow of winds from Tibetan High as the easterly flow concentrates into jet stream centred near about the latitude of Chennai at 150 hPa in July. The jet stream runs from the east coast of Vietnam to the west coast of Africa. Thusthe location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall.Shifting its position east or west causes variation of monsoon activity over India. The Tibetan ‘High’ may sometimes shift much to the west of its usual position. In such a situation, the monsoon may extend further westward into Pakistan and in extreme cases into north Iran, though such a westward position of the Tibetan ‘High’ would be against its having origin in the heating effect ofthe Tibetan Plateau.

  • What is Mascarene high? How does it influence monsoon rainfall?

Mascarene High is a high-pressure area that is found around Mascarene Islands (in south Indian Ocean) during monsoon period. This is responsible for cross-equatorial flow through south Arabian Sea and it acts as southern hemispheric linkage. The variation in the intensity of High Pressure causes monsoon surges across equatorial flow. These surges are responsible for heavy rains along the west coast.

  • What is Somali jet?

Somali jet is low level (1 to 1.5 km asl) inter hemispheric cross equatorial flow of air, attains Jet speed at the west end of monsoon regime along the east coast of Africa. This Jet originates near Mauritius and northern part of Madagascar in the southern Hemisphere. This jet reaches Kenya coast (at about 3ºS) covers the plains of Kenya, Ethiopia and to Somali Coast at about 9ºN)

During May, it moves further into eastern Africa, then into Arabian sea and reaches west coast of India in June. It attains maximum strength in July. Short period (8-10 days) fluctuations are observed in Low Level Jet stream. Its strengthening gives rise to strong monsoon over peninsular India.

  • What is Tropical easterly Jet? How does it influence the rainfall?

South of the sub-tropical ridge over Asia, the easterly flow concentrates intojet stream centred near about the latitude of Chennai at 150 hPa in July. This is Tropical easterly jet. Thejet stream runs from the east coast of Vietnam to the west coast ofAfrica. Over Africa, the location is at 10° N. Normally, the jet is at an accelerating stage from the South China Sea to south India and decelerates thereafter.The location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall.TEJ weakens to less than 50 knots over India in September. During Breakmonsoon conditions TEJ moves northwards up to latitude 20ºN.

  • What is the difference between monsoon depression and depression forming in pre-monsoon season and post-monsoon season?

The depressions which form in the monsoon season are called the monsoon depressions. These are low pressure areas with two or three closed isobars (at 2 hPa interval), which cause most of monsoon rains. These can be of Bay origin, Land origin or Arabian Sea origin. Their shape is roughly elliptical and its horizontal extension is about 1000’s of Kms of surface. Its vertical extension is about 6-9 kms.

Monsoon depression is cold core system (central temperature colder than environment) over surface and in the lower levels and warm core in upper levels (central temperature warmer than environment). The Maximum wind strength and intensity can be noticed at the levels of 0.9km or 1.5 km. The monsoon depressions tilt southwards with height and if monsoon depression is moving westward, the heavy rainfall is mainly concentrated in the SW quadrant.Due to the high vertical wind shear present during South west monsoon season, monsoon depressions are generally do not intensify into cyclonic storms.

The depressions forming in pre-monsoon season and post-monsoon season intensify into a cyclonic storm.The average diameter of Post monsoon storms is about 1200 km whereas in pre-monsoon season it is about 800 km, however intensity does not depend on size. The cyclonic storm is a warm core phenomenon where the temperatures at the centre are warmer than the surrounding (areas) regions. The maximum warming occurs at the 300 hPa level.

  • Why don’t we get cyclones during main monsoon months like July and August?

Tropical cyclogenesis requires several favourable precursor environmental conditions. Warm Ocean waters (of at least 26.5℃ throughout sufficient depth at least on the order of 50 m). Relatively moist layers near the height of 5 km. Non-negligible amount of Coriolis force, pre-existing near surface disturbance. Low values of vertical wind shear between the surface and upper troposphere.

In July and August winds on the surface are westerly/south-westerly to the south of monsoon trough and south easterly/ easterly to its north and are generally stronger over the seas than the Land areas. The upper winds are westerly/south-westerly to the south and south easterly/ Easterly to the north of this trough region. Westerly winds increase with height and reach a maximum speed of 20-25 knots between 900 to 800 hPa levels. Easterly winds strengthen with height from 200 hPa reaching a maximum at 100 hPa. Speeds are between 60 to 80 knots over peninsula at 150 /100 hPa level or even at lower height (around 200 hPa) in the southern latitude. This results in high values of vertical wind shear which is unfavourable for Tropical cyclogenesis. So, we don’t get cyclones during main monsoon months like July and August.

  • What is off shore trough?

During monsoon season a shallow trough of low pressure is observed (on sea level surface chart) along west coast of India. This is known as off-shore trough.This type of system quite frequently develops off the west coast of India, anywhere from north Kerala to south Gujarat, during the period of southwest monsoon, and is responsible for the strengthening of the monsoon in terms of rainfall, in the adjacent parts of the coastal belt.

  • What is off shore vortex?

West coast of India has an orographic barrier in the form of Western Ghats. These mountains are oriented in north south direction and approximately 1000 km in length and 200 km in breadth. When monsoon winds strikes the mountains, on many occasions they do not have enough energy to climb over Western Ghats. On such occasions they tend to be deflected round the mountains and return current forms the off-shore vortex. These vortices are responsible for the occurrence of heavy to very heavy rainfall over west coast during monsoon season.

  • How does rainfall vary during monsoon season? Is there any periodicity.

During monsoon, considerable variability in rainfall is seen with space and time. The following are reasons which contribute to this.

Onset, Advancement and Withdrawal of monsoon. They decide the duration, period of monsoon current at different places.

Position of monsoon trough: It can oscillate 5º to north and 5º to south within 24 hours. If this trough is in south of normal position, strong monsoon conditions are observed over India. If this trough is in north of normal position or if it runs to foothills of Himalayas or not seen at all, then break monsoon conditions are observed. Synoptic systems like cyclonic circulations, lows, depressions move along trough and contribute to rainfall.

Formation and movement of synoptic systems and number of days of systems:

Low frequency oscillations considerably change the rainfall distribution over different parts of India. 40-day mode or northward propagation of maximum cloud zone from equator to 30ºN. This mode is also seen as northward propagation of trough and ridges in wind field with periodicity of 0.75º of longitude per day.

Westward propagating biweekly oscillation of 14 to 15 days.

  • What is synoptic mode of variation?  How does it influence rainfall?

Low frequency oscillations considerably change the rainfall distribution over different parts of India. Synoptic mode variation has a periodicity of 3-7 days. It is mainly due to the formation of low pressure systems and its movements over Indian land mass. Under its influence, central Indian region receives good amount of rainfall.

Westward propagating biweekly oscillation of 14 to 15 days.Trough lines and low-pressure systems, ridges and high-pressure systems propagate in sequence from east to west with a periodicity of 2 weeks (14 to 15 days). This is known as Quasi-biweekly oscillation. When a trough or low-pressure area propagates on a particular area that area will experience an enhanced precipitation and when ridge or high-pressure passes over particular area, it will lead to suppressed rainfall or no rainfall over a particular area.

  • What is Madden Julian Oscillation? How does it influence rainfall?

The Madden Julian Oscillation (MJO) is one of the most important atmosphere-ocean coupled phenomena in the tropics, which has profound influence on Indian Summer Monsoon. The MJO is the leading mode of tropical intra-seasonal climate variability and is characterized by organization on a global spatial scale with a period typically ranging from 30-60 days, which was discovered by Madden and Julian in 1971 in a published paper. It has the following characteristics: –

  1. MJO is a massive weather event consisting of deep convection coupled with atmospheric circulation, moving slowly eastward over the Indian and Pacific Oceans.
  2. MJO is an equatorial traveling pattern of anomalous rainfall that is planetary in scale.
  3. Each cycle lasts approximately 30–60 days. Also known as the 30-60 day oscillation, 30-60 day wave, or intra-seasonal oscillation (ISO).
  4. The MJO involves variations in wind, sea surface temperature (SST), cloudiness, and rainfall.

Based on the place of convective activity the period of MJO is divided into 1-8 phases with each phase roughly last for 7 to 8 days. Since the MJO is the most important mode of tropical intra-seasonal variability with potentially important influences on monsoon activity in the Asian regions on extended range time scale (beyond 7 days to on1 month), the capability of statistical or numerical models in capturing MJO signal is very crucial in capturing the active/break cycle of monsoon.

  • How does monsoon vary from year to year? Is there any periodicity?

Year to year variation of monsoon rainfall over the large number of years is known as the interannual variability of monsoon. Periodicity of monsoon is largely controlled by the global ocean atmospheric phenomena like El nino Southern oscillation (ENSO).

  • What are the main factors governing interannual variation of south west monsoon?

Interannual variations are the variations on the annual cycle of the monsoon producing anomalously wet or dry years. The major factors governing interannual variation of south west monsoon are El ninoSouthern oscillation(ENSO) and Indian Ocean Dipole (IOD). Other contributing factors are North Atlantic Oscillation (NAO) and Pacific Decadal Oscillations (PDO).

  • How do we monitor monsoon? 
  1. Monsoon is being monitored by IMD using various techniques as given below.
  2. Continuous monitoring of surface and upper air meteorological observations
  3. Real time monitoring of the monsoon using remote sensing techniques like satellite and Radars.
  4. Analysis of the different meteorological charts.
  5. Guidance from various national and international weather forecasting models at different spatio-temporal scales.
  • What are the observational tools used for monitoring monsoon?

The observational tools used for monitoring monsoon are:

  1. Synoptic observations of various meteorological parameters plotted on appropriate charts viz., mean sea level pressure chart, wind observations at constant pressure levels, geopotential heights, temperature, etc.
  2. Auxiliary charts prepared out of derived parameters, for example, dew point temperature, pressure tendency, anomaly charts of pressure, maximum & minimum temperatures etc.
  3. Satellite imageries
  4. Satellite bulletins
  5. Various derived products from satellite observations, viz. Cloud Top Temperatures, Cloud Motion Vector (CMV) winds, water vapour derived winds, Outgoing Longwave Radiation (OLR), Quantitative Precipitation Estimates (QPE), divergence-convergence patterns of lower & upper levels, wind shear tendency, etc.
  6. The AWS plotted charts and other products of numerical weather prediction models available on the ftp server.
  7. Some more products of numerical weather prediction models available on Internet from IMD and NCRMWF and other worldwide centres such as UKMO, ECMWF, COLA, NOAA, NOGAPS, JTWC, etc.
  8. Ships and buoy observations
  9. Weather Radar and Doppler Weather Radar Observations.
  10. Current Weather Observations (CWOs), Aircraft Reports (AIREPs)
  • How do we predict monsoon in different spatial and temporal scale?

Prediction of monsoon is done by IMD in different spatial and temporal scales. It varies from country as a whole to district wise in spatial scales and from a seasonal forecast to nowcast in temporal scale.

Seasonal forecast for monsoon rainfallissued based on long range forecast (LRF)in the month of April for the entire country. This forecast update in the month of May for the entire country and also for its broad homogenous regions. LRF of monsoon will give a general picture of seasonal and monthly rainfall for the coming monsoon season.

Extended range forecast of IMD issued in every Thursday of the week give forecast for a period extending from about 10days to 30 days in advance for the entire country. This forecast helps in the forecasting of active-break cycle of monsoon, formation of monsoon lows and depressions.

IMD issues short to medium range forecast for 5 days based on various national and international weather forecasting model guidance and expertise from the scientists. This forecast is being used by various stakeholders for planning their routine activities.

Different meteorological centresissuenowcastfor heavy rainfall during monsoon season with validity up to 6 hours.

  • What is long range forecast of monsoon?

As per the World Meteorological Organization (WMO) definition, long range forecast is defined as the forecast from 30 days’ up to one season’s description of averaged weather parameters.  The monthly and seasonal forecast comes under long range forecast.

  • What is extended range forecast of monsoon?

Extended range Forecast is a forecast for a period extending from about 10days to 30 days in advance. With regard to the extended range time scale it is the time scales between medium range (About a week in the tropics) and seasonal scale and extends up to a period of one month. The extended range time scale of monsoon is also called the intra-seasonal time scale or active-break cycle of monsoon. The synoptic scales systems and other oscillations like Madden Julian Oscillation (MJO) influence the monsoon in this time scale. The monsoon forecast in this time scale is most difficult as it is neither a complete initial value problem (like the short to medium range prediction) nor a complete boundary value problem (like the seasonal forecast) but perhaps the most important of all time scales for economic and agricultural sectors. It makes difficult to predict in this time scale since the time scale is sufficiently long so that much of the memory of the atmospheric initial conditions is lost, and it is probably too short so that the variability of the ocean is not large enough.

  • What is forecast accuracy of ERF?

With regard to the skill of monsoon prediction in this time scale based on the operational coupled models runs at IMD it shows very useful guidance about the monsoon onset, monsoon withdrawal, active and break phases of monsoon and active-break transitions reasonably well. With regard to the quantitative verification, on average, it shows significant skill up to about three weeks for all India rainfall. Over the homogeneous regions of Indian it shows significant skill up to 2 to 3 weeks over Central India, monsoon zone of India &amp; Northwest India. Over the south peninsula and northeast India it shows significant skill up to 2 weeks. On smaller spatial domain like meteorological subdivision level forecast skill it shows useful skill up to 2 weeks.

  • What is short to medium range forecast of monsoon?

Short range forecast is forecast valid for a period up to 3 days and the medium range forecast is valid for 3 to 10 days. This forecast is useful for planning agricultural activities, disaster management, town planning etc.

  • What is forecast accuracy of short to medium range forecast, especially heavy rainfall?

Accuracy of the short to medium range forecast is quite good in capturing the rainfall distribution, heavy rainfall events and formation of the synoptic systems up to 3-4 days in advance. Accuracy of the forecast decreasing beyond 5 days due to error propagation in initial condition.

  • What is SEFS?

SEFS stands for Statistical Ensemble Forecasting system. It is a statistical model used by IMD for the long range forecasting of South west monsoon season rainfall over the country as a whole. Forthisa set of 8 predictors (given in below Table) that having stable and strong physical linkage with the Indian south-west monsoon rainfall is used.

Details of the 8 predictors used for SEFS

No.PredictorUsed for forecasts inCorrelation Coefficient (1981-2010)
1.Europe Land Surface Air Temperature Anomaly (January)April0.42
2.Equatorial Pacific Warm Water Volume Anomaly (February + March)April-0.35
3.SST Gradient Between Northwest Pacific and Northwest  Atlantic (December +January)April and June0.48
4.Equatorial SE India Ocean SST  (FEB)April and June0.51
5.East Asia MSLP                                     (FEB + MAR)April and June0.51
6.NINO 3.4 SST (MAM+(MAM-DJF) Tendency) June-0.45
7.North Atlantic MSLP (MAY)                           June-0.48
8.North Central Pacific Zonal Wind Gradient  850 hPa (MAY)                        June-0.57
Geographical domains of the predictors used in the statistical ensemble forecasting system for the seasonal rainfall forecast

The geographical domains of the predictors are shown in the above figure.  For the April SEFS, first 5 predictors listed in the Table are used. For June SEFS, the last 6 predictors are used that include 3 predictors used for April forecast. The standard errors of the 5–parameter and 6- parameter SEFSs were taken as ±5% and ±4% respectively. According to this forecasting system, the forecast for the seasonal rainfall over the country as a whole is computed as the ensemble average of best few models out of all possible models constructed using two statistical methods; multiple regression (MR) technique and projection pursuit regression (PPR) – a nonlinear regression technique.  In each case, models were constructed using all possible combination of predictors. Using ‘n’ predictors, it is possible to create (2n-1) combination of the predictors and therefore as many number of models. Thus with 5 (6) predictors respectively for April (June) SEFS, it is possible to construct 31 (63) models.

  • What is MMCFS?

MMCFS stands for Monsoon Mission Coupled Forecasting System. It is a coupled dynamical model developed under Monsoon Mission Project. The original model frame work of CFS was developed by the National Centers for Environmental Prediction (NCEP), USA.  This model was modified to provide better forecast over Indian monsoon region for different spatial and temporal resolutions through mission mode research work by the Indian Institute of Tropical Meteorology (IITM), Pune in collaboration with various climate research centers from India and abroad. The latest high-resolution research version of the coupled model has been implemented in High Performance Computer (HPC) at Indian Institute of Tropical Meteorology (IITM), Pune. IMD uses Monsoon Mission Climate Forecast System (MMCFS) model for preparing operational forecast of rainfall and temperature along with statistical models. More model details are available at http://imdpune.gov.in/Clim_Pred_LRF_New/Models.html

  • What is forecast accuracy of LRF for the Southwest Monsoon Seasonal Rainfall over the country a whole?

The LRF for all India season rainfall was reintroduced in 1988 using 16 parameter power regression and parametric models. IMD introduced new state of art LRF models during 2003 & 2007 following review of old forecasting system in 2002.  The performance of the operational long range forecast for the season rainfall over the country as whole for the period 1988-2019 is shown in Figure below.  During the period the absolute error was ≥ 10% of LPA in 7 years with highest in 1994 (21%) followed by 2002 (20%).

The average absolute error (difference between forecast and actual rainfall) during the last 13 years (2007 -2019) during which forecast was prepared using new Statistical Ensemble Forecasting system (SEFS)was 6.25% of LPA compared to the average absolute error of 8.91% of LPA during the 13 years (1995 -2006) just prior to that period. During 1994-2006, the forecast within the ±8% of actual values during 8 years.  Within these 8 years, forecast was within ±4% during 3 years. On the other hand, during 2007-2019, the forecast was within the ±8% of actual values during 8 years with forecast within ±4% during 5 years.This clearly indicates improvement made in the operational forecast system in the recent 13 years period compared to earlier 13 years period.It is not possible to have 100% success for forecasts based on statistical models. The problems with statistical models are inherent in this approach and are being faced by forecasters worldwide.

Performance of Operational Forecast of Monsoon by India Meteorological Department during 1988-2019
  • What is Monsoon Mission?

Monsoon Mission is a national programme launched by Ministry of Earth Sciences (MoES) with a vision to develop state-of-the-art dynamical prediction system for the monsoon rainfall in different time scales. The mission supports focused research by national and international research groups with definitive objectives and deliverables to improve models in the short, medium, extended and seasonal range scales through setting up of a framework for generating dynamical forecasts and improving skill of forecasts. It also supports observational programs which will helps the better understanding of the atmospheric processes related to monsoon. The main objectives of this mission are   

  1. To improve Seasonal and Intra-seasonal Monsoon Forecast
  2. To improve Medium Range Forecast.

Indian Institute of Tropical Meteorology (IITM), Pune, India Meteorological Department (IMD), Indian National Centre for Ocean Information Services (INCOIS), Hyderabad and National Centre for Medium Range Weather Forecast (NCMRWF), Noida are the major participating institutions in Monsoon Mission.

  • How do we declare onset and advance of monsoon?

The onset of the broad-scale monsoon occurs in many stages and                                       represents a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.

At present IMD uses a new-criteria adopted in 2016 for declaring the onset of monsoon over Kerala which was based on the daily rainfall of 14 stations over Kerala and neighbouring area along with wind field and Outgoing Longwave Radiation (OLR) over southeast Arabian Sea. The new criteria emphasize on the sharp increase in rainfall over Kerala along with setting up of large-scale monsoon flow and extension of westerlies up to 600 hPa.  However, IMD declares monsoon onset/progress dates for other regions operationally in a subjective manner considering the sharp increase in rainfall and its characteristic sustenance for a few days and associated changes in the atmospheric circulation features.

The guidelines to be followed for declaring the onset of monsoon over Kerala and its further advance over the country are enlisted below:

a) Rainfall

If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy,

Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery,Kannur, Kasargode and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the 2nd day, provided the following criteria are also in concurrence.

b) Wind field

Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN and Long. 55ºE to 80ºE. The zonal wind speed over the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/satellite derived winds.

c) Outgoing Longwave Radiation (OLR)

INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE. Further Advance of Monsoon over the Country a) Further advance be declared based on the occurrence of rainfall over parts/sectors of the sub-divisions and maintaining the spatial continuity of the northern limit of monsoon, further advance be declared.

The following auxiliary features may also be looked into:

b) Along the west coast, position of maximum cloud zone, as inferred from the satellite imageries may be considered.

c) The satellite water vapour imageries may be monitored to assess the extent of moisture incursion.

Northern Limit of Monsoon (NLM)

Southwest monsoon normally sets in over Kerala around 1st June. It advances northwards, usually in surges, and covers the entire country around 8th July(More details are available at http://www.imdpune.gov.in/Clim_Pred_LRF_New/Reports.html). The NLM is the northern most limit of monsoon up to which it has advanced on any given day.

  • How do we define withdrawal of monsoon?

Like the onset criteria, the criteria for the withdrawal of monsoon have also undergone changes. The current operational criteria used by IMD for declaring the withdrawal from extreme north-western parts of the country was adopted in 2006 andconsist of the following major synoptic features which will be considered only after 1st September,

i) Cessation of rainfall activity over the area for continuous 5 days.

ii) Establishment of anticyclone in the lower troposphere (850 hPa and below)

iii) Considerable reduction in moisture content as inferred from satellite water vapour imageries and tephigrams.

Further withdrawal from the country is declared keeping the spatial continuity, reduction in moisture as seen in the water vapour imageries and prevalence of dry weather for 5 days. SW monsoon is withdrawn from the southern peninsula and hence from the entire country around15thOctober, when the circulation pattern indicates a change over from the south-westerly wind regime.More details are available at http://www.imdpune.gov.in/Clim_Pred_LRF_New/Reports.html.

  • Is there any impact of climate change on monsoon?

Yes. Several studies have attributed the rising trend in the frequency and magnitude of the extreme rainfall events and decreasing trend in moderate rainfall events during monsoon season over central Indian region to climate change and natural variability.

  • What is its impact on monsoon rainfall over different parts of the country?

Based on recent studies it has been observed that the summer monsoon precipitation (June to September) over India has declined by around 6% from last fifty years, with notable decreases over the Indo-Gangetic Plains and the Western Ghats. It is also observed that there has been a shift in the recent period toward more frequent dry spells and more intense wet spells during the summer monsoon season. Over central India, the frequency of daily precipitation extremes with rainfall intensities exceeding 150 mm per day increased by about 75% during the recent decades.

  • How Climate Change does influences heavy rainfall activity?

Temperature of the earth is increasing rapidly due to anthropogenic greenhouse gas emissions. Thermodynamically, warm air holds more moisture as compare the dry air. According to Clausius-Clapeyron equation, the capacity of air to hold moisture increases by 7% for each degree of warming. Studies indicate that, in a changing climate, heavy rainfall events are expected to rise due to abundance of the moisture due to warming.

  • What is its impact on monsoonal low-pressure systems?

Several studies have shown significant decreasing trend in the frequency of Monsoon Depressions over the east coast of India in the recent decades. Some studies showed significant increasing trend in the frequency and duration of monsoon lows, whereas the number of lows intensifying into depressions are observed to be decreasing.

  • What are future projections on monsoon rainfall?

Simulations of Earth’s climate in future decades (typically until 2100) based on assumed scenarios of the concentrations of greenhouse gases, aerosols, and other atmospheric constituents that affect the planet’s radiative balance is called climate projections. Studies show that all-India summer monsoon mean rainfall is likely to increase moderately in future. It is also projected that the lengthening of the season due to late withdrawal. On interannual timescales, it is speculated that severity and frequency of both floods and droughts might increase noticeably in future climate.

  • What are the vagaries of monsoon?

Monsoon brings relief to dry and parched land in the form of rain, and affects Indian agriculture in a very substantial measure. The impact of monsoon on Indian economy is more pronounced. The Indian farmer has to put up a temperamental nature a many occasion in past. Excessive rain leads to floods in certain areas, while little or no rain in other parts bringing drought and famine resulting in acute distress to millions. Such fluctuations in rainfall have engaged the attention of our people and to make considerable effort to avert these calamities. There are many legends in our land of worship to the rain God for averting famines, and prayers are offered for pacifying the turbulent rivers of India. The Indian poets have sung about the rainy season in prose and verse.

  • How do we define floods and droughts?

A great flow of water, especially, a body of water rise in, swelling and over flowing land usually thus covered. Generally, flood occurs due to heavy rainfall in the catchment area but some time it occurs due to upstream discharge/ dam failure.

A flood that occurs in a short time (Usually less than six hours) of heavy or excessive rainfall, dam or levee failure is called flash flood.

Drought:

Drought is the consequence of a natural reduction in the amount of precipitation over an extended period of time, usually  a season or  more  in  length,  often associated  with other climatic factors (viz. high  temperatures, high  winds  and low relative humidity) that  can  aggravate  the severity  of  the  drought event.

There are four types of droughts:

  1. Meteorological Drought
  2. Hydrological Drought
  3.  Agricultural Drought
  4. Socio-Economic Drought

Meteorological Drought:

According to India Meteorological Department, meteorological drought over an area is defined as a situationwhen the seasonal rainfall received over the area is less than 75% of its long term average value.    It is further classified as “moderate drought” if the rainfall deficit is between 26-50% and “severe drought” when the deficit exceeds 50% of the normal.

Hydrological Drought:

Hydrological Drought can be defined as a period during which the stream flowsare inadequate to supply established use of water under a givenwater management system.

Agricultural Drought:

It occurs when available soil moisture is inadequate for healthy crop growth and cause extreme stress and wilting.

Socio-economic drought:

Abnormal water shortage affects all aspects of established economy of a region.  This in turn adversely affects the social fabric of the society creating unemployment, migration, discontent and various other problems in the society. 

Thus, meteorological, hydrological and agricultural drought often leads to what is termed as Socio-economic drought.

  • Which areas are mainly affected by heavy rainfall and dry spells?

Densely populated urban areas are mainly affected by the heavy rainfall due to chances of urban flooding. Land slide prone hilly areas are also affected by heavy rainfall. On the other hand rainfed areas of the agriculture sector are strongly affected by the dry spells.   

  • Whether lightning occurs during monsoon season?

Lightening is mainly associated with the convective clouds. Monsoon clouds are mainly stratiform. Therefore, lightening generally do not occur during active phase of the monsoon season. However, the convective activity during break spell of the monsoon may lead to formation of convective clouds and hence the lightening.

  • How IMD doessupport flood management?

IMD provides real time rainfall situation and intensity as well as rainfall forecast for different temporal and spatial scales to support flood management.

  • What is cloud burst? Can it be predicted? Which area mainly gets cloud burst?

If 10 cm rainfall is received at a station in one hour, the rain event is termed as cloud burst.  It is very difficult to predict the cloud bursts due to its very small scale in space and time. To monitor or nowcast (forecasting few hours lead time) the cloud burst, we need to have dense radar network over the cloud burst prone areas or one need to have a very high resolution weather forecasting models to resolve the scale of cloud burst. Cloud bursts do occur at plains, however, mountainous regions are more prone to cloud bursts due to orography.

  • What is break and active monsoon spells?

After southwest monsoon gets established over Central India in July, copious rainfall is received over large areas of the country with maximum over central India. During the peak monsoon rainfall months (July and August) of the season, the monsoon trough shifts north and south about its normal positioncausing large scale rainfall variation over the country both in terms of spatial and temporal scales. The intervals of drymonsoon conditions during which the large-scale rainfallover the monsoon trough zone (the zone between which themonsoon trough fluctuates north and south wards) is interruptedfor several days in July and August are known as thebreaks. On the other hand, the intervals between spells of dry monsoon conditions when the rainfall is higher than normal are known as active spells.  Break in the monsoon rainfall was defined as the situations when the trough of low pressure was not seen on the surface chart and the easterlies were practically absent in the lower tropospheric levels up to about 1.5 km above sea level for more than 2 days.

  • What are the criteria used to declare active and weak monsoon condition?

Criterion for declaring active monsoon condition over a meteorological sub division is

  1. Rainfall 1 ½ to4 times the normal.
  2. The rainfall in at least two stations should be 5 cm, if that sub-division is along the west coast and 3 cm, if it is elsewhere.
  3. Rainfall in that sub-division should be fairly widespread to widespread. (over the land area)
  4. Wind speed is between 23 to 32 knots(over the Sea)

Criteria for declaring weak monsoon condition over a meteorological sub division is

  1. Rainfall less than half the normal (over the land area)Wind speed upto 12 knots (over the Sea)
  • What is rainstorm?

Rainstorm is a storm characterised by substantial heavy rainfall. It is an extreme rainfall event experienced over a particular area for a particular period, in association with various weather systems of different spatial scales (Monsoon, Thunderstorms, cyclonic storm etc.)A rainstorm of any considerable duration typically consists of spurts of high-intensity rain punctuated by variable periods of low-intensity rain.Many times it has been observed that rainstorms lead to floods and landslides.

  • How can a common man get information on monsoon?

       Information on monsoon is readily available and updated daily on our website:   https://mausam.imd.gov.in. Various mobile apps are available for users such as Meghdoot, Damini, Rainalarm and the weather information is also hosted on Umang app of Government of India. In addition to this farmers can get agro advisories through sms. They can register for the service by registering onhttp://imdagrimet.gov.in/farmer/FarmerRegistrationFrontpage/welcome.php

  • What are the special forecasts provided for agriculture during monsoon season?
  1. India Meteorological Department (IMD) in active collaboration with ICAR, State Agricultural Universities and other institutes is rendering the weather forecast based Agromet Advisory Services (AAS), under GraminKrishiMausamSewa (GKMS) scheme, to the farmers at district level.
  2. Under this scheme, medium range weather forecast at district level is generated for eight weather parameters, viz., rainfall, maximum temperature, minimum temperature, morning and evening relative humidity, wind speed, wind direction and cloud cover.Based on this forecast, Agromet Advisories are prepared by the Agromet Field Units (AMFUs) located at State Agricultural Universities, institutes of ICAR and IIT etc., in collaboration with State Departments of Agriculture and communicated to the farmers to take decision on day-to-day agricultural operations.
  3. Based on the past weather conditions and regular extended range forecasts (ERF), Agromet advisories are being prepared and issued on every Friday by ICAR-CRIDA in collaboration with IMD.
  4. In addition to above, IMD monitors weather aberrations and issues alerts and warnings to the farmers from time to time under GKMS scheme. SMS-based alerts and warning for extreme weather events like cyclone, floods, hailstorm, delayed arrival of monsoon, long dry spells etc. along with suitable remedial measures are issued to take timely operations by the farmers. Such alerts and warnings are also shared with State Department of Agriculture at State level and also with respective districts in various States for the effective management of calamity.
  • What are the forecast products provided for river in flood management?

In flood management India Meteorological Department is providing QuantitativePrecipitation Forecast (QPF) for river sub basins of India for Day 1, Day 2 and Day 3.IMD also monitoring rainfall and volume of water for cumulative period of 1-week for101 river sub basins of India and using Extended Range Forecast providing rainfalland volume of water for 101 river sub basins for week 1, week 2, week 3 and week4.

  • What does IMD do for monitoring and forecasting of urban flooding?

IMD is providing real-time rainfall situation and rainfall intensity with its highlydense AWS/ARG network at major urban cities. The AWS/ARG network is beingincreased to include more urban cities. Also with Doppler Weather Radar andnowcasting it is providing expected rainfall intensities and warning if any in the majorcities of India to avoid urban flooding. In addition to existing services on urban flooding, IMD is starting Impact based forecast (IBF) for major cities from monsoon-2020 onwards. However proper urban drainage system is thekey issue in urban flood management.

  • What is IFLOWS? How does it work?

IFLOWS is a monitoring and flood warning system that will be able to relay alerts of possible flood-prone areas anywhere between six to 72 hours in advance.  The primary source for the system is the amount of rainfall, but the system also factors in tidal waves and storm tides for its flood assessments.

The system has provisions to capture the urban drainage within the city and predict the areas of flooding. The system comprises seven modules- Data Assimilation, Flood, Inundation, Vulnerability, Risk, Dissemination Module and Decision Support System.

The system incorporates weather models from the National Centre for Medium Range Weather Forecasting (NCMRWF), India Meteorological Department (IMD), field data from the rain gauge network of 165 stations set up by Indian Institute of Tropical Meteorology (IITM), BMC and IMD. It has been launched for Mumbai city on 12th June 2020.

  • What are the gap areas in monsoon monitoring and forecasting?

Monsoon forecasting was a grand challenging problem for a very long time and significant progress is made after Monsoon Mission program was launched by Ministry of Earth Sciences, Government of India. Forecasts have improved significantly in recent decade withlong lead times (short range forecasts -3 to 5 days, Extended range forecasts up to 3 weeks and long range forecasts 2 to 4 months lead time). Major gap areas of monsoon forecasting are

1. Systematic biases in mean state of monsoon in present day weather/climate models (Dry and cold bias in Coupled models and wet bias in atmospheric models).

2. If present day models get mean state correctly then they miss to capture reasonable interannual variability vice versa.

3. Interconnections of Indian summer monsoon and Indian Ocean SST is not correct in present day models.

Monitoring of boundary layer and upper air is very much essential to address the above gap areas and at present we have very few observations in the boundary layer and upper air. Enhancing the sampling in these areas over India will significantly reduce the systematic biases in the present day models.Extension and development of observational network will helpful for the better monitoring of monsoon especially for extreme rainfall events.

  • What are the science issues?
  1. How and why the interannual variability of monsoon is controlled beyond El Nino and Indian Ocean Dipole (Equinoo)?
  2. How to improve the synoptic variability (first building block of interannual variability of monsoon) in present day climate models?
  3. What is required to represent clouds accurately in the present weather forecasting and climate models?
  • What is the future plan of IMD for monsoon monitoring and forecasting?
  1. Operationalising the Impact Based Forecast (IBF) for major cities.
  2. Exploring the use of Artificial Intelligence and Machine Learning techniques(AIML) for weather services during monsoon season.
  3. Increasing the number of Automatic weather stations (AWS) and Automatic rain gauge stations (ARG).
  4. Enhanced and sustained observations in the monsoon region particularly in the boundary layer and upper air.
  5. Weather/climate models will be using very high resolution coupled models (Short range: 5km globally and 1 km locally; Extended and long range forecast: 25 km) with improved physics constrained by observations to improve the accuracy of weather/climate predictions including cyclones. The models also will employ Artificial Intelligence and Machine learning techniques to improve the accuracy of the forecasts.
  6. Multi-model ensemble forecasting for the short to medium range using various models of the MoES institutions.
  7. Monitoring of monsoon for South-Asia region.
  8. Operationalisation of Multi Model Ensemble (MME) technique for seasonal prediction.
  9. Development of integrated mobile app for weather information.
  10. Expansion of the Doppler Weather radar (DWR) network.
  11. Enhancement of the in-house research activities related to monsoon.

Rainfall Estimation using Image Processing and Regression Model on DWR Rainfall Product for Delhi-NCR Region

Observed rainfall is a very essential parameter for the analysis of rainfall, day to day weather forecast and its validation. The observed rainfall data is only available from five observatories of IMD; while no rainfall data is available at various important locations in and around Delhi-NCR. However, the 24-hour rainfall data observed by Doppler Weather Radar (DWR) for entire Delhi and surrounding region (up to 150 km) is readily available in a pictorial form. 

A recent research conducted at India Meteorological Department aimed at estimating the rainfall at desired locations using hydrological products of the Doppler Weather Radar located at IMD Headquarters in Lodhi Road, New Delhi. The study was conducted by Dr. Kuldeep Srivastava (Scientist-E & Head, RWFC, N.Delhi) and Mr. Ashish Nigam (Scientific Assistant, RWFC, N.Delhi)

In this research, firstly, the rainfall at desired locations was estimated from the precipitation accumulation product (PAC) of the DWR using image processing in Python language. After this, a linear regression model using the least square method was developed in R language. Estimated and observed rainfall data of year 2018 (July, August and September) was used to train the model. After this, the model was tested on rainfall data of year 2019 (July, August and September) and validated. The paper has been published in the Journal of Atmospheric Science Research and can be found at the link: https://ojs.bilpublishing.com/index.php/jasr/article/view/1859

Using the above, 24-hour accumulated rainfall is being estimated at 35 different locations in and around Delhi-NCR (including Noida, Greater Noida, Ghaziabad, Meerut, Gurugram, Faridabad, etc.) and it is being updated on RMC New Delhi website from 1 June 2020 onwards, daily at 09:30 AM. (Website: http://rmcnewdelhi.imd.gov.in/) 

These products are very useful for Weather Forecasters, Media & general public.

Station NameRainfall recorded by observatory/AWS (in mm)Rainfall estimated using the newly developed model (in mm)
Palam48.642.2
Ridge46.630.6
Ayanagar35.230.0
Pusa1513.7
Delhi University51.658.5
Najafgarh51.1
Noida24.2
Gr. Noida12.6
Ghaziabad33.7
Meerut43.245.6
Gurugram115.7
Faridabad3425.4
Rainfall Comparison for 5th July 2020 between the observed rainfall and the rainfall estimated using the newly developed model

Precipitation Accumulation (PAC) Product, dated 5 July 2020, generated by the Doppler Weather Radar, situated at IMD HQ, Lodhi Road, New Delhi

Lightning: Myths and facts

Myth #1 – Lightning never strikes twice in the same place.

Fact: Lightning often strikes the same place repeatedly, especially if it’s a tall, pointy, isolated object. The Empire State Building was once used as a lightning laboratory because it is hit nearly 25 times per year, and has been known to have been hit up to a dozen times during a single storm.

Myth #2 – Lightning only strikes the tallest objects.

Fact: Lightning is indiscriminate and it can find you anywhere. Lightning may hit the ground instead of a tree, cars instead of nearby telephone poles, and parking lots instead of buildings.

Myth #3 – If you’re stuck in a thunderstorm, being under a tree is better than no shelter at all.

Fact: Sheltering under a tree is just about the worst thing you can do. If lightning does hit the tree, there’s the chance that a “ground charge” will spread out from the tree in all directions. Being underneath a tree is the second leading cause of lightning casualties.

Myth #4 – If you don’t see rain or clouds, you’re safe.

Fact: Lightning often strikes more than three miles from the thunderstorm, far outside the rain or even the thunderstorm cloud. Though infrequent, “bolts from the blue” have been known to strike areas as distant as 10 miles from their thunderstorm origins, where the skies appear clear.

Myth #5 – A car’s rubber tires will protect you from lightning

Fact: True, being in a car will likely protect you. But most vehicles are actually safe because the metal roof and sides divert lightning around you—the rubber tires have little to do with keeping you safe. Convertibles, motorcycles, bikes, open shelled outdoor recreation vehicles and cars with plastic or fiberglass shells offer no lightning protection at all.

Myth #6 – If you’re outside in a storm, lie flat on the ground.

Fact: Lying flat on the ground makes you more vulnerable to electrocution, not less. Lightning generates potentially deadly electrical currents along the ground in all directions—by lying down, you’re providing more potential points on your body to hit.

Myth #7 – If you touch a lightning victim, you’ll be electrocuted.

Fact: The human body doesn’t store electricity. It is perfectly safe to touch a lightning victim to give them first aid.

Myth #8 – Wearing metal on your body attracts lightning.

Fact: The presence of metal makes very little difference in determining where lightning will strike. Height, pointy shape and isolation are the dominant factors in whether lightning will strike an object (including you). However, touching or being near metal objects, such as a fence, can be unsafe when thunderstorms are nearby. If lightning does happen to hit one area of the fence—even a long distance away—the metal can conduct the electricity and electrocute you.

Myth #9 – A house will always keep you safe from lightning.

Fact: While a house is the safest place you can be during a storm, just going inside isn’t enough. You must avoid any conducting path leading outside, such as electrical appliances, wires, TV cables, plumbing, metal doors or metal window frames. Don’t stand near a window to watch the lightning. An inside room is generally safe, but a home equipped with a professionally installed lightning protection system is the safest shelter available.

Myth #10 – Surge suppressors can protect a home against lightning.

Fact: Surge arresters and suppressors are important components of a complete lightning protection system, but can do nothing to protect a structure against a direct lightning strike. These items must be installed in conjunction with a lightning protection system to provide whole house protection.

Collected from: https://www.iii.org/article/lightning-safety-10-myths-and-the-facts

Webinar on Thunderstorms and Lightning

A joint training program webinar was conducted on 14 July 2020 (10:30 AM to 01:00 PM) by National Institute of Disaster Management (NIDM) and India Meteorological Department (IMD) to sensitize participants regarding assessment, forecast, preparedness and mitigation of Hydro meteorological Disasters in the form of Thunderstorms and lightning. The program was inaugurated by Shri Nityanand Rai (Hon. Minister of State, Ministry of Home Affairs, Government of India) in the presence of Dr. N. Rajeevan (Secretary Ministry of Earth Sciences), Dr. M. Mohapatra (Director General of Meteorology, India Meteorological Department) and  Maj.Gen. N.K.Bindal (ED NIDM). Dr Thiruppugazh (Addl. Secretary, NDMA), Prof Surya Prakash (NIDM) and Dr S.Sen Roy (NWFC, IMD) were the resource persons for the meet. 

The webinar also aims to highlight scientific and engineering endeavours aimed at addressing critical gaps in knowledge about these phenomena and understanding adverse effects of thunderstorms & lightning as well as available resources for timely response and recovery. Experts also believe that the severity and frequency of thunderstorm/dust storms are expected to rise in years ahead due to rising global temperature. The increase in occurrence and severity is a wake-up call for all agencies to take appropriate action for prevention, preparedness and mitigation in order to save lives, livestock, property and infrastructure. It becomes a challenge for disaster managers to take preventive and mitigation measures through preparation of action plan.

Kindly follow the YouTube link below for details and presentation in the webinar: https://www.youtube.com/watch?v=9Zv1Rms78dU

Action – Before, during and after a Thunderstorm event

Before Thunderstorm and Lightning

To prepare for a thunderstorm, you should do the following:

· Do remember that vivid and frequent lightning indicates the probability of a strong thunderstorm. · To begin preparing, you should build an emergency kit and make a family communications plan.

· Remove dead or rotting trees and branches that could fall and cause injury or damage during a severe thunderstorm.

· Postpone outdoor activities.

· Remember the 30/30 Lightning Safety Rule: Go indoors if, after seeing lightning, you cannot count to 30 before hearing thunder. Stay indoors for 30 minutes after hearing the last clap of thunder.

· Secure outdoor objects that could blow away or cause damage.

· Get inside a home, building, or hard top automobile (not a convertible). Although you may be injured if lightning strikes your car, you are much safer inside a vehicle than outside.

· Remember, rubber-soled shoes and rubber tires provide NO protection from lightning. However, the steel frame of a hard-topped vehicle provides increased protection if you are not touching metal.

· Shutter windows and secure outside doors. If shutters are not available, close window blinds, shades or curtains.

· Unplug any electronic equipment well before the storm arrives.

During Thunderstorms and Lightning

If thunderstorm and lightning are occurring in your area, you should:

· Avoid contact with corded phones and devices including those plugged into electric for recharging. Cordless and wireless phones not connected to wall outlets are OK to use.

· Avoid contact with electrical equipment or cords. Unplug appliances and other electrical items such as computers and turn off air conditioners. Power surges from lightning can cause serious damage.

· Avoid contact with plumbing. Do not wash your hands, do not take a shower, do not wash dishes, and do not do laundry. Plumbing and bathroom fixtures can conduct electricity.

· Stay away from windows and doors, and stay off porches.

· Do not lie on concrete floors and do not lean against concrete walls.

· Avoid natural lightning rods such as a tall, isolated tree in an open area.

· Avoid hilltops, open fields, the beach or a boat on the water.

· Take shelter in a sturdy building. Avoid isolated sheds or other small structures in open areas.

· Avoid contact with anything metal-tractors, farm equipment, motorcycles, and bicycles.

· If you are driving, try to safely exit the roadway and park. Stay in the vehicle until the Strong rain ends. Avoid touching metal or other surfaces that conduct electricity in and outside the vehicle.

After a Thunderstorm or Lightning Strike

If lightning strikes you or someone you know, call for medical assistance as soon as possible. The following are things you should check when you attempt to give aid to a victim of lightning:

·Breathing – if breathing has stopped, begin mouth-to-mouth resuscitation.

·Heartbeat – if the heart has stopped, administer CPR.

·Pulse – if the victim has a pulse and is breathing, look for other possible injuries. Check for burns where the lightning entered and left the body. Also be alert for nervous system damage, broken bones and loss of hearing and eyesight.

After the storm passes remember to:

· Never drive through a flooded roadway. Turn around, don’t drown!

· Stay away from storm-damaged areas to keep from putting yourself at risk from the effects of severe thunderstorms.

· Continue to listen to a local radio and television stations for updated information or instructions, as access to roads or some parts of the community may be blocked.

· Help people who may require special assistance, such as infants, children and the elderly or those with access or functional needs.

· Stay away from downed power lines and report them immediately.

· Watch your animals closely. Keep them under your direct control.

RAINBOW: AN OPTICAL PHENOMENA

Rainbow is one of the most spectacular light shows observed on the earth. The formation of rainbow is as much as an optical phenomenon, as it is a meteorological phenomenon in the atmosphere. Generally, the rainbow can be observed during morning or evening hours, when the rain is falling on the one part of the sky & the sun is shining on the another. It causes due to the reflection and refraction of sunlight by the rain droplets suspended in the air.

The sunlight consists different wavelengths (380-750 nm) in the visible light corresponds to their colours: red, orange, yellow green, blue indigo & violet. The violet light has least wavelength (380-450nm) while, red light has longer wavelength (520-750 nm).

As shown in figure 1, when sunlight enters from air to denser medium water, a part of it gets reflected & remaining gets refracted or split into its component colors. Generally, the rays of light refracts twice within the droplet. The violet light of the ray (shorter wavelength) refracts the most and red light (longer wavelength) the least. Although most of this light passes right on through the drop and then reflect off the back of the drops. The ray bend again while emerging out the drops. During this optical process, each color light bends differently from others due to slight difference in wavelength and each color ray emerges from the drop at a slightly different angle. For red light, the angle is 42° from the beam of sunlight and for violet light, it is 40° (figure 1). The light leaving the drop dispersed into a spectrum of colors from red to violet and appears as a rainbow.

  1. Shape of rainbow:

The shape & brightness of rainbow depends upon the size of water droplets and the relative location of sun & observer. Brightest rainbow can be seen when the water droplets are large, usually right after a rain shower. If the sun is very low in the sky, either just before sunset or just after sunrise, it can be seen as half circle. The higher the sun is in the sky, lesser the rainbow will be seen.

Generally, the actual shape of rainbow is circular, however it can be seen as half circle or a part of it on the earth surface. The only way to see the full circle of a rainbow in the sky from an airplane and have the sun behind the observer.

2. Double rainbow:

Sometimes, a larger secondary rainbow with its colors reversed can be seen above the primary rainbow. Usually this secondary bow is much dimmer than the primary one. It is caused when sunlight enters the raindrops at an angle that allows the light to make two internal reflections in each drop. Each reflection weakens the light intensity and results the dimmer secondary bow. The reversal of the color order causes the way the light emerges from each drop after going through two internal reflections. Double rainbow spotted in Jaipur on 7th  June, 2020 just after the rain shower in the evening (Figure 2).

Tropical Cyclones & IMD Warning (Part-2)

1. What are the pre-cyclone/during the cyclone/post cyclone responsibilities of a common man?

  • Steps to be taken before the cyclone:
  1. Check houses, secure loose tiles by cementing wherever necessary, repair doors and windows.
  2. Check the area around the house -remove dead or dying trees, anchor removable objects like lumber piles, loose bricks, garbage cans, sign-boards, loose zinc sheets etc.
  3. Keep some wooden boards ready so that glass windows can be boarded.
  4. Keep a hurricane Lantern filled with kerosene, flash light and enough dry cells.
  5. Promptly demolish condemned buildings.
  6. Those who have radio sets should ensure that the radio is fully serviceable in the case of transistors an extra set of batteries should be kept handy.

Steps to be taken during the cyclone:

  1. Keep your radio on and listen to latest weather warnings and advisories from the nearest All India Radio station. Pass the information to others.
  2. Avoid being misled by rumors. Pass only the. Official information you have got from the radio to others.
  3. Get away from low lying beaches or other locations which may be swept by high tides or storm waves. Leave sufficiently early before your way to high ground gets flooded. Do not delay and run the risk of being marooned.
  4. If your house is out of danger from high tides and flooding from the river, and it is well built, it is then probably the best place during weather and storm. However, please act promptly if asked to evacuate.
  • Be alert for high water in areas where streams of rivers may flood due to heavy rains.
  • Board up glass windows or put storm shutters in place. Use good wooden planks Securely fastened. Make-shift boarding may do more damage than none at all. Provide strong suitable support for outside doors.
  • If you do not have wooden boards handy paste paper strips on glasses to prevent splinters flying into the, house.
  • Get extra food, specially things which can be eaten without cooking or with very little preparation. Store extra drinking water in suitable covered vessel.
  • If you are in one of the evacuation areas, move your valuable articles to upper floors to minimise flood damage.
  • Have hurricane lantern, flash lights and/or other emergency light in working condition and keep them handy.
  • Check on everything that might blow away or be torn loose. Kerosene tins, cans, agricultural implements, garden tools, road signs and other objects become weapon of destruction in strong winds. Remove them and store them in a  covered room.
  • Be Sure that a window or door can be opened on the lee side of the house i.e. the side opposite the one facing the wind.
  • Make provisions for children and adults requiring special diets.
  • If the centre of’ ‘eye’ of the storm passes directly over your place, there will be a lull in the wind and rain, lasting for half an hour or more. During this period stay in safe place. Make emergency repairs during the lull period if necessary, but remember that strong wind will return suddenly from the opposite direction, frequently with even greater violence.
  • Be calm. Your ability to meet emergency will inspire and help others.

2. What are the causes of disaster during cyclone?

The dangers associated with cyclonic storms are generally three fold.

  • Very heavy rains causing floods
  • Strong wind
  • Storm surge

Let us discuss each separately:

  • The rainfall associated with a Cyclone vary from system to system even with the same intensity. Record rainfall in a cyclonic storm has been as low as trace to as high as 250 cms. It has been found that the intensity of rainfall is about 85 cms/day within a radius of 50 kms and about 35 cms/day between 50 to 100 kms from the centre of the storm. Precipitation of about 50 cm/day is quite common with a C.S. This phenomenal rain can cause flash flood.
    • The strong wind speed associated with a cyclonic storm. (62-88 kmph) can result into some damage to kutcha houses and tree branches likely to break off. Winds of a severe Cyclonic storm (89-117 kmph) can cause uprooting of trees, damage to pucca houses and disruption of communications. The wind associated with a very severe Cyclonic storm and super cyclonic storm can uproot big trees, cause wide spread damages to houses and installations and total disruption of communications. The maximum wind speed associated with a very severe Cyclonic storm that hit Indian coast in the past 100 years was 260 kmph in Oct, 1999 (Paradip Super cyclone).
    • The severest destructive feature of a tropical storm is the storm surge popularly called tidal waves. The coastal areas are subjected to storm surge and is accentuated if the landfall time coincides with that of high tides. This is again more if the sea bed is shallow. Storm surge as high as 15 to 20 ft. may occur when all the factors contributing to storm surge are maximum. This storm tide inundates low lying coastal areas which has far reaching consequences apart from flooding. The fertility of land is lost due to inundation by saline water for a few years to come.

3. When does a coastal station start experiencing bad weather associated with a Cyclone?

Coasts come under the influence of bad weather in the form of heavy rain, gale winds (exceeding 65 kmph) when the cyclone moves closer to the coast within 200km. Heavy rainfall generally commences about 9-12 hours before cyclone landfall. Gale force winds commence about 6-9 hours in advance of cyclone landfall. Maximum storm surge may appear at or near the landfall time.

4. What is a Storm Surge?

Storm Surge is an abnormal rise of sea level as the cyclone crosses the coast. Sea water inundates the coastal strip causing loss of life, large scale destruction to property & crop. Increased salinity in the soil over affected area makes the land unfit for agricultural use for two or three seasons.

Storm surge depends on intensity of the cyclone (Maximum winds and lowest pressure associated with it and Coastal bathymetry (shallower coastline generates surges of greater heights).

5. What is the vulnerability our coastline from the point of view of storm surge potential?

Entire Indian coast can be categorized into 4 zones

  • Very high risk zones (Surge height > 5m)
  • High risk Zone (Surge height between 3-5m)
  • Moderate risk zone (Surge height between 1.5 to 3m)
  • Minimal risk zone (Surge height < 1.5m)

Accordingly

  • The coastal areas and off-shore islands of Bengal and adjoining Bangladesh are the most storm-surge prone (~ 10-13m) – VHRZ
  • East coast of India between Paradip and Balasore in Orissa (~ 5-7m) – VHRZ
  • Andhra coast between Bapatla and Kakinada holding estuaries of two major rivers Krishna and Godavari (~ 5-7m) – VHRZ
  • Tamilnadu coast between Pamban and Nagapattinam (~ 3-5m) – HRZ
  • Gujarat along the west coast of India (~ 2-3m) –MRZ.

Tropical Cyclones & IMD Warning

1. Tropical cyclone and its annual frequency

The tropical cyclones develop over the warm waters of the Bay of Bengal and the Arabian Sea during the months of May-June and October-November with severe intensity.

The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5. The frequency is more in the Bay of Bengal than in the Arabian Sea, the ratio being is 4:1.

The terms “hurricane” and “typhoon” are region specific names for a strong “tropical cyclone”. Tropical cyclones are called “Hurricanes” over the Atlantic Ocean and “Typhoons” over the Pacific Ocean.

A tropical cyclone (TC) is a rotational low-pressure system in tropics when the central pressure falls by 5 to 6 hPa from the surrounding and maximum sustained wind speed reaches 34 knots (about 62 kmph). It is a vast violent whirl of 150 to 800 km, spiraling around a centre and progressing along the surface of the sea at a rate of 300 to 500 km a day.

2. What is the energy potential of a Tropical Cyclone?

Tropical Cyclone can be compared to a heat engine. The energy input is from warm water and humid air over tropical oceans. Release of heat is through condensation of water vapour to water droplets/rain. Only a small percentage (3%) of this released energy is converted into Kinetic energy to maintain cyclone circulation (wind field). A mature cyclone releases energy equivalent to that of 100 hydrogen bombs.

  • Wind speed criteria for different rain bearing systems form over the ocean
SystemAssociated wind speed Knots (Kmph)
Low pressure area<17(<31)
Depression17-27 (31-49)
Deep Depression (DD)28-33 (50-61)
Cyclonic Storm (CS)34-47 (62-88)
Severe        Cyclonic       Storm (SCS)48-63 (89-117)
Very       Severe         Cyclonic Storm (VSCS)64-89 (118-166)
Extremely Severe Cyclonic Storm (ESCS)90-119 (167-221)
Super Cyclonic Storm≥ 120 (≥ 222)
Wind speed criteria for different rain bearing systems form over the ocean

3. How do the cyclones form and intensify?

In the tropics, weak pressure waves move from east to west. These are  called easterly waves. Under favourable situation, a low pressure area forms over the area of an easterly trough. This gives rise to low level convergence. If the sea is warm (sea surface temperature ≥ 26.50 C) and there is sufficient upper level divergence i.e air is blown off at higher levels from the area of low pressure, the pressure gradually falls. Low level convergence coupled with upper level divergence gives rise to vertical motion taking moist air upwards. These moistures condense at higher levels (middle troposphere) and give out latent heat of condensation. Due to release of heat of condensation the area warms up resulting into further fall in pressure. This process continues and a low pressure system gradually intensifies into a cyclonic storm.

4. What is the size of a tropical cyclone over the north Indian Ocean?

The size of a Tropical Cyclone over Indian seas varies from 50-100 km radius to 2000 km with an average of 300 –600 km.

5. What is the life period of cyclones?

Life period of a tropical cyclone over the north Indian Ocean is 5-6 days. It will have hurricane intensity for 2-4 days as against 6 days of global average.

6. What is the normal movement of a Tropical Cyclone?

Tropical cyclones move as a whole. They casually move west- northwestwards or northwestwards in the northern hemisphere. The average speed is 15-20 kmph (360-480 km per day). They may change their direction of movement towards north. During this change their speed of movement decreases to 10 kmph or even less. A larger fraction of such storms later turn towards northeast and move northeastwards very fast at a speed of 25 kmph or more.

7. How are Tropical Cyclones monitored by IMD?

IMD has a well-established and time-tested organization for monitoring and forecasting tropical cyclones. A good network of meteorological observatories (both surface and upper air) is operated by IMD, covering the entire coastline and islands. The conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite systems. INSAT imagery obtained at hourly intervals during cyclone situations has proved to be immensely useful in monitoring the development and movement of cyclones.

8. How track prediction is done in IMD?

IMD has one of the best forecasting system for predicting tropical cyclones, using high resolution weather prediction models including global, regional and cyclone specific models.

The ensemble forecast systems introduced in the recent past provides probabilistic guidance for track and intensity of cyclones and depressions. Regional models like Hurricane Weather Research Forecast (HWRF) have also been implemented to generate forecasts at very high resolution of 2km. IMD is also planning to make the ocean atmosphere coupled Hurricane Weather Research & Forecast model operational which has the potential to predict intensity more accurately.

9. What is our accuracy of landfall prediction?

Probability of correct forecast decreases with increasing forecast validity period. Mean forecast errors for 12, 24, 48 and 72 hours are about 49, 81,129 and 165 km respectively, which are comparable to corresponding figures of other centres like National Hurricane Centre, Miami, which monitor Atlantic Hurricanes; Typhoon Warning Centre, Tokyo, which monitors Typhoons of Northwest Pacific etc.

10. What is the organizational set up in IMD for Cyclone forecasting and Warning?

The Cyclone Warning Organization in India has a 3-tier system to cater to the needs of the maritime States. These are: Cyclone Warning Division set up at IMD Head Quarters to co-ordinate and supervise cyclone warning operations in the country and to advise the Govt. at the apex level; Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata and Cyclone Warning Centres at Visakhapatnam, Ahmedabad, Bhubaneswar and Thiruvanathapuram

11. What is 4-stage warning system for Tropical Cyclones?

Expectations of Disaster Managers are longer lead time and improved accuracy of landfall forecast. But the present state of art has limitations to make the above requirements go hand in hand. Lead time depends on the formation and duration of cyclone itself which may vary considerably from one cyclone to another. However, since pre-monsoon cyclone season of 1999, IMD introduced a 4-Stage warning system to issue cyclone warnings to the disaster managers. They are as follows:

(1)    Pre-Cyclone Watch

Issued when a depression forms over the Bay of Bengal irrespective of its distance from the coast and is likely to affect Indian coast in future. The pre-cyclone watch is issued by the name of Director General of Meteorology and is issued at least 72 hours in advance of the commencement of adverse weather. It is issued at least once a day.

(2)    Cyclone Alert

Issued atleast 48 hours before the commencement of the bad weather when the cyclone is located beyond 500 Km from the coast. It is issued every three hours.

(3)    Cyclone Warning

Issued at least 24 hours before the commencement of the bad weather when the cyclone is located within 500 Km from the coast. Information about time /place of landfall are indicated in the bulletin. Confidence in estimation increases as the cyclone comes closer to the coast

(4)    Post landfall outlook

It is issued 12 hours before the cyclone landfall, when the cyclone is located within 200 Km from the coast. More accurate & specific information about time /place of landfall and associated bad weather indicated in the bulletin. In addition, the interior distraction is likely to be affected due to the cyclone are warned in this bulletin.

12.  Who are the recipients of Cyclone Warnings?

Warnings are issued for general public, fishermen, farmers and different categories of users such as central and state government officials responsible for disaster mitigation and relief, industrial and other establishments located in the coastal areas, ports, coastal shipping, railways, aviation, transport, communication and power authorities.

13.  How a common man gets information about a cyclonic storm?

Local AIR broadcast hourly (or more frequently) bulletins in local language as well as in Hindi and English. The bulletins give the location of the Cyclonic storm, its direction of movement, place and time of landfall and details of adverse weather expected over the areas likely to be affected by the storm. AIR, New Delhi issues bulletins thrice in a day giving similar information. Apart from that, the cyclone warning messages are sent to the collectors of the districts likely to be affected and the chief secretary of concerned state. The state Govt. takes necessary steps to inform the local population through their machinery such as police wireless etc. They make necessary arrangement for evacuation from coastal area and for removal of the population to other places.

14.  How does IMD keep liaison with State officials?

Area Cyclone Warning Centres (ACWCs) and Cyclone Warning Centres (CWCs) maintain liaison with the concerned state Governments in state and district levels on cyclone related activities. The cyclone warning bulletins are  communicated to the Chief Secretary, Revenue Secretary, Special Relief Commissioner, State control room, State Disaster Management Authority and concerned district collectors every three hourly. In addition, the Chief Secretary is personally briefed by Director, ACWC/CWC regularly. Before the cyclone season, ACWC/CWC organizes the precyclone preparedness meeting under the chairmanship of Chief Secretary where all the high state Govt. officials from various departments participate.