In terms of frequency, recent thunderstorm and dust storm activity was higher than normal, a top meteorologist said in an interview, adding that the intensity was also high, but not record-breaking

Recent thunderstorms have killed hundreds of people across India (Photo by Bogdan Radu)

Vast swathes of India have recently been buffeted by high-intensity thunderstorms and dust storms that led to hundreds of deaths and millions in property damage. M. Mohapatra, additional director general of meteorology, National Weather Forecasting Centre, India Meteorological Department (IMD), explained the reasons behind the intense dust storms and thunderstorms this year. Edited excerpts from an interview to

How is a thunderstorm formed?

Thunderstorm is a group of weather events. A thunderstorm is always accompanied by thunder and lightning, usually with strong gusts of wind, heavy rain, and sometimes with hail. The lifecycle of a thunderstorm may vary from half an hour to three hours.

The genesis of a thunderstorm is dependent on four factors. The first among them is intense heating. Lower level atmosphere and surface of earth should be hot. That is why thunderstorms activity is at its maximum during the summer season. Because of heating, the parcel of air becomes lighter and leads to low density of atmosphere.

The second parameter is moisture availability. If there is moisture, air becomes moist and hot, moist air is lighter than dry air and rises. As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapour it contains begins to cool, releases heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing and various types of ice particles can be created from freezing liquid drops.

The third parameter for thunderstorm formation is instability in the atmosphere. It is explained through lapse rate, which means change of temperature with height.

But we don’t get thunderstorms everyday in the summer.

Even if all these three parameters — intense heat, moisture availability and instability in the atmosphere — are there daily, we do not experience thunderstorms daily. And, here the fourth parameter comes into picture, which is the “trigger” that leads the air to move up rapidly. This triggering comes when there is a weather system. It may be a trough line, or a cyclonic circulation, or a Western Disturbance.

This triggering varies from place to place. In north India and northwest India, the trigger this time was the Western Disturbances. But, in south India, there are no Western Disturbances, but still it is experiencing thunderstorms.

M. Mohapatra, director general of meteorology, National Weather Forecasting Centre, India Meteorological Department

In south India, there is wind discontinuity. The Bay of Bengal gets anti-cyclonic circulation and easterly winds come from east to west. Similarly, the Arabian Sea may have an anti-cyclonic circulation and westerly winds from west to east. These easterly and westerly winds converge on peninsular India and trigger the air to move up rapidly.

What about thunderstorms in northeast India?

Northeast India faces maximum thunderstorms because moisture-laden winds come from the Bay of Bengal and there are Garo-Khasi hills, Myanmar hills and the eastern parts of Himalaya. These winds collide and there is orographic uplifting of winds (due to higher elevation). Because of this, there is very intense thunderstorm activity over northeast India in the months of April, May and June, which can also cause floods and landslides. Thunderstorms in northeast India continue for a longer period of time, but aren’t reported much. Because this year people in and around Delhi got affected, there is a lot of noise around thunderstorms.

Why did we have very strong thunderstorms this year in north India and northwest India?

This year we have had thunderstorm activity over different parts of the country, especially in association with the Western Disturbances. Activity over northwest India has been quite above normal this year, mainly because of the Western Disturbances that acted as trigger. Since April, seven Western Disturbances have passed over northwest India.

So far in May, three such disturbances have passed over northwest India and all were very intense, for instance, on May 2, May 7, and May 13. In terms of frequency, the thunderstorm activity is higher than normal. The intensity is also high, but not record-breaking. Highest wind gust over Delhi was recorded at 141 kmph (kilometre per hour) on May 10, 2006. Next highest was 122 kmph on May 30, 2014. This year, the highest wind gust has been 107 kmph on May 13, which is third highest since 2000.

How does the IMD forecast thunderstorms?

Thunderstorm is a small-scale phenomenon and has a life cycle of up to only three hours. It has a dimension of up to 20 km to 30 km, and therefore, its detection is difficult. Automatic weather stations (AWS) provide some basic parameters such as wind speed, wind direction, temperature, pressure etc.

The second tool is satellite, but it is looking from 36,000 km height. And, it takes about half an hour to capture the image, and another half an hour to process the data. So, by the time a person is looking at satellite imagery at the IMD website, it is already one hour late. Hence, a satellite cannot capture initiation of thunderstorm, unless it is a large-scale thunder activity.

The third tool is Doppler Weather Radar, which is a good tool and takes observation every 10 minutes and can find out the occurrence of thunderstorms. Therefore, for better monitoring, we need a wider network of radars in the country.

To forecast thunderstorms, we utilise current weather observations from our observatories, automatic weather stations, satellite observation, and radars to define what is happening where in case of thunderstorms. Any observational instrument doesn’t give forecast. We have to develop expert tools to provide forecast based on these observations. And, for this some statistical software is used that go for extrapolation. But statistical software has limitations.

What are these statistical software and models used for forecasting thunderstorms?

We cannot predict occurrence and intensity of thunderstorms from numerical models well in advance. At present, we have a global model with a resolution of 12 km and a regional model with resolution of 3 km. With these models, we can predict the area of occurrence broadly.

Apart from these two models, there are some more models of other countries — a total of 10 models — that are taken into consideration every day at 10.30 a.m. by the IMD. All our forecasters across the country examine and discuss the output of these models through a video conference daily from 10.30 a.m. to 12 noon.

Based on consensus, we go for five days (120 hours) forecast. This is a general forecast issued four times in a day from our head office in Delhi and twice a day from our state offices. This forecast gives potential area for warning and is meant for preparedness and not immediate warning.

What about issuing an alert on the day of a severe weather activity?

On the day of occurrence, when we find out about certain developments, we start nowcasting. Nowcasting is valid for the next two to three hours, thus it gives only a limited lead-time. This nowcast, which is at the district level, is provided to relief commissioners, state control rooms, district collectors, disaster management units etc. This alert is specific for a district.

The five-days forecast is for a meteorological subdivision. Nowcast is more specific with time of occurrence, intensity, wind speed etc. Nowcasting is issued any time when we are expecting a severe weather event.

Does the IMD issue nowcast alert to the public?

At present, IMD is not issuing nowcasting bulletin to the general public. This is not our mandate. But, in a recent meeting (after the May 2 thunderstorms and destruction), it was decided by the NDMA (National Disaster Management Authority) that as soon as IMD issues any severe weather warning, it should be flashed to the mobile phone service providers so that the maximum number of people can receive the warning.

To begin with, the NDMA is planning to test it out with the BSNL (Bharat Sanchar Nigam Limited, the state-run telecom operator). After the testing phase, it will be implemented shortly. And, that is the only solution for general public to be informed apart from the media such as All India Radio, Doordarshan and FM radio.

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