Informist, Friday, May 20, 2022

By Preeti Bhagat and Saji George Titus 

NEW DELHI – What was the wettest region is not so wet any more. What was driest is not as dry too. 

Monsoon rain over eastern India, among the world's wettest regions, has been decreasing over the years, says Mrutyunjay Mohapatra, director general of meteorology of the India Meteorological Department.

"If you look at the rainfall distribution in different regions, you will find that rainfall in certain areas is decreasing and in certain areas it is increasing," says Mohapatra in an interview to Informist.

In 12 of the last 20 southwest monsoon seasons, rains over northeast India were deficient. This even as the overall rains in the country have remained more or less constant over the last 100 years, with a decrease in one area offset by an increase in other parts, he says.

While rainfall over Jharkhand and the northeastern states is decreasing, western Rajasthan, considered a desert region, is seeing an increase, says Mohapatra.

"If you look at rainfall for last 100 years, you will find that for the amount of rainfall for the country as a whole, there is no trend."

"It is a good thing that we do not have any trend, irrespective of the impact of climate change," Mohapatra says.

Climate change, however, is increasing the frequency and intensity of extreme weather events in India like heatwave, heavy rainfall and severe cyclones.

The impact of climate change is, however, apparent when once sees higher frequency and intensity of extreme weather events in India like heat waves, heavy rainfall and severe cyclones.

"Climate change certainly has an impact on extreme weather," the weather bureau chief says. "It is found globally, and also in Indian region, that because of climate change, the frequency and intensity of extreme events like heat waves or heavy rainfall is increasing."

"Long term data suggests that frequency and intensity (of extreme weather) is increasing. That means there is an increasing trend, but that doesn't mean every year you will have increased frequency."

The frequency of very heavy rainfall is increasing, especially in the central part of the country, Mohapatra says.

Following are edited excerpts from the interview with Mohapatra:

Q. You have predicted monsoon onset this year to be five days ahead of normal. Will the early onset have any impact on overall coverage and rains?

A. There is no one-to-one relationship between onset of monsoon over Kerala and the performance of monsoon for the country as a whole. It can enhance the rainfall activity in the beginning of the season. But that doesn't mean that the progress of monsoon will be early for the entire country.

Also, it doesn't mean that monsoon rainfall will be higher for the entire country if it comes four days early. Monsoon is an evolutionary process. The amount of rainfall finally depends on many intra-seasonal variations and many large-scale forces acting on the monsoon.

Q. Your first forecast was almost a month ago. How has been the trend since then? Have things improved since the first forecast?

A. I cannot say that now. We will bring our updated forecast in last week of May.

Q. When is the monsoon expected to cover the entire country?

A. There is no such forecast for that. Normally, the monsoon sets in on Jun 1, and then it takes about more than a month to cover the entire country. The normal date to cover the entire country is Jul 8. There is no direct relationship that if it has come early in Kerala it will come early in Rajasthan too.

Q. We have seen extreme weather this year--very harsh summer and winter. Is this because of climate change, or is it just a one off event?

A. Climate is changing and certainly there is enhanced increase in temperature in recent years, which we popularly call global warning. Climate change certainly has an impact on extreme weather. It is found globally, and also in India, because of the climate change, the frequency and intensity of extreme events like heat waves or heavy rainfall are increasing.

Cyclones are not increasing, but extreme severe cyclones are increasing over the Arabian Sea. While there is an overall trend, it is embedded with the climate variability.

So sometimes you may have below-normal temperature, sometimes above-normal temperature. The impact of climate changes doesn't mean that every year temperature will rise or every year you will have heatwave conditions. For example, if you look at last two years, the frequency and intensity of heat waves was quite less and this year it has been quite high. These are all part of climate variability.

If you really want to say that it is an impact of climate change, then you have to see the long term data. Long term data suggests that frequency and intensity is increasing. That means there is an increasing trend, but that doesn't mean that every year you will have increased frequency.

Q. Will extreme weather this year have an impact on the monsoon?

A. At present, we predict monsoon with a dynamic model, which takes into account atmospheric processes, land surface processes and oceanic processes. Prior to that there was a statistical model, using six parameters. For example, one of the parameters is Eurasian snow cover in winter months, which has an inverse relationship with the monsoon. So if snow cover is more, then monsoon rainfall is expected to be less. You can understand the physics, if the snow cover is more, the temperature contrast is less and hence monsoon will be less.

Similarly, there is a relationship with temperature over the Pacific Ocean--that is El Nino and La Nina. Those six parameters, which have a strong co-relation with monsoon, are identified and used to predict the rainfall. Only one parameter cannot decide whether monsoon will be good or bad. It is a combination of parameters because there are many forces that are acting.

Monsoon is a large-scale process. It is a planetary process, which extends from Japan to the US. So you cannot decide performance of monsoon based on one or two parameters. It is a combined effect. Based on extremes that occurred in December and April, we cannot say there will be more rainfall. It is not advisable to come to a conclusion based on only one or two parameters.

Q. You have changed the normal rainfall for monsoon this year. The normal rainfall is now lower by 1.6% than earlier. Is it a decadal cycle or monsoon rainfall is generally coming down?

A. If you look at rainfall for last 100 years, you will find that for the amount of rainfall for the country as a whole, there is no trend. Monsoon rainfall is random. There is no such trend. It is a good thing that we do not have any trend irrespective of the impact of climate change. For the country as a whole, there is not much change in the total monsoon rainfall in the last 100 years. It is almost the same.

But if you consider decade-wise, then you will find that it shows a decadal behaviour, that is an epochal behaviour. For 3-4 decades, you will find that monsoon is active, followed by 3-4 decades where rainfall will be less. If we look at data from 1970s, we will find that it has entered into a negative epoch, that is, rainfall is in the deficient side or below-normal side. That is why if you look at 1970s, 1980s, 1990s, the average rainfall is in the negative side. So when we calculate the normal based on the data of recent years, certainly it is based on data of the negative epoch, and you find that normal is slightly less than earlier. But that does not really mean it is a trend. It is because of the decadal variability of the monsoon.

Q. But if you look at region wise it is definitely changing, isn't it?

A. Certainly. If you look at the rainfall distribution in different regions, you will find that rainfall in certain areas is decreasing and in certain it is increasing. Most importantly, frequency of very heavy rainfall activity is increasing in the country, especially in the central part.

Q. Will that mean more floods?

A. When you have this type of activity, when frequency of light rains or moderate rains is decreasing and frequency of heavy rains is increasing. That means if it is not raining, it is not raining at all and whenever it rains, it rains heavily.

Q. Which are the areas where rainfall is going down, and which are the areas where it is going up?

A. If you look at western part of the country, like western Rajasthan, rainfall is increasing. In eastern parts like Jharkhand and northeastern states, it is decreasing.

Q. I'm a layman, is it because rainfall belt is moving towards the west?

A. There is no shift in the monsoon rainfall belt and also there is no shift in the duration of the monsoon. Every year, around Jun 1, you will find monsoon arrives in Kerala. There is no spatial or temporal shift. But still it needs further investigation to find out the causes for this type of behaviour. There needs to be more fact finding based on data on regional weather.

Q. I remember interviewing one of your predecessors many years ago. One of the things he said was monsoon is a phenomenon which we have not completely understood. Do we know enough about monsoon to predict it?

A. That was 12 years back. Now our knowledge is improving. At that time, we did not have the coupled forecasting system. At that time, we did not have forecasts based on so many numerical models. At that time, we did not have the extended range forecast for next four weeks, and now we have the extended range forecast every week. At that time, we did not have district-level forecast based on western dynamic models for agriculture. So there has been significant improvement in last 10-12 years in terms of the numerical modelling capability, in terms of high performance computing, and in terms of sectoral applications.

Forecast accuracy has improved. Accuracy of early warning hazard forecasts has increased by 40-50% during the last five years. Similarly, if I compare the lead period, there is a net gain of 4 days in last 5 years. For example, my forecast accuracy was 60% earlier for 24-hour forecast, now my Day five forecast has the accuracy of 60% for heavy rainfall. That means whatever forecast was issued five years back one day, now similar forecasts are being issued up to 5 days.

Recently we introduced the impact-based forecast. We are telling at present what the weather will do tomorrow instead of simply telling what the weather will be tomorrow. So there is a significant improvement.

Q. Your short term forecasts are brilliant. Your cyclone forecasts are almost on dot, but your long-range forecasts leave much to be desired. Last year was fine, but your forecasts for 2020 were a disaster.

A. Last year except one forecast for the month of August, all other forecasts were correct. Quantity-wise seasonal forecast becomes difficult because the longer the period, I should have that type of memory in the parameters which are used for the forecast.

There is still scope to improve. Earlier, we did not have dynamical climatic forecast system, which we used last year and this year. We were using Monsoon Mission Climate Forecast System till 2020. In 2021, we decided single model cannot give accuracy, so we shifted to multi-model ensemble. We went through all available global climate forecast systems, and chose five best models based on their historical performance. Based on those models, we developed a multi-model ensemble scheme. That was implemented last year and is being used for providing forecast for monthly temperatures, heat waves, etc. Whatever we have predicted has been occurring. The same model has been used for the forecast of monsoon this year. This is better than the single model or the statistical model.

Q. Are you using artificial intelligence and machine learning in your forecasts?

A. Earlier people tried to apply artificial intelligence and machine learning for the long range forecasts. It did not work. However, improved versions have come and people are trying. AI/ML works better for shorter-period forecasting like nowcasting. IMD is working with many other agencies like Indian Institutes of Technology, and universities for developing this. Certainly there is scope and it is the future.

It is a double-engine concept. One engine will be numerical weather prediction models that will be based on observational systems. Based on that we will do computing and modelling and that will go on improving over the years. The other engine will be AI/ML. That will be based on the huge data that is available with us. Both will go parallelly. Both will complement each other and based on forecasts will be generated in future. That is the plan for next five years.

Q. How does your long range forecasting system compare with models in other countries?

A. Long range forecasts are not issued by many countries. If you consider short-to-medium range, we are at par with all leading countries. We have got a very good modelling system and our forecast accuracy is about 75-80% for heavy rainfall and 85% for thunderstorms 24 hours ahead. Cyclone forecasts are pinpointed and appreciated by even global bodies. Forecast accuracy on heatwave and cold wave are about 92%. For all severe types of weather, forecast accuracy is quite high. Still there are challenges. For example, lightning. Number of people dying because of cyclones has decreased, but number of people dying due to lightning has not decreased. Every year about 25,000-30,000 people die in the country because of lightning.

We started lightning forecasting in 2019. Only 5-6 countries provide lightning forecasts. There is still scope to improve on lightning detection and warning.

Similarly, there is scope to improve heavy rainfall warning. Heavy rainfall accuracy, as I told you is about 75% at meteorological subdivision level. But at the district level or location specific level, the forecasting needs further improvement. This is another challenge we are working on. That will be possible only if augment the observational network. We have about 33 radars, and we are planning to have 67 radars by next 5 years. Similarly, we paln to increasing automatic weather stations in the next 2-3 years.

Q. Will cloudbursts also be covered by this?

A. Cloudburst is a micro-scale phenomenon, it occurs over one or two villages within a few minutes or hours. Therefore, detection itself is a challenge and hence prediction would be a challenge. We are augmenting the radar systems in areas that are prone to cloudburst like the western Himalayas. Cloudburst as such cannot be predicted, but it will detect intense thunderstorms.

Q. Is it right to say Cyclone Asani has helped this year's monsoon advance?

A. I will not say so because Cyclone Asani was not a very intense system, and it weakened into a deep depression before going to the coast. Monsoon is a large-scale system and cyclone occurs for just 2-3 days. Once the transient system moves away, the environment comes back to original. Actually the advance occurred because of westerly winds and cross-equatorial flow increased.

Q. Cyclones during summer months are rare, is that right?

A. I will not say rare, I will say their frequency is less. The Bay of Bengal-Arabian Sea is unique compared to other ocean basins. In other ocean basins there is only cyclone season, but in our case we have two seasons--pre-monsoon and post-monsoon. If you look at frequency, out of five cyclones, one occurs during pre-monsoon and four occur in post-monsoon season.

Q. Is there a chance of the current heat wave becoming worse?

A. No. Heat wave was worse in April. In May, heat wave has been restricted to northwest and adjoining central India. Certainly, it is not going to spread to larger area because eastern part and northern peninsula, they are all getting thunderstorms.

Q. Is there a chance of temperatures touching 50 degrees celsius in the times to come?

A. May is one of the warmest months. The period prior to the onset of monsoon, temperature rises in northwest India. Therefore, the period from May 15 to Jun 15 is the warmest period in northwest India. Normal itself in northwest India will be 42-44 degree celsius in some places. But if you ask whether it will cross 50 degrees celsius we cannot say pinpointed like that. We give forecasts for next 5-7 days. It will not be a new record if it touches 50 degrees celsius also. The highest record is 52.6 degree celsius long back in Rajasthan.

We have given the prediction for May that it will be above normal. Whether it will cross a particular temperature or not, that type of prediction is not there. That prediction is given only in short to medium range.  

Q. You have predicted Indian Ocean dipole to be negative, which is associated with lower rains over India...

A. In the majority of years, La Nina and El Nino play a major role compared with Indian Ocean dipole. But there are certain years like 1997, which was an El Nino year, but Indian Ocean dipole played a good role. So only one parameter cannot decide the fate of monsoon. You will have to consider all parameters together. La Nina is good and Indian Ocean dipole is going into negative, so the ultimate impact could be neutral and 

therefore, normal monsoon.

Q. How is it to be director General of IMD? If rains fail, people blame IMD, if there is a heatwave they blame IMD...

A. Weather forecasting is a nascent science. It is a growing science globally. At the same time, tropical region has its own challenges because of the variations in convections, more frequency in weather events. Also, it is a data sparse region. We have oceans on three sides and great Himalayas to the north.

We must continuously enhance our capability and upgrade our observational systems, modelling systems, early warning systems and whatever be the accuracy that should be converted into forecasts. We should tell the facts and limitations so that people understand it. If I say that my forecast is 60% correct, people will understand.  End

Edited by Vandana Hingorani

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