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Pitter Patterns: IISc study finds increase in area receiving extreme rainfall in the last four decades

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2 Aug 2019
Pitter Patterns: IISc study finds increase in area receiving extreme rainfall in the last four decades

Extreme and unpredictable rainfall patterns in recent years have resulted in devastating floods and severe droughts across the country. These events have resulted in the loss of numerous lives, shutdown of cities and disruptions of agricultural activities. Regular and accurate measurement and analysis of such rainfall pattern are crucial for predicting and preparing for the coming rainfall-related disasters. In a recent study, researchers from the Indian Institute of Science, Bengaluru, have used a non-traditional method to measure extreme rainfall events. This approach is more accurate than what is currently used by meteorologists and researchers.

When scientists quote events of ‘extreme rainfall’, how do they measure them? Meteorologists often divide an area of study into a grid of squares, count the number of squares with rainfall and estimate the amount of rain in each of these squares. They measure extreme rainfall events by counting the number of squares having precipitation above a certain level. This level, measured in millimetres of rain, is chosen based on the typical rainfall patterns in the study area.

In the current study, the researchers have found that simply counting the number of squares with rainfall may be inaccurate because this approach considers precipitation that occurred in neighbouring squares as different, individual events. However, in reality, such extreme rainfall is mostly a single event that spans over a large area. Hence, the error in counting it as many events can lead to overestimation of such rainfall events and ignores its impact.

However, how does the size, or area, of an extreme rainfall matter? “If a large area experiences extreme rainfall on the same day, the risk of flood and other calamities increase substantially as compared to spatially scattered extremes. Thus, it is important to understand the spatial characteristics of rainfall extremes over India,” explains Prof. Arindam Chakraborty from IISc, who led the study.

The researchers collated daily rainfall data of the summer monsoon months (June to September) from 1951–2015, available from the India Meteorological Department (IMD). They then used a technique called “connected component analysis”, in which an algorithm is used to identify objects close to each other. This allowed them to identify rainfall events that are close to each other. Through this, they were able to understand the number and frequency of extreme rainfall events in the country over the past 60 years. The threshold for categorising ‘extreme rainfall’ was established according to the typical rainfall received in a particular area. For example, in some parts of the Western Ghats it was 150 millimetres of rain per day, whereas, for Central India, where rainfall is relatively lower, the threshold was 50 millimetres per day.

The study found that since 1980, the average area covered by extreme rainfall events has been increasing, but the number of these events has remained the same. This pattern, however, is not revealed by counting neighbouring grids as separate rainfall events. Thus, their method, the researchers opine, not only accurately estimates the number of extreme rainfall events, but also reveals the increasing size of such events.

Additionally, researchers also analysed data on the air pressure changes over the years in the study area. They observed that regions with extreme rainfall also had low air pressure, which leads to higher precipitation. This is because low pressure causes air to rise up and condense water vapour to form clouds and rainfall. The study shows that areas with excessive rainfall and low air pressure have experienced more flooding, cyclones and warmer temperatures. They also found that more significant extreme rainfall events are caused by abnormally low pressure due to global air pressure patterns. Hence, climatic conditions across the world influence intense rainfall in India, they say.

The findings show that we need to understand rainfall patterns in India on a global scale with a new and more accurate technique of measuring extreme rainfall. Such methods can help in “predicting extreme rainfall events a few days in advance and assessing flood risk due to such events,” says Prof Chakraborty. An understanding of rainfall patterns is also essential for city planning and resource management during natural calamities.

Currently, there is a lack of nation-wide information on the impact of climate change and extreme weather conditions. Research in climatic conditions is vital to take preventive measures and to deal with the consequences of climate change. In their future studies, the researchers plan to look at how wind and air pressure circulation during the monsoon season affect the size of extreme rainfall events and temperatures.

“Once we know the mechanism, we can have reliable predictions in the future and prepare for the consequences of extreme rainfall events” concludes Prof Chakraborty. 

This article has been run past the researchers, whose work is covered, to ensure accuracy.