Researchers at Indian Institute of Tropical Meteorology, Pune have coined a new term — mini-cloud burst — to define rainfall in excess of 50 mm in two consecutive hours. While extreme rainfall of over 200 mm in 24 hours translates to only 16 mm rainfall per hour, the intensity of rainfall is far more in the case of mini-cloud burst.
With significant increase in short-duration heavy rainfall events during summer monsoon at many places in India, scientists at Pune’s Indian Institute of Tropical Meteorology (IITM) have coined a new term to define such events — mini-cloud burst. Mini-cloud burst is defined as rainfall in excess of 50 mm in two consecutive hours.
Based on hourly rainfall data from 126 stations between 1969 and 2015 the researchers found an average 200 mini-cloud burst events occurring every year in India. Between 1988 and 2007, there were around 265 mini-cloud burst events.
The Indian Meteorological Department already recognises could burst —heavy rainfall, irrespective of the amount, in the steep slope mountainous regions of Himalayas leading to flash floods and rainfall over 100 mm per hour in other places. In contrast, mini-cloud burst over 50 mm rainfall in two hours is indicative of torrential downpour but of lesser intensity than cloud burst.
Currently, on a 24-hour basis, IMD classifies rainfall as heavy (over 60.5 mm), very heavy (over 130 mm) and extremely heavy (over 200 mm). In a paper published in the International Journal of Climatology, Dr. Nayana Deshpande and others from IITM explain the rationale for classifying rainfall of over 50 mm in two hours as mini-cloud burst. While extreme rainfall of over 200 mm translates to only 16 mm rainfall per hour, the intensity of rainfall is far more in the case of mini-cloud burst, they say. Also, compared with extreme rainfall, the rate of water accumulation exceeding absorption and probability of flash floods are thrice more in the case of mini-cloud burst.
Over most parts of India, the highest recorded rainfall in two hours is 100-150 mm, and the locations (other than the mountainous regions of Himalayas) that have recorded rainfall of over 150 mm in two hours are those that also experience cloud burst. So in these locations, the mechanism responsible for heavy rainfall “persists for more than an hour”.
The study found that mini-cloud burst events are “very common” at the foothills of Himalayas. While the west coast records more than three events per season, the Indo-Gangetic plains and Saurashtra receive two per season. At just one per season, Rajasthan and States to the east of Western Ghats (rain shadow region) experience the least number of mini-cloud burst events.
Gujarat coast, east coast of Odisha, some parts of Uttar Pradesh and northeast India, and Indo-Gangetic plain receive more than 70 mm during a mini-cloud burst event. For the rest of India, the amount of rainfall per event is 50-70 mm.
The researchers found mini-cloud burst events occur in June over the Western Ghats, and central India and the foot hills of the Himalayas experience them in July and August. The frequency is low in the month of September.
The foothills of Himalayas and the west coast experience heavy rainfall for two hours from midnight to noon, while central India receive such spells from noon to 6 pm. South India receives short, heavy spells from 6 pm to midnight.