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Decrease in aggregate of extreme rainfall in NE: study
AJIT PATOWARY

 
 GUWAHATI, June 23 - The aggregate of extreme rainfall events over the NE region of India has shown a significant decreasing trend. This has been observed by Rahul Mahanta and Debojit Sarma of the Department of Physics, Cotton University and Amit Choudhury of the Department of Statistics, Gauhati University while studying the meteorological data obtained from the India Meteorology Department (IMD) for a period of 31 years between 1971 and 2001.

Rahul Mahanta, while talking to this correspondent maintained that the findings of their study still hold good. No virtual change has been noticed so far in this context, he asserts.

The findings of the three university teachers were published in a jointly authored article – Heavy rainfall occurrences in northeast India – published in the online edition of the International Journal of Climatology in 2012 (wileyonlinelibrary.com).

The three teachers said in the article that during the study period, the convective potential energy (CAPE) shows a decreasing trend whereas the convective inhibition energy (CINE) shows an increasing trend in the region. This is reasonable, they asserted, arguing that it happened due to the decreasing trend in the heavy rainfall events over the region.

The north-eastern part of India is characterised by unique weather systems. The southwest monsoon is responsible for a large volume of its annual rainfall. Its June to September monsoon seasonal rainfall, which is about 151.3 cm, is much larger than the all India average of 86.5 cm. Its normal rainfall between April and October is about 227 cm with a standard deviation of 38 cm and it forms around 80 per cent of the region’s annual rainfall.

Significantly, the North East is one of the Indian regions with low variability of the seasonal rainfall. The variation in seasonal rainfall here is 10 per cent. The rainfall it experiences in the month of July is the highest for the southwest monsoon season and it gradually decreases thereafter. For the purpose of studying the heavy rainfall events of the region, the months of April and May are also to be considered, even as the monsoon season starts here usually in the last week of May or in the first week of June every year, they said.

From mid-May to early October, with abundant moisture, potential instability and the presence of mountainous terrain, high rainfall (HRF) days are frequent and all the stations record HRF (>15 cm d-1) during the period. During the monsoon season, specially in June-July, these events are more widespread than in other months, followed by August.

It seems the orographic (relating to mountains, especially with regard to their position and form) effects are important in determining the spatial distribution of HRF occurrences with a pronounced high altitude maximum, especially during the summer months under the southwesterly monsoon flow.

After the summer-autumn transition, HRF days are almost non-existent under the northeasterly monsoon flow. The HRF events are infrequent during the winter months because of stable atmospheric stratification with low moisture content. Severe thunderstorm (TS) accompanied by HRF events starts in mid-April and early May and becomes more frequent in late summer and early autumn.

During the analysis period, HRF occurrences were widespread and d-1 dominated by HRF events of intensity 30 cm after the onset of summer monsoon. For the period April-May-June-July-August-September-October, almost 50 per cent of the total rainfall days are comprised of thermal instability, which is maximum in September and both HRF events and TS activities reach their peak in July.

Thus, the fact that even with large CAPE values fewer TSs or HRF events have occurred, showing that factors other than CAPE are also important in the genesis of such systems over the region. Apart from high CAPE, operation of certain trigger mechanism is necessary to release this energy, which in turn helps the development of cumulonimbi clouds. Possible trigger mechanism can be solar heating in the lower layers of the atmosphere, low level horizontal convergence, sufficient wind shear, etc, observed the three teachers.

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