GUWAHATI, Sept 22 - Between 1995 and 2000, thunderstorm activities in Northeast India were high during the April-May pre-monsoon period. But now, these phenomena seem to have lingered in the region up to the month of September. This is a finding of a CSIR-sponsored study project carried out by the Electronics and Atmospheric Research Group (EARG) of the Gauhati University (GU) Physics Department in collaboration with the National Aerospace Laboratory (NAL), Bengaluru, between 2008 and 2013.
The project � Massoscale Climate Modelling of Monsoon and Pre-Monsoon thunderstorm in the NE Indian Region was approved by the CSIR in 2008 under the New Millennium Indian Technology Initiative Programme. However, the results of the project were also influenced by another project carried out by the GU group earlier.
The project led by Prof Ananda Kumar Barbara and Prof Minakshi Devi, has led to the assumption till now that if monsoon rainfall is less than average, then the favourable condition for triggering a thunderstorm, like temperature, humidity and instability in the atmosphere, remain present.
Moreover, it has found a relation between the sea surface temperature and precipitation in the NE region. It has also found that the Bay of Bengal has more favourable condition for the growth and development of cyclones, compared to the Arabian Sea. This can be attributed to the physical and thermodynamic situations of the Bay, said Prof Barbara and Prof Devi.
There are three corridors for entry of moist and hot wind to the NE Indian region. One of them is through the western side � that is, Bihar, Odisha, etc. The second one is through the Patkai area, while the third one is directly from the Bay of Bengal.
The cyclones in the Bay of Bengal usually result in maximum rainfall activities in Agartala, followed by Imphal and Guwahati in a row. Dibrugarh usually receives the least amount of rainfall caused by the Bay of Bengal cyclones. But, it all depends on the cyclonic tracks through which the moist-laden winds travel, said the GU professors.
The GU EARG is now able to make prediction of the precipitation patterns and their magnitude in different parts of the region, three to four days ahead of their occurrence by using the Varsha model it has been running under the project. As for example, the prediction it had made in the case of the 2009 ALIA cyclone came out to be correct when compared with the India Meteorology Department (IMD) data. However, the Varsha model overstated the magnitude of rainfall in this case, said the GU professors.
In the case of the May 2010 Laila Cyclone, the GU group�s prediction that it would result in very little or no rain over the NE region, was also found to be correct. This prediction was also made three to four days ahead of the occurrence of the cyclone.
Significantly, during the months of March and April, presence of aerosol plays a very significant role in the formation of cloud condensation nuclei, which leads to precipitation and thunder. While moisture comes from the Bay of Bengal, aerosol may migrate from the western parts of the country to create the above situation, said the GU professors.
