Study of Nor'westers and Development of Prediction Techniques in Bangladesh during the Pre-Monsoon Season

Abstract

This dissertation is an attempt to study the nor'westers/thunderstorms over Bangladesh and their associated meteorological conditions with a view to finding the possible causes of formation together with the determination of the critical criteria of a number of meteorological parameters. The study includes the spatial and temporal distribution of mean monthly and seasonal nor'westers/thunderstorms and thunderstorm days over Bangladesh. This statistical analytic overview will be helpful to identify the climatological risk area for the nor'westers/thunderstorms and thunderstorm days over the country. The inter-annual variability and trends of the frequency of thunderstorms have also been studied, which reveals that there exists 3-7 years cycles of variation in the monthly and seasonal frequency of thunderstorms over Bangladesh. The country-averaged monthly and seasonal frequency of thunderstorms during the pre-monsoon season has increasing trends. The rate of increase in the frequency of thunderstorms are 0.1912, 0.1059 and 0.2365/year in March, April and May respectively and 0.4566/year during the pre-monsoon season. The spatial distribution of the trends in the frequency of thunderstorms has been studied too. The surface and upper air synoptic conditions, minimum temperature and surface humidity on the dates of occurrence of nor'westers in Bangladesh have been studied and the favourable conditions for the occurrence of nor'westers have been determined. Nor'westers have been found to occur over Bangladesh near the trough line at the surface. The causes of early nor'westers in Bangladesh have been found out. The study reveals that two air masses of different temperatures are found to be present over Bangladesh and adjoining West l3engal specially at 0000 UTC on the dates of occurrence of nor'westers and they form a discontinuous line. Sometimes more than one discontinuous line was present over the region. This indicates that 'pseudo-frontal' activity takes place over Bangladesh on the dates of occurrence of nor'westers. When this pseudo-frontal activity is found in the morning, severe thunderstorms/nor'westers occur in the evening or early night. Nor'westers have been found to occur near the point of intersection of axes of moist and thy areas. The meteorological conditions associated with two tornadoes over Saturia and Tangail on 26 April 1989 and 13 May 1996 have been studied extensively. The tornadoes occurred near the point of intersection of the axes of moist and dry zones. The positive area on Tephigrarn on the dates of occurrence was bigger than that on the previous day, indicating more instability of the troposphere on the dates of occurrence of the tornadoes. The available instability energy of the troposphere increased significantly on the dates of occurrence of tornadoes and these increases were 90.931003x 106 Joule m-2 and 44.75421 x 105 Jm-2 in case of Saturia and Tangail tornadoes respectively as compared to that on the previous days. This tremendous quantity of instability energy was responsible for the occurrence of the tornadoes over Saturia and Kalihati on 26 April 1989 and 13 May 1996.

The rawinsonde observations at 0000 UTC over Dhaka, Chittagong and some stations of India have been utilized to compute different instability indices, water vapour and its fluxes, precipitable water content and different energy components of the atmosphere and their fluxes prior to the occurrence of nor'westers. The criteria of different instability for the occurrence of nor'westers have been determined. Some of the instability indices have been modified in order to have better criteria of these instability indices. For severe to very severe nor'westers, the critical values of MCT, MVT, MTT, MSWI, MKI and MET at 0000 UTC over Dhaka are: MCT ≥ 22°C, MVT ≥ 28°C, MTT ≥ 54°C, MSWI ≥300, MKI ≥ 48°C and MET< -9 Joule gm-1 respectively. For 5 cases of tornadoes, the critical values of the indices are: MCT ≥ 24°C, MVT > 30°C, MTT ≥ 55.0°C, MSWI ≥ 500, MET < -9 and MKI >58°C. Satellite and radar cloud imageries have been used to study the movement of nor'westers over Bangladesh and to estimate vertical velocity associated with nor'westers from radar imageries. The available instability of the troposphere gives a better indication for the occurrence of nor'westers. The higher the available instability energy of the troposphere, the greater is the possibility of occurrence of nor'westers, provided the other conditions especially the synoptic conditions are favourable. The vertical wind shear on the dates of occurrence of nor'westers has also been studied. Simulated average surface wind fields and the rainfall fields for 24 hrs over the region bounded by Lat. 08-32° N and 70-110° E have been obtained by using the NCEP data. The wind fields indicate the surface circulation patterns favourable for the occurrence of nor'westers in Bangladesh and the rainfall fields for 24 hrs over Bangladesh and adjoining area give a good signature for the rainfall occurrence due to nor’westers in Bangladesh. The forecasting techniques for the prediction of nor'westers and associated winds and rainfall have been developed. Four linear multiple regression equations for computation of maximum gusty wind speed associated with nor'westers have been developed. The R2 values for the regressions are 0.2771, 0.3498, 0.3493 and 0.2433 which correspond to the correlation co-efficient of 0.52640, 0.59 144, 0.59102 and 0.49325 respectively, which are not statistically significant. The computed values of maximum gusty wind speed have statistically significant correlation with the actual maximum gusty wind speed. The regression equations have been verified with data of current year (2005) and will be useful for the operational meteorologists for the computations of maximum gusty wind speed associated with nor'westers. Simple linear regression equations and linear multiple regression equations have also been developed for the statistical prediction of 24 hrs rainfall over Dhaka, country averaged 24 hrs rainfall over Bangladesh and 24 hrs maximum rainfall over Bangladesh due to nor'westers. The correlation co-efficients corresponding to these regression equations are statistically significant. These equations have also been verified with the data of current year. The synoptic and statistical techniques developed for the forecasting of nor'westers and associated gusty wind and rainfall will be useful in operational purposes.