On the sensible heat energy, latent heat energy and potential energy of the troposphere over Dhaka before the occurrence of nor’westers in Bangladesh during the pre-monsoon season

Abstract

Attempts have been made to study the different energy components of the troposphere prior to the occurrence of nor’westers in Bangladesh during the pre-monsoon season. The energy components of the troposphere on the dates of occurrence of nor’westers have also been compared with the energy components of non-occurrence day, weekly and monthly normal values. During the pre-monsoon season, the sensible heat (SH) and latent heat (LH) content of the troposphere increase as the season progresses due to the warming of the troposphere and availability of moisture respectively. The SH and LH of the troposphere on the dates of occurrence of nor’westers increases significantly from near the surface to about 300-200 hPa and about 900 hPa level to about 400 hPa or more respectively in most of the cases, although it has inter-layer variations. There are some exceptions especially in the lower troposphere where there exists decrease in LH of the troposphere at 0000 UTC. These exceptions may be due to the availability of less moisture in the lower atmosphere on some occasions early in the morning. It has also been found that the SH and LH of the troposphere on the dates of occurrence of nor’westers become significantly above weekly normal from near the surface to about 300-200 hPa and in the lower troposphere respectively in most of the cases. The SH is found to be below normal in the layer between 850 and 650 hPa levels and significantly above monthly normal in the upper troposphere, having maximum in the layer 400-300 hPa. The LH is significantly above the monthly normal in the lower troposphere, and it becomes below monthly normal in the mid-troposphere. During the pre-monsoon season, mean monthly potential energy (PE) of a parcel of air in the troposphere decreases in the lower troposphere upto about mid-troposphere and then increases in the upper troposphere as the season progresses. The PE has a decreasing tendency near the surface on the dates of occurrence of nor’westers in many cases and then it shows erratic behaviour in the vertical. The variation of PE on the dates of occurrence of nor’westers from the weekly normal is minimum in the lower atmosphere and does not show any definite trend onwards.