Mesoscale simulation of a flood producing rainstorm over Rangamati, Bangladesh using MM5 model

M.N Ahasan,M.A.M Chowdhury,D.A Quadir

doi:10.4038/sljp.v15i0.6824

Abstract

Mesoscale simulation of a flood producing rainstorm of 21 June 2004 over Rangamati, Bangladesh has been carried out using the MM5 Model. The model performance was evaluated by examining the different predicted and model derived parameters. The MM5 model suggests that the rainstorm over Rangamati was resulted by the large scale weather circulation associated with active conditions of southwest summer monsoon over the head Bay of Bengal which generated favourable conditions for developing the severe mesoscale convections. The strong confluence of southwesterly low level flow transporting large amount of moisture (which vertically extended up to 350 hPa) from the Bay of Bengal towards southeast Bangladesh and its neighbourhoods appears to be one of the striking features. An elongated vorticity maximum of the order of 30-40×10 -5 s -1 was observed across the Rangamati. A well-defined area of low level convergence (-25×10 -5 s -1 ) is associated with strong divergence (25×10 -5 s -1 ) in the high level (200 hPa) has provided favourable conditions for strong convection. The model simulated realistic pattern of rainfall over Rangamati as compared with TRMM observations. Considering the quantitative comparison of the domain average rainfall with TRMM observations, it is found that the model over predicts by 12% as compared with TRMM observed rainfall.

Highlights

Rangamati (22.53°N, 92.20°E) – the southeastern hilly city of Bangladesh is situated on the western slopes of Mizo hills and Arakan Mountains (Prasad, 2005)
The intersection of the area of convergence by the southwesterly low level wind is a significant feature favourable for severe convective activity. This strong low level convergence associated with strong outflow in the high level (200 hPa) [Figure 5(d) and Figure 8(b)] has provided favourable conditions for strong convection and production of very high rainfall
Considering the quantitative comparison of the domain average rainfall with TRMM and Bangladesh Meteorological Department (BMD) observations, it is found that the model over predicts rainfall by 12% as compared with TRMM

Summary

INTRODUCTION

Rangamati (22.53°N, 92.20°E) – the southeastern hilly city of Bangladesh (latitudes 20°34 ́-26°38 ́N and longitudes 88°01 ́-92°41 ́E) is situated on the western slopes of Mizo hills and Arakan Mountains (Prasad, 2005). Prasad (2005) has carried out the study on Monsoon forecasting with a limited area numerical weather prediction systems[9] He identified some heavy rainfall events from the summer monsoon season of 1987, 1997 and 2002 over Bangladesh and northeast India. He investigated the synoptic situations responsible for occurring the heavy rainfall using FSU limited area model and he forecasted the events. Ahasan et al, (2011, 2013) carried out the research work on simulation of a heavy rainfall event on 14 September 2004 over Dhaka, Bangladesh and on 11 June 2007 over Chittagong, Bangladesh respectively using MM5 Model[10,11] He identified the some synoptic situations responsible for occurring the heavy rainfall over Dhaka and Chittagong, Bangladesh. The data used, experimental setup of the model and methodology of the study are provided

Data used

RESULTS AND DISCUSSION

CONCLUSIONS