Study on the Genesis of Low Intensity Tropical Cyclones in the Bay of Bengal using WRF Model

Abstract

In the present study, Weather Research and Forecasting (WRF) model is used to study the genesis of TC Rashmi -2008, Nisha -2008, Bijly -2009 and Viyaru -2013 with low intensity which formed on the Bay of Bengal as a depression on 03 UTC on 25 October 2008, 09 UTC on 25 November 2008, 09 UTC on 14 April 2009 and 09 UTC on 10 May 2013. Latter they intensified up to cyclonic storm. Four cases are also studies when there is no cyclone and named as normal case. The model is configured in single domain, 9 km horizontal grid spacing with 290×316 grids in the west-east and north-south directions and 32 vertical levels and run the model using KF cumulus and YSU planetary boundary layer along with Final Reanalysis (FNL) data (1o x1o) from National Centre for Environment Prediction (NCEP), USA as initial and lateral boundary conditions (LBCs). Model is run in two steps for cyclone: firstly the formation of depression and secondly the intensification up to cyclonic storm. For the first step, model is run with 96, 72, 48 and 24 hour to trace the formative stage as depression and for the second step, model is also run for 24 hour to test the formation/intensification as cyclonic storm. For the normal cases, model is run with 96, 72, 48 and 24 hour. Wind speed (WS) at 10m, Vertical wind shear (VWS) between 850-200hPa, Vertical profile of Horizontal wind (VHW), Vertical profile of Vertical wind (VVW), Relative Vorticity (RV) (850 hPa), Sea Surface Temperature (SST), Mean Sea Level Pressure (SLP), Relative Humidity (RH), Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) obtained from cyclones and normal cases are computed. Then, for first step, values obtained from cyclone cases are compared with those obtained from normal cases and available data. Then, for second step, values obtained from cyclone cases are discussed and compared with available observed data. Significant difference between the parameters obtained from cyclone cases and normal cases are obtained. Results suggest that circular shape of wind, close circle of SLP are observed in cyclone cases without exception. RH exists from surface to middle levels. It may say that model simulates the depression and cyclonic stages with more or less intensity with considerable lead time