Study on Convective and Non-Convective Rain of Different Heavy Rainfall Events in the Premonsoon Season using WRF-ARW Model

Abstract

In the present study the Advanced Research WRF (ARW) v3.8.1 model have been used to simulate the heavy rainfall events in the southeastern regions of Bangladesh during 4, 6 and 7 May 2013 and southern region during 15-16 May 2013. The initial and boundary conditions are drawn from the global operational analysis and forecast products of National Center for Environmental Prediction (NCEP-FNL) available at 1o ×1o resolution. The model was configured in single domain, 6 km horizontal grid spacing with 161×183 grids in the eastwest and north-south directions and 30 vertical levels. Time step of integration is set to 30 seconds for maintaining computational stability as the model uses third-order Runge-Kutta time integration scheme. In this research, four different microphysics (MP) schemes i.e., Lin et al., WSM6, Thompson and WDM6 and four different cumulus parameterization (CP) schemes i.e., Kain-Fritsch (KF), Tiedtke, Zhang-McFarlane (ZM) and Multi-scale Kain- Fritsch (MSKF) are used to simulate the heavy rainfall events. In this research convective rain and non-convective rain have been studied to observe the effect of these rainfalls on total rainfall and area average rainfall, relative humidity, vertical velocity, reflectivity and vorticity have been predicted and analyzed. The different verification methods such as threat score (TS), equitable threat score (ETS), bias score (BS) and standard deviation (SD) have been computed for accuracy of forecast. Tiedtke scheme has simulated lower rainfall in combination with all chosen MPs in all Bangladesh and also heavy rainfall area on 4, 6 and 7 May. KF scheme coupling with all MPs have simulated similar average rainfall all over Bangladesh on 4 May and WDM6 coupling with KF, ZM and MSKF schemes have simulated almost similar amount of rain in the heavy rainfall region on 7 May. The model simulated heavy rainfall region is shifted from Khepupara to Mongla on 15 May all MPs and CPs and on 16 May, observed maximum rain at Barisal and Patuakhali matched but higher rain area is found at Khepupara. Simulated convective rain position matched with the observed maximum rain on 15 May but the amount is not significant for all MPs and CPs and on 16 May insufficient Convective Rain have simulated. Maximum amount of rain is found mainly non-convective for all MPs and CPs. On 16 May Lin et al. scheme coupling with all CPs have simulated almost similar results in the heavy rain area and all over Bangladesh all MPs gives higher results. WSM6 and WDM6 schemes coupling with ZM and MSKF schemes gives the better performance on the basis of Threat Score, Equivalent Threat Score and Bias Score on 4, 6 and 7 May 2013 and during 15- 16 May 2013.