Impact of PBL on the Simulation of Heavy Rainfall Events in the Monsoon Season Over Bangladesh Using WRF-ARW Model

Abstract

In the present study the Advanced Research WRF (ARW) model version 3.2.1 has been used to simulate the heavy rainfall events of 27-29 July 2009, 15-16 August 2009, 26-27 June 2010 and 7-8 September 2011 over Bangladesh during monsoon season. To simulate the heavy rainfall events Lin et al. microphysics in combination with Kain-Fritsch (KF) cumulus parameterization (CP) scheme in a nested configuration has been used. In this study, six different planetary boundary layer (PBL) parameterizations schemes have been used to study the heavy rainfall events in monsoon season. The different PBL schemes have been considered are YSU, MYJ, QNSE, MYNN3, ACM2 and BouLac. The model domains consist of 9 km outer and 3 km inner domain horizontal resolution with 28 vertical sigma levels. NCEP FNL data have been used for the initial and lateral boundary condition. The WRF model has been run 72 hours for the heavy rainfall event 27-29 July 2009 and 48 hours for all other cases. Sensitivity experiments have been conducted with the WRF model to test the impact of Planetary Boundary Layer (PBL) schemes in capturing the extreme weather event. The rainfall, wind speed, relative humidity, accumulated upward heat flux, accumulated upward latent heat flux, downward long wave flux, outgoing long wave radiation, reflectivity, PBL thickness, kinematics and thermodynamic characteristics have been studied to identify the effect of PBL schemes on different heavy rainfall events. The simulated rainfall is maximum at the position where the outgoing long wave radiation, Accumulated upward heat flux and downward long wave flux are minimum. The simulated rainfall is maximum at the position where the reflectivity and the accumulated upward latent heat flux are also maximum. The simulated relative humidity at 850 hPa level is almost 98- 100% at different places over the country where maximum rainfall has been simulated. The simulated rainfall has been compared with observed rainfall of Bangladesh Meteorological Department (BMD) and TRMM 3B42RT rainfall. For the simulation of four heavy rainfall events with four initial conditions, the BouLac PBL scheme has given the better result. After BouLac PBL scheme, YSU and ACM2 have given the better result for the simulation of heavy rainfall events.