Assessments of the WRF model in simulating 2021 extreme rainfall episode in Malaysia

An episode of extreme monsoonal flood event has severely affected the East and West coast of Peninsular Malaysia from 16th to 18th December 2021. The extreme rainfall was documented to be associated to Tropical Depression 29 and Typhoon Rai. In addition, biomass burning aerosols were suspected to be capable of intensifying the precipitation. Thus, the main causes of this extreme event are studied with model evaluation being carried out with biomass burning as one of the possible reasons and variables. From the sensitivity analysis on the PBL scheme for the model physics, QNSE scheme is tested to be the best scheme to simulate the episode compared with MYJ and ACM2 and used in the model assessment. The performances of ARW (WRF-ARW), BB (WRF-Chem with biomass burning), and NOBB (WRF-Chem without biomass burning) have been assessed in the reproduction of the precipitation pattern and tropical depression. Simulation results indicate that ARW shows an overall better performance for most meteorological variables with better performance in reproducing the surface-level pressure and wind speed. Model scenarios of ARW and BB produced similar tropical depression spatial distributions but differ in magnitude, where the tropical depression in ARW is stronger during the study period over East coastline. All models overestimate the precipitation intensity, but ARW is much better correlated with observation data followed by NOBB and BB. The findings show that biomass burning aerosols have only a minor impact on intensifying or delaying the rainfall event. Therefore, tropical depression over Peninsular Malaysia is shown to be the main causation to this extreme event in 2021. The model could be applied for the future flood risk management in Malaysia to provide information on decision making.