Abstract
Background: Numerical weather and climate models rely on the use of microphysics schemes to simulate clouds and produce precipitation at convective scales. It is important that we understand how different microphysics schemes perform when simulating high impact weather to inform operational forecasting. Methods: Simulations a heavy rainfall event from 17-20 February 2017 over Botswana were made with the Weather Research and Forecasting (WRF) model using four different microphysics schemes. The schemes used were the Weather Research and Forecasting Single Moment 6-class scheme (WSM6); Weather Research and Forecasting Single Moment 5-class scheme (WSM5); Stony Brook University scheme (SBU-YLIN); and Thompson scheme. WSM5 is considered as the least sophisticated of the four schemes, while Thompson is the most sophisticated. Simulations were initialized and forced by the Global Forecast System (GFS), and configured with a grid spacing of 9km over an outer domain and 3km for a nested inner domain without the convection parameterization. The simulations were produced using the University of Botswana and the Centre for High Performance Computing (CHPC) High Performance Computing (HPC) systems. Results: WSM5 and WSM6 simulations are mostly similar; the presence of graupel in WSM6 did not result in large differences in the rainfall simulations. SBU-YLIN simulated the least amount of rainfall, followed by Thompson. All the schemes captured the north-south rainfall gradient observed on 17 February, but with all simulations rainfall is simulated slightly south of where it was observed. All the schemes overestimated rainfall on 18 February over the central parts of Botswana, and underestimated rainfall on 19 February over most of Botswana. Conclusions: Simulations with different microphysics looked more similar to each other, than to observations. Future studies will test WRF configurations including a single nest over Botswana to determine the best configuration for operational forecasting by the Botswana Department of Meteorological Services.
Highlights
Extreme weather events can have both positive and negative impacts on society
The Global Precipitation Measurement (GPM) and Tropical Applications of Meteorology using SATellite (TAMSAT) datasets are considered as high resolution satellite based rainfall estimates, with TAMSAT developed for the African continent
The Weather Research and Forecasting (WRF) model is used in this study to simulate a heavy rainfall event that took place over Botswana resulting in flooding over parts of the country, including Gaborone
Summary
Extreme weather events can have both positive and negative impacts on society One such event is the Ex-Tropical cyclone Dineo which resulted in torrential rainfall over Botswana with flooding in some areas, while filling up the Gaborone dam which was at its lowest in over 35 years of its existence[1]. The south-western parts of the country receive the lowest amount of rainfall annually (250mm), while the northern parts receive the highest (600 mm)[7] This part lacks the current status of the impacts of cloud microphysical schemes on precipitation forecasts or simulations. Methods: Simulations a heavy rainfall event from 17-20 February 2017 over Botswana were made with the Weather Research and Forecasting (WRF) model using four different microphysics schemes.
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