Simulation of a dust-and-rain event across the Red Sea using WRF-Chem

We present simulations of a series of dust-and-rain events in July/August 2020 that caused significant loss of life and property over northeast Africa and the Red Sea coast using convection-permitting Weather Research and Forecasting-Chem simulations at 1.5 km horizontal grid spacing coupled with prognostic dust and aerosol scheme together with a double-moment cloud microphysical scheme. The model generally captures the optical properties and vertical profiles of dust but has difficulty reproducing the seasonal dust plumes that originate over northeast Africa and reach the western Arabian Peninsula through the Tokar gap. Overlapping of synoptic events with local, diurnal-scale, convective features caused continuous rainfall over the study domain in July/August 2020. The model well simulates the synoptically-driven clouds over the ocean (similar to 4-8 km) and daytime deep convective clouds over the land but cannot reproduce cumulus clouds over the ocean advected from land, which leads to some biases in simulating peak rainfall rates. Although our simulations do not explicitly consider aerosol particles as ice nuclei, the model reproduces the cirrus clouds found at similar to 10-15 km height during the study period. Simulations with a planetary boundary layer (PBL) scheme better represented the diurnal profile of surface temperature and winds, as well as the rainfall distribution, than without using a PBL scheme. Dust generally enhanced rainfall during the synoptic rain event through indirect effects but suppressed rainfall during sea-breeze-induced convective events through dust direct effects by weakening the sea-breeze circulation.