Effect of dust on rainfall over the Red Sea coast based on WRF-Chem model simulations
Authors: Parajuli Sagar Prasad , Stenchikov Georgiy L. , Ukhov Alexander , Mostamandi Suleiman , Kucera Paul A. , Axisa Duncan , Gustafson Jr. William I. , Zhu Yannian 
Published: April 2022

Abstract. Water is the single most important element of life. Rainfall plays an important role in the spatial and temporal distribution of this precious natural resource and it has a direct impact on agricultural production, daily life activities, and human health. One of the main elements that govern rainfall formation and distribution is atmospheric aerosol, which also affects the Earth’s radiation balance and climate. Therefore, understanding how dust compositions and distributions affects the regional rainfall pattern is of crucial, particularly in regions with high atmospheric dust loads such as the Middle East. Although aerosol and rainfall research has garnered increasing attention both as an independent and interdisciplinary topic in the last few decades, the details of various direct and indirect pathways by which dust affects rainfall are not yet fully understood. Here, we explored the effects of dust on rainfall formation and distribution as well as the physical mechanisms that govern these phenomena, using high-resolution WRF-Chem simulations (~1.5 × 1.5 km) configured with an advanced double-moment cloud microphysics scheme coupled with a sectional 8-bin aerosol scheme. Our model-simulated results were realistic, as evaluated from multiple perspectives including vertical profiles of aerosol concentrations, aerosol size distributions, vertical profiles of air temperature, diurnal wind cycles, and spatio-temporal rainfall patterns. Rainfall over the Red Sea coast is mainly caused by warm rain processes, which are typically confined within a height of ~ 6 km over the Sarawat mountains and exhibit a strong diurnal cycle that peaks in the evening at approximately 6 pm local time under the influence of sea breezes. Numerical experiments indicated that dust could both suppress or enhance rainfall. The effect of dust on rainfall were calculated as total, indirect, and direct effects, based on 10-year August-average daily-accumulated rainfall over the study domain covering the eastern Red Sea coast. For extreme rainfall events (domain-average daily-accumulated rainfall of ≥ 1.33 mm), the total (6.05 %), indirect (4.54 %), and direct effects (1.51 %) were all positive (enhancement). At a 5 % significance level, the total and indirect effects were statistically significant whereas the direct effect was not. For normal rainfall events (domain-average daily-accumulated rainfall < 1.33 mm), the indirect effect enhanced rainfall (4.76 %) whereas the direct effect suppressed rainfall (-5.78 %), resulting in a negative net suppressing effect (-1.02 %), all of which were statistically significant. We investigated the possible physical mechanisms of the effects and found that the dust direct effects were mainly caused by the scattering (absorption) of solar radiation by dust. The surface cooling (warming) induced by scattering (absorption) weakens (strengthens) the sea breeze circulation, which decreases (increases) the associated landward moisture transport, ultimately suppressing (enhancing) rainfall. Our results have broader scientific and environmental implications. Specifically, although dust is considered a problem from an air quality perspective, our results highlight the important role of dust on sea breeze circulation and associated rainfall over the Red Sea coastal regions. Our results also have implications for cloud seeding and water resource management.