Effect of dust on rainfall over the Red Sea coast based on WRF-Chem model simulations
Authors: Parajuli Sagar P. , Stenchikov Georgiy L. , Ukhov Alexander , Mostamandi Suleiman , Kucera Paul A. , Axisa Duncan , Gustafson Jr. William I. , Zhu Yannian 
Published: July 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 important 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 affect the regional
rainfall pattern is crucial, particularly in regions with high
atmospheric dust loads such as the Middle East. Although aerosol and
rainfall research has garnered increasing attention as both 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 km × 1.5 km) configured with an advanced double-moment cloud
microphysics scheme coupled with a sectional eight-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 18:00 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 was 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 net
effect of dust on rainfall was positive or enhancement (6.05 %), with the
indirect effect (4.54 %) and direct effect (1.51 %) both causing
rainfall increase. 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 rainfall suppression by dust direct effects was mainly
caused by the scattering of solar radiation by dust. The surface cooling
induced by dust weakens the sea breeze circulation, which decreases the
associated landward moisture transport, ultimately suppressing rainfall. For
extreme rainfall events, dust causes net rainfall enhancement through
indirect effects as the high dust concentration facilitates raindrops to
grow when the water vapor is sufficiently available. 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.