Effect of dust on rainfall over the Red Sea coast based on WRF-Chem model simulations

Abstract

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.

Simulation of a Dust‐And‐Rain Event Across the Red Sea Using WRF‐Chem

“…We also run simulations with and without dust to understand the effect of dust on rainfall during the case study. We conduct an additional set of simulations without aerosol-radiation interaction in order to calculate the direct and indirect effects of dust on rainfall as in Parajuli et al (2022b). The dust effects were calculated from the above two sets of simulations, the first being with and without dust simulations (F1 and F2) that included both direct and indirect effects and the second set being with and without dust (F3 and F4) simulations with restricted aerosol-radiation interactions (aer_ra_feedback = 0).…”

Editorial: Atmospheric dust: How it affects climate, environment and life on Earth?

“…Dust particles can further affect the rainfall patterns through various direct and indirect pathways (Koren et al, 2005;Jin et al, 2015;Parajuli et al, 2022). Changes in rainfall patterns can have broader and long-term consequences because the entire biosphere directly depends upon rainfall for its survival.…”

“…The direct and indirect effects of dust on rainfall was quantified recently by Parajuli et al (2022). The study which used WRF-Chem simulations over the Red Sea coastal region showed different responses of dust on rainfall for extreme and normal rain events.…”

“…properties during the past decades due to advances in observational techniques and model development. Global and regional weather/climate models are used to simulate the emission of dust and its interactions with the climate (Chen et al, 2014;Kok et al, 2014;Jin et al, 2015;Francis et al, 2021a;Singh et al, 2021;Parajuli et al, 2022). However, the large spatiotemporal heterogeneity of dust sources, from giant sand dunes to small ridge and furrows of the agricultural field, from short-lived dust devils for several minutes to global dust transport for several weeks (Uno et al, 2009;Francis et al, 2020), makes it extremely challenging to represent the dust cycle (emission, transport, and deposition) in climate models.…”

Aerosol-cloud-radiation interaction during Saharan dust episodes: The dusty cirrus puzzle

“…Hui et al, 2008) and through a modulation of synoptic-scale and mesoscale atmospheric circulations (e.g. Jin et al, 2021;Parajuli et al, 2022). Not being able to predict dust-induced cirrus formation can lead to large forecast errors for the solar irradiance at the surface, even in day-ahead forecasts.…”