Direct and indirect effect of dust on rainfall

Rainfall affects the distribution of surface water and groundwater resources, which are declining over the Middle East and North Africa (MENA) due to overexploitation. Dust is the dominant aerosol type in desert regions, and it can affect regional water resources by modulating rainfall distributions. Cloud seeding experiments are being considered to augment the availability of freshwater resources in the region. In this context, we investigate and quantify dust's direct and indirect effect on rainfall over the Red Sea coast using high-resolution (1.5x1.5 km) WRF-Chem model simulations.

We observed different dust impacts on rainfall for extreme and regular rain events. Dust enhanced net rainfall for extreme events (~6%) but suppressed (~-1%) for regular rain events. For extreme rainfall events, the net effect was primarily driven by indirect effects, which enhanced rainfall by about +4.6%. This is because synoptic features usually cause extreme rain events, and the high dust concentration facilitates raindrops to grow in the presence of sufficient moisture. For regular rainfall events, the net effect was governed by direct effects, which suppressed rainfall by about -5.8%. The result was suppression because the dust's direct effect depends upon the local circulation, the sea breezes in our case. Dust scatters shortwave radiation, cools the land surface, and thus weakens the sea breeze circulation, bringing more moisture to the land and suppressing the rainfall.

Because background dust aerosols are essential for rain formations, our results strongly indicate that cloud seeding may be less effective when the atmosphere is already seeded with dust aerosols, a typical environment in desert regions.