A study of aerosol impacts on clouds and precipitation development during an extreme rainfall event over Far-North Cameroon using WRF-Chem

Abstract

An unusual extreme rainfall occurred in far-north Cameroon on August 2020. The monthly accumulated rainfall observed, exceeded 274 mm and the main rainfall time occurred during the third week of this month with a total accumulated rainfall of 80 mm. The Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) was used to investigate the effects of aerosols on the cloud properties, radiative flux, and precipitations over this region. Three sensitivity tests were designed by varying the anthropogenic emissions. Increased aerosol number concentration has resulted in more numerous cloud droplets of smaller size, through which the optical properties of clouds have been changed. Both shortwave and longwave cloud forcing are decreased in more polluted tests while the net radiative cloud forcing is increased causing an overall warming effect. The cloud and ice water content increased while the rainwater content decreased in the more polluted tests. It is found that; cloud fraction is reduced in more polluted atmospheres. The domain-averaged accumulated rainfall in the clean and polluted tests decreased compared to that in the control test and the maximum accumulated rainfall increased in more polluted experiments. Nevertheless, changes in the rate of precipitation are observed, such that under a polluted atmosphere light rain is increased, while both moderate and heavy rain are weaker, confirming the fact that a decrease in cloud droplet size can reduce precipitations.