Sensitivity of low-level clouds and precipitation to anthropogenic aerosol emission in southern West Africa: a DACCIWA case study

Abstract

Abstract. During the West African summer monsoon, pollutants emitted in urbanized coastal areas modify cloud cover and precipitation patterns. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field campaign provided numerous aircraft-based and ground-based observations, which are used here to evaluate two experiments made with the coupled WRF–CHIMERE model, integrating both the direct and indirect aerosol effect on meteorology. During one well-documented week (1–7 July 2016), the impacts of anthropogenic aerosols on the diurnal cycle of low-level clouds and precipitation are analyzed in detail using high and moderate intensity of anthropogenic emissions in the experiments. Over the continent and close to major anthropogenic emission sources, the breakup time of low-level clouds is delayed by 1 hour, and the daily precipitation rate decreased by 7.5 % with the enhanced anthropogenic emission experiment (with high aerosol load). Despite the small modifications on daily average of low-level cloud cover (+2.6 %) with high aerosol load compared to moderate, there is an increase by more than 20 % from 14:00 to 22:00 UTC on hourly average. Moreover, modifications of the modeled low-level cloud and precipitation rate occur far from the major anthropogenic emission sources, to the south over the ocean and to the north up to 11∘ N. The present study adds evidence to recent findings that enhanced pollution levels in West Africa may reduce precipitation.

The Impact of Aerosols on the Stratiform Clouds over southern West Africa: A Large-Eddy Simulation Study

“…Using COSMO-ART model in a simulation for 2-3 July 2016 case, Deetz et al (2018) found that under the influence of the Maritime Inflow (MI, cold air) from Guinean Gulf, stratus-stratocumulus transition are susceptible to the aerosol direct effect resulting in a spatial shift in the MI front and a temporal shift of the cloud transition. Over SWA and influenced by anthropogenic emission sources, the breakup time of LLSCs can be delayed by one hour and daily precipitation rate can decrease by 7.5% according to Deroubaix et al (2022). Moreover, semi-direct and indirect effects were studied together by varying respective magnitude or forcing emission of anthropogenic aerosols but they were not examined separately.…”