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.
Highlights
In southern West Africa (SWA), the population is rapidly increasing, driving up anthropogenic emissions (AE) (Liousse et al, 2014)
This study presents numerical modeling experiments conducted with WRF-CHIMERE in combination with aerosol and cloud observational datasets from the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) campaign, which are described in Sect
We investigate four aerosol types (BC, organic carbon (OC), ammonium, nitrate), which are of prime importance due to their optical and hygroscopic properties (Carslaw et al, 2010)
Summary
In southern West Africa (SWA), the population is rapidly increasing, driving up anthropogenic emissions (AE) (Liousse et al, 2014). During the West African monsoon (WAM) period (June–September), in addition to local pollution emissions, aerosols from two remote sources are transported toward the Guinean coast, namely mineral dust from the north and biomass burning aerosol (BBA) from Central Africa (e.g., Flamant et al, 2018a) These different sources contribute to the high aerosol load over the SWA and have deleterious human health impacts (Bauer et al, 2019). Depending on their optical and chemical properties, aerosols influence the local meteorology through direct (and semi-direct) effects via the absorption and scattering of radiation, and via hygroscopic aerosol enhancing cloud droplet number concentrations, thereby decreasing droplet sizes (e.g., Haywood and Boucher, 2000; Carslaw et al, 2010). An adequate representation of these effects is critical for general circulation models (Fan et al, 2016; Seinfeld et al, 2016), especially over West Africa, where clouds in the lowermost troposphere are not well reproduced during the WAM season (Hannak et al, 2017)
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