Numerical simulations of aerosol radiative effects and their impact on clouds and atmospheric dynamics over southern West Africa

Abstract

Abstract. Southern West Africa (SWA) is undergoing rapid and significant socioeconomic changes associated with a massive increase in air pollution. Still, the impact of atmospheric pollutants, in particular that of aerosol particles, on weather and climate in this region is virtually unexplored. In this study, the regional-scale model framework COSMO-ART is applied to SWA for a summer monsoon process study on 2–3 July 2016 to assess the aerosol direct and indirect effect on clouds and atmospheric dynamics. The modeling study is supported by observational data obtained during the extensive field campaign of the project DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) in June–July 2016. As indicated in previous studies, a coastal front is observed that develops during daytime and propagates inland in the evening (Atlantic inflow). Increasing the aerosol amount in COSMO-ART leads to reduced propagation velocities with frontal displacements of 10–30 km and a weakening of the nocturnal low-level jet. This is related to a subtle balance of processes related to the decrease in near-surface heating: (1) flow deceleration due to reduced land–sea temperature contrast and thus local pressure gradient, (2) reduced turbulence favoring frontal advance inland and (3) delayed stratus-to-cumulus transition of 1–2 h via a later onset of the convective boundary layer. The spatial shift of the Atlantic inflow and the temporal shift of the stratus-to-cumulus transition are synergized in a new conceptual model. We hypothesize a negative feedback of the stratus-to-cumulus transition on the Atlantic inflow with increased aerosol. The results exhibit radiation as the key player governing the aerosol affects on SWA atmospheric dynamics via the aerosol direct effect and the Twomey effect, whereas impacts on precipitation are small.

An evaluation of operational and research weather forecasts for southern West Africa using observations from the DACCIWA field campaign in June–July 2016

“…() and Deetz et al . (). In this area models tend to overestimate clouds, associated with too dry and too cold conditions near the surface, particularly after the monsoon onset, when the main rain band has moved northwards towards the Sahel and when coastal upwelling is well established.…”

Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes

“…Most of our knowledge on the processes relevant for LLCs is based on numerical simulations, as high-quality observational data are scarce in this region. Studies by Schrage and Fink (2012), Schuster et al (2013), Adler et al (2017) and Deetz et al (2018) suggest that processes spanning from the microscale to the synoptic scale are important such as horizontal cold air advection, orographic lifting, lifting related to gravity waves and shear-generated vertical mixing underneath the axis of a nocturnal low-level jet (LLJ). There is evidence that the horizontal cold air advection is related to the southwesterly monsoon flow, which transports maritime air from the Gulf of Guinea northwards over land.…”

The role of aerosol-cloud interactions in linking anthropogenic pollution over southern West Africa and dust emission over the Sahara

“…They quantified the important impact of aerosol on shortwave radiation (a decrease of -20 W m −2 ) whereas the impact 70 on longwave radiation was found to be negligible. In the continuity of this work, Deetz et al (2018b) showed that the amount of aerosols over southern West Africa impacts the dynamics of the daytime coastal moist front generated along the Gulf of Guinea and the intensity of its inland propagation, this feature modulating the transport of anthropogenic aerosols emitted at the coast (Deroubaix et al, 2019).…”