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.
Highlights
Atmospheric aerosol particles are highly relevant in terms of weather, climate and human health
Stevens and Feingold (2009) and Fan et al (2016) emphasize the need to analyze aerosol indirect effect (AIE) with a dependence on cloud regimes with fine-scale models to explicitly resolve the interacting processes rather than using global models with parameterizations. This is supported by the study of Marsham et al (2013), which reveals that the West African monsoon (WAM) representation in the UK Met Office Unified Model shows fundamental differences between realizations with explicit and parameterized moist convection
We showed that Atlantic inflow (AI) affects the entire Southern West Africa (SWA) domain through the course of the day via cold air advection, the nocturnal low-level jet (NLLJ) that can be found in the AI postfrontal area, and convergence-induced convection and precipitation
Summary
Atmospheric aerosol particles are highly relevant in terms of weather, climate and human health. The modeling study of Lau et al (2017) focuses on the impacts of aerosol– monsoon interactions on variability over the northern Indian Himalaya foothills during the summer of 2008 They highlight the fact that the aerosol direct effect (ADE), i.a. mineral dust transport and radiative-heating-induced dynamical feedback processes, have major impacts on the largescale monsoon circulation. Stevens and Feingold (2009) and Fan et al (2016) emphasize the need to analyze AIE with a dependence on cloud regimes with fine-scale models to explicitly resolve the interacting processes rather than using global models with parameterizations This is supported by the study of Marsham et al (2013), which reveals that the West African monsoon (WAM) representation in the UK Met Office Unified Model shows fundamental differences between realizations with explicit and parameterized moist convection. The study concludes with a summary and evaluation of the findings (Sect. 7)
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