The role of aerosol–radiation–cloud interactions in linking anthropogenic pollution over southern west Africa and dust emission over the Sahara

Abstract

The aerosol direct and indirect effects are studied over west Africa in the summer of 2016 using the coupled WRF-CHIMERE regional model including aerosol-cloud interaction parameterization. First, a reference simulation is performed and compared with observations acquired during the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) field campaign which took place in June and July 2016. Sensitivity experiments are also designed to gain insights into the impact of the aerosols dominating the atmospheric composition in southern west Africa (one simulation with halved anthropogenic emissions and one with halved mineral dust emissions). The most important effect of aerosol-cloud interactions is found for the mineral dust scenario, and it is shown that halving the emissions of mineral dust decreases the 2 m temperature by 0.5 K and the boundary layer height by 25 m on a monthly average (July 2016) and over the Saharan region. The presence of dust aerosols also increases (decreases) the shortwave (longwave) radiation at the surface by 25 W m −2 . It is also shown that the decrease of anthropogenic emissions along the coast has an impact on the mineral dust load over west Africa by increasing their emissions in the Saharan region. It is due to a mechanism where particulate matter concentrations are decreased along the coast, imposing a latitudinal shift of the monsoonal precipitation and, in turn, an increase of the surface wind speed over arid areas, inducing more mineral dust emissions.

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

“…Even if both state-of-the-art models and satellite products have difficulties in retrieving the precipitation in SWA, this comparison suggests that the model predicts too many days of low precipitation rate, as it has already been shown for the WRF model (Menut et al, 2019).…”

“…The modifications in low-level clouds and precipitation affect in turn the agricultural activities in this region, and could also have implications for the transition from stratiform to convective clouds (Lohou et al, 2020). Furthermore, the reduction of precipitation in southern West Africa due to the anthropogenic aerosol emitted along the coast, changes the liquid water transported towards the Sahel, and modifies the winds and soil humidity, and as a consequence dust emissions, as shown by Menut et al (2019). The present study adds evidence to the emerging hypothesis that, during the West African monsoon, increasing anthropogenic aerosol pollution in southern West Africa has already caused a precipitation reduction (Pante et al, 2020).…”

“…The indirect effect is taken into account thereby transferring four fields of CHIMERE to the microphysics scheme of WRF: (i) aerosol size distribution, (ii) bulk hygroscopicity of internally mixed aerosols, (iii) ice nuclei, and (iv) deliquesced aerosols (Tuccella et al, 2019). Over West Africa, the high aerosol load makes the integration of the aerosol effects an important steps toward the understanding of feedbacks on the meteorology during the monsoon (Menut et al, 2019).…”

“…3). The observed mean vertical profiles are composed of three vertical parts: (i) from the surface to 1 km amsl, close to saturation (> 90 %), which is the monsoon layer, (ii) from 1-2 km amsl, RH close to 90 % and low wind speed, which is a directional Menut et al (2019).…”

“…The latest generation of Numerical Weather Prediction models integrates the aerosol effects on meteorology (Baklanov et al, 2014). Over Africa, Menut et al (2019) have shown -with the online coupled WRF-CHIMERE model -that a decrease of anthropogenic emissions along the Guinean coast imposes a northward shift of the monsoonal precipitation associated with an increase of the surface wind speed over arid areas in the Sahel, inducing more mineral dust emissions.…”