Aerosol influences on low-level clouds in the West African monsoon

Jonathan Taylor ,Adam Vaughan ,Paul Connolly ,T W Choularton ,Michael J Flynn ,Alfons Schwarzenböeck ,Peter Hill ,James Lee ,Joël Brito ,J R Dorsey ,Régis Dupuy,Phil Rosenberg,Cyrille Flamant,Cyrielle Denjean,Thierry Bourriane,Peter Knippertz,Ian Crawford,Hugh Coe,Daniel Sauer,Valéry Catoire,Valerian Hahn,Barbara Brooks,Sophie L Haslett,Christiane Voigt,Keith Bower

doi:10.5194/acp-19-8503-2019

Abstract

Abstract. Low-level clouds (LLCs) cover a wide area of southern West Africa (SWA) during the summer monsoon months and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of in situ measurements of cloud microphysics from the region, taken during June–July 2016, as part of the DACCIWA (Dynamics–aerosol–chemistry–cloud interactions in West Africa) campaign. This novel dataset allows us to assess spatial, diurnal, and day-to-day variation in the properties of these clouds over the region. LLCs developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Regional variation in LLC cover was largely orographic, and no lasting impacts in cloud cover related to pollution plumes were observed downwind of major population centres. The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities but were not statistically significant across the region. A significant fraction of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the Southern Hemisphere. To assess the relative importance of local and transported aerosol on the cloud field, we isolated the local contribution to the aerosol population by comparing inland and offshore size and composition measurements. A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated cloud drop number concentration by 13 %–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the Southern Hemisphere is likely to dampen any effect of these increased local emissions on cloud–aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African summer monsoon.

Highlights

During the summer monsoon in June–September, large areas of southern West Africa (SWA) are covered by lowlevel clouds (LLCs), which form overnight and thicken in the morning, before breaking up in the early afternoon (van der Linden et al, 2015)
Sessment of cloud drop number concentration (CDNC) based on aircraft data acquired during the DACCIWA field campaign, as well as parcel modelling, showed that local emissions had a reduced effect on CDNC on the regional scale, as biomass burning pollution transported from the Southern Hemisphere dominated regional aerosol concentrations (Haslett et al, 2019b)
This transported pollution caused a high baseline CDNC inland, putting the clouds in a regime where they had a reduced susceptibility to any further increases in aerosol, and minimising the impact any enhancements in CDNC are likely to have on cloud radiative effects and precipitation

Summary

Introduction

During the summer monsoon in June–September, large areas of southern West Africa (SWA) are covered by lowlevel clouds (LLCs), which form overnight and thicken in the morning, before breaking up in the early afternoon (van der Linden et al, 2015). Many climate models struggle to accurately represent LLCs (Hannak et al, 2017), and measurements of LLCs (and their radiative interactions with higher-level cloud layers) are a key uncertainty in the quantification of the overall cloud radiative effect in SWA (Hill et al, 2018). The population of SWA is expected to undergo large increases (United Nations, 2017), leading to corresponding increases in emissions of anthropogenic pollution (Liousse et al, 2014). Such increases may affect dynamics and cloud microphysics in the region, and it is of interest to determine any impact on the regional climate such as changes in cloud cover and precipitation (Knippertz et al, 2015b)

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