TY - JOUR
T1 - Aerosol influences on low-level clouds in the West African monsoon
AU - Taylor, Jonathan W.
AU - Haslett, Sophie L.
AU - Bower, Keith
AU - Flynn, Michael
AU - Crawford, Ian
AU - Dorsey, James
AU - Choularton, Tom
AU - Connolly, Paul J.
AU - Hahn, Valerian
AU - Voigt, Christiane
AU - Sauer, Daniel
AU - Dupuy, Régis
AU - Brito, Joel
AU - Schwarzenboeck, Alfons
AU - Bourriane, Thierry
AU - Denjean, Cyrielle
AU - Rosenberg, Phil
AU - Flamant, Cyrille
AU - Lee, James D.
AU - Vaughan, Adam R.
AU - Hill, Peter G.
AU - Brooks, Barbara
AU - Catoire, Valéry
AU - Knippertz, Peter
AU - Coe, Hugh
N1 - © Author(s) 2019
PY - 2019/7/4
Y1 - 2019/7/4
N2 - 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 sensi- tivity 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.
AB - 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 sensi- tivity 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.
UR - http://www.scopus.com/inward/record.url?scp=85060993873&partnerID=8YFLogxK
U2 - 10.5194/acp-19-8503-2019
DO - 10.5194/acp-19-8503-2019
M3 - Article
AN - SCOPUS:85060993873
SN - 1680-7316
VL - 19
SP - 8503
EP - 8522
JO - Atmospheric Chemistry and Physics
JF - Atmospheric Chemistry and Physics
IS - 13
ER -