Sensitivity of a continental night‐time stratocumulus‐topped boundary layer to varying environmental conditions

Abstract

Large‐eddy simulation of a nocturnal stratocumulus‐topped boundary layer in a continental midlatitude environment has been performed to examine the sensitivity of the cloud to a number of different environmental parameters. The simulations showed that the stratocumulus cloud was strongly affected by the presence of an overlying free tropospheric cirrus cloud (FTC), in agreement with previous studies of marine nighttime stratocumulus. When introducing an FTC with an optical thickness of 2, stratocumulus liquid water path decreased by 30%. Enhancing the optical thickness of the FTC to 8 further decreased the liquid water path by almost 10%. The presence of an FTC decreased the cloud‐top radiative cooling which decreased the turbulent mixing in the boundary layer, so that the liquid water content and cloud depth were reduced. The sensitivity of the stratocumulus cloud to an overlying FTC was found to be affected by the moisture content in the free troposphere. When a clear positive or negative moisture gradient above the inversion was imposed, and an overlying FTC with an optical thickness of 8 was introduced, the stratocumulus cloud LWP decreased by more than 40%. Furthermore, the effect of changes in free tropospheric moisture content and an overlying FTC on the stratocumulus cloud properties was found to be nonlinear; the combined response was in general weaker than the two responses added together. The modeled response to changes in cloud condensation nuclei (CCN) concentrations was found to be non‐significant, unless the CCN concentrations were so low that drizzle was induced (∼50 cm−3).

An overview of the diurnal cycle of the atmospheric boundary layer during the West African monsoon season: results from the 2016 observational campaign

“…The LLJ, which is linked to the north-south pressure gradient associated with the Saharan heat low (Parker et al, 2005;Lothon et al, 2008;Abdou et al, 2010), occurs frequently in this region and is found approximately in the same layer as the LLCs. It is unclear what role middle and upper-level clouds play in nocturnal cloud formation, their morning dissolution and in the growth of the cloud-topped daytime boundary layer in the following day (Leung et al, 2016). Aerosols, mainly emitted from the urban agglomerations along the West African coast, and their north-eastward transport with the monsoon flow, are also suspected to impact cloud characteristics in southern West Africa (Knippertz et al, 2015a).…”

A meteorological dataset of the West African monsoon during the 2016 DACCIWA campaign

“…With respect to the latter process, shear-induced turbulence in the upper part of the LLJ and in the lower part of the African Easterly Jet (AEJ) could, depending on the position of the AEJ, mix the wet monsoon flow and the dry Saharan air layer above. Prior to DACCIWA it was not clear to what extent the nocturnal LLSC formation, their morning dissolution and the growth of the cloud-topped daytime ABL on the following day depend on free tropospheric moisture content and middle and upper-level clouds 10 , 11 . Aerosols emitted from the main metropolitan areas on the Gulf of Guinea coastline (Abidjan, Accra and, Lagos) and advected by the south-westerly monsoon flow also influence LLSC macro-physical characteristics in southern West Africa by affecting cloud microphysical properties such as cloud droplet and ice crystal formation and size-distribution 3 , 12 .…”

Observations on aerosol optical properties and scavenging during cloud events

“…from biogenic sources, and a general increase in particle size which can also likely be attributed to condensation of semi-and non-volatile biogenic compounds on pre-existing particles. Over the boreal region, biogenic compounds do have a significant effect on the aerosol load during the warm months, and their emissions increase with increasing temperature (Liao et al, 2014). Overall, an increase in the biogenic contribution, decrease in the anthropogenic contribution (BC), and ageing of long-range BC increase the scavenging efficiency of scattering material.…”