Abstract
Abstract. Data derived from instruments on board the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat satellites as well as meteorological parameters from reanalysis are used to explore situations when moist aerosol layers overlie stratocumulus clouds over the southeast Atlantic during the biomass burning season (June to October). To separate and quantify the impacts of aerosol loading, aerosol type, and humidity on the radiative fluxes (including cloud top cooling), the data are split into different levels of aerosol and moisture loadings. The aerosol classification available from the CALIPSO products is used to compare and contrast situations with pristine air, with smoke, and with other (non-smoke) types of aerosols. A substantial number of cases with non-smoke aerosols above clouds are found to occur under similar meteorological conditions to the smoke cases. In contrast, the meteorology is substantially different for the pristine situations, making a direct comparison with the aerosol cases ambiguous. The moisture content is enhanced within the aerosol layers, but the relative humidity does not always increase monotonously with increasing optical depth. Shortwave (SW) heating rates within the moist aerosol plumes increase with increasing aerosol loading and are higher in the smoke cases compared to the non-smoke cases. However, there is no clear correlation between moisture changes and SW absorption. Cloud top cooling rates do not show a clear correlation with moisture within the overlying aerosol layers due to the strong variability of the cooling rates caused by other meteorological factors (most notably cloud top temperature). No clear influence of aerosol type or loading on cloud top cooling rates is detected. Further, there is no correlation between aerosol loading and the thermodynamic structure of the atmosphere nor the cloud top height.
Highlights
Stratocumulus clouds have a cooling effect on Earth’s climate due to their strong reflection of incoming solar radiation and their relatively small effect on the outgoing longwave radiation
Thereafter, we will analyze the influence of the aerosol layer and its composition on the radiative heating profiles and examine the main drivers of any influence: aerosol type, loading, or moisture (RH, qv)
Selecting a greater number of aerosol situations, we will analyze the relationship between the aerosol optical depth and the free tropospheric moisture observed in our data and compare it with previous studies
Summary
Stratocumulus clouds have a cooling effect on Earth’s climate due to their strong reflection of incoming solar radiation and their relatively small effect on the outgoing longwave radiation. Large-eddy simulations and radiative transfer calculations have shown a reduction of the stratocumulus top longwave (LW) cooling due to a downward LW flux increase caused by the water vapor accompanying the aerosol layer (Yamaguchi et al, 2015; Zhou et al, 2017; Deaconu et al, 2019) This effect, combined with an increase in the atmospheric stability due to shortwave (SW) absorption by the aerosols, may decrease the entrainment rate (Deaconu et al, 2019), which impacts the deepening of the boundary layer and the transition from stratocumulus to cumulus (Wood, 2012).
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