Aerosol impacts on subtropical low-level clouds: a satellite and modelling perspective

Sammanfattning: Complex interactions between aerosols, clouds, and radiation impact Earth's climate. However, several aspects of these interactions remain uncertain, which has led to extensive research over the last decades. This thesis explores some unresolved aspects by focusing on subtropical low-level stratocumulus (Sc) clouds, which have a significant cooling effect on climate. The clouds are also sensitive to varying aerosol conditions, which can influence their formation, properties, and lifetime. Clouds over the South East Atlantic have been studied in detail, using both numerical modeling and satellite observations, to shed light on the interactions between aerosols, clouds, and radiation. This geographical region displays a large and semi-permanent Sc cloud deck and is also subjected to meteorological conditions that bring large amounts of light-absorbing aerosols from biomass fires over the African continent. The biomass-burning plumes also bring enhanced levels of moisture, and the individual influence of the aerosols and the moisture on the low-level cloud properties have been investigated.The analysis of satellite retrievals showed a radiative impact (sensitive to aerosol composition and aerosol optical depth) of moist aerosol layers in the free troposphere over the South East Atlantic; however, it was not possible to observe a clear influence of these humid aerosol layers on the underlying low-level clouds. Aerosol-radiation interactions were implemented in a large eddy simulation (LES) code that was used to model stratocumulus to cumulus transitions (SCT) in weather situations where moist absorbing aerosol layers were in contact with low-level clouds and mixed into the marine boundary layer (MBL). In these simulations, the heating by the absorbing aerosol within the MBL affected the persistence of the Sc clouds by accelerating the SCT, especially during daylight and broken cloud conditions. However, the humidity accompanying the absorbing aerosol was also found to be important --  it reduced the deepening of the MBL when located above the Sc deck and delayed the SCT when in contact with clouds. Furthermore, the additional moisture resulted in a radiative cooling effect that was comparable to the radiative cooling effect caused by the aerosol itself. The simulated SCTs were found to be mostly driven by increased sea surface temperatures, regardless of aerosol conditions. This result was different compared to two other LES models where the SCT was driven by drizzle under the same low aerosol conditions. On a larger scale, it was found that an explicit description of aerosol-cloud interactions in a climate model led to smaller differences between the simulated and mean observed values of the shortwave cloud radiative effect compared to when a non-interactive parameterization was used.