Mean structure and diurnal cycle of low clouds over the Southeast Atlantic: Insights from satellite observations and climate model simulations

The diurnal cycle of Southeast Atlantic boundary layer clouds is described with the use of satellite observations and multi-scale modeling framework (MMF) simulations during austral spring (September-November). Hourly resolution cloud fraction (CF) and cloud top height (HT) are retrieved from Meteosat-9 radiances using algorithms adapted from MODIS observations for the Clouds and Earth's Radiant Energy System (CERES). Liquid water path is obtained from the University of Wisconsin microwave satellite climatology. The MMF simulations use a 2-D cloud-resolving model (CRM) that contains an advanced third order turbulence closure to explicitly simulate cloud physical processes in every grid column of a general circulation model. The CRMs replace all cloud-related parameterizations in the Community Atmosphere Model Version 3.5. Twelve vertical levels below 700 hPa are used in order to better resolve boundary layer clouds. The model accurately reproduces the marine stratocumulus spatial extent and CF. In addition, the model shows that the mean cloud cover spatial variability is primarily explained by the boundary layer decoupling strength, defined as the cloud base height and lifting condensation level differences, whereas a boundary layer shoaling accounts for a coastal decrease in CF. A westward boundary layer deepening is reproduced in both satellite observations and simulations. Nevertheless, the model overestimates the liquid water path 50% more than satellite observations. The observed diurnal cycles in CF and HT are relatively modest, with fractional changes smaller than 15%, and 7%, respectively. The model suggests that small diurnal cycles are linked to a strong temperature inversion. Minima in CF, HT, and liquid water path around 15:00 local time over most of the domain indicate that the diurnal cycle is mainly driven by solar radiative heating. Differences between the Southeast Atlantic and Southeast Pacific regimes will be also discussed.