Środowiskowe Seminarium Fizyki Atmosfery

It is believed the shallow boundary-layer clouds, such as subtropical stratocumulus and shallow cumulus, hold the key to the Earth climate and climate change. This is because of their strong impact on solar radiation and relatively insignificant effect on Earth thermal (longwave) radiation. In particular, the so-called "climate sensitivity", that is, the change of the mean surface air temperature in a general circulation model (GCM) in response to the increase of the greenhouse gases in the atmosphere, critically depends on changes to the boundary layer clouds. It follows that understanding interactions among various processes that affect macroscopic (or bulk) properties of shallow convective clouds, such as the cloud fraction, cloud depth, albedo, precipitation, etc., is of significant importance. Such understanding is essential for the development of advanced representations of such clouds in contemporary GCMs.

This lecture will review microphysical (i.e., cloud and precipitation) processes argued in the past to affect macroscopic properties of boundary-layer clouds and will discuss results of numerical simulations based on the RICO model intercomparison case to investigate if such effects indeed play any role in shallow convection. In general, model simulations suggest that the answer to this question is mostly negative, that is, macroscopic properties of shallow precipitating convective clouds seem unaffected by microphysical processes, with the obvious exception of the cloud albedo due to the so-called Twomey effect. Limitations of model simulations will be discussed and future work in this area will be suggested.