Atmospheric Physics, Chemistry and Climate

Clouds and Convection

In present day general circulation models (GCM), clouds are considered to be subgrid scale processes because turbulence generated by clouds and convection occurs at much smaller scales than the smallest resolvable scale by a GCM grid. For example, cloud scale updrafts and downdrafts are of the order of tens to hundreds of meters, whereas GCM grids have horizontal grid spacing around 100 km.

As a result, clouds and convection must be parameterized in GCMs.

Parameterizations are mathematical representations of a chemical or physical process, using variables resolved by a general circulation model and obtained from observation, theory, and optimization.

We develop parameterizations that represent the effect of convection and stratiform clouds in GCMs. To help in the development of these parameterizations, we use high resolution models that can explicitly simulate cloud-scale motions, such as CSRM (cloud system resolving model) and LES (large-eddy simulation).





Figure: Large-eddy simulation of a stratocumulus cloud. The gray shaded surface depicts the cloud boundary and the colored vertical cross section shows the vertical velocity. This simulation is based on the second research flight (RF02) of the DYCOMS-II (Second Dynamics and Chemistry of Marine Stratocumulus) experiment. It was conducted over the Pacific south-west off Los Angeles, CA in July 2001.