A Simple Conceptual Model of Lower Troposphere Stability and Marine Stratus and Stratocumulus Cloud Fraction

We examine the importance of lower troposphere stability for Marine Stratus and Stratocumulus (MSC) cloud cover variations over subtropical oceans on monthly and seasonal timescales. Our approach is to consider a two-layer model with moist denser boundary layer air topped by dry lighter free air beneath a trade wind inversion at around 700 mb. When a large amount of dry static energy is transported from above and latent heat released from ocean surface, cloud is developed to radiate back the extra energy into space . This physical picture is supported by ISCCP FD data, ERA-40 reanalysis, and NCEP-NCAR reanalysis.The variation of dry static energy transport and surface latent heat release leads to the variation of cloud top radiative forcing, which is a function of low cloud cover.

A diagnostic low cloud cover scheme is developed based on the above physical pictures, which is a nonlinear function of atmospheric stability and large scale subsidence. Use ERA-40 and ISCCP-FD data as input, the scheme reproduces well the seasonal variation of low cloud cover in the subtropical MSC regions.The new scheme simulates a much better seasonal variation of low cloud cover during 1990 to 2000 than the NCAR CAM3 empirical linear low stratiform cloud cover scheme near the Peruvian regions. NCAR CAM3.1 simulation results using the new scheme will be presented in the talk.