Composite Structure of Plumes in Stratus-topped Boundary Layers

Abstract

Knowledge of convective plumes within the clear convective boundary layer (CBL) is quite advanced owing to direct measurements, tank experiments, and large-eddy simulation studies. As a result, modeling of the CBL is relatively successful. Progress for the stratus-topped boundary layer (STBL), however, is slow. This study compares the plume structure of the surface-heated CBL with that of the cloud-top-cooled STBL in the hope of extending our knowledge of the CBL to the STBL. A conditional sampling technique is applied to the STBL flow fields that are generated through large-simulations, so that the structures of typical updrafts and downdrafts may be derived. For the purpose of comparing the surface-heated CBL and the cloud-top-cooled STBL, an idealized STBL is simulated where the turbulence is maintained solely by cloud-top radiative cooling. The principal difference between the CBL and this idealized STBL lies in the origin of the plumes: primary plumes in the CBL are generated at a rigid surface, while those in the STBL are generated at the entraining interface. It was found that in the idealized STBL, the compensating updrafts are nearly as strong as the top-cooling-generated downdrafts, and they contribute a significant amount to the heat, moisture, and momentum transports. This differs very much from the CBL, where the compensating downdrafts are much weaker than the surface-heating-generated updrafts and contribute much less to the transports. The mechanism that results in such an asymmetry between the CBL and STBL is examined, and suggestions on how the asymmetry affects the entrainment process are made.