A Model for Entrainment into a Cloud-Topped Marine Boundary Layer. Part I: Model Description and Application to a Cold-Air Outbreak Episode

Abstract

A model is presented for the growth and evolution of a cloud-topped marine boundary layer. In this model the entrainment rate is determined from the turbulence kinetic energy (TKE) budget. It is assumed that the TKE budget can be partitioned according to whether each process produces TKE or converts it into potential energy, and that dissipation is proportional to production. This leads to an entrainment relationship which is considerably different than used in previous cloud-topped models. This model is used to study an episode of cold-air outbreak over Lake Ontario during the International Field Year for the Great Lakes (IFYGL). The model reproduces changes in potential temperature and dew point as the air crossed the lake and the associated time variation of these parameters at the down-wind shore with an accuracy of better than 1°C. Model and measured soundings closely match, especially with respect to the presence and location of such features as cloud layers. Depth of the mixed layer also was generally well modeled. Use of divergences measured by the lakewide IFYGL buoy network did not give good agreement with the data. It is believed that this indicates that mixed-layer depth is sensitive to divergences at a smaller scale than the size of the lake.