Simulation of Entrainment at the Top of the Atmospheric Boundary Layer for Use in General Circulation Models (GCM)

摘要

A scheme for handling condensation and clouds and entrainment simulation with a well mixed boundary layer in general circulation models is developed. Entrainment simulation is related to an entrainment velocity, the magnitude of which is a function of the cloud water content. Unrealistic condensation in the middle of a well mixed layer is avoided by not allowing condensation to appear unless the model level is in the immediate proximity of the inversion base. The scheme was tested in a one-column version of a global model. A column situated over the ocean is considered and data are chosen such that surface fluxes of heat and moisture are minimized to make the radiative cooling at the cloud tops the main motor for the turbulent motion. The results from 48 hr integrations indicate that the scheme renders a realistic evolution: the boundary layer depth increases with time, a cloud deck with realistic liquid water concentration is present at the top of the boundary layer but practically no precipitation reaches the ground.