DIURNAL VARIATION OF THE PLANETARY BOUNDARY LAYER IN A MESOSCALE MODEL.

Abstract

A two-dimensional mesoscale atmospheric model is presented and used to study unsteady dynamic processes occurring in the planetary boundary layer (PBL) driven by diurnal heating at the ground. The model reproduces turbulent fluxes of heat and momentum both by explicitly modeling resolvable eddies and by employing a single parameterization at all levels of the model to represent vertical fluxes caused by subgrid-scale eddies. The unsteady behavior of horizontally-averaged profiles of temperature and velocity respond quite realistically to the diurnally varying heat flux at the ground. The spatial variation of predicted atmospheric quantities shows a great deal of resolved eddy activity during the day with a significant remnant persisting through the night at higher levels of the PBL. These eddies account for the predominant means of vertical heat and momentum transfer away from the surfaces. Temporal variation of vertical heat and momentum profiles shows boundary layer activity to be confined to a few hundred meters at night while extending up to a kilometer during the day.