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, particularly with regard to the time variation of lapse rates and the occurrence times of maximum and minimum temperatures at various levels in the lower boundary layer. 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 with the model realistically reproducing the unsteady behavior of heat fluxes in the PBL. 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. A weak heat flux source at the ground with an amplitude of 10% of the maximum daytime heating produced a nocturnal heat island some 60 m high with a maximum city-country temperature contrast of ?1C.