Mesoscale Boundary Layer Evolution over Complex Terrain. Part I. Numerical Simulation of the Diurnal Cycle

Abstract

The continuous development of a meso-?-scale boundary layer over sloping terrain upwind of a high mountain barrier was simulated through a complete diurnal cycle using a nonhydrostatic boundary-layer model. The simulation detailed the evolution of a 500?800-m deep nocturnal boundary layer containing 1?3 m s?1 thermal circulations in the region upwind of a high ridge. Shear between the 5 m s?1 gradient level winds above the boundary layer and the mesoscale thermal circulations maintained the turbulent mixing of cold air upward against the stable stratification. The nocturnal boundary layer is replaced the following morning by a growing convective boundary layer containing 3?5 m s?1 warm thermal flows under its base. A multiple layer structure appears during the morning transition with the coexistence of the synoptic, nocturnal and developing daytime wind systems. As the morning progresses, the downwind edge of the stable layer slowly retreats back toward lower elevations while the convective layer grows under its base. By 5 h after sunrise, the morning transition is complete. Comparisons of the model simulation with field data show that the model accurately simulates the diurnal development of the mesoscale boundary layer.