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

Abstract

{T}he continuous development of a meso-scale boundary layer oversloping terrain upwind of a high mountain barrier was simulated througha complete diurnal cycle using a nonhydrostatic boundary-layer model.{T}he simulation detailed the evolution of a 500-800 m deep boundarylayer containing 1-3 m/s thermal circulations in the region upwindof a high ridge. {S}hear between the 5 m/s gradient level winds abovethe boundary layer and the mesoscale thermal circulations maintainedthe turbulent mixing of cold air upward against the stable stratification.{T}he nocturnal boundary layer is replaced the following morningby a growing convective boundary layer containing 3-5 m/s warm thermalflows under its base. {A} multiple layer structure appears duringthe morning transition with the coexistence of the synoptic, nocturnaland developing daytime wind systems. {A}s the morning progresses,the downwind edge of the stable layer slowly retreats back towardlower elevations while the convective layer grows under its base.{B}y 5 h after sunrise, the morning transition is complete. {C}omparisonof the model simulations with field data show that the model accuratelysimulates the diurnal development of the mesoscale boundary layer.