Experimental and numerical study of the modifications of mixed-layer structure by inhomogeneous surface fluxes and secondary circulations

摘要

The problem of scale interactions in the planetary boundary layer (PBL), i.e., how forcing mechanisms of differing spatial and temporal scales affect the properties of the PBL, has been a subject of considerable interest for some time. One aspect of this problem that has received increasing attention in recent years is the representation, in mesoscale and general circulation models. of the effects of subgrid-scale inhomogeneities in surface properties. Differences in sensible and latent heat fluxes between adjacent areas can result in secondary circulations. modifications to the boundary layer structure, and the transport of heat and moisture from one area to another. Despite their potential included on local PBL properties, such effects can not be resolved by numerical models whose scale is too coarse; thus, suitable parameterizations that account for these phenomena are needed. In this paper the authors present some results of wind and temperature measurements over an area with significant variations in surface fluxes on scales of O(10 km). They then use fine resolution numerical simulations to show how synoptic, topographical, and thermal forcing combine to affect the properties of the PBL in the region. Specifically, they consider the relative importance of topography and land-use differences on the generation of secondary circulations and on the depth of the mixed layer, and they show how these effects are modified by synoptic wind fields. Implications for coarser resolution models are also discussed.