Diagnostic Downscaling of Large-Scale Wind Fields to Compute Local-Scale Trajectories

Abstract

This paper describes a simple method, based on routine meteorological data, to produce high-resolution wind analyses throughout the planetary boundary layer (PBL). It is a new way to interpolate wind measurements. According to this method, high-frequency information from surface wind measurements is extrapolated to greater heights by assuming that the vertical shear of the horizontal wind, that is, the differential vertical wind profile, is horizontally more homogeneous than the wind profile itself. Under this assumption, it is sufficient to combine high-resolution surface wind measurements with low-resolution vertical profiles of differential winds?for which high-resolution measurements usually do not exist?to yield high-resolution wind analyses throughout the PBL. The method can thus be viewed as a diagnostic downscaling of large-scale wind fields. Downscaling works best during daytime within a homogeneous air mass and in flat terrain. A validation against sodar wind measurements demonstrates that downscaling actually improves large-scale wind fields. A comparison of trajectories calculated from large-scale wind fields, from downscaled wind fields, and from wind fields produced by a conventional diagnostic wind field model, with daytime constant level balloon flights, again shows that the downscaled wind fields are most accurate.