Verification of boundary layer wind patterns in COSMO-REA2 using clear-air radar echoes

Abstract

The verification of high-resolution meteorological models requires highly resolved validation data and appropriate tools of analysis. While much progress has been made in the case of precipitation, wind fields have received less attention, largely due to a lack of spatial measurements. Clear-sky radar echoes could be an unexpected part of the solution by affording us an indirect look at horizontal wind patterns: regions of horizontal convergence attract non-meteorological scatterers such as insects; their concentration visualizes the structure of the convergence field. Using a two-dimensional wavelet transform, this study demonstrates how divergences and reflectivities can be quantitatively compared in terms of their spatial scale, anisotropy (horizontal), and direction. A long-term validation of the highly resolved regional reanalysis COSMO-REA2 against the German radar mosaic shows surprisingly close agreement. Despite theoretically predicted problems with simulations in or near the “grey zone” of turbulence, COSMO-REA2 is shown to produce a realistic diurnal cycle of the spatial scales larger than 8 km. In agreement with the literature, the orientation of the patterns in both datasets closely follows the mean wind direction. Conversely, an analysis of the horizontal anisotropy reveals that the model has an unrealistic tendency towards highly linear, roll-like patterns early in the day.