Comparison of spectral characteristics of precipitation from radar estimates and COSMO-model predicted fields

Abstract

The spectral characteristics of hourly precipitation fields are studied for a number of meteorological events, classified depending on the precipitation process involved, namely stratiform, convective or mixed stratiform–convective. The study focuses on the comparison between spatial spectral characteristics of observed precipitation fields, obtained from hourly radar estimates, and modelled hourly predictions of the same field by the non-hydrostatic model COSMO. The results show that the power spectra of radar hourly precipitation are characterised by invariance within ranges of horizontal spatial scales that are different for the three classes of events. COSMO reproduces some basic characteristics of the radar spectra in all cases. Nevertheless, in stratiform events, COSMO spectra present a well-defined scale break at about 15 km with no counterpart in radar data. It is suggested that this model feature is related to the presence of spurious horizontal smoothing introduced by the semi-lagrangian advection scheme for precipitation. Smoothing affects all scales up to the maximum length scale of precipitation and horizontal advection by the wind. An analysis of wind intensity in the lower troposphere over the region supports this interpretation. Discrepancies in precipitation spectra between radar and COSMO data in convective events are interpreted as a consequence of the inadequacy of the model resolution for a correct representation of convection.