Climatology of the Vertical Profiles of Polarimetric Radar Variables at X Band in Stratiform Clouds

The Quasi-Vertical Profiles (QVP) methodology for processing and displaying polarimetric radar data has demonstrated significant benefits for our understanding of microphysical processes in stratiform clouds. The approach also allows to identify layers of enhanced differential reflectivity Z_DR and specific differential phase K_DP above the freezing level with high vertical resolution and accuracy. Accurate measurements of K_DP are particularly important because enhanced values can be used for nowcasting precipitation enhancement near the surface. Combined with the measurements of the radar reflectivity factor Z, such K_DP estimates can be used to significantly improve estimates of ice water content IWC and snow rate S compared to traditional Z-based estimates. Such advantages have been first demonstrated at S band at which K_DP in snow is generally low. Since K_DP at X band is about three times higher than at S band, X-band observations are particularly valuable to further explore K_DP measurements for an improved classifcation and quantification of ice and snow. We present QVP statistics of 40 stratiform events observed with the polarimetric X-band radar in Bonn, Germany. An anti-correlation between Z_DR and K_DP in the dendritic growth layer (DGL) and a strong dependency on the cloud top temperature have been noticed. The K_DP enhancement within the DGL is often accompanied with short-term sagging of the melting layer caused by riming or its longer-term lowering possibly associated with cooling due to melting if the amount of snow aloft is high (see Figure). We quantify the range of variability of the polarimetric radar variables in the melting layer at X band including backscatter differential phase δ and K_DP, which has never been examined in this region. Backscatter differential phase δ contains information on particle size and the degree of riming. In order to separate information contained in δ and K_DP, first the depth of the melting layer is determined and then the QVP of the differential phase shift Φ_DP is interpolated between the values below and above the melting layer to estimate K_DP. We present relations between K_DP in the DGL and time-lagged surface precipitation enhancement indicated by simultaneously increased Z and differential reflectivity Z_DR towards the surface. Mostly, positive trends in K_DP in the DGL concur with an increase in melting layer depth and δ indicating larger aggregates. Snow rate estimates based on Z and K_DP aloft are derived using the dense rain gauge network of the city of Bonn and the assumption that water mass fluxes just above the freezing level and below the melting layer do not differ much.