The transition from bulk to spectral bin microphysics schemes within numerical weather prediction (NWP) models promises more realistic simulations of cloud resolving processes and finally an improving of weather forecasting quality. However the needed size distributions of water drops and ice particles were parameterized from rare observations, typically from ground based rain distrometers or aircraft in-situ measurements. The current work introduces a retrieval method to derive size distributions from synergetic use of vertical pointed Ka-band and polarimetric C-band radar. The method is based on using the full height-resolved Doppler spectra instead of mean values of reflectivity and radial velocity inside the radar bin volume. Within a Mie and T-matrix based radar forward operator, Doppler spectra were simulated from assumed size distributions, taking into account the attenuation at Ka-band. In an iterative way, the parameters of distributions were varied until differences between simulated and observed radar profiles could minimized. Additional data e.g. from radiosounding and SODAR wind profilers were used to estimate and minimize the most relevant error sources expected from vertical air motion and turbulence. First results were achieved from a case study of 8th July 2007 during the Convection and Orographically Induced Precipitation Study (COPS)
