Observations of KDP in the ice region of precipitating clouds at X-band and C-band radar frequencies

Positive values of specific differential phase shift (KDP) in the ice region of precipitating clouds have been recently shown to be linked to increased surface precipitation rates at S-band (Kennedy and Rutledge, 2011). The regions of enhanced KDP are observed near the -15C isotherm and are likely related to the growth of plate-like crystals. The observed values of specific differential phase shift are only few tenths of deg/km at S-band, but due to the KDP scaling with frequency, are expected to be higher and more easily detectable above noise at higher frequency. This study aims at analysing the phenomenological behavior of KDP in the ice region of mesoscale precipitation systems simultaneously observed at C-band and X-band radar frequencies. The polarimetric C-band and X-band systems managed by Arpa Piemonte have collected several cases in the North-West Italy subalpine region. Of particular interest are the high resolution RHI scans collected by the X-band system (ARX), showing KDP values above 1.5 deg/km in the mid-tropospheric layer 5-7 km height, corresponding to temperatures in the range –9C to –20C. Electromagnetic scattering simulations are also performed in order to verify the hypothesis about the microphysical origin of these specific polarimetric signatures.