Polarimetric Radar Observations in the Ice Region of Precipitating Clouds at C-Band and X-Band Radar Frequencies

Abstract

ata collected by C-band and X-band radars in northwestern Italy are analyzed to study the behavior of the polarimetric variables in the ice region of precipitating clouds, with special emphasis on the specific differential phase Kdp. It is found that stratiform precipitation, irrespective of the precipitation type at the ground and as opposed to convective systems, is characterized by well-pronounced positive differential reflectivity Zdr and Kdp values near the model-predicted ?15°C isotherm. The regions of enhanced Zdr and Kdp are likely related to the growth of dendrite crystals in the region where the difference between the saturation vapor pressure over water and the saturation vapor pressure over ice is greatest. Coincident C-band and X-band measurements, in conjunction with electromagnetic scattering simulations, demonstrate that Kdp scales with frequency, indicating that the ice particles in the vapor deposition preferential growth zone are Rayleigh scatterers. Peak values around 2.0° and 3.5° km?1 are observed at C band and X band, respectively. Most noteworthy is that an extended analysis of hourly and daily vertical profiles of C-band data over 1 year has shown that Kdp observations around the ?15°C temperature level in stratiform precipitation are well correlated (0.8) with the reflectivity in the underlying rain layer.