Polarimetric observations of precipitating and non-precipitating clouds

The KOUN dual-polarization radar (located in Norman OK) served as the prototype for the future upgrade of the WSR-88D radars. It is very sensitive so that reflectivities of -14 dBZ produce unity signal to noise ratios at 25 km. Thus, detection of these and weaker signals is possible through appropriate processing of returns. Here, we present the polarimetric radar data from stratiform and nonprecipitating clouds, obtained with this radar. Collected data offer insights into poorly investigated bulk properties of ice crystals. The vertical profiles of polarimetric variables (reflectivity, differential reflectivity Z_DR, cross-correlation coefficient ρ_hv, and differential phase Φ_DP) are examined. Several clouds exhibit maxima of Z_DR of 2-3 dB near their tops. This is followed by a gradual decrease to a few tenths of a dB at the cloud bottom, suggesting that initially Z_DR follows the growth of the ice crystals. As the size of the plates increases in the horizontal direction, Z_DR increases and so does (ever slightly) their fall speed. It is hypothesized that beyond a certain size aggregation becomes the dominant growth process which causes the gradual decrease of Z_DR. The location of the maximum Z_DR coincides with the location of the minimum in ρ_hv, which is about 0.95. A one-dimensional model of ice crystal growth and aggregation is used to explain the observed profiles of the measured polarimetric values.