S-Band radar dual polarization observations of winter storms

The polarimetric S-Band CSU-CHILL radar has collected data during a number of winter season storm events in the northeastern Colorado area. Three dimensional sampling of these echo systems was done using various combinations of volumetric PPI scans and slow elevation rate RHI scans; the transmit polarization was alternated between horizontal and vertical on a pulse to pulse basis.

These observations show that small (approximately 0.2 - 0.6 deg / km) positive specific propagation differential phase shift (Kdp) areas are often detectable at altitudes several km above the ground (environmental temperatures of ~ -15 to -25 deg C). Co-polar reflectivity levels in these positive Kdp areas are on the order of 20 dBZ. The height at which the most positive differential reflectivity (Zdr) occurs is generally somewhat above the maximum Kdp level. The positive Kdp and Zdr magnitudes both tend to decrease towards the surface; indicating that particle aggregation is in progress.

This paper will examine the organization of the above-mentioned polarimetric data fields in several of the winter storm events that have been observed with the CSU-CHILL radar. The results of an electromagnetic scattering (T-matrix) model will be used to help interpret the polarimetric data patterns.