Polarimetric radar signatures of dendritic growth zones within Colorado winter storms

X-band polarimetric radar observations of winter storms in northeastern Colorado on 20-21 February, 9 March, and 9 April 2013 are examined. These observations were taken by the Colorado State University-University of Chicago-Illinois StateWater Survey (CSU-CHILL) radar during the Front Range Orographic Storms (FROST) project. The polarimetric radar moments ofreflectivity factor at horizontal polarization Z_H, differential reflectivity Z_DR, and specific differential phase K_DP exhibited a range of signatures at different times near the -15°C temperature level favored for dendritic ice crystal growth. In general, K_DP was enhanced in these regions with Z_DR decreasing and Z_H increasing toward the ground, suggestive of aggregation(or riming). The largest Z_DR values (~3.5-5.5 dB) were observed during periods of significant low-level upslope flow.Convective features observed when the upslope flow was weaker had the highest K_DP(>1.5° km^-1) and Z_H (>20 dBZ) values. Electromagnetic scattering calculations using the generalized multiparticle Mie method were used to determine whether these radar signatures were consistent with dendrites. Particle size distributions (PSDs) of dendrites were retrieved for a variety of cases using these scattering calculations and the radar observations. The PSDs derived using stratiform precipitation observations were found to be reasonably consistent with previous PSD observations. PSDs derived where riming may have occurred likely had errors and deviated significantly from these previous PSD observations. These results suggest that this polarimetric radar signature may therefore be useful in identifying regions of rapidly collecting dendrites, after considering the effects of riming on the radar variables.