Radar-derived structural and precipitation characteristics of ZDR columns within warm-season convection over the United Kingdom

Analyses of the radar-observed structure and derived rainfall statistics of warm-season convection developing columns of enhanced positive differential reflectivity (ZDR) over England’s southwest peninsula are presented here. Previous observations of ZDR columns in developing cumulonimbus clouds over England were rare. The observations presented herein suggest otherwise, at least in the southwesterly winds over the peninsula. The results are the most extensive of their kind in the UK; the data were collected using the National Centre for Atmospheric Science dual-polarization X-band radar (NXPol) during the COnvective Precipitation Experiment (COPE). In contrast to recent studies of ZDR columns focused on deep clouds that developed in high-instability environments, the COPE measurements show relatively frequent ZDR columns in shallower clouds, many only 4-5 km deep. The presence of ZDR columns is used to infer an active warm rain process has contributed to precipitation evolution in convection deep enough for liquid and ice growth to take place. Clouds with ZDR columns were identified objectively in three COPE deployments, with both discrete convection and clouds embedded in larger convective complexes developing columns. Positive ZDR values typically extended to 1-1.25 km above 0°C in the columns, with ZDR ≥ 1 dB sometimes extending nearly 4 km above 0°C. Values above 3 dB typically occurred in the lowest 500 m above 0°C, with coincident airborne measurements confirming supercooled raindrops’ presence. Statistical analyses indicated that the convection which produced ZDR columns was consistently associated with the larger derived rainfall rates compared to the overall convective population sampled by the NXPol during COPE