Use of ZDR columns for early detection of severe convection within the operational radar network of the United Kingdom

A number of studies in the UK, Southern Great Plains and Southeastern USA have highlighted the presence of certain radar signatures prior to the onset of or during severe convection. One type of such radar signature is a differential reflectivity (ZDR) column, which is defined as a vertical columnar region of enhanced ZDR that extends above the freezing level. Several field campaigns synthesising polarimetric radar and in-situ measurements confirmed that such columns are collocated or near strong updrafts, which would loft large supercooled millimetre-sized drops into the convective storm. Previous works using a single research radar in Oklahoma have also exploited the usefulness of detecting ZDR columns for informing nowcasters of severe convection.

The UK Met Office has fully upgraded all 15 C-band radar as of January 2018 to have polarimetric capabilities. From this network, we extend the Met Office 3D radar composite to include ZDR and were able to observe ZDR columns. We then investigated the operational potential of using ZDR columns for nowcasting severe convection. For this study over the UK, severe convection is defined as total storm-integrated accumulation of more than 200mm of rainfall. An algorithm for automatic detection of ZDR columns was developed, based on ZDR >= 1.0dB and ZH >= 10dBZ. Across three case days, detected ZDR columns were found to precede severe convection in tracked convective cells with a lead time of up to 20 minutes. Requiring maxima above 1.4dB and 30dBZ of ZDR and ZH respectively was an appropriate second condition for all three cases although the sensitivity of this result varied across case days. The ability to detect ZDR columns in operational radar data with a useful lead time prior to severe convection is a promising development towards improved nowcasting of severe convection in the UK.