Identification of ZDR columns for early detection of severe convection in southern England
Authors: Lo Chun Hay Brian , Stein Thorwald H. M. , Westbrook Chris D. , Scovell Robert W. , Darlington Timothy , Lean Humphrey W. 
Published: March 2022

&lt;p&gt;Various studies in the UK, 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 (&lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt;) column, which is defined as a vertical columnar region of enhanced &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; that extends above the freezing level. Several field campaigns synthesising radar and in-situ measurements confirmed that such columns contain large supercooled millimetre-sized droplets lofted into convective storms and are in, or near strong updrafts. Recent work using a single research radar in Oklahoma also exploited the usefulness of detecting &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns for informing nowcasters of severe convection.&lt;/p&gt;&lt;p&gt;The goal of this study is to identify potential severe convective events in the UK mostly for cases over the summer season using polarimetric radar measurements. The UK Met Office has fully upgraded all 18 C-band radars since January 2018 with full dual-polarisation operational capability. From this network, we derive a 3D radar composite, which provides large coverage on the order of 1000 km for monitoring potentially hazardous weather. Environmental conditions are also investigated prior to and during the onset of convection to understand the effectiveness in &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns as precursors of severe convection depending on synoptic regime.&lt;/p&gt;&lt;p&gt;Using past cases, we track storm cells using maximum reflectivity in the column and identify whether the cells contain &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns, where a &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; column is identified based on a 3D volume thresholded by reflectivity (&lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt;) and &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt;. For nowcasting of severe storms, with &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt; &gt; 50 dBZ, we find optimal &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt; and &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; thresholds of around 30 dBZ and 2.0 dB respectively existing within &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns. This agrees with past literature and physical understanding indicating a low concentration of large super-cooled water droplets within &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns explained by condensation-coalescence processes, especially during early stages of convective development. In contrast, other works may show &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns associated with areas of high &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;H&lt;/sub&gt;, suggesting detection of such columns in more mature stages of a storm. Algorithm performance in identifying &lt;em&gt;Z&lt;/em&gt;&lt;sub&gt;DR&lt;/sub&gt; columns for early detection of severe convection and its optimal parameters vary with synoptic regime.&lt;/p&gt;