Characteristic of ZDR Columns in a High-Resolution Numerical Weather Prediction Model

Previous studies have highlighted the presence of observed radar signatures prior to the onset of or during severe convection. One example is a differential reflectivity (ZDR) column, which is defined as a vertical columnar region of enhanced ZDR that extends above the freezing height. Although this phenomena has been simulated in other works for storms in the USA Southern Great Plains and north-western Germany, there is still little literature on this. It is therefore of interest to study ZDR columns for convective storms in the UK.

The aim of this study is to formulate a suitable polarimetric radar forward operator for simulating ZDR from Unified Model output. In addition to the current single moment microphysical scheme in the UK Met Office Unified Model, there is now capability to run a double moment scheme (CASIM: Cloud AeroSol Interacting Microphysics). The Met Office has fully upgraded all C-band radars since January 2018 with full dual-polarisation operational capability. This allows for comparison between observed ZDR from UK radars with simulated ZDR for a convective event of interest.

Scattering modelled using Rayleigh-Gans theory was tested for various droplet shapes and compared against T-matrix calculations. Mie resonance effects resulted in up to around 1.0dB differences in simulated ZDR between the scattering theories, which is significant for the identification of ZDR signatures. We thus chose to implement our forward operator using PyTmatrix and carried out sensitivity tests on using several drop aspect ratio models and drop canting distribution widths. A suitable set of assumptions was then implemented into a forward operator in the form of a lookup table converting model grid rain mass fractions and rain number concentrations into reflectivity (ZH) and ZDR.

A case study of widespread deep convection over southern England is simulated at 1500m and 300m resolution, testing both microphysics schemes. Within convective cells, ZDR column heights are found to reach 1km above the freezing level. Within identified cells, we find that ZDR column intensity, height and width values in the single moment scheme are higher than those in the double moment scheme. These findings are complemented by an object-based evaluation against the Met Office 3D radar composite (which includes ZDR) using contoured frequency by altitude diagrams.