Abstract/Summary

Columns of enhanced differential reflectivity (ZDR) are indicative of the suspension and lofting of large raindrops and hence can be used as a proxy for intense updrafts in radar observations. The UK operational weather radars were upgraded since January 2018 to measure ZDR. The Met Office has introduced a new double moment microphysics scheme. Both allow the study of ZDR columns for nowcasting and forecasting, respectively. The primary aims of the thesis are to assess the potential of ZDR columns in predicting severe convection within the UK and to evaluate simulated ZDR columns within the UK’s operational weather model. A 3D composite of ZDR is produced from the operational radar data. Evaluation against research radar data indicates that ZDR columns can reliably be detected in the composite at a horizontal resolution of 1 km. A ZDR column detection algorithm for the composite is developed. Detected columns are found to precede severe convection in tracked convective cells with lead times up to 20 minutes for three case days. The variety in thresholds giving optimal performance and skill in the early detection of severe convection across case days can be explained by the presence of hail in the cell. A polarimetric radar forward operator is constructed for the simulation of rain contribution to ZDR within the Met Office Unified Model (MetUM). The implemented T-matrix scattering method accounts for non-Rayleigh scattering. Simulated ZDR within the single moment (Wilson-Ballard) and the double moment (CASIM) microphysics schemes within MetUM simulations of 300-metre grid length are evaluated with radar observations. Simulated ZDR columns are wide, deep, and intense within Wilson-Ballard simulations, whereas such columns are rare in CASIM simulations, in contrast with the abundance of ZDR columns in observations. Although CASIM captures well the ZDR-ZH relationship for rain, updrafts in simulations coupled to CASIM are considerably weaker.