Polarimetric radar signatures of the Pentecost supercell 2014 in Germany

The 2014 Pentecost weekend storms in Europe were a series of severe supercell storms which followed a heatwave in early June 2014. Outbreaks of severe weather were reported from these storm developments with the worst damages occurring over the German state of North Rhine-Westphalia on 9 June, where the storm was described as one of the most violent in decades by the German weather service (DWD). The polarimetric signatures (wall, forward overhang, and echo-free vault) and the development into a bow echo characterize the storm as a high precipitation supercell. During this event six fatalities, wind gusts up to 150km/h, hail and a flash flood in Duesseldorf has been reported.

A national 3D composite of polarimetric moments covering Germany with 1km horizontal, 250m vertical, and 5 minutes temporal resolution has been generated, which represents an ideal data base for studies of the polarimetric signatures of this impressive event. 10 C-band radars from the DWD radar network, recently upgraded to polarimetry, have been included.

Vertically extensive columns of enhanced differential reflectivity Z_DR exceeding the 0°C level identify location, shape and strength of the updraft in polarimetric radar measurements. The so-called Z_DR-column product based on the 3D composite has been derived for the Pentecost event. Elevated Z_DR-columns in the heading anvil predict the intense bow echo with an impressive unprecedented lead-time of 1 hour in advance (20-30 minutes lead-time can be expected for deep convective summer events in Oklahoma). Possible explanations for the fact that these hanging Z_DR-columns ahead of the storm do not reach the ground will be considered and include evaporation, stronger updraft at the lower levels or dry air aloft with strong winds, which act to erode cloud water and to transport hydrometeors downwind in the thunderstorm anvil.

Even though it can be stated that all Z_DR-columns identified throughout the event are associated with updrafts, the correlation analyses between the Z_DR-column product and surface rainfall for the entire event reveal that only a small fraction of elevated updrafts are sustainable to produce any meaningful precipitation. Although new convective cells may or may not develop in the zones of elevated updrafts, the detection of the updrafts zones is valuable for aviation safety, because these areas of initial updrafts usually do not exhibit significant radar reflectivity but may have strong impact on airplanes due to localized and sudden vertical air motions.

The national 3D polarimetric radar composite represents an unprecedented data base to study the polarimetric signatures of this remarkable anvil. A small tool based on the composite allows arbitrary cross-sections of all polarimetric moments at any time during the day. The composite-based cross-sections are complemented by high-resolved genuine and reconstructed RHIs of the 10 nation-wide C- band radars and the polarimetric X-band radar in Bonn. Also highly-resolved quasi-vertical profiles (QVP's) derived from PPI scans measured at high antenna elevations monitor the temporal evolution of the anvil.

Generally, the Z_DR-column product has a great potential for nowcasting. The study contributes towards its practical utilization and thus distinction between updraft zones sustainable to produce precipitation and turbulence. The second focus is a detailed description of polarimetric features of the remarkable anvils, which are still poorly described by existing NWP models and play an important role in radiation transfer.