Refining and validating radar based hail identification by using Helsinki Testbed observations

Radar hail signatures at C-band are ambiguous. In several recent and not so recent studies, it was reported that differential reflectivity values of rain-hail mixtures could be higher than 5 dB. This result is contrasting with S-band radar observations where differential reflectivity values close to 0 dB are expected. Due to a high spatial and temporal variability of hailstorms, it is very difficult to provide sufficient ground-truth observations to validate these measurements. Furthermore, for the same reason, performance of current hydrometeor classification algorithms is very rarely quantified in terms of such metrics as for example probability of hail detection. In this study we utilize a unique dataset collected at Helsinki Testbed to address the above-mentioned problems.

The Helsinki Testbed was established in 2005 jointly by the Finnish Meteorological Institute and Vaisala. As a part of the testbed instrumental setup more than 60 Vaisala WXT 510 weather transmitters were installed in the Helsinki Metropolitan area. The weather transmitters are capable of precipitation intensity measurements, as well as of discrimination between rain and hail. Since the establishment of the testbed, the transmitters have reported more than 100 hail hits. This is a unique dataset since it is not only providing records of hail occurrences, but also provides exact location and times of those events. Our preliminary comparison of radar observations and the WXT hail reports, show a good agreement between those observations. A more detailed study of this dataset will be presented at the conference.