Combining GNSS meteorology and Instability Indices for derivation of classification functions in South Central Bulgaria.

Severe weather events, such as intense precipitation, hail and thunderstorms, are common summer phenomena in Bulgaria and are associated with large economic losses. An active hail suppression is taking place in North West and South Central Bulgaria over the dense agricultural regions from May to September. A joint venture between Sofia University "St. Kliment Ohridski'' and the Bulgarian Hail Suppression Agency contributed to the project “Balkan-Mediterranean real-time severe weather service” (BeRTISS, 2017-2020). As part of the BeRTISS project, a pilot operational service was established by exploiting GNSS tropospheric products in support of safety, quality of life and environmental protection in the region. To facilitate the service, in 2018 a GNSS network with 12 reference stations was installed and since February 2020, the Sofia University GNSS Analysis Center provides operational near-real time products. First results of combining GNSS derived Integrated Water Vapour (IWV) and Instability Indices (InI) are reported for Sofia plain for the period May-September (2010 - 2015). Based on statistical regression analysis, classification functions are obtained that contribute to the thunderstorm forecasting skill. The majority of the classification functions combining IWV and InI are reported with the best performance in May, followed by June and September. In this work, IWV and InI classification functions are presented for South Central Bulgaria. The first results for the period May-September 2020-2021 indicate the probability of detection 0.71 for IWV, 0.85 for InI, and 0.89 for IWV and InI combined. While false alarms decreased from 0.42 for IWV to 0.39 for InI and 0.28 for IWV and InI combined. These results will be implemented in the GNSS Strom Demonstrator web portal.