Precipitation Classification Functions for Northwest Bulgaria: GNSS IWV and Instability Indices

Precipitation monitoring during the convective storm season May-September is conducted routinely by the Bulgarian Hail Suppression Agency (HSA). Since 2020, in Northwest Bulgaria data from four stations of the Global Navigation Satellite Systems (GNSS) have been processed in near-real time mode and vertically Integrated Water Vapour (IWV) has been computed in operational manner. As a part of a storm nowcasting demonstrator GNSS IWV and Instability Indices (InI) thresholds are implemented for Sofia and Central Bulgaria. In this work site-specific classification functions are computed for Northwest Bulgaria. A GNSS derived monthly IWV threshold separates well precipitation (P) and no precipitation (nP) groups in July, August and September. Probability of detection is between 77–100% for July and 78–85% for August. For July the false alarm ratio scores are high in the range 30–66%, which limits the use of IWV. Classification functions based on InI and IWV have the best performance with an increase of probability of detection score by 16% in July, 23% in August, and 20% in September and decrease of false alarm ratio score by 23% in July, 22% in August, and 8% in September.