Severe weather analysis with real-time GNSS tropospheric products and radar reflectivity

In the last decade, advancement of GNSS processing resulted in high-quality tropospheric products in real-time with a temporal resolution of 15 min and latency of 10 min, suitable for severe weather nowcasting. In Bulgaria a convective Storm Demonstrator (Storm Demo) is built using real-time GNSS tropospheric products and Instability Indices in support of public weather and hail suppression services. The Storm Demo targets the development of service featuring GNSS products for two regions with hail suppression operations in Bulgaria, where thunderstorms and hail events occur between May and September, with a peak in July. The Storm Demo real-time Precise Point Positioning processing is conducted with the G-Nut software for 12 ground-based GNSS stations in Bulgaria. This study examines the benefits of using sub-hourly real-time GNSS tropospheric data to improve the nowcasting of convective storms in Bulgaria. A total of 3 weeks with severe weather in July-August 2023 were selected and analysed using GNSS derived Integrated Water Vapour (IWV) and weather radar data. In the period 2-7 July, for all days when IWV values are above the thunderstorm threshold of 32.2 kg/m2, high values of radar reflectivity height are registered for periods between 7 and 12 hours. The height of radar reflectivity factor of 45 dBZ over zero-degree isotherm (ΔH45) is a widely used predictor for possible severe convective storm development. Diurnal cycle of IWV for the period 6-16 UTC was also analysed in conjunction with ΔH45 to distinguish between frontal and thermal convection development.