Convective indices and their trends on days with observed large and severe hail in Serbia

Hail is a weather phenomenon that can cause significant damage to material goods, crops, infrastructure, and motor vehicles, resulting in human injuries. Large and severe hail is often associated with severe convective storms, which are strongly related to atmospheric instability and the formation of intense thunderstorms. Predicting these thunderstorms, including their initial timing, location, and intensity, remains one of the most challenging aspects of modern weather forecasting. Current numerical weather forecast models often fall short in resolution, making accurate forecasting difficult. Therefore, meteorologists use convective indices as additional tools to predict thunderstorm development; these indices are considered valuable predictors for forecasting the occurrence of thunderstorms.

Convective indices are calculated from radiosondes' vertical temperature and water vapour profiles. In this research, we analysed 13 convective (or stability) indices derived from radiosonde measurements collected at the meteorological station Košutnjak in Belgrade, Serbia (φ = 44°46′ N, λ = 20°25′ E, h = 203 m above sea level) during days when at least one rocket-launching station, as part of the hail suppression system of the Republic Hydrometeorological Service of Serbia (RHMZ), reported occurrences of large and severe hail. Data for the warm season (April to October) at 12 UTC was gathered from 2002 to 2020. The term 12 UTC was selected based on the fact that approximately 96% of all hail events in Serbia, recorded from 1975 to 2009, occurred between 12:00 and 24:00 local time (UTC + 1). To statistically assess if there is a monotonic upward or downward trend, the Mann–Kendall test was used. If there is a trend, its magnitude is calculated using the Sen’s slope.

Recently, discussions about extreme weather conditions have increased. In response, we have focussed our attention on convective indices related to the occurrence of large and severe hail, defined as hailstones with a diameter of 21 mm or more. During the period analysed, days with such extreme hail accounted for 20% of all hail days. From 2005 to 2020, we identified significant monotonic trends in six out of thirteen convective indices: a decreasing trend for the Lifted Index (LI) and Boyden Index (BI), and increasing trends for the Severe Weather Threat Index (SWEAT), K Index (KI), Totals (TT), and Convective Available Potential Energy (CAPE). We could not conclude that days that meet the previously established threshold criteria for stability indices are becoming more frequent.

The number of hail days featuring hailstones with diameters between 21 mm and 30 mm and between 36 mm and 50 mm has been decreasing over time. Yet, the calculated coefficients of determination for both linear regression equations (R² = 0.108 and R² = 0.068, respectively) indicate that these trends are not significant. The number of days with hailstones between 31 mm and 35 mm also did not increase significantly (R² = 0.024). The overall frequency of extreme hail days has therefore not increased.

 

This research was supported by the Science Fund of the Republic of Serbia, No. 7389, Project „Extreme weather events in Serbia - analysis, modelling and impacts” - EXTREMES