Synoptic and mesoscale conditions of deep moist convection during the cold season in Croatia

In this study, thunderstorm activity during the cold part of the year was analyzed based on Thunderstorm Intensity Index (TSII) data on a predefined grid with a mesh of 3 km × 3 km in Croatia. The study covered a five-year period from 2016 to 2020, focusing on the months from October to March. The goal of the research was to conduct a spatial and temporal analysis of thunderstorm activity and determine the synoptic and thermodynamic conditions under which it occurs. The analysis aims to provide an overview of the fundamental characteristics, thereby improving the understanding of deep moist convection in the cold part of the year, which poses a significant challenge in operational weather forecasting due to its lower frequency and more difficult intensity assessment.

On surface synoptic charts the occurrence of surface frontal disturbances is detected and using 500 hPa synoptic charts the upper-level weather regime is determined. Thermodynamic and kinematic parameters are calculated from radiosonde profiles from stations in San Pietro Capofiume, Udine, Brindisi, Pratica di Mare (Italy), Zagreb and Zadar (Croatia), using the thundeR free software package.

A total of 296 convective days were selected for analysis from the observed period. The results indicate that synoptic forcing plays a significantly greater role in the development of convection during the cold part of the year compared to the warm part, while the dominant upper-level flow regime is southwesterly at the leading side of the upper level trough. The obtained values of Convective Available Potential Energy (CAPE) in the cold part of the year are much lower than those in the warm part. Additionally, most thunderstorms develop under conditions of strong vertical wind shear, indicating that the atmospheric environment conducive to winter thunderstorms is predominantly a high shear – low CAPE environment.