The continental and regional synoptic background favorable for hailstorms occurrence in North-Eastern Romania

Abstract

The current study aims to portray the specific weather patterns associated with hail falls in north-eastern Romania. This was done using multiple hail records data covering a long period of time (1981–2020). COST733 software enabled us to assess objectively the atmospheric circulation classification types for the middle troposphere (500 hPa) and also for the ground level (sea level pressure) over the study region. Based on these, we have identified 5 major weather patterns (MPs) which explain up to 85% of the recorded hail events over the region. In terms of wind vector direction and speed, driving the hailstorm cells, these MPs are represented by two main groups of synoptic patterns. The first one is characterized by weak advective patterns (cut-off lows, north-easterly anticyclonic flow), while the second one is characterized by strong advective patterns (westerly, south-westerly, and north-westerly atmospheric flows). Further, these major patterns are split into 16 combined circulation types (CCTs), as indicated by the atmospheric circulation at the ground level. Our analysis showed that in May and June hailstorms are determined especially by weak advective patterns over north-eastern Romania, while from June to August the strong advective patterns are more common for these extreme weather events. The atmospheric environment associated with these synoptic conditions, as indicated by instability parameters, is also described using a series of atmospheric parameters: sea pressure level, 500 hPa geopotential field, 700 hPa wind vector, and precipitation amount at the continental scale. This analysis indicates that hail occurs in a regional atmospheric environment characterized by a MUCAPE >500 J/kg and high wind shear in the lower troposphere. The results obtained are meant to become a useful tool in regional hail forecast.

Characteristics of Convective Parameters Derived from Rawinsonde and ERA5 Data Associated with Hailstorms in Northeastern Romania

“…Atmospheric instability conditions are strongly influenced by synoptic patterns in the region. A recent study [42] using the same hail data found the highest MUCAPE characterising the north-easterly anticyclonic flow. There are numerous situations where LI values indicate a moderate instability but CAPE indicates a low stability, which can be attributed to the existence of temperature inversions or an inhibitory layer.…”

“…Data from reported hail events used in this study were used in previous studies [37,42], and originate from three different sources: Romanian National Meteorological Administration (RNMA); European Severe Weather Database (ESWD); and the media and local newspapers. If a hailstorm is recorded at one location from the study area on a particular day, we define it as a hail day for this station.…”