The Putna-Vrancea Natural Park (PVNP) is one of the wildest mountain areas in Romania and is home to internationally important species of flora and fauna. Although the park’s area is covered by forests, the last two centuries have seen several major transformations. The aim of this article is to present a historical approach to the development of habitat areas within the PVNP, with an attempt to show both the situation before and after the establishment of its protection regime. The changes in the forest area were identified using available cartographic material from 1789, 1895, 1957, 1975, 2005, and 2018. The analysis of forest distribution according to different classes of natural factors (altitude, slope, soil type, and lithology) highlighted the early 20th century when forest cover decreased at low altitudes and slopes. However, anthropogenic factors were responsible for almost all changes. The specific socio-economic system of the Vrancea Mountains, according to which the forest was and still is owned by the inhabitants of several villages, led to periods of deforestation: burning to increase the area of pastures and the unsustainable exploitation of wood by some companies that had been granted this right by the owners. In the 1960s and 1970s, the mountain pastures were reforested, and in the last 20 years, the mountain pastures have been transformed into forests. Although the use of old maps for this type of analysis has certain limitations, the results obtained provide insight into the causes of the transformation of forest landscapes in some areas of the PVNP.

Temperature inversions represent complex phenomena, specific for depression forms of relief which introduce changes in vertical zoning (Apăvăloae, Apostle, Pîrvulescu, 1986 by stable air stratification (Erhan, 1981), because of their morphological features, this is due to the interdependence of general movement and characteristics of the active surface, being the cause of their production (Apăvăloae, et. al., 1984, 1986, 1987). The generation of thermal inversions, their intensity and frequency is a consequence of altitude topography, morphology, fragmentation, orientation and degree of closure to external drives. (Apăvăloae, Apostle, Pîrvulescu, 1988). Thermal inversions represent complex phenomena, specific to depression landforms that introduce changes within the vertical zoning (Apăvăloae, Apostol, Pîrvulescu, 1986) by stable air stratification (Matveev, 1958, Erhan, 1981, Yinghui Liu, 2002) where vertical thermal gradient is negative (Matveev, 1958), due to their morphological features, also as a consequence of the interdependance of general circulation and characteristics of the active surface, the cause of their generation (Apăvăloae, et. al., 1984, 1986, 1987).

The present paper analyzes 549 severe weather events reported to the ESWD (European Severe Weather Database) that caused large hail in the territory of Romania. Values of atmospheric instability indices have been analyzed for these episodes using data from Bucharest and Budapest sounding stations. For a period of 140 days with episodes of large hail, 24 instability indices were analyzed to describe the atmospheric conditions of the main daily convective activity. The mean values for most indices characterize an unstable atmospheric environment. Of the indices that measure potential instability, VT (vertical totals index) and TT (totals index) had values that described a conductive atmospheric environment for the development of hailstorms. In addition, the interquartile values of LIV (lifted index using virtual temperature) had values lower than zero. For SWEAT (severe weather threat index) and CAPEV (convective available potential energy index using virtual temperature), only the values in the 75th percentile describe a very unstable environment (according to the literature). Strong linear correlations were registered between several pairs of indices such as CAPEV-LIV and SWEAT-SI that can be used for the operational forecast of hail.

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.