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The lower pressure of water vapor of the outer eastern and south-eastern Carpathians, as well as in Dobrogea, determines within these areas an average annual number of days of under 2 days. In sheltered areas from Bârlad Plateau the average multi-annual value decreases even below 0,5 days. Hail events that affect the territory of Moldova are, in most cases, caused by thunderstorms during May and June (frequency of over 60 %). We had analysed in the present paper a representative number of statistical severe convective events causing hailstorms and material damage between 1990-2013 period. Values of atmospheric instability indices have been analyzed for these episodes as CAPE (Convective available potential energy), SWEAT (severe weather threat), Li (lifted index), KI (K index), TT (total totals) to evaluate their medium range forecast under the hail production conditions. These indicators measure the degree of atmospheric instability providing information about possible occurrence of clouds with large vertical extent. The most powerful correlations have been recorded in the case of parameter LI, for which the daily mean value was used. The status of these indices is assessed for both the days with hail, but also on previous intervals with hail to assess their capacity to forecast this extreme weather phenomenon.

This paper uses the comparative method in order to highlight the thermal anomalies, the impact of atmospheric circulation and the air masses through specific situations. Our preliminary study employs radio sounding data from Bucharest-Băneasa weather station, and this method is applied for an altitude interval bounded by Earth’s surface and the geopotential surface of 900hPa. Thus, the results have been obtained from radio soundings up to the 900 hPa level, which corresponds to an altitude of 1000 meters, because above this level, there is a tendency towards generalization due to the decreased influence of the subjacent active surface and the impact of the particularities of the general atmospheric circulation above Bucharest. The aim of the study was to explain the temperature distribution through statistical shape/form indices and to measure their degree of flattening and asymmetry. Processing the radio – soundings data complemented, when not measured, by the remote sensing data available after 2007, involved the statistical modeling of time series for the cold season (October to March), which meant analyzing the temporal variations and the prognosis for the evolution of meteorological and climatic elements. A comparison of the values taken by different parameters of the analyzed thermal regime could not be achieved without a more detailed presentation of the thermal regime up to 900 hPa, by entirely taking into account thermal anomalies, exceptional thermal gradients and the altitude/height of isotherms directly involved in the analyzed phenomenon, and also by directly correlating these elements with the characteristic synoptic situations.

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. [ABSTRACT FROM AUTHOR]

Increasing temperature values in altitude, as a deviation from normal law, generates stability of air masses. The most typical situations occur outside the Carpathian chain. In particular cases, when the maximum barometric prevail on large European territories, absolute thermal inversions occur, which are characterized by large thickness, intensity, duration and amplitude. These types of thermal inversions block water vapour at lower levels and produce changes in atmospheric moisture regime by forming mists and low stratiform clouds. Geostatistical analysis of this climatic risk phenomena for aeronautical activity was carried out by processing temperature and moisture database from 7 aerological stations and analysis of synoptic maps. The results revealed a strong connection between the two meteorological phenomena, with the highest frequency in winter. [ABSTRACT FROM AUTHOR]

The present work analysed a statistically representative number of severe convective events which caused hail and material damage during 1990-2015. For these episodes heights of 0°C, -6°C, -10°C and -20°C isotherms were analyzed. In the layers between these four isotherms, the formation and growth of hailstones occur. The height of these isotherms in the period preceding hail is essential in weather forecast of this phenomenon. Lifted index values recorded before hail events are also correlated with height isotherms. The altitudinal differences between the height of -6°C and -20°C isotherm and freezing level which represents intervals of initiation and formation of hail are identified. Not at last the values of the meteorological indices used to forecast the potential formation and hail falling in extracarpathian Moldavia are presented in details. [ABSTRACT FROM AUTHOR]