A climatology and trend of hail events in Romania are presented using hail data spanning the years 1961–2014. Hail observations from weather stations and model reanalysis data were used to document the spatial and temporal distributions, variabilities, and environments of hail events. The results show that hail occurs most frequently in mountainous areas, while the smallest average number of hail days per year is found in the southeast. Herein, the convective season was defined as April–September, given that 94.2% of all mean monthly hail days were identified in this period. During the convective season the hail events prevail with most of these occurring in the afternoon and evening hours between 1000 and 1800 UTC. The severe hail events occur, overall, between 1400 and 1600 UTC, while in the southwest severe hail occurs later between 1600 and 1900 UTC. The spatial distribution of the convective parameters is consistent with the spatial distribution of hail days, revealing that hail is not favored in southeastern Romania, but in the rest of the country. The trend analysis of mean hail days per year disclose that 55.2% of all stations show a statistically significant upward trend, 3.8% show a statistically significant downward trend, while 40.9% show no statistically significant trend. A correlation between the variability of hail days per year and the variability in the occurrence of low pressure systems of Atlantic origin exists, the latter generating low pressure systems over the Mediterranean Sea that supply eastern Europe with the moist air needed for convection.

ABSTRACT:Severe storms that produce hail of significant amount or size carry a high risk, being responsible for damage at the ground. Hail spawned from these storms can affect crops, automobiles and buildings. A total of 52 hail events that occurred in May 2013 in southern Romania are used to investigate the relation between radar-derived products and damage produced by hail. Two case studies are also presented to highlight the methodology used in this study. Three-dimensional single polarization weather radar data were used to derive composite reflectivity, echo top heights, vertically integrated liquid, vertically integrated liquid density and hail kinetic energy flux to detect the hail clouds. Time integration and spatial distribution of these radar products were produced in order to capture the swath, intensity and longevity of the hailstorms. Hail and damage reports were used to link the radar variables to the effects of hailstorms at the ground. Hail size information was arbitrarily broken down into bins of reported hail to investigate the relation between hail size and radar parameters. The results show that the areas where hail and damage were reported are well captured by the footprints and magnitude of the radar variables. The average values of the radar parameters corresponding to hail size bins increase with the increase of hail diameter. A steeper increasing trend characterizes the vertically integrated liquid density and hail kinetic energy flux. The results show a good agreement between weather radar data and surface reports.

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Abstract. Weather radar measurements are used to study the climatology of convective storms and their characteristics in the transboundary Prut River basin. The Storm Cell Identification and Tracking (SCIT) algorithm was used to process the volumetric reflectivity measurements, in order to identify, characterize, and track the convective storm cells. The storm attribute table output of the algorithm was used to separate the convective from the stratiform storm cells, by applying a simple selection criterion based on the average vertically integrated liquid (VIL) values. The radar-derived characteristics of convective storms were used to document the spatial and temporal distributions and storm properties in terms of duration, distance travelled, movement direction, and intensity. The results show that 94.3 % of all convective storm cells were detected during May–August, with the peak in July. The peak time for convective storm cells' occurrence was in the afternoon and evening hours between 10:00 and 18:00 UTC. The median duration of a convective storm was 42 min, the median distance travelled was 23 km, and the median movement speed was 7.7 m s−1. The average movement of storms varied with months, but overall most convective storms move from the south-west and south–south-east. Also, the analysis shows that the longer-lasting convective storms were the most intense. The spatial distribution of the convective cells reveals yearly variation patterns and hotspots but also highlights the limitations of radar measurement at longer distances. Reanalysis data suggest that low values of sea level pressure over the Black Sea can act as a dynamical driver of convective storms in the analysed area.