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Past research has primarily focused on tornado outbreak intensity; however, this study presents an updated ranking index scheme that provides intensity ranks for both hail and tornado-dominant outbreaks. All outbreaks spanning 1960 - 2022 were obtained using a 24-hour kernel-density-based approach to map the severe weather report density. Notably, secular trends in the annual means of many of these variables (such as the number of hail and wind reports) showed a significant upward trend until 2010, after which that trend flattened. Thus, these fields were detrended using support vector regression that better fit these parameters' underlying annual time series. The resulting indices delineate between tornado and hail-dominant outbreaks, allowing further investigation into mixed-mode outbreaks and synoptic-scale precursors of these unique outbreak modes. It also provides an objective measure of outbreak intensity which can be useful when assessing potential future impacts from events with similar meteorological characteristics.

One of the primary missions of the National Weather Service (NWS) is to use a network of more than 150 NEXRAD radar installations to monitor weather for threats to life and property. Large hail produced by severe thunderstorms is a major focus of this mission. An algorithm known as the Maximum Estimated Size of Hail (MESH) algorithm is in operational use to diagnose the presence and size of hail. This study aims to use dual-Doppler observations as well as the MESH algorithm to test the idea that storms that rotate produce larger hail. Previous studies have used polarimetric radar products to detect the presence of large hail and dual-Doppler methods have been used to study embryonic hail, but no research has tested the theory of hail and rotating storms with observational evidence. A set of 59 case studies was gathered; each included a hail report submitted by a trained weather spotter or NWS employee and complete radar observations through the depth of a storm from two radars. The radar observations were resampled to a three-dimensional Cartesian grid and a dual-Doppler analysis was run on each case study. A strong correlation (stronger even than the MESH algorithm) was found between measured vorticity and hail size, lending credence to the idea that rotating storms do indeed have a higher ceiling for hail production. However, no correlation was found between MESH error and rotation. Further research will be required to evaluate whether or not this relationship can be used to augment the MESH algorithm so as to improve its skill. / Master of Science / The National Weather Service (NWS) uses a nationwide network of long-range weather radars to monitor the atmosphere over the United States. One of the primary applications of that network is to protect life and property by identifying and monitoring storms capable of producing large hail. The probable size of hail is estimated using an algorithm on raw radar data. This study uses multi-radar analysis methods to establish a metric for storm rotation, which is used in conjunction with the operational hail size algorithm to test the theory that rotating storms produce larger hail. A set of 59 case studies, each composed of data from a pair of radars as well as a measured hail size from a storm spotter, was gathered and analyzed. Analysis resulted in three values per case study: actual hail size, predicted hail size, and storm rotation intensity. Storm rotation and actual hail size were found to be strongly correlated, lending credence to the idea that rotation increases a storm’s hail production ceiling. However, no strong correlation was found between the operational algorithm’s error and storm rotation. Additional research will be required to determine if the relationship between rotation and actual hail size can be used to improve the skill of current operational algorithms.

The thesis deals with testing of radar-based hail detection methods and their use in hail climatology of Czechia. Data from C-band Doppler radars were used. There is described the testing of the selected algorithms in the first part of the thesis. The algorithms were chosen according to the data availability. On the basis of 25 well documented hail events, suitable threshold values were found to form hail criteria. Because of the hail events considered, the work concentrates on severe hail with diameter from about 2 cm. In addition, available data from Germany (Baden-Württemberg) were included in testing the detection criteria. The best performing criteria were used to form a new combined criterion (COMBI). The second part of the work applies the hail criteria, in particular the COMBI criterion. The performance of the COMBI criterion is shown in four case studies. Then the COMBI criterion was determined from the radar data of the six years 2007-2012. All the days from the months May to August were evaluated. The point maximum of about one day with severe hail was found for Czechia. For the most of the Czech territory the value was not higher than a quarter of a day in a year. In the seasonal distribution the maximum of hail days belongs to June. And the diurnal distribution showed the maximum of severe hail...

ABSTRACT Hail poses a substantial threat to life and property in the state of Florida. These losses could be minimized through better understanding of the relationships between atmospheric variables that impact hail formation in Florida. Improving hail forecasting in Florida requires analyzing a number of meteorological parameters and synoptic data related to hail formation. NOAA archive data was retrieved to create a database that was used to categorize text files of hail days. The text files were entered into the National Oceanic and Atmospheric Administration Earth System Research Laboratory website to create National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis maps of atmospheric variables for Florida hail days as well as days leading to the hail event. These data were then analyzed to determine the relationship between variables that affect hail formation, in general, across different regions and seasons in Florida using Statistical Product and Service Solutions. The reasoning for the differing factors affecting hail formation between regions, seasons and hail sizes were discussed, as well as forecasting suggestions relating to region and month in Florida. The study found that the majority of all hail that occurs in Florida is during the wet season. A low Lifted Index, high Precipitable Water and lower than average Sea Level Pressure, in most cases, is present during hail days in Florida. Furthermore, results show that Vector Wind magnitude increases as hail size increases. Additionally, several atmospheric variables useful to studying hail events, such as Lifted Index, Precipitable Water, Sea Level Pressure, Vector Wind and Temperature have significant correlations with each other depending on the region and season being observed. Strong correlations between low Lifted Index, high Precipitable Water values and the occurrence of hail events are discussed, as well as the relationship between temperature anomalies at various pressure levels and the occurrence of hail events.

M.Sc. (Geography) / Hailstorms are well-known phenomena in the summer rainfall region of southern Africa where they cause extensive damage - especially in the agricultural sector. This thesis examines the geography and climatology of hail in the Transvaal. It deals with three main issues, namely a) the spatial and temporal characteristics of hail days (HDs); b) rainfall and atmospheric conditions prevailing during hail events; and c) the geography of hail damage as it pertains to maize. In the Transvaal, hail day frequency (HDF) increases with altitude and latitude in a non-linear (exponential) manner. Variations in altitude, as reflected in the diverse physiography of the area, account for most of the spatial and temporal variations in hail occurrence. Seven hail regions can be distinguished which differ from one another in terms of the onset times of hail, its seasonal occurrence and annual HDF patterns. In general, most hailstorms occur during November with the peak onset time varying between 16:00 and 20:00. The most notable finding concerning rain - hail interrelationships, is that rainfall and HDF appear to be inversely related, years/months with high hail incidence being dry and vice versa. Daily and seasonal precipitation characteristics also differ between 'high hail years' (HHYs; dry) and 'low hail years' (LHYs; wet). For instance, during HHYs, the peak hail season is delayed while the rain season peaks earlier. Moreover, while the frequency of convective systems remains nearly the same during HHYs as in 'normal' years, the average precipitation area and the volumetric production decreases significantly. These anomalies appear to be the result of changes in the large-scale circulation patterns (as reflected by the transition from baroclinic to quasi-barotropic conditions) which influence the precipitation from mesoscale convective systems. It is likely that the Southern Oscillation plays a role in these changes, particularly during November and December. However, although these Southern Oscillation Index (SOl) - HDF associations are weak, they are appreciably stronger than those between the SOl and rainfall. Rainfall characteristics on HDs differ from those of non-hail rain days in that, on HDs, more rain falls and the rain-bearing systems are more extensive. In general, atmospheric conditions are less stable, and the humidity level is higher, on HDs than on other days. Furthermore, HDs are characterized by warm north easterly winds near the surface but cold, dry south westerlies at the 600 hPa level. Above this the south westerlies become progressively stronger