Atmospheric circulation patterns associated with hail events in Lleida (Catalonia)

Although the advance of powerful computers has improved the outputs of meteorological models, a good synoptic classification can be very useful for weather forecasting. Therefore, the Meteorological Service of Catalonia is working on drawing up an accurate classification for extreme events. Hail events on the Lleida plain (inland Catalonia) were chosen to test different classifications. Since 1990 the Associació de Defensa Vegetal-Terres de Ponent has been collecting information on hailstorms and the damage caused in this area. Only the hail events with high impact were used to obtain the main synoptic patterns. A manual and an objective classification were carried out using the ERA-Interim reanalyses. The manual classification was done with the same variables (mean sea level pressure, temperature at 850 hPa and geopotential at 500 hPa) and the same domain as used in the objective classification. The methodology of the objective classification consisted of three steps. Firstly, a principal component analysis was used to reduce the dimension of the variables. Then, cluster analysis was applied to the component scores to obtain the atmospheric patterns. Finally, a discriminant analysis of the clusters was performed in order to improve the classification and to evaluate its goodness. Nine synoptic patterns were obtained in both classifications.

Research Highlights
► We presented an objective methodology to obtain and validate a synoptic classification of hailstorm events.
► This objective methodology was based in multivariate techniques: principal components, cluster and discriminant analysis.
► We also compared it with the subjective classification we carried out.
► Nine patterns were obtained in both methodologies.
► The objective classification seems to be suitable for our purposes, however, needs to be applied to other severe events to test its effectiveness.