A comprehensive analysis of hail events in Portugal: Climatology and consistency with atmospheric circulation

Abstract

A first hail climatology in mainland Portugal is provided, based on a network of 15 meteorological stations and over the period of 1971–2009 (39 years). Three hail sub-classes based on the SYNOP codes were considered (small hail, hail without thunder and hail with thunder). Furthermore, hail occurrences were also compared with thunderstorm, convection and precipitation days. Hail events are more frequent in northern Portugal during winter and spring (peaking in April). Both thunderstorm and convection events present a maximum in April–May and a secondary peak in autumn (October). This secondary peak is frequently associated with convective storms without hailfall at the ground, as no clear autumnal maximum is recorded in the hail frequency, particularly in the case of small hail and hail without thunder. The typically higher temperatures in October than in February–April may lead to excessively high-freezing level heights that hamper hailfall at the surface. The dynamical mechanisms underlying hail occurrence in mainland Portugal were also analysed using eight three-dimensional weather types. The atmospheric conditions associated with three weather types: Western European Trough, Blocking and Scandinavian Trough were responsible for ca. 75% of the hail events throughout Portugal. Overall, hail events are favoured either by extra-tropical depressions, with cold front passages over Portugal, or by upper-level troughs/lows over western Iberia, but preferably occurring in the period from December to May. These dynamical drivers promote instability conditions, which were also diagnosed by the convective available potential energy and total-totals index.