Intense Tornadoes in a Region of Low Convective Available Potential Energy : Insights from Western Continental Europe

Abstract

Whereas Convective Available Potential Energy (CAPE) is well known for its role in supercell development in the continental United States, its significance in Europe, a region of low CAPE, is yet to be confirmed. Here we focus on tornado occurrences on the western fringes of the European continent, with data from Portugal, Spain, France, Belgium, and The Netherlands. Data compilation from multiple sources produced a total inventory of 1675 tornadoes spanning the period 1940–2022. An investigation using ERA-5 data was achieved on the basis of nine single-altitude-level variables such as CAPE, atmospheric temperature and pressure, as well as five other parameters at different pressure levels including Δ°C at 0–1000 m and wind shear at 0–1 and 0–6 km – thereby providing a comprehensive thermodynamic overview of regional atmospheric conditions at the time of high-intensity tornado events. Analysis reveals that 71% (n = 176) of intense tornadoes (i.e., >F2 and >EF2) occurred under CAPE values of less than 300 J/kg. Findings about the spatial distribution of intense tornadoes occurring under conditions of low atmospheric instability also reveal a seasonal component. Tornadoes in areas more remote from direct Atlantic or Mediterranean’s influence tend to occur predominantly in summer, albeit in relatively limited numbers (n = 32). In contrast, maritime areas exhibit a higher frequency of tornado occurrences, particularly during autumn and winter (totals of 60 and 51, respectively). Three subregions, identified using cluster analysis, were studied to provide a geographically more focused understanding of the conditions that contribute to tornado formation. Local topography and other climatic characteristics were considered as potential factors. The average CAPE in the western continental Europe study area for the period 1990–2022 was 405 J/kg, i.e., 1.8 times lower than over the coterminous United States. In Europe, maximum values rarely exceed 3500 J/kg, whereas CAPE values above 3500 J/kg occur several times a year in the U.S. We hypothesize that wind shear may be a key factor in these patterns. We also propose a European CAPE scale suited to the regional conditions that influence thunderstorm development.