Tornadic Environments in the Iberian Peninsula and the Balearic Islands based on ERA5 Reanalysis

Abstract

A dataset of 907 tornado and waterspout events recorded from 1980 to 2018 was built to study convective environments in the Iberian Peninsula and Balearic Islands (western Mediterranean). The events were grouped into different categories, distinguishing waterspouts and tornadoes that were stratified by intensity according to the Fujita (F) scale and the Enhanced Fujita (EF) scale. The analysis separated the north-east (NE) and south-west (SW) subareas in the region of study, which present different seasonal cycles. For each event, atmospheric profiles from the ERA5 reanalysis data were used to determine convective available potential energy (CAPE), storm-relative helicity (SRH), vertical wind-shear (WS), the Universal Tornadic Index (UTI), and the product of wind-shear and the square root of two times CAPE (WMAXSHEAR). Results showed that the NE events are mostly associated with higher CAPE and lower helicity and wind-shear than the SW events. Thus, a significant number of SW tornadoes are associated with high-shear, low-CAPE environments. Moreover, the low-shear, high-LCL tornado environment, which is common inland during warm-season, is more usual in the NE subarea. Composite parameters such as the UTI and WMAXSHEAR06 are good discriminators between significant and weak tornado events, although WMAXSHEAR06 presents some limitations for the SW events due to low CAPE and weak differences in the WS (0–6 km) between the (E)F1 and (E)F2+ events. This weakness was resolved by using the 0–3 km WS instead of the 0–6 km WS when calculating the WMAXSHEAR. A new threshold for WMAXSHEAR03 is proposed (500 m2⋅s−2) to distinguish between significant and non-significant tornado environments. Finally, the Szilagyi Waterspout Nomogram, originally developed for the Great Lakes of North America, was successfully tested in the forecasting of waterspout formation for the first time in the western Mediterranean area, although the technique should be adapted to correctly detect cool-season mid-latitude waterspouts.