A provisional climatology of cool-season convective lines in the UK

A 7-year climatology of cool-season convective lines in the UK has been constructed using archived radar rainfall data. 103 lines meeting basic size, duration and intensity criteria were identified over the period September 2003 to February 2010. Of these, 87% occurred in association with frontal systems (mostly cold fronts). The remaining 13% occurred within polar-maritime air masses to the rear of frontal systems. Annual frequencies were highest over inland areas of southern and central England (5.0–7.5 events per 10,000 km2, per year) and decreased markedly to the north (< 1.0 events per 10,000 km2, per year over north and west Scotland). Tornado and Storm Research Organisation (TORRO) tornado data revealed that 27.2% of lines produced at least one tornado. The environment of tornadic and non-tornadic lines was investigated by analysis of available observations, from which representative values of various environmental parameters were obtained. Tornadic lines were found to be associated with significantly larger pre-line surface-based CAPE, line-normal forward motion, cross-line horizontal wind veer, and cross-line temperature decrease. Furthermore, a tendency for tornadic lines to occur near the core, or under the left exit region, of mid-level jets was apparent.