A climatology of convective available potential energy in Great Britain

Abstract

Deep moist convection (DMC) requires three ingredients: instability, moisture and lift. One measure that incorporates two of these, instability and moisture, is convective available potential energy (CAPE). A 10-year climatology of CAPE over Great Britain is presented covering the period 1 June 2002-31 May 2012, based on a 9-km grid spacing implementation of the Weather Research and Forecasting (WRF) model, with two-way interactive nesting. Appropriate tests are carried out to verify model reliability by comparing simulated and observed CAPE. CAPE is found to be highly variable both spatially and temporally, the highest values being produced during Spanish plume events. A strong relationship is confirmed between surface temperature and CAPE magnitude, the highest CAPE across Great Britain during this period locally exceeding 3000 J kg-1. In an average year, 15 days produce CAPE in excess of 500 J kg-1 somewhere in Great Britain, 4 days > 1000 J kg-1 and 1 day > 1500 J kg-1. Three main CAPE seasons are identified: 'land dominated CAPE' between April and September, 'sea dominated CAPE' between September and January and 'low CAPE' from January to April. The southern North Sea witnesses significant CAPE all year round because of a combination of favourable synoptic situations, including warm air plumes in spring/summer and cold air incursions over warmer seas in winter. CAPE is not a direct predictor of thunderstorm incidence, due in part to the confounding effect of convective inhibition (CIN). However, at the annual scale, when comparing against an existing days of thunder climatology, we observe a close correspondence with >500 J kg-1 CAPE frequency.