A radar-based climatology of tropopause folds and deep convection

Tropopause folds have been shown to enhance or inhibit convective storms, but how does a forecaster know which will happen in any given situation? How does the structure of tropopause folds affect the occurrence, location and morphology of convection? As a start to answering these questions, we constructed a 5-yr (2006--2010) radar-based climatology of tropopause folds and convective storms for Wales, United Kingdom. Based on the continous, high-resolution data from a VHF wind-profiling radar located at Capel Dewi, 183 tropopause folds were identified. A maximum of tropopause-fold cases occurred in January with a secondary maximum in July. A clear correspondence was found between the occurrence of folds and the North Atlantic Oscillation index, with a positive phase of the oscillation associated with an increased number of folds in winter 2006--2009, and springs 2007 and 2009. Based on the data from the weather radar network operated by the UK Met Office, a radar-based climatology of convective storms was developed, resulting in 685 cases. Unlike much of the US where a clear annual cycle peaking in the warm season occurs, convective storms over Wales display a relatively consistent occurrence throughout the year, except for a strong minimum in late winter and early spring. Multicellular lines were the most common (43%) with a monthly distribution that peaks in October, followed by isolated cells (33%) most common in May--September and nonlinear clusters (24%) with a maximum in November--January. Convective storms are associated with 57% of the tropopause folds identified in this study. The monthly distribution of these cases reveal a significant seasonal cycle with a maximum in December. The tropopause folds were then classified according to their location with respect to an upper-level trough into four categories: east of the trough, underneath the tropopause fold on eastern side of the trough, underneath the trough and underneath the fold on the western side of the trough. From the 55 tropopause folds observed underneath the tropopause fold on the eastern side the trough, 67.3% were associated with convective storms. The convective mode in this region was dominated by multicellular lines (41.8%). From the 128 the tropopause folds observed underneath the fold on the western side of the trough, 32.8% were associated with a convective storm. In this region the convective mode was dominated by isolated cells (23.4%). These results show that the stronger the synoptic forcing, the more likely storms are to be more highly organized. Future work aims to better understand the reasons for this result.