Secondary initiation of multiple bands of cumulonimbus over southern Britain. I: An observational case-study

SUMMARY Special observing facilities have been assembled in southern England as part of the Convective Storm Initiation Project (CSIP) to study the mesoscale and convective-scale processes that determine precisely where warm-season convective showers will break out. This paper reports the results of a case-study during the pilot field campaign of CSIP in July 2004. One purpose of the pilot project was to demonstrate the value of various observational facilities and to evaluate the usefulness of a variety of analysis and synthesis techniques. Amongst other things, the case-study demonstrates the utility of high-resolution imagery from the Meteosat Second Generation satellite for tracking the early stages of the convective clouds, and of a new clear-air scanning radar at Chilbolton for mapping both the top of the boundary layer and the initialgrowth of the convective cells that penetrate it. The particular event studied involved the triggering of convection that developed into three parallel arcs of showers and thunderstorms. The first arc was triggered along the leading edge of the outflow (density current) from an earlier cluster of showers, but the convection in the second and third arcs was triggered by a different mechanism. The paper describes in detail the way in which this convection broke through the stable layer, or lid, at the top of a boundary layer of variable depth. The strength of the lid decreased and the depth of the boundary layer increased with timeas a result of diurnal heating, but the precise locations where convection finally broke through were determined by the spatial variability in boundary-layer depth. The analysis suggests that a wave-like modulation of the boundary-layer depth of amplitude 150 m, perhaps due to a gravity-wave disturbance from the earlier cluster of showers, had a greater influence on where the convection was triggered than the modest hills (typically 200 m high) in southern England.