INITIATION OF CONVECTION FORCED BY A DESCENDING DRY LAYER AND LOW LEVEL MOIST CONVERGENCE: A CASE STUDY OF CSIP IOP 9

A narrow line of convective showers formed over southern England in the afternoon of 18 July 2005 and was observed by a large number of instruments deployed as part of the Convective Storm Initiation Project (CSIP). The showers formed in the region behind a cold front and beneath two apparently descending dry layers. One of the dry layers formed as a folded feature associated with a tropopause depression that was located to the north-west of the UK, and had formed 2 days earlier from a breaking Rossby wave over the Atlantic. The second dry layer formed as a fragment of the original fold due to cyclonic rotation around the depression. This fragment was also responsible for the observations of descending layers when ascent would otherwise have been seen behind the cold front. As the layers descended over the UK the upper-level cold front, associated with the lowermost dry layer, overran higher w air nearer the surface and resulted in convective, or potential, instability. Combined with a surface convergence line formed by flow around upstream orography, these factors were sufficient to force convection up to around 6 km where it was eventually halted by the uppermost dry layer. Here, we present observations from CSIP and model data to illustrate the unique processes involved in forcing the convection in this case. This extended abstract is based on a manuscript (Russell et al., 2008b) that has been submitted to Tellus A