A case study of a large patch of billows surmounted by elevated convection

A region of large-amplitude Kelvin-Helmholtz billows, of area 2000 km , was observed in a region of moderate rain at the rear of a mesoscale convective system on a day during the Convective Storm Initiation Project in southern England. The mesoscale convective system (MCS) was characterized by elevated convection above a layer of cool air, separated by a sloping statically stable layer characterized by strong wind shear. Doppler radar observations showed that the large patch of billows was situated within this shear layer and that it persisted over the 2 h period of observation, maintaining its position with respect to the MCS and producing surface pressure perturbations of ±0.3 hPa. Potentially unstable air above the sloping shear layer reached its level of free convection while ascending above this layer. The resulting elevated convection was in the form of cells whose spacing appeared to be influenced by the underlying billows. Results from large-eddy model simulations are shown to support the hypothesis that, although the large-scale ascent alone would have triggered the elevated convection, the billows exerted a secondary forcing effect on the convection.