Idealised modelling studies of secondary initiation observed during CSIP

The Convective Storm Initiation Project (CSIP) aims to improve our understanding of the processes controlling both primary and secondary convective initiation in the maritime mid-latitude climate of the UK. We present idealised modelling studies of secondary initiation by a cold-pool outflow and convectively generated gravity waves, for a case observed during CSIP pilot field campaign. On the 10th of July 2004 a cold pool outflow was observed initiating an arc of cumulus, which quickly developed into cumulonimbus, downwind of the generating storm (the "parent storm"). Three further arcs, one less distinct than the other two, were initiated further downwind of this first arc. These later arcs also went on to give significant precipitation, but were not initiated by cold pool outflows.

The model results strongly support the hypothesis that gravity waves, generated by the "parent storm", were responsible for initiating the later three arcs of convective showers, whilst the wind-shear profile led to convective initiation on only the downwind side of the cold-pool outflow of the "parent storm". The fastest two wave modes generated by the storm suppressed convection and slower modes were responsible for the convective initiation. The timing of these processes and the asymmetry (with the largest effects downwind of the "parent storm") agree with the observations. Model runs with reduced resolution suggest that a non-hydrostatic high resolution (approximately 1 km) numerical weather prediction model should be able to capture the secondary initiation processes, assuming that the generating storm had been forecast/nowcast correctly. Additional cases from the main CSIP field campaign will also be presented. These include at least one other case where the effects of convectively generated waves were significant.