Observations of convection initiation "failure" from the 12 June 2002 IHOP deployment

Observations of the development of cumulus convection, which reached depths of several kilometers but failed to develop into sustained, precipitating, cumulonimbus clouds-an event the authors term "convection initiation failure"-are presented from the 12 June 2002 International H2O Project (IHOP) case. The investigation relies heavily on remote and in situ data obtained by mobile, truck-borne Doppler radars, mobile mesonets, mobile soundings, and stereo cloud photogrammetry. Data collection was focused in northwestern Oklahoma near the intersection of an outflow boundary and dryline. Thunderstorms developed along the dryline during the late afternoon approximately 40 km east of the domain intensively observed by the ground-based observing systems. Farther west, within the region of dense observations analyzed herein, cumulus congestus clouds formed along an outflow boundary. Multiple-Doppler wind syntheses revealed that the boundary layer vertical velocity field was dominated by thermals rather than by circulations associated with the mesoscale boundaries. In spite of this observation, deep cumulus cloud development was confined to the mesoscale boundaries. Trajectories into the deep cumulus clouds that developed along the outflow boundary were much more vertical than those entering the shallow cumulus clouds observed away from the outflow boundary. It is hypothesized that the role of the outflow boundary in promoting deep cumulus cloud formation was to promote updrafts that were less susceptible to the dilution of equivalent potential temperature, which controls the potential buoyancy, vertical velocity, and depth that can be realized by the clouds. It is also hypothesized that the lack of a persistent, spatially continuous corridor of mesoscale ascent along the outflow boundary and associated moisture upwelling contributed to convection initiation failure along the outflow boundary.