The Process of Thunderstorm Generation by Bores

The relative importance for thunderstorm initiation of direct lifting of conditionally unstable air by bores compared to the upward mixing of low-level moisture by bore circulations is examined. A large number of lidar and radar profiling systems are used in this effort, in conjunction with ultra-high resolution numerical models. Profiling instrumentation used includes moisture measurements from airborne Differential Absorption Lidar and ground-based Raman lidar; wind measurements from Doppler lidar; aerosol measurements from backscatter lidar; additional information from 10-cm Doppler weather radars, FM-CW radar, and boundary layer wind profilers; and profiles of temperature and moisture retrieved from a variety of other remote sensing systems including microwave radiometer.

The presence of bores and their attendant vertical circulations was readily detected by the lidar and radar systems. Results showed that the bores wafted moist air up to the middle troposphere and weakened the capping inversion, thus reducing inhibition to deep convection development. Application of parcel displacement profiles derived from the Doppler lidar and wind profiler analyses to representative pre-bore soundings permitted assessments to be made of the effects of the bore and gravity current passage on the atmosphere. These calculations suggest that the strong bore-induced lifting was insufficient to trigger the storms, but in the one cold frontal case, the dual lifting provided by the bore and the subsequent gravity current that made it possible for low-level parcels to reach their level of free convection. In a more general sense, it is found that even though bores can produce strong lifting, this may not be sufficient to trigger deep convection if the lifting is confined to too shallow a layer and/or is of insufficient duration, or the pre-bore environment contains a very strong capping inversion with limited moisture.