A Mesoscale Gravity Wave Event Observed during CCOPE. Part III: Wave Environment and Probable Source Mechanisms

Abstract

Synoptic and special mesoscale observations taken during the Cooperative Convective Precipitation Experiment (CCOPE) are used to describe the multiscale environment of a gravity wave event, understand the wave-environment interactions that led to the development of severe thunderstorms, and asses possible wave-generation mechanisms. The storms formed sequentially as a packet of gravity waves propagated across a stationary thunderstorm outflow boundary. Convection developed most rapidly in that part of the mesonetwork in which existed the combination of relatively high parcel buoyant energy, weak restraining inversion, strong storm downdraft potential, and substantial vertical wind shear (associated with a mesoscale jet streak). Synoptic-scale analysis reveals that the waves were excited north of a stationary front and within the right exit region of the jet streak as it approached a stationary ridge in the 300 mb height field. Strong indications of unbalanced flow were diagnosed within the gravity wave source region. Hence, it is suggested that the propagation of the jet streak toward the ridge resulted in the shedding of a gravity-inertia wave packet in a association with a geostrophic adjustment process, which in turn triggered severe thunderstorms along the preexisting outflow boundary. A shear instability analysis conducted upon a representative CCOPE sounding shows that the vertical shear associated with the jet also could have served as a wave energy source, since a wave critical level was found at which the calculated Richardson number fell to a value Ri?¼. Additional analyses indicate that the observed waves were nondispersive and hydrostatic and that vertical energy propagation was impeded by a wave duct associated with the presence of the critical level and lower-tropospheric static stability. The highly coherent nature of the waves, which persisted for many horizontal wavelengths, is explained by this ducting mechanism. These results would seem to point to both geostrophic adjustment and shear instability as plausible wave source mechanisms. It is conjectured that the observed waves were generated by geostrophic adjustment processes, additional energy was supplied through interaction with the critical level, and their coherence maintained through the ducting mechanism.