The life cycle of an undular bore and its interaction with a shallow, intense cold front

The evolution of a bore and attendant prefrontal features associated with the passage of a shallow, intense surface cold front across the Southern Plains is examined using high-resolution surface and remote sensing observations combined with output from a high-resolution numerical model simulation. Observations show that as the front propagated into south-central Kansas and north-central Oklahoma, it encountered an ambient wave duct and an undular bore formed at the front. At Lamont, Oklahoma, the passage was manifest as the leading edge of coincident temperature and moisture gradients preceded by a wind shift and pressure trough. By the time the feature progressed southeastward to Okmulgee, Oklahoma several hours later, its passage more closely resembled that of a classical cold front as the leading edge of the temperature and moisture gradients were more nearly coincident with the wind shift, though all still slightly trailed the pressure trough. Thus, a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relationship to the parent surface front occurred in a short period of time.

The details of this evolution are examined using output from a fine-scale numerical simulation, performed using the Weather Research and Forecasting Model (WRF). The analysis shows that the circulation associated with the bore was responsible for the development of the pressure trough and wind shift ahead of the leading edge of the surface baroclinic zone. The substantial mixing accomplished by the bore circulation vertically redistributed boundary layer moisture eventually eroding a low-level stable layer that was a component of the wave duct that originally supported propagation of the bore. With the elimination of the wave duct, the bore rapidly decayed allowing the surface baroclinic zone to merge with the wind shift to produce the more classical cold frontal structure observed over east-central Oklahoma.