Frontal-Wave-like Evolution in Some Mesoscale Convective Complexes

Abstract

In some mesoscale convective complexes (MCCs) the convective regions assume a configuration that resembles a frontal wave, while in other systems the configuration is not classifiable. We examined the evolution of the internal structure and flow of two MCCs that were part of an episode of systems that propagated along a stationary front, and in which the convective activity evolved from a chaotic to a frontal-wave-like pattern. The development of mesoscale vorticity and its possible relationship to the spatial patterns are explored. Each system developed two dissimilar convective bands that together formed an open-wave pattern. The greatest number of convective clusters and the greatest midlevel convergence and high-level divergence were found in a core region (the ?apex? of the wave) where the bands intersected. The north-south convective line was less enduring than the core convection or the cast-west band; it evolved from outflow boundaries of the early storms preceding the mesosystem. A conceptual model of a frontal-wave-like MCC is developed from Doppler radar and rawinsonde observations. Three airstreams are included in the model: a relatively warm and saturated, ascending flow from the apex into the rear half of the cloud shield; a dry midlevel inflow into the southern flank of the system, in which both ascending and descending motions are observed; and cool inflow into the northern flank of the stratiform cloud below 6 km. It is found that the frontal-wave configuration is apparently not related to the development of vorticity on the scale of the mesoscale cloud system. Rather, the north-south convective line may evolve ahead of the outflow of the early storms as it propagates into the most unstable air. It is hypothesized that the unidirectional profile of vertical shear led to an asymmetric distribution of stratiform cloud north of the convective core region. The asymmetry may have suppressed a stronger, more prolonged convergent wind, which inhibited a stronger response of the rotational wind.