The Evolution of Vortical Patterns and Vortices in Mesoscale Convective Complexes.
The evolution and flow structure of four mesoscale convective complexes (MCCs) that manifested some form of vortical pattern of convective rainfall were investigated with datasets from the PRE-STORM field program. Intersecting convective bands that resembles an extratropical, frontal -wave cyclone evolved in the growth stage of three cases. A spiral pattern also emerged in one case, while the fourth developed a comma-shaped occlusion. A strong divergent wind response to the mesoscale heat source was observed in all systems, but only a weak rotational wind developed in the frontal wave systems. A meso-alpha -scale vortex was probably not responsible for the frontal wave pattern; rather the pattern resulted from the intersection of a mesoscale cold front advancing into the most unstable air with an existing stationary front. The convectively active core of the MCCs propagated with the intersection. When upper level directional shear spread a stratiform cloud in more than one direction around the core, and when intense convection developed well south of the core and the front, the eventual spin-up of a vortex was fostered. In other cases, advection of the stratiform cloud north of the core may have hindered spin-up because a dry mind-level inflow beneath the cloud developed counter to the direction of rotation. A conceptual model of the mesoscale frontal-wave cyclone employs the conveyor belt model of the extratropical cyclone. The warm conveyor belt is accelerated from the core upward and rearward in a jet within the former stationary frontal zone; the dry airstream converges from all possible directions beneath the more heavily precipitating part of the stratiform cloud; the cool conveyor belt on the north side descends from the 4 to 7 km layer into the core region. Conditional symmetric instability may have initiated mesoscale overturning in a 1 km deep layer in the low levels of the stationary frontal zone. The conceptual models presented here for a weakly rotating (frontal wave) MCC and a strongly rotating MCC are compared with conceptual models of larger-scale extratropical cyclones and smaller -scale supercell thunderstorms.
- Pub Date:
- Physics: Atmospheric Science