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Evolution of vortical patterns and vortices in mesoscale convective complexes




Fortune, Michael A., author

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The evolution and flow structure of four mesoscale convective complexes (MCCs) that manifested some form of vortical pattern of convective rain all were investigated with datasets from the PRE- STORM field program. Intersecting convective bands that resembled 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 mesa-a-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 mid-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.


Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1989.

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Convective clouds
Clouds -- Dynamics


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