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A Long-Lived Mesoscale Convective Complex. Part II: Evolution and Structure of the Mature Complex

Peter J. WetzelDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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William R. CottonDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Ray L. McAnellyDepartment of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Abstract

An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean.

Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs found by other authors are consistent with those studied here. In addition, significant similarities were found between the structure of these MCCs and developing tropical cloud clusters. It is concluded that the MCCs analyzed here are basically tropical in nature.

A number of previously unreported features are found common to the two MCCs studied here. Among thew are a 50 kPa divergence/convergence couplet, hypothesized to be an adjustment of the flow around an “obstacle,” and a ring of convergence at 20 kPa surrounding the large circular, divergent anvil region. Also, the high-speed upper-tropospheric outflow in the vicinity of the MCCs is shown to be shallow, indicating that the effect of these systems on the upper-tropospheric flow, in terms of changes in total kinetic energy, may not be as large as implied in previous work. Finally, computations show that while the two MCCs generated vertical velocities comparable to those associated with cyclogenesis, they transported virtually no heat meridionally, suggesting that MCCs are primarily driven by buoyant forces.

Abstract

An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean.

Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs found by other authors are consistent with those studied here. In addition, significant similarities were found between the structure of these MCCs and developing tropical cloud clusters. It is concluded that the MCCs analyzed here are basically tropical in nature.

A number of previously unreported features are found common to the two MCCs studied here. Among thew are a 50 kPa divergence/convergence couplet, hypothesized to be an adjustment of the flow around an “obstacle,” and a ring of convergence at 20 kPa surrounding the large circular, divergent anvil region. Also, the high-speed upper-tropospheric outflow in the vicinity of the MCCs is shown to be shallow, indicating that the effect of these systems on the upper-tropospheric flow, in terms of changes in total kinetic energy, may not be as large as implied in previous work. Finally, computations show that while the two MCCs generated vertical velocities comparable to those associated with cyclogenesis, they transported virtually no heat meridionally, suggesting that MCCs are primarily driven by buoyant forces.

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