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The Evolution of an Oklahoma Dryline. part II: Boundary-Layer Forcing of Mesoconvective Systems

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  • 1 Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman 73019
  • | 2 Field Observing Facility, National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

Data from the National Severe Storms Laboratory surface mesonetwork are objectively analyzed to give insight into processes that contributed to the development of three tornadic mesoconvective systems near the 8 June 1974 Oklahoma dryline. Storm cells constituting each of the systems form over recurring zones of convergence within 20 km of the dryline. Different mechanisms appear to force the individual convergence zones.

Storms of the first system appear simultaneously only after the establishment of a pressure trough just cast of a zone of convergence 15 km east of the dryline. The convergence zone intensifies and progresses eastward with the storms; meanwhile, a second convergence zone appears at the dryline in response to apparent storm-induced pressure systems trailing the storms. The fact that deep convection did not occur over the second zone is attributed to static stabilization caused by mesoscale unsaturated downdrafts in the upper troposphere. Storms of the second system develop in a consecutive manner over a third set of convergence anomalies that originally appeared at the dryline and subsequently propagated northeastward. These propagating disturbances have gravity wave characteristics. Formation of the third system, a solid squall line, is related to a frontogenetic circulation about a progressing cold front as it encountered the abundant moisture present at the stalled dryline.

It is concluded that precursor conditions to severe convective occurrences can be determined from surface mesoscale analysis and, moreover, provide considerable insight into mechanisms that produce low-level convergence.

Abstract

Data from the National Severe Storms Laboratory surface mesonetwork are objectively analyzed to give insight into processes that contributed to the development of three tornadic mesoconvective systems near the 8 June 1974 Oklahoma dryline. Storm cells constituting each of the systems form over recurring zones of convergence within 20 km of the dryline. Different mechanisms appear to force the individual convergence zones.

Storms of the first system appear simultaneously only after the establishment of a pressure trough just cast of a zone of convergence 15 km east of the dryline. The convergence zone intensifies and progresses eastward with the storms; meanwhile, a second convergence zone appears at the dryline in response to apparent storm-induced pressure systems trailing the storms. The fact that deep convection did not occur over the second zone is attributed to static stabilization caused by mesoscale unsaturated downdrafts in the upper troposphere. Storms of the second system develop in a consecutive manner over a third set of convergence anomalies that originally appeared at the dryline and subsequently propagated northeastward. These propagating disturbances have gravity wave characteristics. Formation of the third system, a solid squall line, is related to a frontogenetic circulation about a progressing cold front as it encountered the abundant moisture present at the stalled dryline.

It is concluded that precursor conditions to severe convective occurrences can be determined from surface mesoscale analysis and, moreover, provide considerable insight into mechanisms that produce low-level convergence.

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