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Formation of Mesoscale Lines of Pirecipitation: Severe Squall Lines in Oklahoma during the Spring

Howard B. BluesteinSchool of Meteorology, University of Oklahoma, Norman, OK 73019

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Michael H. JainWeather Alert Radar Network (WARN), Wellston, OK 74881

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Abstract

Four distinct kinds of severe, mesoscale convective-line development are identified in Oklahoma during the spring based on the analysis of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma. The primary classes of fine formation are broken line, back building, broken areal and embedded areal. Each is described in detail, along with illustrative examples. Comparisons are made with other observations and with numerical model simulations. The former two classes of line formation have been previously documented, while the latter two have not. Only the broken-areal squall line has been realistically simulated numerically.

The environment for each of the types of line development was determined from data from the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. It was found that broken-line formation tends to occur along cold fronts in a multicell environment, while back building occurs along any boundary in a supercell environment. The former formation is associated with a steering level with respect to cell motion, while the others are not. A steering level with respect to line motion exists around 6 or 7 km MSL in all cases. Cells within back-building squall lines have high relative helicity, like supercells, while cells within broken-line squall lines have low relative helicity. Most lines were oriented approximately 40° to the left of the pressure-weighted vertical shear vector in the troposphere, along the pressure-weighted vertical shear vector in the lowest 1 km and at a large angle to the shear somewhere in the lower portion of the middle troposphere.

Abstract

Four distinct kinds of severe, mesoscale convective-line development are identified in Oklahoma during the spring based on the analysis of an 11-year period of reflectivity data from the National Severe Storms Laboratory's 10-cm radar in Norman, Oklahoma. The primary classes of fine formation are broken line, back building, broken areal and embedded areal. Each is described in detail, along with illustrative examples. Comparisons are made with other observations and with numerical model simulations. The former two classes of line formation have been previously documented, while the latter two have not. Only the broken-areal squall line has been realistically simulated numerically.

The environment for each of the types of line development was determined from data from the standard National Weather Service surface and upper-air networks and from special rawinsonde launches. It was found that broken-line formation tends to occur along cold fronts in a multicell environment, while back building occurs along any boundary in a supercell environment. The former formation is associated with a steering level with respect to cell motion, while the others are not. A steering level with respect to line motion exists around 6 or 7 km MSL in all cases. Cells within back-building squall lines have high relative helicity, like supercells, while cells within broken-line squall lines have low relative helicity. Most lines were oriented approximately 40° to the left of the pressure-weighted vertical shear vector in the troposphere, along the pressure-weighted vertical shear vector in the lowest 1 km and at a large angle to the shear somewhere in the lower portion of the middle troposphere.

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