The Transition Zone and Secondary Maximum of Radar Reflectivity behind a Midlatitude Squall Line: Results Retrieved from Doppler Radar Data

Scott A. Braun Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Robert A. Houze Jr. Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

Thermodynamic and microphysical retrieval techniques are applied to dual-Doppler synthesized air motion fields for a midlatitude squall line, which passed through the Oklahoma-Kansas Preliminary Regional Experiment for the Stormscale Operational and Research Meteorology Program (PRE-STORM) observational array in Kansas and Oklahoma on 10–11 June 1985. The retrieved pressure and potential temperature fields are consistent with surface network and sounding data, while the retrieved microphysical fields show the characteristic secondary maximum of radar reflectivity in the stratiform region and the band of low reflectivity, or transition zone, lying between the leading convective line and the secondary maximum.

The retrieved fields indicate the processes producing the secondary maximum and transition zone minimum of radar reflectivity more quantitatively than has been possible in previous studies. The primary processes accounting for these features of the radar reflectivity pattern were 1) the substantial increase in precipitation mass concentrations by vapor deposition within the region of mesoscale ascent in the stratiform region and the increase in particle size resulting from the strong aggregation of ice particles above the bright band in the region of the secondary band, 2) the suppression of growth in the middle to upper level descent just behind the convective region, which enhanced the minimum of radar reflectivity in that zone, and 3) the trajectories of ice particles detrained from the convective line, which qualitatively accounted for the general location of the secondary band. Additional insights into these processes are discussed.

Abstract

Thermodynamic and microphysical retrieval techniques are applied to dual-Doppler synthesized air motion fields for a midlatitude squall line, which passed through the Oklahoma-Kansas Preliminary Regional Experiment for the Stormscale Operational and Research Meteorology Program (PRE-STORM) observational array in Kansas and Oklahoma on 10–11 June 1985. The retrieved pressure and potential temperature fields are consistent with surface network and sounding data, while the retrieved microphysical fields show the characteristic secondary maximum of radar reflectivity in the stratiform region and the band of low reflectivity, or transition zone, lying between the leading convective line and the secondary maximum.

The retrieved fields indicate the processes producing the secondary maximum and transition zone minimum of radar reflectivity more quantitatively than has been possible in previous studies. The primary processes accounting for these features of the radar reflectivity pattern were 1) the substantial increase in precipitation mass concentrations by vapor deposition within the region of mesoscale ascent in the stratiform region and the increase in particle size resulting from the strong aggregation of ice particles above the bright band in the region of the secondary band, 2) the suppression of growth in the middle to upper level descent just behind the convective region, which enhanced the minimum of radar reflectivity in that zone, and 3) the trajectories of ice particles detrained from the convective line, which qualitatively accounted for the general location of the secondary band. Additional insights into these processes are discussed.

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