Dual-Doppler Radar Analysis of a Midlatitude Squall Line with a Trailing Region of Stratiform Rain

Bradley F. Smull Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195

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Robert A. Houze Jr. Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195

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Abstract

The mesoscale structure of a squall-line system that passed over Oklahoma on 22 May 1976 is investigated by dual-Doppler radar analysis. The mature storm consisted of a leading line of deep convection, which exhibited organized multicellular structure, trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level. These contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels, which has been termed the “transition zone.” While conventional and single-Doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm, the dual-Doppler analysis demonstrates the pervasiveness of these features over much of the squall line's length.

The structure of a midtropospheric maximum of rearward, system-relative flow crossing the system is particularly well described by the dual-Doppler data. This mesoscale current originated ahead of the storm. gained strength while passing through the convective line, spanned the transition zone, and extended to near the back edge of the stratiform region. It strongly influenced precipitation growth and radar echo structure by promoting the transfer of ice particles from convective cells across the transition zone into the trailing stratiform region. Deep, intense updrafts occurred in association with convective cells along the leading edge of the system. Convective downdrafts were apparently active both in the lower troposphere, where thermodynamic data showed they were a source of air feeding the leading gust front, and at upper levels, where the Doppler analysis indicated they were forced by convergence of air detrained from the tops of the updrafts with slower moving ambient air. Horizontal momentum transported vertically by convective motions converged at midlevels, accelerating parcels rearward and so bolstering the front-to-rear flow.

Profiles of radar-derived mean vertical motion confirm the presence of a mesoscale updraft overlying a mesoscale downdraft in the transition and trailing stratiform regions. The mean descent in the lower troposphere was particularly deep and intense in the transition zone and may have contributed to the decreased reflectivity values observed there.

Abstract

The mesoscale structure of a squall-line system that passed over Oklahoma on 22 May 1976 is investigated by dual-Doppler radar analysis. The mature storm consisted of a leading line of deep convection, which exhibited organized multicellular structure, trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level. These contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels, which has been termed the “transition zone.” While conventional and single-Doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm, the dual-Doppler analysis demonstrates the pervasiveness of these features over much of the squall line's length.

The structure of a midtropospheric maximum of rearward, system-relative flow crossing the system is particularly well described by the dual-Doppler data. This mesoscale current originated ahead of the storm. gained strength while passing through the convective line, spanned the transition zone, and extended to near the back edge of the stratiform region. It strongly influenced precipitation growth and radar echo structure by promoting the transfer of ice particles from convective cells across the transition zone into the trailing stratiform region. Deep, intense updrafts occurred in association with convective cells along the leading edge of the system. Convective downdrafts were apparently active both in the lower troposphere, where thermodynamic data showed they were a source of air feeding the leading gust front, and at upper levels, where the Doppler analysis indicated they were forced by convergence of air detrained from the tops of the updrafts with slower moving ambient air. Horizontal momentum transported vertically by convective motions converged at midlevels, accelerating parcels rearward and so bolstering the front-to-rear flow.

Profiles of radar-derived mean vertical motion confirm the presence of a mesoscale updraft overlying a mesoscale downdraft in the transition and trailing stratiform regions. The mean descent in the lower troposphere was particularly deep and intense in the transition zone and may have contributed to the decreased reflectivity values observed there.

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