Finescale Vertical Structure and Evolution of a Preconvective Dryline on 19 June 2002

Benjamin D. Sipprell University of Wyoming, Laramie, Wyoming

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Bart Geerts University of Wyoming, Laramie, Wyoming

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Abstract

High-resolution airborne cloud radar data and other International H2O Project datasets are used to describe the vertical structure of an unusual prefrontal dryline. This dryline, observed in northwestern Kansas on 19 June 2002, first progressed eastward and tilted toward the west, and later became more stationary and reversed its tilt, toward the moist side. The convective boundary layer (CBL) depth difference also reversed: only in the later phase did the dry-side CBL become deeper than on the moist side. Echo and single/dual-Doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient. Both this gradient and the solenoidal circulation reversed in the later phase. Simultaneously, confluence toward the dryline increased, resulting in an increasing moisture gradient as well as a deepening CBL in the dryline convergence zone. It is speculated that the baroclinically generated horizontal vorticity contributed to this CBL deepening, as the sign of this vorticity was opposite to that of the low-level wind shear on the opposite side of the dryline in both phases. Deep-convective initiation appears to have resulted from this local CBL deepening, leading to a total elimination of convective inhibition near the dryline.

Corresponding author address: Bart Geerts, Department of Atmospheric Sciences, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

Abstract

High-resolution airborne cloud radar data and other International H2O Project datasets are used to describe the vertical structure of an unusual prefrontal dryline. This dryline, observed in northwestern Kansas on 19 June 2002, first progressed eastward and tilted toward the west, and later became more stationary and reversed its tilt, toward the moist side. The convective boundary layer (CBL) depth difference also reversed: only in the later phase did the dry-side CBL become deeper than on the moist side. Echo and single/dual-Doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient. Both this gradient and the solenoidal circulation reversed in the later phase. Simultaneously, confluence toward the dryline increased, resulting in an increasing moisture gradient as well as a deepening CBL in the dryline convergence zone. It is speculated that the baroclinically generated horizontal vorticity contributed to this CBL deepening, as the sign of this vorticity was opposite to that of the low-level wind shear on the opposite side of the dryline in both phases. Deep-convective initiation appears to have resulted from this local CBL deepening, leading to a total elimination of convective inhibition near the dryline.

Corresponding author address: Bart Geerts, Department of Atmospheric Sciences, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

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