An Observational Study of the Dryline

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  • 1 National Oceanic and Atmospheric Administration, National Severe Storms Laboratory, Mesoscale Research Division, Boulder, Colorado
  • | 2 National Severe Storms Laboratory, Norman, Oklahoma
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Abstract

This study presents analyses of data collected in the vicinity of a cloud-free dryline that occurred in western Oklahoma on 24 May 1989. Observations reveal sharp contrasts across the quasi-stationary, north-south dryline during midafternoon. Of greatest significance is a pronounced gradient of virtual potential temperature, although horizontal convergence and vorticity also maximize at the dryline.

The environment of the 24 May dryline is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both sides of the dryline. The dryline resembles a “mixing zone” containing varying proportions of hot, dry air to the west side and warm, moist air from the lowest 200 m within 10 km to the east of the dryline. The mixing zone slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Saturation-point analysis indicates that the mixing zone is characterized by a single mixing-line structure defined by the respective quasi-homogeneous air masses on either side of the dryline.

Dynamical analysis reveals that near-surface westerly flow is accelerated upward and over relatively cool air above the surface by an elevated low pressure region at the dryline. Flow accelerations are nonhydrostatic at the dryline, while the flow is in hydrostatic balance both to the west and to the east of the dryline. Magnitudes of the inertial, pressure, and Coriolis accelerations are comparable to the east of the dryline, implying a considerable ageostrophic flow component as well as a quasigeostrophic linkage between the low-level jet and the west-east horizontal pressure gradient.

Abstract

This study presents analyses of data collected in the vicinity of a cloud-free dryline that occurred in western Oklahoma on 24 May 1989. Observations reveal sharp contrasts across the quasi-stationary, north-south dryline during midafternoon. Of greatest significance is a pronounced gradient of virtual potential temperature, although horizontal convergence and vorticity also maximize at the dryline.

The environment of the 24 May dryline is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both sides of the dryline. The dryline resembles a “mixing zone” containing varying proportions of hot, dry air to the west side and warm, moist air from the lowest 200 m within 10 km to the east of the dryline. The mixing zone slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Saturation-point analysis indicates that the mixing zone is characterized by a single mixing-line structure defined by the respective quasi-homogeneous air masses on either side of the dryline.

Dynamical analysis reveals that near-surface westerly flow is accelerated upward and over relatively cool air above the surface by an elevated low pressure region at the dryline. Flow accelerations are nonhydrostatic at the dryline, while the flow is in hydrostatic balance both to the west and to the east of the dryline. Magnitudes of the inertial, pressure, and Coriolis accelerations are comparable to the east of the dryline, implying a considerable ageostrophic flow component as well as a quasigeostrophic linkage between the low-level jet and the west-east horizontal pressure gradient.

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