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The Influence of Topography and Lower-Tropospheric Winds on Dryline Morphology

Steven E. PeckhamDepartment of Meteorology, Texas A&M University, College Station, Texas

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Louis J. WickerDepartment of Meteorology, Texas A&M University, College Station, Texas

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Abstract

The effects of topography, wind shear, and zonal wind speed on dryline formation and evolution are investigated using a three-dimensional nonhydrostatic mesoscale model. Rather than conduct a case study a parameter study was performed to examine factors that control the depth and strength of the dryline circulation.

This study reveals that the potential for convective storm formation is greatest in those environments in which the cross-dryline flow is weak above the dryline location. This results in boundary layer flow nearly parallel to the north–south-oriented dryline boundary. Under these conditions, the subsiding westerly flow and elevated residual layer formation do not strengthen the capping inversion above the eastern convective boundary layer. In addition, moist air parcels from near the surface are able to reside within the dryline updraft resulting in higher midtropospheric relative humidity subsequently increasing the likelihood of convective storm initiation.

As downslope westerly flow strengthens above the dryline subsidence warming increases and the capping inversion east of the boundary lowers. In response to the subsidence pressure falls occur east of the dryline. Consequently, the east–west horizontal pressure gradient weakens reducing the convergence and frontogenetical circulation along the dryline. The depth of the vertical circulation decreases and air parcels resident within the dryline updraft are detrained at lower altitudes.

Corresponding author address: Dr. Steven E. Peckham, Department of Atmospheric Science, University of Illinois at Urbana–Champaign, 105 S Gregory St., Urbana, IL 61801.

Email: peckham@atmos.uiuc.edu

Abstract

The effects of topography, wind shear, and zonal wind speed on dryline formation and evolution are investigated using a three-dimensional nonhydrostatic mesoscale model. Rather than conduct a case study a parameter study was performed to examine factors that control the depth and strength of the dryline circulation.

This study reveals that the potential for convective storm formation is greatest in those environments in which the cross-dryline flow is weak above the dryline location. This results in boundary layer flow nearly parallel to the north–south-oriented dryline boundary. Under these conditions, the subsiding westerly flow and elevated residual layer formation do not strengthen the capping inversion above the eastern convective boundary layer. In addition, moist air parcels from near the surface are able to reside within the dryline updraft resulting in higher midtropospheric relative humidity subsequently increasing the likelihood of convective storm initiation.

As downslope westerly flow strengthens above the dryline subsidence warming increases and the capping inversion east of the boundary lowers. In response to the subsidence pressure falls occur east of the dryline. Consequently, the east–west horizontal pressure gradient weakens reducing the convergence and frontogenetical circulation along the dryline. The depth of the vertical circulation decreases and air parcels resident within the dryline updraft are detrained at lower altitudes.

Corresponding author address: Dr. Steven E. Peckham, Department of Atmospheric Science, University of Illinois at Urbana–Champaign, 105 S Gregory St., Urbana, IL 61801.

Email: peckham@atmos.uiuc.edu

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