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Long-Lived Mesoconvective Vortices and Their Environment. Part II: Induced Thermodynamic Destabilization in Idealized Simulations

S. B. TrierNational Center for Atmospheric Research, Boulder, Colorado*

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C. A. DavisNational Center for Atmospheric Research, Boulder, Colorado*

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W. C. SkamarockNational Center for Atmospheric Research, Boulder, Colorado*

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Abstract

Idealized numerical simulations are used to quantify the effect of quasi-balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in Part I. Maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly, near the radius of maximum tangential vortex winds. The location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear, and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90° to the left of downshear for strong vortices in weak ambient vertical shear. Although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear, maximum upward displacements are associated with large vortices and occur in approximately average vertical shear for MCV environments.

The simulations suggest that in larger and stronger than average MCVs, the lifting that results from the MCV being embedded in a weakly baroclinic environment is, alone, sufficient to saturate initially moist and conditionally unstable layers immediately above the boundary layer. The horizontal location of the resulting thermodynamic instability is approximately coincident with the maximum lower-tropospheric upward displacements. Since in the absence of sustained deep convection the vortices develop substantial vertical tilt, the destabilized region in the lower troposphere lies nearly underneath the vortex center at its level of maximum strength, consistent with observations that redevelopment of organized, long-lived (e.g., t ≥ 6 h) deep convection is most often found near the midtropospheric MCV center. This location for convectively induced stretching of preexisting vertical vorticity is optimal for maintaining the vortex against the deleterious effect of differential advection by the ambient shear.

Corresponding author address: Dr. Stanley B. Trier, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000.

Email: trier@ucar.edu

Abstract

Idealized numerical simulations are used to quantify the effect of quasi-balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in Part I. Maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly, near the radius of maximum tangential vortex winds. The location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear, and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90° to the left of downshear for strong vortices in weak ambient vertical shear. Although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear, maximum upward displacements are associated with large vortices and occur in approximately average vertical shear for MCV environments.

The simulations suggest that in larger and stronger than average MCVs, the lifting that results from the MCV being embedded in a weakly baroclinic environment is, alone, sufficient to saturate initially moist and conditionally unstable layers immediately above the boundary layer. The horizontal location of the resulting thermodynamic instability is approximately coincident with the maximum lower-tropospheric upward displacements. Since in the absence of sustained deep convection the vortices develop substantial vertical tilt, the destabilized region in the lower troposphere lies nearly underneath the vortex center at its level of maximum strength, consistent with observations that redevelopment of organized, long-lived (e.g., t ≥ 6 h) deep convection is most often found near the midtropospheric MCV center. This location for convectively induced stretching of preexisting vertical vorticity is optimal for maintaining the vortex against the deleterious effect of differential advection by the ambient shear.

Corresponding author address: Dr. Stanley B. Trier, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000.

Email: trier@ucar.edu

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