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The Formation of Small-Scale Atmospheric Vortices via Horizontal Shearing Instability

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  • 1 Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, and NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma
  • 2 NOAA/National Severe Storms Laboratory, Norman, Oklahoma
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Abstract

Motivated by high-resolution observations of small-scale atmospheric vortices along near-surface boundaries, this study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries. The series of dry, constant potential temperature simulations are initialized with a north–south-oriented constant-vorticity shear zone and north–south periodic boundary conditions.

In all simulations, the shear zones develop wavelike perturbations that eventually roll up into discrete vortices. These vortices have features resembling those observed in many laboratory and numerical studies (i.e., instabilities developed into elliptical cores connected by vorticity braids that precess and contain pressure minima in their centers). To assess the instability mechanism, the results are compared to linear theory. Excellent agreement is found between predictions from linear theory for the wavenumber of maximum growth as a function of shear zone width and growth rate as a function of shear zone vorticity, suggesting to a very good first approximation, horizontal shearing instability (HSI) is responsible for the growth of initial small perturbations. It is also found that predictions of linear theory tend to extend well into the nonlinear regime.

Finally, preferred regions of cumulus formation are assessed by including moisture in four simulations. Maximum updrafts and simulated cumuli tend to form along the periphery of cores and/or along the braided regions adjacent to the cores. Because of the important modulating effect of misocyclone development via HSI and subsequent moisture transport, cumulus spacing and size/depth are also dependent on the shear zone width and vorticity.

Corresponding author address: Dr. Michael S. Buban, Atmospheric Turbulence and Diffusion Division, NOAA/OAR/Air Resources Laboratory, 456 S. Illinois Ave., Oak Ridge, TN 37830. E-mail: michael.buban@noaa.gov

Abstract

Motivated by high-resolution observations of small-scale atmospheric vortices along near-surface boundaries, this study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries. The series of dry, constant potential temperature simulations are initialized with a north–south-oriented constant-vorticity shear zone and north–south periodic boundary conditions.

In all simulations, the shear zones develop wavelike perturbations that eventually roll up into discrete vortices. These vortices have features resembling those observed in many laboratory and numerical studies (i.e., instabilities developed into elliptical cores connected by vorticity braids that precess and contain pressure minima in their centers). To assess the instability mechanism, the results are compared to linear theory. Excellent agreement is found between predictions from linear theory for the wavenumber of maximum growth as a function of shear zone width and growth rate as a function of shear zone vorticity, suggesting to a very good first approximation, horizontal shearing instability (HSI) is responsible for the growth of initial small perturbations. It is also found that predictions of linear theory tend to extend well into the nonlinear regime.

Finally, preferred regions of cumulus formation are assessed by including moisture in four simulations. Maximum updrafts and simulated cumuli tend to form along the periphery of cores and/or along the braided regions adjacent to the cores. Because of the important modulating effect of misocyclone development via HSI and subsequent moisture transport, cumulus spacing and size/depth are also dependent on the shear zone width and vorticity.

Corresponding author address: Dr. Michael S. Buban, Atmospheric Turbulence and Diffusion Division, NOAA/OAR/Air Resources Laboratory, 456 S. Illinois Ave., Oak Ridge, TN 37830. E-mail: michael.buban@noaa.gov
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