Near-ground rotation in simulated supercells: On the robustness of the baroclinic mechanism

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Abstract

This study addresses the robustness of the baroclinic mechanism that facilitates the onset of surface rotation in supercells by using two idealized simulations with different microphysics parameterizations and by considering previous results. In particular, the importance of ambient crosswise vorticity relative to baroclinically generated vorticity in the development of near-ground cyclonic vorticity is analyzed. The storms were simulated using the CM1 model in a kinematic base state characterized by a straight-line hodograph. Atrajectory analysis spanning about 30 min was performed for a large number of parcels that contribute to near-surface vertical-vorticity maxima. The vorticity along these trajectories was decomposed into barotropic and nonbarotropic parts, where the barotropic vorticity represents the effects of the preexisting, substantially crosswise horizontal storm-relative vorticity. The nonbarotropic part represents the vorticity produced baroclinically within the storm. It was found that the imported barotropic vorticity attains a downward component near the surface, while the baroclinic vorticity points upward and dominates. This dominance of the baroclinic vorticity is independent of whether a single-moment or double-moment microphysics parameterization is used. A scaling argument is offered as explanation, predicting that the baroclinic vertical vorticity becomes increasingly dominant as downdraft strength increases.

Original languageEnglish
Pages (from-to)4929-4942
Number of pages14
JournalMonthly Weather Review
Volume143
Issue number12
DOIs
StatePublished - 2015

Keywords

  • Atm/Ocean Structure/Phenomena
  • Circulation/Dynamics
  • Forcing
  • Models and modeling
  • Numerical analysis/modeling
  • Physical meteorology and climatology
  • Supercells
  • Tornadogenesis
  • Vorticity

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