Deformation Frontogenesis: Observation and Theory

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  • 1 Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado, Boulder, Colorado
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Abstract

A low-level deformation frontogenesis event that occurred during the Stormscale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST) field program is analyzed in the context of semigeostrophic theory. The observed evolution and vertical structure of the potential temperature and alongfront wind fields are compared to that predicted by both numerical and analytical solutions of the semigeostrophic equations initialized at the onset of the deformation frontogenesis. The model solutions provide relatively accurate predictions of the surface potential temperature distribution 5 h later, when the frontogenesis ended. The point along the front with the steepest potential temperature gradient is observed to move closer to the point with the highest relative vorticity by an amount that is in rough agreement with the model prediction. Vertical profiles of potential temperature from soundings show a nearly mixed layer below ∼400 m that cannot be predicted by the inviscid solutions, but there is good agreement with inviscid theory above this level. The observed profiles of alongfront wind are characterized by a low-level jet with maximum speed at the level of the inversion, and the vertical shear below the jet maximum is opposite that predicted by the thermal wind equation. The semigeostrophic model does appear to depict this frontogenesis event in the upper layer, while the lower layer is dominated by surface drag and shear-induced turbulent mixing.

Abstract

A low-level deformation frontogenesis event that occurred during the Stormscale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST) field program is analyzed in the context of semigeostrophic theory. The observed evolution and vertical structure of the potential temperature and alongfront wind fields are compared to that predicted by both numerical and analytical solutions of the semigeostrophic equations initialized at the onset of the deformation frontogenesis. The model solutions provide relatively accurate predictions of the surface potential temperature distribution 5 h later, when the frontogenesis ended. The point along the front with the steepest potential temperature gradient is observed to move closer to the point with the highest relative vorticity by an amount that is in rough agreement with the model prediction. Vertical profiles of potential temperature from soundings show a nearly mixed layer below ∼400 m that cannot be predicted by the inviscid solutions, but there is good agreement with inviscid theory above this level. The observed profiles of alongfront wind are characterized by a low-level jet with maximum speed at the level of the inversion, and the vertical shear below the jet maximum is opposite that predicted by the thermal wind equation. The semigeostrophic model does appear to depict this frontogenesis event in the upper layer, while the lower layer is dominated by surface drag and shear-induced turbulent mixing.

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