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Matt Hitchman

Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison

On the Similarity of Upper Troposheric / Lower Stratospheric Potential Vorticity Dipoles Above Tropical and Midlatitude Deep Convection

Room 811 AOSS, February 20, 2017, 3:30 PM

Abstract

Simulations of the effects of deep convection on the structure of potential vorticity (PV) in the upper troposphere and lower stratosphere (UTLS) have shown that a common signature in the presence of ambient horizontal vorticity is a horizontal PV dipole (e.g., Chagnon and Gray 2009). Idealized "balanced vortex" studies of PV dipoles in the fluid dynamics community have explored gravity wave radiation in the adjustment process. Here the relationship between convection and PV structures in the UTLS in tropical cyclone Talas and the extratropical "Super Tuesday" cyclone is investigated with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). Dipoles of potential temperature in the UTLS are interpreted as an upward deflection of the ambient flow over the updraft (cold), followed by subsidence in its lee (warm), aligned with the wind direction. PV dipoles larger than +/-20 PVU are identified, with typical vertical and horizontal extents of ~ 3 km and ~ 200 km, and lifetimes up to 12 hours. Confirming Chagnon and Gray, it is found that horizontal PV dipoles are related to vortex tilting, where horizontally-oriented vorticity associated with vertical shear of the ambient wind is bent into a horseshoe shape by the updraft, yielding a PV dipole. This suggests that theta dipoles are perpendicular to PV dipoles and that "low PV lies to the left of the wind shear", or in the case of tropical cyclones, "low PV lies radially outward". Mesoscale jets occur between the dipoles, which oppose the ambient anticyclonic flow. During the extratropical transition of Talas, convective PV anomalies evolved under synoptic-scale deformation into a pair of PV streamers, which modified the midlatitude westerly jet.



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