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

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). “Balanced vortex” studies of PV dipoles in fluid dynamics 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 of 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, creating a new kind of steady-state configuration.

During the extratropical transition of Talas, convective PV anomalies evolved under synoptic-scale anticyclonic deformation into a pair of PV streamers, which modified the midlatitude westerly jet. The formation of a pair of PV streamers during the Super Tuesday storm in cyclonic deformation also formed a pair of PV streamers which accentuated the jet.

In a slight departure from Chagnon and Gray, who studied PV production within individual convective towers, it is suggested that the diabatic processes which are responsible for the synoptic-scale ambient vertical wind shear allow for vertical tilting by local updrafts, which impinge upon the base of the stratosphere, creating large PV dipoles.