A potential vorticity diagnosis of the relationship between upper-level troughs and the severe convective regime

Potential vorticity diagnostics are applied to determine how changes in the structure and amplitude of upper-tropospheric mobile troughs affect conditions relevant to severe convective development. First, suitable techniques are devised for: (1) modifying the PV distribution associated with the troughs; (2) retrieving thermodynamic soundings and hodographs from the balanced flow; (3) computing convective parameters from the balanced flow distributions and (4) interpreting balanced flow and convective parameter changes attributable to idealized upper-level PV modifications in the presence of background PV gradients.; Perturbations are introduced to the upper-level PV distribution associated with two significant tornado outbreaks, and the impact on the convective parameter space explored. It is found that CAPE is increased (reduced) when upper-level troughs are amplified (weakened). The impact of upper-level PV changes on the shear parameters is more complex, but the storm-relative helicity is reduced (increased) downshear relative to the large-scale vertical shear vector of an amplified (weakened) upper-level trough. On the basis of these results, a conceptual model linking upper-level PV modifications to the SCR is presented.