The Influence of Upper-Tropospheric Potential Vorticity on Convective Morphology

Extensive research has illuminated the roles that buoyancy and vertical wind shear have in determining convective mode. The goal of this study is to examine synoptic forcing, as measured by the form of the waves on the dynamic tropopause and the strength of the resultant temperature advection, together with the environmental variables that are currently used to differentiate storm type. Logistic regression is used to make this discrimination and provides the preliminary results. First, multicellular lines and isolated rotating cells are associated with weaker synoptic forcing. Second, where an upper-level feature is present, multicellular convection (lines and clusters combined) is favored relative to isolated storms when there is stronger synoptic forcing, the shape of the upper-level potential vorticity (PV) feature is elongated, and there is a more southerly component in the deep-layer (0–6 km) mean wind. Third, when an upper-level feature is present and the convection is multicellular, clusters are favored relative to lines when the upper wave is not cut off (i.e., a wave structure promoting advection), provided that the deep-layer shear is not oriented at an optimal angle relative to a preexisting meridional boundary (as shown by prior research). Finally, if the convection is isolated, rotating storms may be favored for a broad area of PV advection, as measured by the relative equality of the zonal and meridional axes of the advection area, suggesting the importance of broader-scale destabilization. These findings are used to formulate testable hypotheses. Future model-based experiments using PV surgery or PV regression methods are proposed to clearly elucidate the dynamics behind these relationships.