Potential Vorticity Diagnosis of the Severe Convective Regime. Part III: The Hesston Tornado Outbreak

Abstract

Nonlinear balance potential vorticity (PV) inversion is used to diagnose the sensitivity of the severe convective parameter space to the amplitude of a subsynoptic-scale PV anomaly on 13 March 1990, a day on which a significant tornado outbreak impacted the Great Plains. PV surgery is used to both amplify and remove the PV anomaly, and the contemporaneous impact on various convective parameters is subsequently quantified by using piecewise PV inversion to compute the changes in those parameters attributable to each PV alteration. It is found that amplifying the anomaly increases the CAPE by amounts typically ranging from 20% to 30% within the atmospheric columns experiencing the maximum PV increase. Ascent is increased slightly downshear of the PV anomaly, consistent with extant conceptual models governing synoptic-scale forcing for vertical motion. Amplifying the PV anomaly increases deep-layer shear over the southern half of the outbreak region and reduces storm-relative helicity over the northern half, primarily through changes in the estimated storm motion vector. Removing the anomaly produces complementary changes of the opposite sign. Thresholds of several commonly used convective parameters are chosen on the basis of prior empirical studies, and the horizontal displacement of these threshold contours produced by the PV alterations reveals that relatively modest subsynoptic-scale PV changes would not likely change the predominant convective mode during the Hesston outbreak.