The breakdown of balance in low potential vorticity regions: Evidence from a clear air turbulence outbreak

An unsolved question of fluid dynamics is how much gravity wave emission is to be expected from regions of low (anticyclonic) potential vorticity.

To examine this question, we investigate a case of widespread clear air turbulence (CAT) reported over the data-rich upper Midwest United States in October 1996. This outbreak lasted approximately 18 hours and consisted of dozens of reports of moderate-to-severe CAT.

Using satellite imagery and dynamical diagnostics appropriate for unbalanced, gravity wave-generating flows, low-PV dynamics are shown to be closely related to reports and visual indicators of CAT in both time and space. It also appears that dynamical interactions between large-scale cyclonic vortices contributed to the intense smaller-scale ageostrophic dynamics exhibited in the regions of CAT.

This work provides support for the hypothesis that low-PV regions can be the progenitors of vigorous geostrophic adjustment.