Low-Potential Vorticity Air Near the Jet Stream Associated with Clear-Air Turbulence: Two Case Studies

Synoptic- and mesoscale regions of inertial instability (i.e., regions of negative absolute vorticity) commonly occur on the anticyclonic-shear side of upper-tropospheric jet streams and can last for 18 h or more, but their evolution under realistic atmospheric conditions and their potential for generating turbulence are not well-understood. Using observations from the mesosphere-stratosphere-troposphere (MST) radar in Aberystwyth, Wales, complemented by convection-resolving mesoscale simulations carried out with the Weather Research and Forecasting (WRF) model, we study two events during which air near the jet stream with negative potential and absolute vorticity passed over the UK. Both cases occurred during the aftermath of Tropical Storm Karl and in the context of a meridional jet stream with high curvature. During both events, the MST radar showed spectral broadening, indicating turbulence. The WRF simulations allow us to track regions with negative absolute vorticity from their origin to disappearance over the UK, elucidating the mechanisms responsible for generating the inertial instability as well as the dynamical processes occurring near the tropopause over Aberystwyth at the time of the radar observations. We compare the circulations associated with the inertially unstable regions in the WRF simulations to the band- and pancake-like circulations seen in previous idealised model simulations of inertial instability. Cross-sections reveal that the negative absolute vorticity regions occur in layers with much larger horizontal than vertical extent and are associated with weak layering in the cross-jet winds. These layers resemble the ``pancakes” seen in idealised model simulations, but they have the same vertical length scale as the regions of negative absolute vorticity themselves. Undulations on the boundaries of the inertially unstable regions are also associated with perturbations in the wind speed. Time series of winds, potential vorticity, and potential temperature, and hodographs generated from MST radar observations and model data at the radar location, are compared. At large scales, there is generally good agreement between the model and radar winds, and both model and radar hodographs show a general pattern of anticlockwise rotation (downward energy propagation) in the lower troposphere changing to clockwise rotation (upward energy propagation) with height, consistent with an inertia-gravity wave source at tropopause level. We pay special attention to how the circulations associated with the inertially unstable regions are related to turbulence detected by spectral broadening in the MST radar observations during the events.