AbstractAbstractRecent investigations of inertial instability in the upper troposphere reveal elongated, near-synoptic-scale strips of anticyclonic geostrophic absolute vorticity (approximately 100 km × 1000 km) that occasionally appear on the equatorward side of the midlatitude jet streams. Some of these features have been linked to diabatic heating in synoptic and mesoscale phenomena; however, anecdotal evidence suggests inertial instability can also arise in environments characterized by large-scale descent. The classic example arises when northwesterly flow upstream of a large-scale trough is characterized by cold-air advection. In such instances, quasigeostrophic (QG) subsidence associated with anticyclonic geostrophic vorticity advection by the thermal wind is maximized immediately along the jet core. Jet flanks (cyclonic/anticyclonic shear sides) experience strong horizontal gradients in vertical motion, which act to tilt horizontal vorticity into the vertical. It is shown in an exemplary April 2013 case that vortex tilting by QG descent in the equatorward entrance region contributed to an anticyclonic vorticity tendency in flow which was eventually characterized by inertial instability. This unstable region existed over several days, challenging the notion that unstable conditions are inherently short lived. Linearized inertial instability theory suggests that parcels travel through the upper-level jet streak before perturbations associated with the release of inertial instability grow to a meaningful size. This indicates that the described regions of inertial instability are persistent, especially given an ongoing production mechanism.Significance StatementStrongly sheared winds can be associated with inertial instability, the release of which produces wave-like turbulence sometimes hazardous to aircraft. Since this turbulence acts to remove the instability, it is often assumed that these regions are both rare and short lived. Under certain conditions, however, the hazard to aircraft can be maintained on the equatorward side of the midlatitude westerly jet streams for several days. These turbulent conditions are somewhat unique from other aviation hazards in that they typically occur in regions of large-scale descent and, thus, clear skies (i.e., clear-air turbulence).
