On the Role of Inertial Instability in Stratosphere–Troposphere Exchange near Midlatitude Cyclones*

Abstract

Abstract In simulations of midlatitude cyclones with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS), mesoscale regions with large negative absolute vorticity commonly occur in the upper troposphere and lower stratosphere (UTLS), overlying thin layers of air with stratospheric values of ozone and potential vorticity (PV). These locally enhanced stratosphere–troposphere exchange (STE) events are related to upstream convection by tracing negative equivalent potential vorticity (EPV) anomalies along back trajectories. Detailed agreement between the patterns of negative absolute vorticity, PV, and EPV—each indicators of inertial instability in the UTLS—is shown to occur in association with enhanced STE signatures. Results are presented for two midlatitude cyclones in the upper Midwest, where convection develops between the subpolar and subtropical jets. Mesoscale regions of negative EPV air originate upstream in the boundary layer. As they are transported through convection, EPV becomes increasingly negative toward the tropopause. In association with the arrival of each large negative EPV anomaly, a locally enhanced poleward surge of the subpolar jet occurs, characterized by high turbulent kinetic energy and low Richardson number. Isosurfaces of wind speed show that gravity waves emanating from inertially unstable regions connect with and modulate the subpolar and subtropical jets simultaneously. Inertially unstable convective outflow surges can facilitate STE locally by fostering poleward acceleration in the UTLS, with enhanced folding of tropospheric air over stratospheric air underneath the poleward-moving jet.