A New View of Stratospheric Folds and Inertial Instability Near Midlatitude Cyclones

The classical view of stratosphere troposphere exchange (STE) near midlatitude cyclones is grounded in high resolution observations of thin sheets of stratospheric air in the troposphere – one would reasonably conclude that they are “stratospheric intrusions”. We suggest an alternative, kinematic point of view, that on the upstream side of a ridge, stratospheric intrusions commonly result from overriding tropical upper tropospheric air. Case studies using the University of Wisconsin Nonhydrostatic Modeling System (UWNMS) of developing midlatitude cyclones in the upper Midwest show that convection along a cold front between the subtropical and subpolar jets creates inertially unstable air and poleward surges at the level of the subpolar jet. Vertically confined poleward surges overrides stratospheric air, creating the classical signature of a stratospheric intrusion. Poleward surges of air can aid true intrusion due to the induced peripheral circulation, similar to that around a rising warm plume. Lagrangian particle trajectories in midlatitude cyclone case studies with the Weather Research Forecast model support the following picture: air in the “intrusion” moves roughly eastward at constant height, while air from near the subtropical jet moves northeastward over the intrusion in the upper troposphere and lower stratosphere (UTLS). This simple observationally-based view of differential advection avoids the problem of the Sawyer-Eliassen solutions, which disagree with modeled and observed meridional circulations near the jet. Instead of “trying to maintain a balanced state”, the atmosphere in the UTLS near a developing cyclone is distinctly out of thermal wind balance, often accelerates due to inertial instability, and actively converts potential to kinetic energy. Density variations in the meridional plane are consistent with a waterfall, where mixed stratospheric/tropospheric air can subside under tropospheric air. The three dimensionality of Rossby wave breaking on the base of stratosphere is shown to be like a left-breaking ocean wave on the beach (in the NH).