On the Relationship between Inertial Instability, Poleward Momentum Surges and Jet Intensifications near Midlatitude Cyclones

This study explores the role of inertial instability in poleward momentum surges and ‘‘flare ups’’ of the subpolar jet near midlatitude cyclones. Two cases are simulated with the University of Wisconsin Non-hydrostatic Modeling System to investigate the mechanisms involved in jet accelerations downstream of quasi-stationary ‘‘digging troughs.’’ Deep convection along the cold front leads to regions of inertial instability in the upper troposphere, which are intimately linked to jet accelerations. Terms in the zonal and meridional wind equations following the motion are evaluated for a selected air parcel within the inertially unstable region. A two-stage synoptic evolution is diagnosed, which is a characteristic signature of inertial instability. First, meridional flow accelerates following the motion, because of the subgeostrophic zonal flow and strong northward pressure gradient force (a statement of inertial instability). Second, supergeostrophic poleward flow leads to zonal acceleration and a jet flare-up. Inertial instability thus effectively displaces a westerly jet maximum poleward relative to inertially stable conditions. The structure of the poleward surge involves a distinctive ‘‘head’’ of high angular momentum, with the region of inertial instability enclosing the jet maximum and a core of strongly negative potential vorticity inside the surge. Departures from angular momentum–conserving profiles during meridional displacement are interpreted in terms of the pressure gradient force and degree of inertial stability. Inertial instability reduces the resulting zonal wind profilerelative to angular momentum conservation but provides a significant poleward displacement of the resulting zonal wind maximum.