Stratospheric Overfolds and Underfolds Associated with Midlatitude Cyclones as Seen in START-08 Aircraft Flight Data and UWNMS Simulations

The NSF/NCAR Gulfstream V, High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft sampled the upper troposphere and lower stratosphere (UTLS) over North America during April – June 2008 in the Stratosphere-Troposphere Analyses of Regional Transport (START-08) campaign. Numerical simulations with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS) were carried out for each flight to investigate stratosphere – troposphere exchange and air mass pathways associated with midlatitude cyclones. By introducing each 4D flight path into the UWNMS, winds, temperature, and idealized tracers in the model may be compared with aircraft winds, temperature and an array of observed aircraft trace constituents.

Perhaps surprisingly, analysis of particle trajectories and 3D flow in the model does not support the traditional concept that stratospheric folds are caused by a “circulation around the jet”. A new conceptual model is suggested for interpreting stratospheric intrusions as being due to quasi-isentropic differential advection from upstream. An “underfold” often occurs as parcels enter a mid-latitude trough, and is caused by the upward decrease in meridional parcel excursion above the wave amplitude maximum in the upper troposphere. Differential advection in the vertical also explains how the subpolar and subtropical jets can become superimposed. If there is deep convection in the warm-upglide sector to the east of a trough, an “overfold” is likely to occur. This is caused by a poleward surge of inertially unstable upper tropospheric air over stratospheric air which is travelling quasi-isentropically from the west. Tracer-tracer correlation plots for specific flight segments highlight regions of stratospheric, tropospheric, and mixed air, which support our conceptual model. Stratospheric folds are a direct consequence of nearly-isentropic sloping buoyancy, which is also the primary cause of the baroclinic wave and of the jet stream itself.

Figure Caption. Schematic diagrams of differential advection in the UTLS as a primary agent in creating tropopause folds. The effects of sloping convection (baroclinic energy conversion) on folds can be accelerated by inertial instability.