2:00 PM - 2:15 PM
[AAS02-02] On the Formation of Tropopause Folds and Constituent Gradient Enhancement near Westerly Jets
★Invited Papers
Keywords:Tropopause Folds, Subpolar Jet, Extratropical Cyclones, Tracer Transport, Aircraft Observations , Vertical Motions
The role of quasi-isentropic differential advection in creating tropopause folds and strong constituent gradients near midlatitude westerly jets is investigated using the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). Dynamical structures are compared with aircraft observations through a fold and subpolar jet (SPJ) during research flight 4 (RF04) of the Stratosphere–Troposphere Analyses of Regional Transport (START08) campaign. The observed distribution of water vapor and ozone during RF04 provides evidence of rapid transport in the SPJ, enhancing constituent gradients above relative to below the intrusion. The creation of a tropopause fold by quasi-isentropic differential advection on the upstream side of the trough is described. This fold was created by a southward jet streak in the SPJ, where upper tropospheric air displaced the tropopause eastward in the 6–10 km layer, thereby overlying stratospheric air in the 3–6 km layer. The subsequent superposition of the subtropical and subpolar jets is also shown to result from quasi-isentropic differential advection.
The occurrence of low values of ozone, water vapor, and potential vorticity on the equatorward side of the SPJ can be explained by convective transport of low-ozone air from the boundary layer, dehydration in the updraft, and detrainment of inertially unstable air in the outflow layer. An example of rapid juxtaposition with stratospheric air in the jet core is shown for RF01. The net effect of upstream convective events is suggested as a fundamental cause of the strong constituent gradients observed in midlatitude jets, with the aggregate divergence aloft causing upper-tropospheric air to flow over stratospheric air. Idealized diagrams illustrate the role of differential advection in creating tropopause folds and constituent gradient enhancement. The primacy of buoyancy as a causal agent for vertical motion in midlatitude cyclones is emphasized.
The occurrence of low values of ozone, water vapor, and potential vorticity on the equatorward side of the SPJ can be explained by convective transport of low-ozone air from the boundary layer, dehydration in the updraft, and detrainment of inertially unstable air in the outflow layer. An example of rapid juxtaposition with stratospheric air in the jet core is shown for RF01. The net effect of upstream convective events is suggested as a fundamental cause of the strong constituent gradients observed in midlatitude jets, with the aggregate divergence aloft causing upper-tropospheric air to flow over stratospheric air. Idealized diagrams illustrate the role of differential advection in creating tropopause folds and constituent gradient enhancement. The primacy of buoyancy as a causal agent for vertical motion in midlatitude cyclones is emphasized.