The Relationship between Frontal Zones and Airstream Boundaries

1. The Relationship between Frontal Zones and Airstream Boundaries Robert A. Cohen Dept. of Physics, East Stroudsburg University, East Stroudsburg, Pennsylvania D avid M. Schultz CIMMS, University of Oklahoma, and NOAA/National Severe Storms Laboratory, Norman, Oklahoma http://www.esu.edu/~bbq rcohen@po-box.esu.edu

2. Motivation Airstream boundaries are interesting! • Interesting weather occurs along airstream boundaries: fronts, dryline, troughs, outflow boundaries. • Although kinematic frameworks exist to diagnose fronts, such frameworks for airstream boundaries don’t exist.

3. Integrated Contraction Rate r(t) = r(0) e-Ct; t < 0C = (1/t)ln [r(t)/r(0)] where C is the time-integrated contraction rate Cohen and Kreitzberg, MWR, 1997

6. Questions • What is it about the flow that influences the growth and orientation of ASBs? • What does this flow imply about predictability? • What is the relationship between ASBs and frontal zones? • Is there a framework for interpreting ASBs in extratropical cyclones?

8. Instantaneous Rates • Contraction rate along the axis of contraction -σ(ψc) = c = ½ (E–D) • Contraction rate along thermal gradient -σ(β) = ½ (Ecos2β –D)Note: 2-D frontogenesis, F = ½ |Δθ| (Ecos2β –D)[when β = 0, ASB-genesis is associated with frontogenesis] • Contraction rate along inflow axis -σ(χc) = C = ½ [(E2 – ς2)½ – D) Cohen, R. A., and D. M. Schultz, 2005: Contraction rate and its relationship to frontogenesis, the Lyapunov exponent, fluid trapping, and airstream boundaries. Mon. Wea. Rev., 133, 1353–1369.

12. Answers? • What is it about the flow that influences the growth and orientation of ASBs? • The asymptotic contraction axis is useful for determining the orientation of the ASB • Large contraction rates represent a region of ASB-genesis. • Imaginary rates reflect regions of trapped flow • What does this flow imply about predictability? • The paths of individual parcels in the ASB region are highly dependent on initial conditions but the feature itself is relatively stable • In regions characterized by this type of flow (extratropics), error growth is expected to be large but analyses can be good if features can be matched.

13. Answers? • What is the relationship between ASBs and frontal zones? • The flow associated with ASBs is such that the thermal gradient is strengthened when the ASB is parallel to the frontal zone and weakened when perpendicular; So, ASBs don’t necessarily line up with fronts; may line up with thermal ridges and troughs • “The axis of outflowis not a creator of fronts, but a collector of fronts; for though frontogenesis may occur anywhere in the field, the line of frontogenesis will move toward this axis, and the resulting front will be found at this axis, or close to it” (Petterssen 1940, p. 255, his emphasis) • Is there a framework for interpreting ASBs in extratropical cyclones? • Strong cyclones may have a “characteristic” airstream structure but various frontal signatures, which depend on the orientation of this structure relative to the thermal gradient