The Relationship Between Fronts and Airstream Boundaries

Robert Cohen
East Stroudsburg University
East Stroudsburg, PA

David Schultz
NOAA/National Severe Storms Laboratory
Norman, OK

Using novel diagnostics, we will show that fronts and airstream boundaries are separate but related features and that the relationship is governed by the large-scale flow. In particular, fronts and airstreams are most likely to be separate in a diffluent flow pattern (when the axes of dilatation are perpendicular to the isotherms).

To identify the two separate features and demonstrate that they are, indeed, distinct from one another, we use the local temperature gradient (for fronts) and the time-integrated contraction rate (for airstream boundaries). To identify the relationship between them, we employ the frontogenesis function and a new function we call the instantaneous contraction rate (or airstream boundary genesis function). If time allows, a related function, the Lyapunov exponent, will be introduced which, when expressed in terms of the kinematics of the flow, can be used to diagnose the potential for fluid trapping.

The application of these diagnostics is illustrated using idealized nondivergent barotropic vortices and observed oceanic cyclones in diffluent and confluent background flows. The implications for conceptual models of conveyor belts and fronts in midlatitude cyclones will be discussed.