Air Masses and Airstreams

Robert Cohen
East Stroudsburg University
East Stroudsburg, PA

David Schultz
NOAA/National Severe Storms Laboratory
Norman, OK

Many scientists have recognized that the airflow through cyclones can differ between cyclones. Recently, Evans et al. (1994) and Young (1995) have proposed that the degree of confluence and diffluence of the large-scale flow, amonth other factors, can affect the nature of the flow through a cyclone, and hence, its frontal structure. To illustrate how the degree of background confluence of diffluence can affect the airstreams within cyclones, we use the idealized nondivergent barotraopic vortex models of Schultz et al. (1997, in preparation) and the airstream boundary diagnostic of Cohen ahd Kreitzberg (1997). Although an obvious limitation to this work is the absence of vertical motion in the idealized model, a wide range of evolutions can occur, despite this limitation. This also will demonstrate that shearing deformation, not convergence, dominates the formation of airstream boundaries in the real atmosphere, an assertion made by Cohen and Kreitzberg (1997).

Using the control case of a barotropic vortex in the absence of background flow (Doswell 1984), a single airstream boundary is related to the differential rotation rate of the air in the vortex. When the vortex is placed in background diffluence (DIFF in Schultz et al. 1997), the bulk of the warm conveyor belt turns cyclonically around the vortex center and wraps around the cold conveyor belt, resembling the spiral cloud pattern often observed with deeply occluded oceanic cyclones. When the vortex is placed in background confluence (CONF in Schultz et al. 1997), the bulk of the warm conveyor belt turns anticyclonically away from the vortex center, reminiscent of the linearly shaped, polar-fron cloud bands noted by Browning (1990), among others. Finally, the evolution of these idealized airstreams will be compared and contrasted to airstreams calculated from mesoscale model simulations of observed cyclones in the real atmosphere.