Frontal interaction with mesoscale topography

A two-dimensional, hydrostatic, nearly adiabatic primitive equation model i s used to study the evolution of a front passing across topography. Frontog enesis is forced by shearing deformation associated with the nonlinear evol ution of an Eady wave. This study extends previous work by including an upp er-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state. Results for the Eady wave simulations show that the mountain retards and bl ocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advectio n ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstre am. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mount ains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surfa ce front remains coupled with the upper-level PV anomaly. The front moves c ontinuously across the mountain. The net result, regardless of mountain size and shape, is that the front re aches the base of the lee slops stronger, sooner, and with a decreased cros s-front scale compared to the "no-mountain" case. Well downstream of the mo untain, no position change of the surface front is observed.