Influence of Surface Drag on the Evolution of Fronts

Abstract

Surface frontal structure during cyclogenesis, and the sensitivity of this structure to surface friction, is examined. The approach is based on the analyses of simulations using a primitive equation model, with the domain restricted to a sector of one hemisphere, and the physics reduced to surface drag, horizontal diffusion, and dry convective adjustment. The model horizontal resolution is 1.2° latitude ? 1.5° longitude, and there are 21 layers in the vertical. The drag coefficient is varied in simulations with midlatitude jet streams as initial conditions. The extent to which simulations in the adiabatic framework or with highly simplified representations of physical processes succeed in producing features of cyclone evolution emphasized by recent observational analyses is evaluated. Shallow bent-back warm fronts develop in simulations with surface drag coefficients that are zero or representative of ocean surfaces. Horizontal advection, first in strong easterly and later in strong northerly winds, is primarily responsible for the resulting bent-back structure of the warm front. The effect of surface drag on simulated lower-tropospheric wind speeds and frontogenesis is nonuniform. Warm frontogenesis is enhanced in simulations with relatively low surface drag through a feedback process involving vorticity, deformation, convergence, and warm-air advection. Surface drag tends to inhibit warm frontogenesis by decreasing the low-level wind speed and reducing the contribution of warm advection to the feedback. Consistently, a distinct warm front does not develop in the simulation with a surface drag coefficient representative of continental surfaces. Cold frontogenesis, on the other hand, is not very sensitive to surface drag. Further simulations with doubled horizontal resolution (0.6° latitude ? 0.75° longitude), slightly higher baroclinity at lower levels in the initial conditions, and small surface drag produce bent-back fronts that spiral around the surface pressure minimum. These results suggest that there are important differences in the structure of surface fronts associated with marine and continental cyclogenesis.