Frontogenesis over a Mountain Ridge

Abstract

The interaction Of developing two-dimensional cold and warm frontal systems with a mesoscale mountain ridge is examined. The flow of the rotating model atmosphere is assumed to be inviscid, adiabatic, and Boussinesq. The geostrophic momentum approximation is made. An imposed horizontal deformation field forces the frontogenesis. The nonlinear model equations are solved numerically in physical space using terrain-following Coordinates which incorporate the fully nonlinear, lower boundary condition. Comparison of the model results with and without topography enables assessment of the impact of the mountain on the frontogenesis. A scale analysis indicates that the most general problem of frontal interaction with an infinite mountain ridge encompasses a seven-dimensional parameter space. The scale analysis provides justification for die two-dimensional geostrophic momentum approximation and defines an inverse Richardson number as a measure of the importance of the ageostrophic advection. Sensitivity of the model to variations in the orographic and frontal Richardson numbers and the ratio of the orographic and frontal length scales is examined. Flow over the mountain ridge results in retardation of the surface cold front on the upstream side, while rapid advection of the front across the mountain top yields an advancement of the frontal position downstream. The combination of acceleration and deceleration produces a net 100 km advancement of the front far downstream compared with the front-only case. The front is significantly weakened on the upslope side, but reappears stronger in the lee. Aloft, the upper-level front advances a similar distance in the presence of the mountain and undergoes a slight weakening. The general character of the interaction is relatively independent of the initial frontal strength. Superposition of a warm front with the orographic disturbance results in strengthening on the upstream side and weakening in the lee. The retardation and advancement are similar to those of the cold front.