Balanced and unbalanced upper-level frontogenesis

The dynamics of frontogenesis at upper levels are investigated using a hierarchy of three numerical models. They are, in order of decreasing sophistication, the anelastic (AN), the geostrophic momentum (GM), and the quasi-geostrophic (QG) approximations to the full equations of motion. Intercomparison of the broad-scale structure of the upper-level jet-fronts as described by the AN and GM models shows close agreement. In contrast, the QG model exhibits marked differences, related largely to the neglect of vortex tilting in generating vertical vorticity. The GM and QG models assume cross-front thermal wind balance at all times, which is an excellent approximation when the vertical shear in the cross-front geostrophic wind is weak. The unbalanced along-front flow contributes significantly to the dynamics as described by the AN model. -from Authors