Tropopause Folds and Surface Frontal Collapse

A two-layer, deformation-forced, semigeostrophic frontogenesis model is presented in which the lower and upper layers represent the troposphere and stratosphere, respectively. The evolution of an upper-tropospheric front and associated tropopause fold is examined after the formation of a discontinuity at the lower boundary. Recently developed methods for extending the model beyond the time of surface frontal collapse are used to obtain a physically consistent discontinuous solution in the lower troposphere. The tropopause folds thus obtained are compared to those that evolve when the model is integrated beyond the time of collapse at the surface without regard to the unphysical nature of the solution at low levels. Differences between the two solutions in the vicinity of the tropopause appear to be minimal even when the upper- and lower-tropospheric fronts are connected by common isentropes. This suggests that upper-tropospheric frontogenesis is a process largely independent of surface-based dynamics within the framework of the model presented here. The related issue of the differences in lower-tropospheric dynamics between a one-layer rigid-lid model and the two-layer model is also discussed. It is found that subsidence of the tropopause in the two-layer model is responsible for an enhanced cross-front ageostrophic flow at low levels. This result indicates the significance of the upper boundary condition in SG simulations of lower-tropospheric frontogenesis.