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A synoptic example of intense frontogenesis at 500 mb is presented and the underlying mechanism is investigated both from the thermal and dynamic standpoints. It is shown that the basic process in operation is a cross-stream gradient of sinking motion with strongest subsidence at the warm edge of the frontal zone. This condition gives rise, to an intensified horizontal temperature gradient and, because of the generally disregarded “vertical-shear” terms in the vorticity equation, leads to a marked increase in vorticity as well. The Ertel vorticity theorem is applied in order to confirm the above conclusions and also to show that in certain regions the frontal zone was of stratospheric origin, the lower boundary being initially part of the polar tropopause. Finally it is shown how the vertical-shear terms may be incorporated in the Sawyer-Bushby numerical prediction model.
Reed et al. (Thu,) studied this question.