Frontogenesis and the development of secondary wave cyclones in FASTEX

Two major mechanisms of frontogenesis-deformation and shear-are important i n frontal wave cyclone development. Horizontal deformation can suppress the nonlinear wave development. Using an analytic model, Bishop and Thorpe hav e shown that large strain rates inhibit any wave-slope amplification. For r eal cases, this ambient strain can be measured using the vorticity-divergen ce attribution method developed by Bishop. This technique permits us to con firm the crucial role of such strain on the evolution of cases of wave deve lopment during the Fronts and Atlantic Storm Track EXperiment (FASTEX). Horizontal shear in the presence of an along-front thermal gradient is also an important mechanism of frontogenesis. Using an Eady model, Joly and Tho rpe have shown that, in cases of large along-front thermal gradient, fronta l waves have growth rates smaller than the front itself, and thus would not develop. The domain-independent attribution method developed by Bishop is here extended to a geopotential-field partition. This leads, via a nonlinea r balance condition, to the estimation of the ambient along-front potential -temperature gradient. The role of such an along-front potential-temperatur e gradient is discussed, as well as the relative contributions of the two f rontogenesis mechanisms for the FASTEX cases.