Frontogenesis and the development of secondary wave cyclones in FASTEX

Two major mechanisms of frontogenesis‐deformation and shear‐are important in frontal wave cyclone development. Horizontal deformation can suppress the nonlinear wave development. Using an analytic model, Bishop and Thorpe have shown that large strain rates inhibit any wave‐slope amplification. For real cases, this ambient strain can be measured using the vorticity‐divergence attribution method developed by Bishop. This technique permits us to confirm the crucial role of such strain on the evolution of cases of wave development 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 Thorpe have shown that, in cases of large along‐front thermal gradient, frontal 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 nonlinear balance condition, to the estimation of the ambient along‐front potential‐temperature gradient. the role of such an along‐front potential‐temperature gradient is discussed. as well as the relative contributions of the two frontogenesis mechanisms for the FASTEX cases.