SECONDARY CYCLOGENESIS - THE INITIATION PHASE OF A FRONTAL WAVE OBSERVED OVER THE EASTERN ATLANTIC

The initiation phase of a secondary frontal wave in the wake of a prim ary cyclogenesis event over the Atlantic is documented. North of the t railing surface cold front and south-west of the primary cyclone, an i ntense upper-level jet generates a direct and transverse ageostrophic circulation in its entrance region. Evidence is shown for two conflict ing roles of this circulation: first, stabilization of the front throu gh frontogenetic cross-frontal low-level convergence; and second, an i ncrease of the potential for instability of the front through diabatic (e.g. latent-heat release) potential-vorticity (PV) redistribution in the upward branch of the circulation. Following saturation of the pri mary cyclogenesis and the associated along-front north-eastwards shift of the upper-level jet, the low-level branch of the ageostrophic circ ulation behaves like a zipper closing on the front, while in the upwar d branch an anomalous strip of PV builds along the cold front. Isentro pic PV maps show that parcels of stratospheric origin embedded in the downward branch of the ageostrophic circulation (a tropopause fold) ma y have ended in the vicinity of the strip of anomalous low-level PV, p erhaps contributing to an increase in the potential for instability of the surface front. Using the domain-independent attribution method of Bishop, it is shown that the frontal wave forms on a part of the cold front where: first, the along-front stretching rate due to the enviro nmental flow decays below the critical threshold value (0.6 x 10(-5) s (-1)) prescribed by theoretical studies; and second, the across-front frontogenetic component due to the ageostrophic circulation decays. Th is is consistent with semi-geostrophic theory, where the cross-frontal convergence and the stretching deformation are linked through the Saw yer-Eliassen equation. Results add to other recent observational findi ngs: that the potential for instability, the evolving environmental fl ow and the induced ageostrophic convergence each have a crucial role i n differentiating between frontal waves that will grow and those that will be suppressed.