The “striated delta” signature of gravity waves generated near the jet stream during rapid extratropical cyclogenesis

Distinctive, triangular-shaped “striated delta” clouds provide a novel framework for better understanding large-amplitude gravity wave generation near upper-tropospheric jets and fronts during rapid extratropical cyclogenesis, as the transverse cloud-top banding is thought to be the imprint of gravity waves. For 28 striated deltas in the Australian region between May and September 2009, cloud formations have a mean length of 1,034 km, width of 537 km, lifetime of 7 hr and striation wavelength of 74 km. Composite analysis in ECMWF operational analysis data shows that deltas typically occur within the poleward jet exit during baroclinic evolution, located near the axis of inflection between an upstream upper trough and downstream ridge. Candidate gravity wave generation mechanisms in the vicinity of the upper jet coinciding with striated delta development are examined, including large parcel accelerations, nonlinear imbalance within the broadscale flow and deep moist convection. The striated delta shape is partly explained by a triangular pattern of Q-vector convergence and quasi-geostrophic adiabatic vertical motion forcing within the poleward jet exit, although the pattern is displaced a few hundred kilometres upstream.