Curious, triangular-shaped cloud formations called ‘striated deltas’ often accompany rapid extratropical cyclogenesis and feature transverse banding, visible in satellite imagery. A study of 28 striated delta events in the Australian region between May and September 2009 examined the environment in which striated deltas form and investigated the cause of cloud banding.

Composite analysis of all 28 events confirmed that striated deltas typically develop in the poleward jet exit region, near the axis of inflection between an upstream trough and downstream ridge. Further, high resolution numerical simulation in WRF of an individual Tasman Sea rapid extratropical cyclogenesis case study from September 2009 revealed, amongst other notable gravity wave packets, the existence of a familiar delta-shaped gravity wave packet in the lower stratosphere above the jet exit. Ray tracing calculations suggested that vertically-radiating gravity waves originating in the jet may imprint their properties onto cloud in the jet exit and modulate observed striation and convective spacing characteristics within striated deltas.

The gravity wave source is likely associated with regions of flow imbalance within the jet stream during rapid extratropical cyclogenesis, although complete explanation of the dynamical relationship between large-amplitude gravity wave generation within the jet stream, rapid extratropical cyclogenesis and striated delta clouds is expected to be more complicated.