Inertia-Gravity Wave Generation Near Upper Jets in the Australian Region

During rapid extratropical cyclogenesis in the Australian region, distinctive striated, triangular-shaped clouds commonly form on the poleward side of the jet exit near the axis of inflection between an upstream trough and downstream ridge. These clouds are called striated deltas and the striations are shown here to be caused by radiating inertia-gravity waves. An analysis of 28 striated delta clouds shows that the striated deltas have a mean length of 1034 km and width of 537 km, and a striation wavelength of 74 km. Large parcel accelerations, nonlinear flow imbalance and active convection within the striated delta are features of the composite upper-tropospheric environment. Patterns of Q-vector illustrates the unique shape of striated delta clouds to be coincident with a delta-shaped forcing of adiabatic ascent in the poleward jet exit. One of the extratropical cyclones analysed and its associated striated delta is simulated with WRF-ARW both with and without diabatic heating. Both simulations produce pronounced gravity wave packets along the surface cold front, along the downstream upper jet axis and a delta-shaped packet in the lower stratosphere above the jet exit. Ray tracing identifies the source region of the waves in the stratosphere to be the upper jet. Vertically-radiating gravity waves originating in the vicinity of the jet during rapid extratropical cyclogenesis, propagate both upwards and downwards, imprinting striations onto the cloud in the jet exit and setting the spacing between the convective bands.