Forward-backward assimilation, prediction and dynamics for the southeast Australia floods of October 1993

Devastating floods occurred over northern Victoria, southeast Australia on 3 October 1993. Prompted by the significance of the rainfall, and the very poor guidance provided by local and international forecast systems, a study has been made of (a) numerical prediction of the event, and (b) the associated large-scale and mesoscale dynamics. To identify and objectively reduce errors in initial conditions, a forward – backward assimilation system has been developed. The technique utilises backwards running of the assimilating forecast model with modified physical parametrisations, and the observational network over continental Australia, to extrapolate the data over the poorly observed ocean areas to the south and west. The new analyses show enhanced vertical structure and intensity of the (oceanic) cut-off low, and a clearly defined split jetstream over ocean areas to the southwest of the observational network. The forecasts from the new initial conditions are considerably improved. Evidence is presented that downstream development (successive trough-ridge amplification) played an important role in establishing the large-scale environment. Theoretical estimates of group and phase speeds show reasonable agreement with the observed behaviour. The upper-level flow is characterised by an amplifying trough that eventually orientates SW-NE. The resulting forward-tilted cut-off low is associated with very strong divergence (and ascent) on its eastern flank. Lagrangian trajectory diagnostics are used to illustrate this structural change and to relate the evolution of the potential vorticity and the jetstreams. At low levels, the environment is characterised by weak inertial stability and sustained conditional instability. It is postulated that the low inertial stability allows the development of large-scale divergent flows (mostly associated with convection) and the strengthening of a low level jet. This in turn maintains the moisture supply and conditional instability in the rain area, so that a positive feedback is established between the convection and the large-scale flow. The presence of these upper and lower level flow features results in a highly efficient rain system.