COLD FRONTS OVER SOUTHEASTERN AUSTRALIA - THEIR REPRESENTATION IN AN OPERATIONAL NUMERICAL WEATHER PREDICTION MODEL

The aim of this paper is to assess the ability of a numerical weather prediction model to simulate cold fronts over southeastern Australia. A total of nine summertime fronts is studied with the research version of the Australian Bureau of Meteorology's operational numerical weath er prediction model. In each case it is shown that the simulations pro duce a well-defined frontal trough at the current operational resoluti on of 150 km, though in all cases the simulated movement lagged that i n the atmosphere. Model statistics such as skill scores and rms errors have a large degree of spatial organization and tend to be associated with errors in frontal speed more than with poor representation of fr ontal structure. Increasing model resolution to 50 km produces an impr oved frontal structure but does not significantly alter the simulation of frontal position. Various diagnostics including vertical cross sec tions, isentropic relative flow fields and near-surface fields of zeta , \del theta\, vertical velocity, horizontal convergence, Q vectors, a nd the frontogenesis function are presented for the simulated fronts. Consistent structural relationships are shown to exist between these f ields. The front is seen as part of a larger-scale trough extending th rough the depth of the troposphere, and its location and movement occu r in association with significant quasigeostrophic forcing. The line o f maximum cyclonic zeta corresponds most closely to the surface wind s hift line, and this feature represents the most unambiguous means of d efining the front from the model fields. In situations where the manua l analyses gave the front a double structure including a prefrontal tr ough, the numerical analysis-prognosis system combined these into one sharp trough. Cross sections normal to the frontal surface reveal much deeper cold air and a stronger and deeper warm-air jet than the equiv alent east-west sections. Isentropic relative flow diagnostics reveal close agreement with the equivalent diagnostics in the Australian Cold Fronts Research Programme.