Cold Fronts over Southeastern Australia: Their Representation in an Operational Numerical Weather Prediction Model

Abstract

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 weather prediction model. In each case it is shown that the simulations produce a well-defined frontal trough at the current operational resolution of 150 km, though in all cases the simulated movement lagged that in 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 frontal structure. Increasing model resolution to 50 km produces an improved frontal structure but does not significantly alter the simulation of frontal position. Various diagnostics including vertical cross sections, isentropic relative flow fields and near-surface fields of <<zeta>> , <<light vert. bar>> <<upside down delta>> <<theta>> <<light vert. bar>> , vertical velocity, horizontal convergence, Q vectors, and the frontogenesis function are presented for the simulated fronts. Consistent structural relationships are shown to exist between these fields. The front is seen as part of a larger-scale trough extending through the depth of the troposphere, and its location and movement occur in association with significant quasigeostophic forcing. The line of maximum cyclonic <<zeta>> corresponds most closely to the surface wind shift line, and this feature represents the most unambiguous means of defining the front from the model fields. In situations where the manual analyses gave the front a double structure including a prefrontal trough, 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 equivalent east-west sections. Isentropic relative flow diagnostics reveal close agreement with the equivalent diagnostics in the Australian Cold Fronts Research Programme.