Evolution of the Bureau of Meteorology's Global Assimilation and Prediction System. Part 1: Analysis and initialisation

The Australian Bureau of Meteorology's Global Assimilation and Prediction system (GASP) has been progressively upgraded in recent years. A major enhancement was the use of multivariate statistical interpolation (MVSI) instead of the previous univariate (UVSI) method of objective analysis. The methodological differences between MVSI and its predecessor, and the corresponding enhanced data monitoring and quality control procedures, are described. The data assimilation properties of the MVSI system are demonstrated to be superior to the UVSI by: (a) better background fields in the assimilation cycle, (b) smaller adjustments to analysed fields during prediction model initialisation, and (c) a better fit of the initialised fields to the observations. Parallel forecast trials over several periods in different seasons indicate a systematic improvement in medium-range forecast skill when using MVSI analyses.