Mesoscale Rainfall Forecasts over New Zealand during SALPEX96: Characterization and Sensitivity Studies

Abstract

Rainfall diagnostics from 48-h, 20-km mesoscale runs of the RAMS model configured for the New Zealand region have been characterized and compared to forecasts from the U.K. Meteorological Office global model with a view to operational use. The accuracy and precision of these diagnostics and their sensitivity to various model parameters have been determined by conducting several parallel series of experiments for the month-long SALPEX96 observing period (October?November 1996) and by comparing model results with rain gauge data. A detailed validation reveals that the mesoscale configuration of RAMS adds significant value to rainfall forecasts from the global model in situations of heavy orographic rain, particularly when the full RAMS microphysics scheme is used. The higher spatial resolution of the mesoscale model allows a better representation of the steep New Zealand orography and the observed sharp rainfall gradients. The mesoscale model and the global model both overforecast light rain and perform more poorly for light rain than for moderate or heavy rain. In the sensitivity study it is found that snow, graupel, and aggregates provide important enhancement mechanisms for rainfall in the Southern Alps, and modeling processes related to these hydrometeor species improves forecasts in the lee of the Southern Alps (the ?spillover? effect). It is also found that the soil moisture initialization strongly affects forecasts of light rain in our study, and that increasing the size of the mesoscale model domain does not always improve rainfall forecasts in the data sparse New Zealand region. The implications of these findings for future data assimilation work are also discussed.