An evaluation of numerical weather prediction based rainfall forecasts

Abstract

A.Seed@bom.gov.au +61 39669 4591 (A. Seed), P.Steinle@bom.gov.au +61 39669 4848 (P. Steinle) Abstract Assessment of the forecast precipitation is required before it can be used as input to hydrological models. Using radar observations in south-eastern Australia, forecast rainfall from the Australian Community Climate Earth-System Simulator (ACCESS) is evaluated for 2010 and 2011. Radar rain intensities were first calibrated to gauge rainfall data from four research rainfall stations at hourly time steps. It was shown that the Australian ACCESS model (ACCESS-A) overestimated rainfall in low precipitation areas and underestimated elevated accumulations in high rainfall areas. The forecast errors were found to be dependent on the rainfall magnitude. Since the cumulative rainfall observations varied across the area and through the year, the Relative Error (RE) in the forecasts varied considerably with space and time, such that there was no consistent bias across the study area. Moreover, further analysis indicated that both location and magnitude errors were the main sources of forecast uncertainties on hourly accumulations, while magnitude was the dominant error on daily time scale. Consequently, the precipitation output from ACCESS-A may not be useful for direct application in hydrological modelling, and pre-processing approaches such as bias correction or exceedance probability correction will likely be necessary for application of the Numerical Weather Prediction (NWP) outputs.

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