Systematic Errors of the CWB Global Forecast Model PartⅠ: Fundamental Variables and Mean Meridional Circulation

The Central Weather Bureau (CWB) of the Republic of China officially began its NWP operation on July 1, 1988. The global forecast model at CWB is a multi-level grid point model with fourth order potential enstrophy conservation scheme in the horizontal differencing. Based on the forecast data, January and July model climatology from the day-1, day-2, and day-3 forecasts are constructed. Also the simulated mean meridional circulation are investigated. The systematic errors can be identified by comparing the model climatology with the observed analyses. For the January simulation from day-1 forecast of the height field, it is found that the strength of the Aleutian low and the Siberian high are all underestimated. Day-2 and day-3 forecasts show further decrease in intensity of those two dominant systems. The temperature simulation at 850 hPa level indicates that negative mean bias exists in most of the land area in Asia, while positive bias prevails in the oceanic region. The 500 hPa height field fromday-1, day-2 and day-3 simulations all indicate that large negative biases are around southwestern China mountain ranges. In July, the Pacific high is well reproduced in those three forecasts, and the mean bias related to the land-ocean contrast is reduced. The Pacific high shrinking in its N-S range and extending further westward can be identified in the July climatology fromday-2 and day-3 forecasts. Zonal mean structures of the zonal wind and meridional wind in January and July reveal the model's ability to capture the seasonal migration of the general circulation. Day-2 forecast gives better Hadley cell simulation compared with the day-1 forecast. The systematic errors from the CWB model are compared with the errors of other operational models (e.g., ECMWF, NMC). The implications of those mean errors are also discussed.

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