Low Frequency Predictability of Dynamical Extended Range Forecast Experiments

摘要

The tropical intraseasonal oscillation is examined using projections of 5-dayaveraged wind and height data onto the normal modes of a primitive equation model linearized about a basic state at rest and prescribed global temperature. The data set utilized in thesis is from the Dynamical Extended Range Forecast (DERF) Experiment, conducted from January 1986 to March 1987. The examination of zonal wind anomalies in the tropics shows that the forecast model is unable to accurately predict either the amplitude or propagation of the 30-50 day oscillation, even when it is well-defined in the analysis. The forecasted amplitude is always much weaker than what the analysis data indicate. While errors are found in the tropical divergent wind field, the current study does not permit conclusions to be drawn regarding the influences of these errors, if any, on the extratropics. On the other hand, error patterns in the rotational wind fields of the extratropics, especially the Northern Hemisphere, are found to propagate into tropical regions. This best seen in the internal Rossby modes. Extratropical error patterns in the external Rossby modes, which are significantly larger in magnitude, are found to be mostly stationary. However, since these errors extend into tropical regions, the rotational error fields in the tropics closely resemble their extratropical counterparts. (Author) (jd)