An empirical-dynamical extended range forecast model incorporating tropical diabatic forcing

Results of an empirical-dynamical extended range forecast model (EDM) with skill comparable to a state-of-the-art GCM are presented. Notably, it includes tropical diabatic heating as an evolving model variable rather than as an externally specified forcing, and also includes, in effect, the feedback of the extratropical weather systems on the more slowly varying circulation. Both of these features are shown to be important contributors to the model's realism.

At Week 2, the EDM's skill is competitive with that of the nonlinear medium range forecast (MRF) model in use at the National Centers for Environmental Prediction (NCEP). The EDM's skill for Week 3 and beyond appears to be superior to the MRF model. Realtime forecasts for the past winter and summer (available at http://www.cdc.noaa.gov/~crw/lim/) were similarly successful. Notably, EDM forecasts are more accurate over the Tropics and North Pacific, particularly during periods of active tropical heating anomalies. Importantly, this encouraging model performance is not limited to years of El Nino or La Nina episodes in the eastern tropical Pacific.

Examination of the dominant growing singular vectors of the EDM further confirms the importance of tropical heating anomalies associated with El Nino/La Nina and Madden-Julian Oscillation (MJO) episodes in the predictable dynamics of the extratropical circulation. The analysis suggests that without inclusion of tropical heating, weekly averages during winter (summer) may be predictable only about two (one) week(s) in the extratropics, but with tropical heating included, they may be predictable as much as seven (five) weeks ahead. Thus, an operational forecast model, with O(10^6) degrees of freedom, can be outperformed by the EDM, with 37 degrees of freedom, which takes about a second of computer time to run.