Ensembles of winter and summer seasonal simulations have been carried out with an 80 km resolution version of the National Centers for Environmental Prediction (NCEP), Environmental Modeling Center (EMC) Eta model over the North American region. The lateral boundary conditions for the Eta model are prescribed from Center for Ocean-Land-Atmosphere (COLA) R40 atmospheric general circulation model (AGCM) integrations which used observed atmospheric initial conditions and observed global sea surface temperature (SST).
An examination of 15 seasonal winter simulations and 15 seasonal summer simulations shows that the nested model significantly improves the simulations of seasonal precipitation compared to the global model alone. The physical parameterizations, enhanced resolution and better representation of orography in the Eta model produce better simulations of precipitation and its interannual variability. In particular, the precipitation difference between the 1998 drought and 1993 flood over the U.S. was much better simulated by the nested model. The simulations of circulation features were generally as good or better than those from the global model alone.
Estimates of external (SST-forced "signal") and internal (dynamics-generated "noise") variability were made for both the global model and the nested model simulations. Contrary to the expectation that a higher resolution model would have higher internal dynamics-generated variability, the signal, noise and signal-to-noise ratios of the near surface temperature and precipitation fields were generally quite similar between the nested model and the global model simulations. In the winter season, the nested model had larger signal-to noise ratios in both temperature and precipitation than did the global model alone.
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 3 February 1999
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