The reanalysis is done at NCEP (formerly NMC) using T62 (209 km) global spectral model of 28 vertical levels. This is the same model used in the assimilation system, as implemented in the NCEP operational system in December 1994. The model has 5 levels in the boundary layer and about 7 levels above 100 hPa. The lowest model level is about 5 hPa from the surface, and the top level is at about 3 hPa. This vertical structure was chosen so that the boundary layer is reasonably well resolved and the stratospheric analysis at 10 hPa is not much affected by the top boundary conditions.
The details of the model dynamics and physics are described in Development Division (1988), Kanamitsu (1989), and Kanamitsu et al (1991). The model includes parameterizations of all major physical processes, i.e., convection, large scale precipitation, shallow convection, gravity wave drag, radiation with diurnal cycle and interaction with clouds, boundary layer physics, an interactive surface hydrology, and vertical and horizontal diffusion processes. A major difference with the model as described by Kanamitsu et al (1991) is the use of a simplified Arakawa-Schubert convective parameterization scheme developed by Pan and Wu (1994) based on Grell (1993). Pre-implementation experiments showed that the simplified Arakawa-Schubert scheme results in much better prediction of precipitation than the previous Kuo scheme over the continental US, as measured by equitable threat scores over North America. In addition, the precipitation patterns over the tropics are more realistic, with a smoother distribution, and less concentration over tropical orographic features.
Two other recent improvements were also implemented into the reanalysis model. The first is a better diagnostic cloud scheme (Campana et al, 1994) which has resulted in model generated outgoing long-wave radiation (OLR) in much better agreement with observations. The second is a new soil model, based on Pan and Mahrt (1987), which has also resulted in much more realistic surface temperature, and more skillful predictions of precipitation over North America in the summer. These changes to the model were systematically tested by running two months of assimilations in the summer and in the winter, and 25 forecasts from each assimilation. Some tuning of the cloudiness and cloud optical properties were performed to correct systematic temperature and cloudiness errors. The final version of the model also produced good 5-day forecast scores.
For a detailed summary, visit model_summary.