Many computer forecast models divide the atmosphere into a series of vertically layered grid cells. Some numerical models use other techniques to divide the atmosphere, but this approach is the easiest to describe conceptually. The distance between the centers of each of these grid cells is called the grid spacing. The smaller the grid spacing, the better the resolution of the model, and the more detailed the model terrain. Higher resolution models, however, also require more calculations (and hence computer power), and this is what ultimately places a limit on model resolution.
|Source: NOAA/Wikipedia Commons|
NCEP also runs two ensemble modeling systems, the Short-Range Ensemble Forecast (SREF) system and the Global Ensemble Forecast Systems (GEFS). An ensemble modeling system features several forecasts produced by models with somewhat different formulations (e.g., different ways of modeling precipitation processes) or initial analyses. Because several forecasts are produced, ensemble modeling systems are run with larger grid spacings. The idea, however, is to use the spread in the forecasts to better evaluate forecast uncertainty and the range of possibilities during the forecast period. Spaghetti diagrams are often used to examine the diversity of forecasts produced by the ensemble.
|"Spaghetti" plots of 10C 700-mb temperature analyses (top) and |
48-h forecasts (bottom ) from the NCEP SREF.
Products from these modeling systems are available at http://www.ral.ucar.edu/weather, http://www.spc.noaa.gov/exper, http://www.meteo.psu.edu/~gadomski/ewall.html, and http://weather.utah.edu. In the case of the latter, we are only providing the NAM and GFS, but hope to do more in the future.
The modeling systems at NCEP undergo frequent updates and, unfortunately, I do not know of an easily accessible web site that clearly describes the configuration of each of the NCEP modeling systems. Thus, the information above may get stale quickly and it is based on the best information that I could locate. If anyone knows of a single site that clearly describes the configuration of the NCEP models, let me know.
Update: 16 Oct 2011
One of our readers points out that a great site for updated information on the NCEP models is http://www.meted.ucar.edu/nwp/pcu2/. Access requires registration, but it's free.