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The '''Flow-following, finite-volume Icosahedral Model''' (FIM) is a [[numerical weather prediction]] model currently under development at the [[Earth System Research Laboratory]].
The FIM is being developed as a candidate to eventually supplant the [[Global Forecast System]], the United States's current medium-range forecast model. The FIM was originally slated to become operational some time in 2014; the GFS will continue to be run and maintained for several years afterward (much in the same way the GFS and its predecessor, the [[Nested Grid Model]], ran concurrently for several years). The model currently produces similar results to the GFS, but runs slower on the NWS's operational computers. Its three-part name derives from its key features: "flow-following" indicates that its vertical coordinates are based on both terrain and [[potential temperature]] (isentropic [[sigma coordinates]], previously used in the now-discontinued [[rapid update cycle]] model), and "finite-volume" describes the method used for calculating horizontal transport. The "icosahedral" portion describes the model's most uncommon feature: whereas most grid-based forecast models have historically used rectangular grid points (a less than ideal arrangement for a planet that is a slightly [[oblate spheroid]]), the FIM instead fits Earth to a [[truncated icosahedron]], with twelve evenly spaced pentagons (including two at the poles) anchoring a grid of hexagons.
The FIM runs as a multiscale model, with a suffix number indicating the model's horizontal resolution. FIM7 operates at a spatial resolution of approximately 60 km, FIM8 at 30 km, FIM9 at 15km and FIM9.5 at 10km. The FIM7 and FIM8 both run twice daily (0z and 12z) with 6-hour temporal resolution out to 14 days. The FIM9 runs four times daily, also with 6-hour steps, out to 7 days. (FIM9.5 is not currently in operation.)
==External links==
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