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Flow-following, finite-volume Icosahedral Model

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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. 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 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.)


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