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 intendedbeing developed as a candidate to eventually supplant the [[Global Forecast System]], the United States's current medium-range forecast model, when operational (this was originally estimated to be some time around 2014). The model is expected to greatly improve on the GFS, which has historically performed more poorly than its rival models from other countries (particularly the [[Global Environmental Multiscale Model|GEM]] and [[Integrated Forecast System|IFS]]). 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.)
