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 intended 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 followare based on both terrain and [[isentropepotential temperature]]s (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 isruns currentlyas beinga runmultiscale model, with a suffix number indicating the model's horizontal resolution. FIM7 operates at resolutionsa spatial resolution of 10,60 15km, FIM8 at 30 andkm, 60FIM9 kilometersat 15km and FIM9.5 Whenat being10km. runThe regularly,FIM7 theand modelsFIM8 both run twice daily,with(0z forecastsand issued12z) inwith six6-hour intervalstemporal resolution out to 14 days. (onlyThe atFIM9 theruns 30four kmtimes scale;daily, thealso otherwith resolutions6-hour only runsteps, out to 7 days). (FIM9.5 is not currently in operation.)
