The Flow-following, finite-volume Icosahedral Model (FIM) is an experimental
numerical weather prediction
Numerical weather prediction (NWP) uses mathematical models of the atmosphere and oceans to weather forecasting, predict the weather based on current weather conditions. Though first attempted in the 1920s, it was not until the advent of comput ...
model that was developed at the
Earth System Research Laboratories in the United States from 2008 to 2016.
The FIM was developed as a candidate to eventually supplant the
Global Forecast System
The Global Forecast System (GFS) is a global numerical weather prediction system containing a global computer model and variational analysis run by the United States' National Weather Service (NWS).
Operation
The mathematical model is ...
, the United States's current medium-range forecast model. The FIM was originally slated to become operational some time in 2014 but never did so. The model 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
A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circu ...
), the FIM instead fits Earth to a
Goldberg polyhedron with
icosahedral symmetry
In mathematics, and especially in geometry, an object has icosahedral symmetry if it has the same symmetries as a regular icosahedron. Examples of other polyhedra with icosahedral symmetry include the regular dodecahedron (the dual polyhedr ...
, with twelve evenly spaced pentagons (including two at the poles) anchoring a grid of hexagons.
In November 2016, the ESRL announced it was no longer pursuing the FIM as a replacement for the GFS and would be instead developing the FV3, which uses some of the FIM's principles except on a square grid. The FIM will continue to be run for experimental purposes until FV3 commences. The FIM will also be upgraded with some of the same features as FV3 (retaining the icosahedral grid) and coupling to examine the use of the model in the longer ranges.
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 15 km and FIM9.5 at 10 km. Each scale runs on a temporal resolution of 6-hour steps. As of 2017, only the FIM7 (running out 10 days) and FIM8 (running out 14 days) continue to be run daily. The FIM7 also runs four runs between Tuesday and Wednesday each week as a
climate model
Numerical climate models (or climate system models) are mathematical models that can simulate the interactions of important drivers of climate. These drivers are the atmosphere, oceans, land surface and ice. Scientists use climate models to st ...
, with the mean output from those runs issued in one-week intervals; it thus complements the
Climate Forecast System, the only other model in the U.S. government's arsenal that covers that time frame.
References
External links
Official FIM Web site includes forecasts and documentation
{{Atmospheric, Oceanographic and Climate Models
Numerical climate and weather models