Progress report on FIM: a flow-following finite-volume icosahedral model

A flow-following finite-volume icosahedral model (FIM) over the global domain has been developed. The model is based on hybrid isentropic-sigma layers and an icosahedral geodesic grid. Equations are discretized with conservative finite-volume operators. The flow-following isentropic-sigma vertical coordinate is an improved version of the re-gridding and re-mapping scheme which has been successfully used in atmospheric and ocean models such as RUC (Rapid Update Cycle) and HYCOM (HYbrid Coordinate Ocean Model). The icosahedral geodesic grid consisting of hexagonal grid cells with 12 pentagons provides a quasi-uniform coverage of the sphere and allows hierarchical refinement of grid spacing. Tendency terms in the prognostic equations are approximated by the explicit 3rd order Adam-Bashforth time differencing scheme. Monotonicity in the transport scheme is achieved by extending the Zalesak (1979) flux-corrected transport (FCT) scheme to multiple time levels for the 3rd order Adam-Bashforth scheme. Physical parameterizations in FIM match those used operationally by the Global Forecast System (GFS) at the National Centers for Environmental Prediction (NCEP). FIM has been evaluated with the idealized cases of Williamson et. al.(1992) for two-dimensional flows on the sphere and multi-month simulations of thermally forced mid-latitude moist baroclinic wave development with topography for three-dimensional flows. Recently, FIM has started real data simulations using GFS initial conditions. Results from the idealized tests as well as real-data simulations will be shown in the conference.