The Fim-ihycom Model In Subx: Evaluation of Subseasonal Errors and Variability

Abstract

NOAA/ESRL/GSD has produced both real-time and retrospective forecasts for the Subseasonal Experiment (SubX) using the FIM-iHYCOM model. FIM-iHYCOM couples the atmospheric Flow-following finite volume Icosahedral Model (FIM) to an icosahedral-grid version of the Hybrid Coordinate Ocean Model (HYCOM). This coupled model is unique in terms of its grid structure: in the horizontal, the icosahedral meshes are perfectly matched for FIM and iHYCOM, eliminating the need for a flux interpolator; in the vertical, both models use adaptive arbitrary Lagrangian-Eulerian hybrid coordinates. For SubX, FIM-iHYCOM initializes four time-lagged ensemble members around each Wednesday, which are integrated forward to provide 32-day forecasts. While it has already been shown that this model has similar predictive skill as NOAA’s operational CFSv2 in terms of the RMM index, FIM-iHYCOM is still fairly new and thus its overall performance needs to be thoroughly evaluated. To that end, this study examines model errors as a function of forecast lead week (1-4) – i.e., model drift – for key variables including 2-m temperature, precipitation, and SST. Errors are evaluated against two reanalysis products: CFSR, from which FIM-iHYCOM initial conditions are derived, and the quasi-independent ERA-Interim. The week 4 error magnitudes are similar between FIM-iHYCOM and CFSv2, albeit with different spatial distributions. Also, intraseasonal variability as simulated in these two models will be compared with reanalyses. The impact of hindcast frequency (4 times per week, once per week, or once per day) on the model climatology is also examined to determine the implications for systematic error correction in FIM-iHYCOM.