Subseasonal Forecasting Skill with Advanced Coupled Models including the 30km FIM-HYCOM Model

A coupled atmosphere (FIM) and ocean (HYCOM) modeling system (FIM-HYCOM) is a relatively new global coupled model developed for subseasonal to seasonal prediction. Both component models operate on a common icosahedral horizontal grid and use an adaptive hybrid vertical coordinate (sigma-isentropic in FIM and sigma-isopycnic in HYCOM). FIM-HYCOM has been used to conduct 16 years of subseasonal retrospective forecasts (32-day forward integrations), run with 4 ensemble members once per week. Preliminary results from this multi-year hindcast indicate that the forecast skill of many metrics from FIM-HYCOM including 500 hPa heights and the Madden-Julian Oscillation (MJO) is comparable to that of the operational Climate Forecast System (CFSv2) used by the National Centers for Environmental Prediction.

Development and modification of physical parameterizations (GFS-based but with options for other cumulus and land-surface) have allowed useful retrospective experiments with FIM-HYCOM.  For instance, regional biases in cloud cover – and hence surface shortwave radiation flux – are sometimes large in both coupled (FIM-HYCOM) and uncoupled (FIM-only) simulations (also found in other coupled models). Work is ongoing using advanced approaches to subgrid-scale clouds to address this issue.  Sensitivity to shallow and deep convection schemes and their subgrid-scale treatment and to different horizontal resolutions will be presented. Our paper will also include evaluation of blocking, MJO, and sudden stratospheric warming events from these hindcast experiments against observations and model output from CFSv2.