Deterministic and Probabilistic Forecast Skill with an Icosahedral, Vertically Quasi-Lagrangian Coupled Model

A coupled atmosphere (FIM) and ocean (iHYCOM) modeling system FIM-iHYCOM has been developed for subseasonal to seasonal prediction, where both component models operate on a common icosahedral horizontal grid to avoid surface flux interpolation and use adaptive near-isentropic vertical coordinates. Once-per-week subseasonal hindcasts with 4 time-lagged ensemble members over an 18-year period have been carried out with this model at 60km horizontal resolution for NOAA's Subseasonal Experiment (SubX). We will describe the ability of the model to simulate mid-latitude tropospheric blocking frequency, Madden-Julian oscillation patterns, and sudden stratospheric warming events – all of which have been shown to be relevant on subseasonal timescales. The metrics used in this study indicate that both deterministic and probabilistic forecast skill at weeks 3-4 from FIM-HYCOM is comparable to that of several operational models, including NOAA’s operational Climate Forecast System version 2 and the European Centre for Medium-Range Weather Forecasting model.