Quantify the Coupled GEFS Forecast Uncertainty for the Weather and Subseasonal Prediction

The Global Ensemble Forecast System version 12 (GEFSv12) has been implemented into National Centers For Environmental Prediction operations since September 2020, which was uncoupled, but increased the horizontal resolution from 34 to 25 km, increased ensemble members from 21 to 31, and extended forecasts from 16 to 35 days. It significantly improved probabilistic forecast skills in many categories, such as precipitation, tropical storms, Madden-Julian Oscillation (MJO), etc. The improvements resulted from many aspects, including model resolution increase, dynamical core upgrade, advances in hybrid data assimilation and physical parameterizations, and more importantly, from using new stochastic schemes to improve forecast uncertainty. To further improve GEFS's sub-seasonal forecast skill, a coupled GEFS was built up on the Unified Forecast System prototype version 5 that fully couples an atmospheric model with a land surface model, ocean model, ice model, and wave model. A set of coupled GEFS experiments were conducted to test different horizontal resolutions at approximately 50 and 25 km while adjusting the stochastic parameterization schemes for the atmosphere to better represent forecast uncertainties. The experiments were run for a 2-year period from October 2017 to September 2019, with one initialization per week at Wednesday 00 UTC, 11 ensemble members, and were forecasted out to 35 days. The coupled GEFS significantly improves 500 hPa height anomaly correlation in week-1, week-2, and MJO skills compared to the current operational forecast. The forecast spread of tropical wind is greatly reduced by improved stochastic schemes and matches well with the forecast root mean square errors. The correlation of forecast error variance and ensemble variance is improved for the coupled GEFSs. Meanwhile, the spread of MJO has been greatly reduced for the coupled GEFSs to improve the MJO forecast uncertainty. Plain Language Summary The Unified Forecast System is a new community-based, atmosphere-land-ocean-sea ice-wave-aerosol coupled, comprehensive Earth modeling system. A Global Ensemble Forecast System (GEFS; version 12; uncoupled) has been implemented into National Centers For Environmental Prediction operations since September 2020 with 31 members, out to 35 days to cover subseasonal prediction. A new fully coupled GEFS in this investigation has demonstrated an overall better performance around the weather, medium-range, and weeks 3 and 4 time scales. Forecast uncertainties are quantified by adjusting the coefficients and parameters of the stochastic schemes.