APPENDIX B

Details of Seasonal and Subseasonal Forecast Systems

EXAMPLES OF MODELS USED BY OPERATIONAL CENTERS FOR SUBSEASONAL AND SEASONAL FORECASTING

At the National Centers for Environmental Prediction (NCEP), the Climate Forecasting System version 2 (CFSv2) is currently used for both the subseasonal and seasonal (S2S) predictions. CFSv2 is a fully coupled model representing the interaction between the Earth’s atmosphere, oceans, land, and sea ice (Saha et al., 2014). It became operational at NCEP in March 2011. The atmospheric model has a horizontal resolution of T126 (about 100 km) and 64 vertical levels. The ocean component is the Modular Ocean Model developed by the Geophysical Fluid Dynamics Laboratory (GFDL) with 0.5 to 0.25 degree resolution and 40 vertical levels, and with an interactive sea ice model. For the subseasonal forecast (defined by NCEP as days 0-45), 16 members are run every day (4 members run four times a day at 00Z, 06Z, 12Z, and 18Z). The retrospective forecasts are done from 1999 to 2010, four members a day. For the seasonal forecast (defined by NCEP as months 0-9), four runs per day are performed. The retrospective forecasts are constructed with four members run every fifth day for the past 29 years (1982-2010).

The European Centre for Medium-Range Weather Forecasting (ECMWF) utilizes two different systems for the S2S predictions. The operational seasonal forecasting system, known as System 4, was implemented in 2011. The atmospheric model is the ECMWF Integrated Forecast System (IFS) model frozen version 36r4. It has a horizontal resolution of TL255 (~60km) and 91 vertical levels. The ocean component is from the Nucleus for European Modeling of the Ocean (NEMO), with the ORCA1 configuration, which has a 1x1 degree resolution in midlatitudes and enhanced meridional resolution near the equator. The retrospective forecast is done from 1981 to 2010 for 15 members for 7 months initialized with ECMWF Interim Reanalysis (ERA Interim) on the first day of each month. The seasonal forecasts consist of a 51-member ensemble. The ensemble is constructed by combining the 5-member ensemble ocean analysis with sea surface temperature (SST) perturbations and the activation of stochastic physics. The forecasts have an initial date of the 1st of each month, and run for 7 months. For

the subseasonal prediction, ECMWF’s monthly forecasting system is used. The atmospheric model is the same version as ECMWF’s deterministic forecast. The atmospheric model is run at TL639 resolution from day 0 to day 10 and at T319 from day 10 to 32 with 62 vertical levels. The ocean component is also NEMO with the ORCA1 configuration. Fifty-one members run to 46 days twice a week (Monday and Thursday at 00Z). The ocean and atmosphere models are fully coupled, and the retrospective forecasts are constructed with 11 members run at the same day and month as the Thursday real-time forecast over the past 20 years.

Additional details about these and other operational seasonal forecast systems are shown in Table B.1, and Table B.2 provides similar information for subseasonal systems.

TABLE B.1 Forecast and retrospective forecast system characteristics of the 12 Global Prediction Centers (GPC) of the WMO. SOURCE: Adapted from the S2S Research Implementation Plan and http://www.wmo.int/pages/prog/wcp/wcasp/gpc/gpc.php (accessed January 27, 2016).

TABLE B.2 Forecast and retrospective forecast system characteristics for subseasonal forecasting systems from operational centers participating in the WCRP-WWRP Subseasonal to Seasonal Prediction Project. SOURCE: Adapted from s2sprediction.net and https://software.ecmwf.int/wiki/display/S2S/Models, both accessed January 27, 2016.