Seasonal Forecast System 4

Implementation date: 1 November 2011  System 3 ceased to be operational, and System 4 became the new operational forecast system. Forecasts from System 4 will be released at 12 UTC on the 8th of each month. System 3 run up to and including February 2012 according to the schedule at the time (release date on the 15th of each month), but once it ceases to be operational its availability is not guaranteed.  Access to System 3 data is only available via MARS.

Technical description of System 4

System 4 is based on a more recent atmospheric model version (IFS model cycle 36r4), higher resolution forecasts with a higher top of the atmosphere, more members, and a larger hindcast data set (see Table below). System 4 initial perturbations are defined (as in System 3) with a combination of atmospheric singular vectors and an ensemble of ocean analyses. Atmosphere model uncertainties are simulated using the 3-time level stochastically perturbed parameterized tendency (SPPT) scheme and the stochastic back-scatter scheme (SPBS) operational in the EPS (System 3 used only a 1-time version of SPPT). System 4 uses NEMO instead of HOPE as its ocean component (with the same resolution), with initial conditions generated by the Near Real Time (NRT) NEMOVAR suite instead of HOPE/OI. In February, May, August and November, 15 of the 51 members are extended to 13 months.

Key differences between the operational System 3 and the forthcoming System 4:

  System 3 System 4

Ocean model

IFS cycle 31r1 36r4
IFS horizontal resolution TL159 TL255
IFS vertical resolution (TOA) L62 (0.5 hPa) L91 (0.01 hPa)
IFS model unc stoch schemes 1-lev SPPT 3-lev SPPT and SPBS
Fc mem run monthly for 7m 41 51
Fc mem ext to 13m (FMAN) 5 15
Re-forecast years 25 (1981-2005) 30 (1981-2010)
Re-forecast members 11 (7m) / 5 (7-13m) 15 (0-13m)

A description of the technical aspects of changing from System 3 to System 4 is provided in the Technical description of Seasonal Forecasting System 4. This document should be read before the new user_guide.pdf, which gives a reference description of the forecast system and the data archive, but does not include information on the changes from System 3 to System 4.