The real-time VarEPS/monthly forecasting system is a 51-member ensemble of 32-day integrations. The first 10 days are performed with a TL399L62 resolution forced by persisted SST anomalies (updated every 24 hours). After day 10, the model is coupled to the ocean model and has a resolution of TL255L62. The extension of VarEPS to 32 days is performed every Thursday. Before January 2008, the monthly forecasting system was a separate system from EPS. The first operational real-time monthly forecast was realized on Thursday, 7 October 2004.

Atmospheric component: IFS with the same cycle as the deterministic forecast:

CY24R3 from 27/03/2002 to 24/04/2000 (included)

CY25R1 since 24/04/2002 (included)

CY25R4 since 29/01/2003 (included)

CY25R5 (IBM) from 12/03/2003 (included)

CY26R1 from 07/05/2003 (included)

CY26R3 from 08/10/2003 (included)

CY28R1 from 10/03/2004 (included)

CY28R3 since 07/10/2004 (included)

CY29R1 since 07/04/2005 (included)

CY29R2 since 30/06/2005 (included) + new sea-ice treatment*

CY30R1 since 02/02/2006 (included) + change of resolution to 62 vertical levels

CY31R1 since 14/09/2006 (included)

CY31R2 since 14/12/2006 (included)

CY32R2 since 07/06/2007 (included)

CY32R3 since 08/11/2007 (included)

CY32R3V since 13/03/2008 (included) : Switch to VAREPS/Monthly

CY33R1 since 05/06/2008 (included)

CY35R1 since 30/09/2008 (included)

CY35R2 since 12/03/2009 (included)

CY35R3 since 10/09/2009 (included)

CY36R1 since 28/01/2010 (included) + change of resolution to T639/T319

Oceanic component: HOPE (from Max Plank Institute for Meteorology, Hamburg) with a zonal resolution of 1.4 degrees and 29 vertical levels. The ocean has lower resolution in the extratropics, but higher meridional resolution in the equatorial region (0.3 degrees), in order to resolve ocean baroclinic waves and processes which are tightly trapped at the equator.

Coupling: OASIS (from CERFACS, France). The atmospheric fluxes of momentum, heat and fresh water are passed to the ocean every hour. In exchange, the ocean surface temperature (SST) is passed to the atmosphere.

2.2 The Back-statistics:

After 10 days of coupled integrations, the model drift begins to be significant. It displays similar patterns to seasonal forecasting after 6 months of integrations, but with less amplitude. The strategy for dealing with model drift is straightforward. We initialize the ocean, atmosphere and land surface to be as close to reality as possible, and calculate the forward evolution of the system as best we can using numerical approximations of the laws of physics. No "artificial" terms are introduced to try to reduce the drift of the model and no steps are taken to remove or reduce any imbalances in the coupled model initial state: we simply couple the models together and start to integrate forward. The effect of the drift on the model calculations is estimated from previous integrations of the model in previous years (the back-statistics). The drift is removed from the model solution during the post-processing.

An additional motivation for creating a model climatology is that after about 10 days of forecasts, the spread of the ensemble is very large (see, for instance, forecast plumes). Therefore, the probability distribution function (pdf) of the model climatology needs to be evaluated, in order to detect any significant difference between the ensemble distribution of the real-time forecast and climatology.

In the present system, the climatology (back-statistics) is a 5-member ensemble of 32-day VarEPS/monthly integrations, starting on the same day and month as the real time forecast for each of the past 18 years. For instance, the first starting date of the real-time forecast was 27 March 2002. The corresponding climatology is a 5-member ensemble starting on 27 March 1990, 27 March 1991, ..., 27 March 2001. The 5-member ensemble is thus integrated with 18 different starting dates. This represents a total of 90 integrations and constitutes the 90-member ensemble of the back-statistics.

The back statistics are created every week and are ready 3 weeks before the real-time forecasting suite starts.

* new sea-ice treatment: before 30 June 2005, the sea-ice cover was computed from the SSTs produced by the ocean model. Since 30 June 2005, the sea-ice cover is persisted from the atmospheric initial conditions till day 10, then relaxed towards climatology. After day 30, the sea-ice cover is the climatologocal sea-ice cover (from ERA40).

25.01.2010