ECMWF started an experimental programme of monthly forecasting (time range from 10 to 30 days) in March 2002. The monthly forecasting system is operational since October 2004. In March 2008 ECMWF has combined its VAriable Resolution Ensemble Prediction System (VarEPS) and monthly forecasting into a single system. On Thursday of each week, the 00 UTC VarEPS forecast is extended from 15 to 32 days at a resolution of T319 L62 with ocean coupling introduced from day 10.

Why Monthly Forecasting?

Two forecasting systems are currently operational at ECMWF: medium-range weather forecasting and seasonal forecasting. Medium-range weather forecasting produces weather forecasts out to 10 days, whereas seasonal forecasting produces forecasts out to 6 months. The two systems have different physical bases. Medium-range weather forecasting is essentially an atmospheric initial value problem. Since the time scale is too short for variations in the ocean significantly to affect the atmospheric circulation, the ECMWF medium-range weather forecasting system is based on atmospheric-only integrations. SSTs are simply persisted. Seasonal forecasting, on the other hand, is justified by the long predictability of the oceanic circulation (of the order of several months) and by the fact that the variability in tropical SSTs has a significant global impact on the atmospheric circulation. Since the oceanic circulation is a major source of predictability in the seasonal scale, the ECMWF seasonal forecasting system is based on coupled ocean-atmosphere integrations. Seasonal forecasting is also an initial value problem, but with much of the information contained in the initial state of the ocean.

The main goal of monthly forecasting is to fill the gap between these systems and produce forecasts for the time range 10 to 30 days. The time range 10 to 30 days is probably still short enough that the atmosphere retains some memory of its initial state and it may be long enough that the ocean variability has an impact on the atmospheric circulation. Therefore, the monthly forecasting system has been built as a combination of the medium-range EPS and the seasonal forecasting system. It contains features of both systems and, in particular, is based on coupled ocean-atmosphere integrations, as is the seasonal forecasting system.

An important source of predictability over Europe in the 10-30 day range is believed to originate from the Madden Julian Oscillation (MJO) (see, for instance, Ferranti et al 1990). The MJO is a 40-50 day tropical oscillation. Several papers (see, for instance, Flateau et al, 1997) suggest that the ocean-atmosphere coupling has a significant impact upon the speed of propagation of an MJO event in the Indian Ocean and western North Pacific. The use of a coupled system may therefore help to capture some aspects of the MJO variability.

The ECMWF monthly forecasting system has two components:

- The real-time forecasting system

- The back-statistics needed to create a model climatology to calibrate the real-time forecasting system.

The real-time forecasting system

The real-time VarEPS/monthly forecasting system is a 51-member ensemble of 32-day integrations. The first 10 days are performed with a TL639L62 resolution forced by persisted SSt anomalies. After day 10, the model is coupled to the ocean model and has a resolution of TL319L62. 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.

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.

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 climatology sea-ice cover (from ERA40).

18-01-2011