Ensemble generation

The monthly forecasting system is run 51 times from slightly different initial conditions. One forecast, called the control forecast, is run from the operational ocean and atmosphere ECMWF analyses. 50 additional integrations, the perturbed members, are made from slightly different initial atmospheric and oceanic conditions, which are designed to represent the uncertainties inherent in the operational analyses.

Atmospheric perturbations: Same as for EPS:

a) 50 perturbations are produced using the singular vector method. These include perturbations in the extratropics + perturbations in some tropical areas by targetting tropical cyclones.

b) Stochastic physics: in order to take into account the effect of uncertainties in the model formulation, the tendencies in the atmospheric physics are randomly perturbed during the model integrations. The current implementation is a first attempt to simulate random model errors due to parametrized physical processes. It is assumed that these random errors are coherent between the various parametrization modules and over certain space and time scales. The scheme assumes that larger tendencies from the parametrization will have a larger random error component.

Oceanic perturbations:

A set of wind stress perturbations is calculated by taking the difference between different wind stress analyses. 5 ocean assimilations (1 control and 4 perturbed) are produced by randomly picking 2 perturbations from the set of wind stress perturbations for each month of data assimilation and adding them with a + or - sign to the analyzed wind stress. The same perturbations cannot be chosen for 2 consecutive months.

Since the coupled ocean-atmosphere coupling starts at day 10, the ocean model is integrated for 10 days forced by the heat fluxes, wind stresses and P-E produced by each individual member of the first 10 days of the VarEPS integrations. this creates and additional spread in the oceanicx initial conditions.

A more complete description of the wind stress perturbations can be found here.