Large ensemble data assimilation systems with about 20 members are being developed at the Met Office and University of Bonn. The assimilation systems are otherwise quite different. The Met Office reanalysis system will be used for production early 2016 and cover the period from 1978. The University of Bonn will still be developed and the aim is to run a 5 year period for demonstration.

The Met Office has tested different combinations of ensemble and 4-dimensional variational assimilation and there is a report documenting many of the considerations and choices. An Ensemble of hybird 4D-VARs turned out to be the best available option for this regional applications. Observations, model and lateral boundaries have perturbations to create the ensemble spread.

MO ens 4DVar
MO ens 4DVar
Schematic view of the Ensemble 4DVar