The University of Bonn has developed an Ensemble nudging scheme within the COSMO modelling system from DWD. It is a first step to combine a hybrid LETKF (Localised Ensemble Transform Filter) with the nudging scheme.

The ensemble capabilities have been implemented inot the COSMO nudging scheme so that each member is nudged towards perturbed observations. The perturbations have been designed to be random within the assumed range of observation errors. Physical checks have been implemented to ensure that the observations are reasonable and are physically consistent, e.g. with respect to vertical temperature lapse rate.

A summer and a winter period have been tested with the ensemble nudging of perturbed observations and the ensemble spread looks good (so that most events are captured by the ensemble and all members are equally probable). The example below shows that observations are quite uniformly distributed over the bins and not too many are outside of the ensemble spread (bin 1 and 22).