Title: Accelerating and parallelizing minimizations in ensemble and deterministic variational assimilation

Authors: Gerald Desroziers (CNRM/GAME, Meteo-France)
Loik Berre (CNRM/GAME, Meteo-France)

EnKF can be transposed to variational ensemble assimilation, where a set of perturbed variational analyses are performed. In this case, however, there is an evident important additional cost associated to the use of multiple minimizations. The aim of the presentation is to investigate different techniques to reduce the cost of the multiple minimizations that have to be performed.

The use of a preconditioning technique, based on Ritz eigenpairs resulting from a first minimization performed by a combined Lanczos / conjugate-gradient, algorithm is in particular investigated. The possibility to construct more explicitly a new perturbed solution, with Lanczos vectors issued from a single prior unperturbed or perturbed minimization, is also studied. This appears to provide a first significant reduction in the cost of the new minimization.

Finally, a new approach is proposed to generalize the previous idea to the use of a multiple set of Lanczos vectors issued from an ensemble of perturbed assimilations. The application of this procedure to a simplified analysis problem shows encouraging results, as it appears to be a possible way for reducing the global cost of an ensemble variational assimilation. Moreover, this seems to provide an efficient strategy for parallelizing such an ensemble variational assimilation but also the deterministic variational assimilation itself.

An application of the previous approach to the 4D-Var assimilation ensemble associated with the French global prediction system will also be shown.

GMAO Head: Michele Rienecker Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: May 27 2011