Title: Exploring strategies for coupled 4D-Var data assimilation using an idealised atmosphere-ocean model

Author: Polly Smith (University of Reading, UK)
Alison Fowler (University of Reading, UK)
Amos Lawless (University of Reading, UK)

Coupled atmosphere-ocean data assimilation offers a long list of potential benefits, including improved use of near-surface observations, reduction of initialisation shocks in coupled forecasts and generation of a consistent system state for the initialisation of coupled forecasts across all timescales. Strongly-coupled data assimilation presents significant scientific and technical challenges and so several operational centres are developing weakly-coupled assimilation systems as a first step. In this work we use an idealised single column atmosphere-ocean model to assess the expected benefits of moving towards coupled data assimilation in the context of incremental 4D-Var. Our simplified framework retains the key elements of coupling processes in a fully coupled ocean-atmosphere model system without being overly complex. The system has the capability to run both strongly- and weakly-coupled assimilations as well as uncoupled atmosphere or ocean only assimilations, thus allowing a systematic comparison of the different strategies for treating the coupled data assimilation problem. We will present results from a series of identical twin experiments devised to investigate the behaviour and sensitivities of each approach. Overall, our study demonstrates that significant benefits may be expected from coupled data assimilation. When compared to uncoupled initialisation, coupled assimilation is able to produce more balanced initial analysis fields, thus reducing initialisation shock and its impact on the subsequent forecast. Single observation experiments demonstrate how coupled assimilation systems are able to pass information between the atmosphere and ocean and therefore use near-surface data to greater effect. We show that much of this benefit may also be gained from a weakly-coupled assimilation system, but that this can be sensitive to the parameters used in the assimilation.

GMAO Head: Steven Pawson Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: Feb 9 2015