Exploration of advanced ocean data assimilation schemes at NCEP

The first task will be to upgrade NCEP’s Global Ocean Data Assimilation System (GODAS) from the current 3DVar system implemented in 2003 to the ensemble Local Ensemble Transform Kalman Filter (LETKF). GODAS serves as the ocean component of the integrated atmosphere-ocean analysis system, in turn providing the initial conditions for NCEP’s atmosphere/ocean Climate Forecast System, version 2, (CFSv2). The second task will be to combine the 3DVar and LETKF systems to form a hybrid version of GODAS. We will explore the effectiveness of this hybrid system to represent timeevolving local correlations due, for example to fronts or currents, while at the same time maintaining large-scale correlations. An additional task will be to examine the computational efficiency of the hybrid filter relative to its 3DVar and LETKF alternatives.

The proposal will bring together researchers from the University of Maryland experienced in the development and use of LETKF with NCEP researchers who have overseen development of 3DVar-GODAS and the first two versions of the CFS. The rationale for proposed work is: 1) It will result in an upgrade of the ocean analysis system to one that will be analogous to NCEP’s 3DVar-hybrid Gridpoint Statistical Interpolation system (GSI) used for atmospheric analysis. This upgrade will allow the next generation CFS to gain the benefit of a more integrated atmosphere-ocean-land analysis system in which both ocean and atmosphere components use coupled ensemble-based estimates of flux error at the interface. 2) The implementation of LETKF in GODAS will provide NCEP with a more flexible ocean analysis system, for example simplifying the inclusion of new observational data sets like sea surface salinity and providing an error estimate for the ocean state. This flexibility is important to allow NCEP to implement assimilation upgrades to both GODAS (using a MOM-based model) and the eddy-resolving Real-Time Ocean Forecast System (RTOFS) (using a HYCOM-based ocean model). The new development work proposed here will be carried out in NCEP’s computing environment thus facilitating the integration of the resulting system into operations.

This proposal addresses the NOAA Next Generation Strategic Plan’s call to ‘create accurate and reliable estimates of the state of the ocean, including its temperature, salinity, and motion fields for accurate forecasts and assessments’. Likewise it addresses the Climate Program Office Strategic Climate Objective: I Improve Scientific Understanding, specifically the goals of improved monitoring and improving initial conditions to explore ‘useful predictions of climate variability and change for the next one to three decades’. Within the Modeling, Analysis, Predictions, and Projections (MAPP) program 2013 Call the developments described in this proposal address Priority Area 1: Research to Advance Climate Reanalysis and Research Focus 2: Integration among Earth System reanalysis components by improving the integration of the atmospheric and ocean analysis systems.