Title: Hybrid ensemble-4DVAR data assimilation in a coastal ocean model

Author: Ivo Pasmans (Oregon State University)
Alexander Kurapov (Oregon State University)

4DVAR data assimilation implementations in near-real time ocean prediction systems are traditionally carried out as a sequence of estimates in relatively short time windows. Currently the background model error covariance used in these implementations is static in time and homogeneous in space. Such a choice for the background covariance may not be justified in highly dynamic coastal ocean environments, including the shelf area off Oregon/Washington (US West Coast) where the fresh water discharge of the Columbia River into the salty ocean and seasonal coastal upwelling create highly inhomogeneous conditions. Furthermore, the location of the strongest inhomogeneities changes rapidly in response to the wind.

To include the dynamics of the region into the background covariance we have developed and tested a methodology in which the initial condition error covariance for the 4DVAR system is computed based on a localized ensemble of model forecasts. The method has been tested with a 2-km resolution coastal ocean circulation model that includes the Columbia River discharge, uses the Regional Ocean Modeling System (ROMS) as nonlinear forecast model and the AVRORA tangent linear and adjoint system, developed by our group at OSU, to calculate the adjustments to the initial conditions. The test took place during summer upwelling conditions and along-track satellite altimetry, satellite sea-surface temperatures and surface velocity from an array of land-based high-frequency radars were used for assimilation. We find that the spatial structure of the background covariance depends strongly on the river plume location. Next to that, comparison with independent mooring temperature-depth profiles shows that the assimilation of surface data yields improvement in near-bottom temperature on the shelf.

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

Curator: Nikki Privé Last Updated: Feb 9 2015