Ocean altimeter assimilation with observational- and model-bias correction

We implement a combined on-line model- and observation-bias correction scheme in the UK Met Office Forecasting Ocean Assimilation Model (FOAM) Unified Model ocean data assimilation system. The observation bias is designed to estimate the error in the mean dynamic topography that is used for altimeter data assimilation. In future, this mean dynamic topography and its errors may be derived from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission geoid data. The mean dynamic topography field is added to the altimeter data Supplied as sea-level anomalies, giving the absolute dynamic topography. The model-bias scheme separately estimates the remaining bias in the model's sea surface height field. The final unbiased estimate of the absolute dynamic topography is assimilated into the FOAM model by adjusting the subsurface density field Using the Cooper and Haines scheme. Various diagnostics, including the observation minus background statistics, show that both model- and observation-bias correction schemes improve the assimilation results. Combining the schemes provides better results than either used alone. Copyright (C) Royal Meteorological Society and Crown Copyright, 2008