CERA : A coupled data assimilation system for climate reanalysis
Laloyaux, Patrick ; Dee, Dick
Climate reanalysis combines information from past meteorological observations with modern forecast models, using data assimilation techniques originally developed for numerical weather prediction. The resulting reanalysis datasets provide a comprehensive and coherent record of Essential Climate Variables over an extended period of time. ECMWF recently completed the production of ERA-20C, an atmospheric reanalysis spanning the entire 20th century, based on observations of surface pressure and marine winds (Newsletter 141, p. 9). There are now plans for a new 20th century reanalysis, called CERA-20C, in which ocean and atmospheric observations are assimilated simultaneously into a coupled atmosphere–ocean model. Interest in coupled data assimilation is growing. Several research groups and national weather services have begun to use coupled models for forecasting and are developing different ways to initialise the forecasts with observations. The innovative approach developed at ECMWF for the purpose of climate reanalysis is at the cutting edge of data assimilation research. As described in this article, the CERA system has been found to lead to a better fit with observations, an improved use of near-surface measurements and smaller initialisation shock effects.
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