Comparison of Data Assimilation Coupling Strategies for Earth System Models

Over the last decade, the Earth science community developed several prototypes of data assimilation methods for initialization of coupled model systems. These methods range from so-called weakly coupled data assimilation (DA) methods (where only the model guess is coupled but not the data assimilation system) to strongly coupled methods (where both the first guess and the DA are coupled). In this talk, I will present and contrast several prominent strategies for coupling in DA, including weakly coupled, outer loop-coupled, interface coupled, and strongly coupled methods. I attempt to provide guidance on when each of the methods is expected to perform best and what practical limitations we still need to overcome to improve the fidelity of the initial conditions for coupled Earth system models.