A Mutual Information-Based Ensemble Kalman Filter
Ensemble Kalman filters (EnKFs) are widely used for data assimilation in geophysical systems. Among various implementations, the local ensemble transform Kalman filter (LETKF) has gained popularity because of its computational efficiency. However, the deterministic EnKF such as the LETKF is known to be less robust than the stochastic EnKF in strongly nonlinear regimes. We generalize the LETKF such that it contains a stochastic term and includes the stochastic EnKF within it. We adaptively optimize the parameter that determines the weight of the stochastic term based on an identity of mutual information, which is satisfied by the Kalman filter in linear Gaussian systems. As the analysis perturbation equations of EnKFs are decomposed into a system of equations for modes that are uncorrelated with each other, the application of mutual information is easily achieved. The generalized LETKF thus optimized is named the mutual information-based ensemble Kalman filter (MI-EnKF). The MI-EnKF indirectly uses the third- and fourth-order moments of the forecast ensemble through entropy. To speed up calculations of entropy, we create a lookup table based on maximum entropy distributions. We conduct data assimilation experiments using the Lorenz-96 model to confirm the validity of the optimization method of MI-EnKF. When the observation operator is linear, the ML-EnKF shows the same analysis accuracy as the LETKF. When the observation operator is strongly nonlinear, the MI-EnKF is more accurate than both LETKF and stochastic EnKF regardless of ensemble size. Optimizing just the first mode can lead to significant improvements, but positive impacts of increasing the number of optimized modes are not observed unless the ensemble size is large. The optimized parameter values indicate that the optimal EnKF lies between the deterministic EnKF and the stochastic EnKF.