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Data assimilation settings

The experiment setting is similar to Twin Experiment 3, but with three more different ensemble sizes: 16, 32 and 64 members.

Computational time are 127, 290, and 475 hours respectively (i.e. about 1, 2.3, and 4 hour per 3 day simulation or 2.4, 5.6 and  9.1 hours per 7 day simulation).

Results

At assimilation station

At a validation station

Over all locations

Discussions

Generally, it is expected that the accuracy of an EnKF increases with ensemble size. In this experiment, however, there is no clear indication about the trend. It is not always true that EnKF with a larger ensemble size is more accurate than EnKF with a smaller ensemble. It is likely due sample variation; here for each ensemble size the EnKF is run only once. To reduce the effect of sample variation, it is necessary to run the experiments a number of times and to average the results out.

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