PowerPoint Presentation - Methods for dealing with spurious covariances arising from small samples in ensemble data assimilation

Two EnKF approaches

•Serial approach - for each observation, update each model variable (tapering the influence of the observation to zero at a specified distance). Used in NCAR DART.

•Local approach - update each model variable one at a time, using all observations within a specified radius (increasing R with distance between observation and model variable) - we use this approach since it scales well on massively parallel computers

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	So, we have two ways of solving the problem.
	Serial approach makes the most sense when you have few obs and a large state vector
	Local approach makes more sense when you have lots of obs - then it’s much easier to parallelize the problem and it scales well on a MP system. Mathematically, there are differences between the two (the Local approach involves some approximations), but practical experience has shown there is little or no difference in accuracy.