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

Two EnKF approaches

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

ĄLocal 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


	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 itŐs 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.