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3DVAR Changes
Dave Parrish has not included the weak constraint on model time derivatives of divergence because of difficulties with convergence, and the fact that using balanced and unbalanced variables on the eta coordinate seems to cure the problem of poor dynamic balance. Instead of the adjoint, he has used a simpler formulation similar to non-linear normal mode initialization, where the tangent linear model is the linear shallow water equations linearized about a state of rest.