Four‐Dimensional Assimilation In the Presence of Baroclinic Instability

Abstract Current operational assimilation methods have revealed deficiencies in cases of strong baroclinic development. Baroclinic conditions are therefore appropriate for evaluating the potential for improvement which could be achieved through the implementation of a fully four‐dimensional data assimilation. In this paper the behaviour of a variational scheme is investigated for a typical baroclinic instability problem, where a wave develops and retroacts on to the basic zonal flow. the tangent‐linear model is shown to lead to a good approximation of the time evolution of the wave over a range of 48 hours, even at the most intense cyclogenesis period. For the assimilation experiments the twin experiment approach is applied. Only part of the flow evolution is observed, either the zonal component or the eddy component. In either case the method proves successful in reconstructing the unobserved part of the flow, taking advantage of the nonlinear coupling that exists between those components. However, nonlinearities can lead to difficulties when the range of validity of the tangent‐linear model is exceeded or when the cost function exhibits multiple minima.