Examination of the accuracy of a tangent linear model

The accuracy of a tangent linear version of a 3-dimensional mesoscale primitive equation model (the PSU/NCAR MM4) is investigated by comparing its results with those produced by identical perturbations introduced in nonlinear forecasts of the original model. Moist physical processes are not considered in this study. For perturbation magnitudes as large as typical current analysis errors, the perturbation tendencies are shown to be very accurately estimated by the tangent linear model (TLM), with greater relative error in a summer case than in a winter one. The evolutions of perturbations in forecasts out to 72 h are also accurately estimated, although the unperturbed lateral boundary conditions that act to sweep perturbations out of the domain are an artificial means of perturbation constraint. -from Authors