Different approaches to model error formulation in 4D-Var: a study with high-resolution advection schemes

Abstract

All numerical models are imperfect. Weak constraint variational data assimilation (VDA), which provides a treatment of the modelling errors, is studied; building on the approach of Vidard et al. (Tellus, 56A, pp. 177–188, 2004). The evolution of model error (ME) is modelled using ordinary differential equations, which involve a scalar parameter. These approaches were tested using different high-resolution advection schemes. The first set of experiments were constructed to see if it is possible to account for (numerical) discretization error within such a framework. In other set of experiments, a systematic source of modelling error was introduced by deliberately specifying an incorrect value for the Coriolis parameter in the model. Results with observational state at half of the model state resolution, are also presented.We also discuss a method of estimating the scalar parameter in the ME through VDA. In all cases, the inclusion of ME provides reduction in forecasting errors. Also, our experiments indicate that different settings of the model (e.g. using different high-resolution advection schemes) would need different ME formulation. Results presented in this paper could be used to formulate sophisticated ME forms to account for systematic errors in higher dimensional models with complex advection schemes.