A dynamical estimation of forecast error covariances in an assimilation system

Abstract

Although no model error term is specified, the estimation error variances grow according to the dynamics on poorly observed areas. The unbounded error growth found in the Southern Hemisphere has to be limited by a representation of error saturation to account for nonlinearities in the atmosphere. A weak relaxation to climatology is introduced in order to improve the independence on the initial covariances. The behavior of the error variances and correlations is shown to be particularly interesting over and around the oceans, suggesting improvements to analysis schemes and forecast skill models. A comparison with observation minus analysis and forecast statistics provides an estimate of the model error, which is then introduced into the covariance estimation procedure. -from Author