Influence of physical processes on the tangent-linear approximation

Abstract

A comprehensive set of linear physical parametrizations is developed for the tangent-linear andadjoint versions of ECMWF global forecast model. The following processes are described :vertical diffusion, subgrid-scale orographic effects, large scale condensation, long-wave radiationand deep cumulus convection. The accuracy of a tangent-linear model including these processesis examined by comparing 24-h forecasts obtained from an adiabatic tangent-linear version andpairs of non-linear integrations including full physics. It is shown that for finite size perturbations(analysis increments), the inclusion of physics improves the fit to the non-linear model. Theimprovement is largest for specific humidity, where combined effects of vertical diffusion (nearthe surface), large-scale condensation (in the mid-latitudes troposphere) and moist convection(in the lower tropical troposphere) contribute to a better evolution of moisture increments.Simplications have been designed with respect to the operational non-linear physics, mostly toavoid the growth of spurious unstable modes. Computation of singular vectors has revealedthat the linear package does not contain such spurious unphysical structures. This comprehensiveset of linear parametrizations is currently used in the ECMWF operational 4D-Var assimilationsystem.