Tangent linear approximation
From Glossary of Meteorology
tangent linear approximation
An assumption used in applications of tangent linear models and adjoint models that the evolution of small perturbations in nonlinear models may be approximated by tangent linear (and adjoint) equations for finite time intervals.
In forecasts of extratropical synoptic-scale weather systems, this approximation is generally valid for two to three days with dry tangent linear and adjoint models, although qualitatively useful information may be obtained for longer forecast intervals. The accuracy implied by the tangent linear approximation applies equally to the perturbation forecast of a tangent linear model and to the sensitivity provided by the corresponding adjoint model.