Adjoint model

From Glossary of Meteorology

adjoint model

A model composed of adjoint equations that maps a sensitivity gradient vector, xJ(t0) = 𝗟TxJ(t1) , from a forecast time, t1, to an earlier time, t0, which can be the initial time of a forecast trajectory.

J is some scalar measure of the forecast, 𝗟T is a linear adjoint operator, and x is the model state vector. An adjoint model can provide a first-order (tangent linear) approximation to sensitivity in a nonlinear model.
See adjoint equation, adjoint sensitivity, tangent linear equation.