A Hybrid Differential-Ensemble Linear Forecast Model for 4D-Var

A key component of the 4D-Var data assimilation method used widely for numerical weather prediction is the linear forecast model, which is approximately tangent linear to the forecast model. Traditionally this has been based on differentiating the forecast model, though recently some authors have experimented with an ensemble regression technique, the localized ensemble tangent linear model (LETLM). We propose a hybrid of the two, in which a simplified conventional tangent-linear model (e.g., just the dynamical core) is used together with an LETLM-like adjustment every time step to account for the remaining processes (in this example, the parameterized physics). This is much cheaper than the LETLM, and in tests using the Met Office’s linear model performs considerably better than either a pure LETLM (with a very large ensemble) or the existing linear model.