__It is the same as the
scalar formula for ____T__, but now **x** is a model state
vector, with 106-8d.o.f., and **y**o is the set of observations, with 105-9 d.o.f.

**
**

**R** is the observational error covariance,

**B** the forecast error
covariance.

•

In 3D-Var **B** is constant: it does not include “errors of the day” **
**