4-Dimensional Ensemble&endash;Variational (4DEnVar) data assimilation option
Prior to version 3.9, WRFDA supported a 3DEnVar data assimilation option. For Version 3.9, the capability for 4DEnVar assimilation has been added. The procedure for 4DEnVar is very similar to the procedure for 3DEnVar. Unlike 4DVAR, 4DEnVar does not require WRFPLUS, the tangent linear/adjoint model. It is more analogous to the First Guess at Appropriate Time (FGAT) method than full 4DVAR. Like FGAT, 4DEnVar requires multiple first guess files, one for each assimilation time window. In addition, you must have ensemble output for each of the first guess times, and generate ensemble perturbations from each of these sets of ensemble forecasts.
To activate the 4DEnVar option, set the namelist variable use_4denvar=.true. See the users guide for full details about this capability. A tutorial case with a test ensemble can be found here. In this example, the ensemble forecasts were initialized at 2005071512 and valid for 2005071521—2005071603. The 4DEnVar hybrid analysis (wrfvar_output) will be valid for 2005071600.
