A set of initial ensemble members must be prepared before the hybrid application can be started. The ensemble can be obtained from other ensemble model outputs, or you can generate them yourself. This can be done, for example, adding random noise to the initial conditions at a previous time and integrating each member to the desired time. A tutorial case with a test ensemble can be found at http://www2.mmm.ucar.edu/wrf/users/wrfda/download/wrfda_hybrid_etkf_testdata.tar.gz. In this example, the ensemble forecasts were initialized at 2006102712 and valid 2006102800. A hybrid analysis at 2006102800 will be performed using the ensemble valid 2006102800 as input. Once you have the initial ensemble, the ensemble mean and perturbations can be calculated following the steps below: