Raising the model top for an ensemble Kalman filter

Recently, the Canadian Meteorological Center (CMC) introduced a new, staggered, vertical coordinate for the Global Environmental Multiscale model. The new coordinate is thought be more accurate and should facilitate raising the model top further into the stratosphere. As a first step, we raised the model top from 10 hPa (~32 km) to 2 hPa (~43 km) while keeping the horizontal resolution and the number of vertical levels the same as before (400×200 with 58 vertical levels), effectively resulting in slightly reduced vertical resolution. Since the model top has been raised, it has been necessary to change the statistical description of the model error. We moved to a non-separable description provided by the variational system at our center. With the new statistics we have less spread for the wind component and it is evaluated if adding stochastic parameterization can lead to an appropriate increase in ensemble spread. Doubling the ensemble size dealt with the small scales near the surface in the new statistics and also with the low correlations with wind in the stratosphere.