Ensemble Perturbations for Subseasonal CFSv2 Forecasts

Atmospheric and coupled initial perturbations for the NCEP CFSv2 are being generated and analyzed to assess prediction capability gains for the weeks 3 and 4 forecast period. Atmospheric-only perturbations based on the 4D-EnKF were first generated to understand how the forcing exerted by the atmosphere at the initial time is transmitted in the coupled model to the SST and ocean fields. The geographic structures of the spread and the forecast errors were compared. It was found that it takes over a month for the SST errors to resemble the large-scale geographic structure of the spread. Also, the distribution of SST spreads for grid-points over the ocean tend to be sharper and with faster growth rates than the SST error distribution. These results are guiding a method for coupled perturbations such that the evolution of the spread growth across the forecast lead-times approaches the error growth for surface variables. The benefit of increasing the size of the CFSv2 ensemble is also evaluated. It is apparent that larger ensemble sizes produce more reliable forecasts.