Subseasonal forecasting with the CFS Climate Test Bed Experiments

1. Subseasonal forecasting with the CFS • Climate Test Bed Experiments • Augustin Vintzileos and Dave Behringer • Past: The Maritime Continent Prediction Barrier. Impacts of horizontal resolution and atmospheric I.C. on MJO forecast skill. • Present: Impact of high frequency oceanic I.C. on MJO forecast skill. • Future: Proposals to be submitted to CTB. Optimal ways for ensemble generation, Multi-Model-Ensemble MJO forecast

2. Past results: In a series of retrospective forecasts conducted under the CTB we explored the dependence of the MJO forecast skill to horizontal resolution and atmospheric initial conditions These hindcast experiments have shown that: (a) horizontal resolution (at least up to T254) is not improving forecast skill and cannot break through the Maritime Continent Forecast Barrier (b) better atmospheric initial conditions improve MJO forecasts by 3-5 days without nevertheless breaking the Maritime Continent Barrier.

3. Skill for the MJO mode (verification CDAS2) GDAS T62 T126 CDAS2 Persistence T254

4. Current CTB retrospective subseasonal forecasts with the CFS: Rationale: • Using the observed daily OI SST we identified intraseasonal oceanic modes which project to the MJO • The CFS is initialized by GODAS of which: • SST is damped to the weekly Reynolds SST • Contains information from 2 weeks before and two weeks after • As a consequence the amplitude of intraseasonal oscillations in the operational GODAS is weak

5. Standard Deviation of the 20-90 day filtered SST 2002 - 2006 There is much less intraseasonal variability in the operational GODAS than observed. The coupled model in free mode presents more realistic intraseasonal behavior 2002 - 2006

6. In order to introduce intraseasonal modes in the ocean initial conditions we used an experimental version of GODAS in which we restore the temperature of the oceanic mixed layer to the observed daily OI SST. Comparison of operational GODAS (blue) with experimental GODAS (red) The experimental GODAS clearly contains higher frequencies

7. Present retrospective subseasonal forecasts under CTB: • We used the most current version of the GFS (to become operational in spring) at resolution T126L64 coupled to MOM3 • In the control hindcast we used the operational GDAS for initializing the atmosphere (shown to produce the best results) and the operational GODAS. Retrospective forecasts were initialized every 5 days from May 23rd to August 11th from 2002 to 2006. This new version of the CFS has the same forecast skill with the previously used CFS. • We then repeated these experiments using the experimental GODAS

8. Up to lead time of 7 days there is no difference when initializing with operational and experimental GODAS. After day 7 the experimental GODAS is consistently better but the difference is such that we can only claim a marginal improvement Diagnostic studies to follow will address several issues (e.g., how fast and why are the injected high frequency oceanic modes damped)

9. Future experiments: Further investigations of the Maritime Continent Barrier. Subseasonal forecast is by definition probabilistic. Therefore there is necessity for ensemble forecasting. Currently, there is no perturbation technique targeting subseasonal lead times. The MJO is a very difficult phenomenon to simulate and forecast. Each dynamical model has its own ‘view’ of what the MJO is (i.e., orthogonality). It follows that multi-model ensemble subseasonal forecasts is a necessary way to go. These issues are addressed in proposals submitted to the CTB