Mike Fiorino michael.fiorino@noaa Assimilation and Modeling Branch Global Systems Division

1. Recent trends in dynamical medium-range tropical cyclone track prediction and the role of resolution v physics in the ECMWF model Mike Fiorino michael.fiorino@noaa.gov Assimilation and Modeling Branch Global Systems Division Earth System Research Laboratory Boulder, CO 4 March 2009

2. Why medium-range (72-h) track? • Operational – long-range warning is a medium-range track problem • no skill vis-à-vis CLIPER in early 1980s • high skill is required for using improved dynamical intensity guidance – I can’t believe a rapid intensification forecast if the track doesn’t make (physical) sense… • Modeling (& HFIP) • analysis v model error – 72 h is when model errors become dominant • multi-scale dynamics – synoptic scales dominate, but vortex scales can have large impacts • closely follows tropical wind score • MRT is the 500 mb 5-d Anomaly Correlation score of the tropics

3. The importance of modeling… POD of NHEM TC in reanalysis v operations TC POD improvement in 1989 from physics change, ditto for degradation in 1994… 850 mb tropical wind score time when the correlation of forecast and analysis vector wind drops to 70%

4. OBS (not) critical to TC analysis? • POD of “TC” in reanalysis v operations • ERA-15 uses model physics circa 1994 and achieves 1994 operational model POD throughout the 1979-1994 period • TC POD follows the tropical wind score… and the tropical wind score follows the physics… • score improves from 1–7 d • when score reaches 3-4 d, global models began showing MRT skill • model of 1st order importance in analysing the 1st order TC vortex

5. History of Atlantic MRT (72-h) MFE – Models v CON medium-range track (72-h) mean forecast error – two models UKMO (global) and GFDL (limited-area) with long history v ‘best’ or baseline consensus (BCON) 1992-2008 SKILL– percent improvement over the no-skill baseline aid CLIPER (climatology and persistence) CLIPER trend? better databases error cut in half from ~ 280  130 nm 1990s-> 2008 BCON better than models OFCL ~ BCON higher skill in 2008 even though CLIPER error increased  UKMO and GFDL models did very well in 2008….

6. Atlantic %gain(+)/loss(-) v BCON • context for ECMWF results: • 2008 a good year for the models in the LANT • models typically 20-25% worse than BCON in all basins, but more so in WPAC

7. ECMWF resolution v physics changes • 2005-2008 model changes • 1) increase in horizontal resolution from T511  T799 (4020 km) 200602 • 2) “significant changes to model physics” on 20071106, including new formulation of “convective entrainment and relaxation time scales • Tropical wind scores nearly constant

8. ALL TCs – % improvement over BCON ECMWF is 20% better than BCON when most models are 20% worse at the medium range (72 h) • MRT TC skill difference greater than change in tropical wind score • increased resolution (red v orange) improves 72-120 h error • physics change improves forecast at all times • skill relationship (physics > res) consistent between basins, but stronger signal in WPAC and SHEM, more muted in LANT

9. Summary • ECMWF has shattered both TC MRT predictability limit from the 1990s (~130 nm); the skill of consensus; and the (old) PACOM (50, 100) 150 nm requirement • Updated results as of 20090303 in SHEM (TC activity ~ 54% below normal) 72-h MFE: • CONW – 158 nm • ECMWF – 108 nm • +32% gain over BCON; 61 cases

10. My personal (biased) takeaways… • How ECMWF achieved these results is of fundamental importance to HFIP… • improved TC v synoptic-scale forecast? • deterministic (hi-res) v ensembles • can ensembles work until the model(s) achieves the same skill as ECMWF? no, physics 1st • TC vortex analysis • 1st order structure (assimilate the “working best track”) 1st;detailed, hi-res obs 2nd • global v limited-area models • when do errors the global-scales become significant? ~ 36 h 