1. 1 Making upgrades to an operational model : An example Jongil Han and Hua-Lu Pan NCEP/EMC GRAPES-WRF Joint Workshop

2. 2 Recent upgrade in the operational GFS (Effective at July 27 th 2010 12 UTC) Resolution increase: T382L64 (~35km) => T574L64 (~23km) Major physics change: - Shallow and deep convection, PBL schemes - Radiation: - SW: NCEP0 => RRTM (Rapid Radiative Transfer Model) - LW computation frequency: 3 hrs => 1 hr - SW cloud overlap: random => maximum-random overlap - Positive-definite vertical tracer transport scheme (remove negative water vapor) - Minor changes in mountain blocking parameterization

3. 3 Revision of shallow cumulus convection scheme One of long standing problems in the GFS was the systematic underestimation of stratocumulus clouds over off-coast regions in the eastern Pacific and Atlantic Oceans. This problem has been attributed to the shallow convection scheme which uses a turbulent diffusion approach

4. 4 ISCCP Old GFS Low cloud cover (%) (P>680hPa)

5. 5 Seek conditionally unstable layer K K max =5m 2 s -1 Old operational shallow convection scheme in the GFS (Tiedke, 1983) LCL

6. 6 Old shallow convection scheme (Diffusion scheme) New shallow convection scheme (Mass flux scheme) Mass flux analogy(de Roode et al., 2000) : A u (updraft area)=0.5 A d (downdraft area)=0.5 A u ~0.0; A d ~1.0 Environment is dominated by subsidence resulting in environmental warming and drying.

7. 7 Revised shallow convection scheme Use a bulk mass-flux parameterization same as deep convection scheme. Separation of deep and shallow convection is determined by cloud depth (currently 150 mb). Entrainment rate is given to be inversely proportional to height (which is based on the LES studies) and much larger than that in the deep convection scheme. Mass flux at cloud base is given as a function of the surface buoyancy flux (Grant, 2001). This differs from the deep convection scheme, which uses a quasi-equilibrium closure of Arakawa and Shubert (1974) where the destabilization of an air column by the large-scale atmosphere is nearly balanced by the stabilization due to the cumulus.

8. 8 Revised shallow convection scheme Entrainment rate: Siebesma et al.2003: Detrainment rate = Entrainment rate at cloud base c e =0.3 in this study

9. 9 Revised shallow convection scheme Mass flux at cloud base : M b =0.03 w * (Grant, 2001) (Convective boundary layer velocity scale)

10. 10 CTL New shallow convection scheme

11. 11

12. 12 Cloud depth (mb) Deep & shallow Shallow only (<150mb)

13. 13 ISCCP New shallow Old shallow

14. 14 Revised PBL scheme Include stratocumulus-top driven turbulence mixing based on Lock et al.’s (2000) study Enhance stratocumulus top driven diffusion when the condition for cloud top entrainment instability is met Use local diffusion for the nighttime stable PBL rather than a surface layer stability based diffusion profile Background diffusivity for momentum has been substantially increased to 3.0 m 2 s -1 everywhere, which helped reduce the wind forecast errors significantly

15. 15 (Buoyancy reversal term is neglected) MRF PBL Revised model Heat flux C=1.0 where c=0.2 (CTEI condition) (Simplified after Lock et al., 2000) (Moeng et al., 1999) (MacVean and Mason, 1990)

16. 16

17. 17 No stratocumulus top driven diffusion With stratocumulus top driven diffusion Low cloud cover (%)

18. 18 GFS Grid Point Storms (bull’s eye precip) The GFS suffered from grid point storms during the convective season, which was another long standing problem in the GFS forecasts. The old deep convection scheme did not appear to fully eliminate the instability and consequently, an explicit convective ascent occurred on the grid scale, producing unrealistically large precipitation.

19. 19 24 h accumulated precip ending 12 UTC 15 July 2009 Grid Point Storm Observed72 h GFS Forecast

20. 20 Revised deep convection scheme Increase cloud top - Random cloud top selection => single deepest cloud - A convective overshooting is parameterized in terms of cloud work function Increase maximum allowable cloud base mass flux Include the effect of convection-induced pressure gradient force to reduce convective momentum transport (reduced about half)

21. 21 Old GFS deep convection scheme (SAS) SL DL LFC CTOP h hshs Updraft mass flux 0.5 1.0 Downdraft mass flux 1.0 0.05 Entrainment Detrainment Environmental moist static energy 150mb

22. 22 A hshs hchc 0.1A The overshoot of the cloud top is stopped at the height where a parcel lifted from the neutral buoyancy level with energy equal to 10% of the cloud work function (A) would first have zero energy.

23. 23 Revised deep convection scheme Maximum cloud base mass flux [currently 0.1 kg/(m 2 s)] is defined for the local Courant-Friedrichs-Lewy (CFL) criterion to be satisfied (Jacob and Siebesman, 2003); Then, maximum mass flux is as large as 0.5 kg/(m 2 s) for T382 (35km) resolution

24. 24 Revised deep convection scheme Organized entrainment (Betchtold et al., 2008) turb. org. in sub-cloud layers above cloud base

25. 25 Revised packageOld GFS Total precipitation (grid scale+convective)

26. 26 Revised package Old GFS Convective precipitation

27. 27 Revised package 24 h accumulated precipitation ending at 12 UTC, July 24, 2008 from (a) observation and 12-36 h forecasts with (b) control GFS and (c) revised model Total precipitation (grid scale+convective)

28. 28

29. 29 Reduction of convective momentum transport due to convection-induced pressure gradient force (Han and Pan, 2006) c: effect of convection-induced pressure gradient force c=0.0 in the current operational GFS convection scheme c=0.55 in the revision

30. 30

31. 31 #CASES (350) (323) (300) (275) (245) (199) (158) (129) #CASES (257) (230) (194) (163) (135) (91) (55) (27)

32. 32 Medium-range forecast experiments with data assimilation Resolution: T382L64 (about 35km at equator) Test period: June 2 – Nov. 10, 2008 (7-day forecasts at each 00Z cycle), which includes the whole 2008 hurricane season. A spin-up series of forecasts for the previous 19 days has been discarded from the analysis.

33. 33 500 mb height forecast skill

34. 34 NH SH T574 (~23km)

35. 35 500 MB Anomaly Correlation Northern HemisphereSouthern Hemisphere

36. 36 Precip. score over continental US

37. 37 Vector wind error (Tropics)

38. 38 #CASES (359) (331) (307) (281) (254) (205) (161) (131) #CASES (261) (233) (197) (166) (138) (92) (56) (28)

39. 39

40. 40 Turbulence parameterization : moist-process conserved variable mixing (T l and Q t ) Macro and micro physics : Ferrier scheme with partial clouds Consistent cloud fraction formulation for macro-physics, turbulence, and radiation Future plan