Coauthors: Siegfried Schubert, Max Suarez (NASA/GSFC)

1. Diurnal Cycle over the Great Plains in NCEP/GFS Myong-In Lee NASA/GSFC Global Modeling and Assimilation Office Coauthors: Siegfried Schubert, Max Suarez (NASA/GSFC) Jae Schemm, Soo-Hyun Yoo (NOAA/NCEP/CPC) Hua-Lu Pan, Jongil Han (NOAA/NCEP/EMC) Climate Diagnostics and Prediction Workshop, 23-27 October 2006, Boulder, CO

2. Contents 1. Compare Diurnal cycle in three global models (NCEP/GFS, GFDL, NASA) 2. Impact of model resolution 3. Local versus large-scale forcing 4. Focus on GFS and Great Plains • Convection trigger functions • Sensitivity to the convection scheme 5. The global diurnal cycle 6. Summary

3. Model Characteristics

4. Five-member ensembles driven by Climatological SST forcing (1983-2002 avg) Phase (local time) of Maximum Precipitation (24-hour cycle)

5. GP Obs(HPD) GFDL NCEP NASA SE (spread) (ensemble mean) Diurnal Cycle of Rainfall – Ensemble Mean and Spread

6. Sensitivities to the Model Resolution 2 Deg 1 Deg 1/2 Deg (Lee et al. 2006, JC)

7. GFS T62 Conv Precip Ratio Diurnal variation of the convective precipitation Mostly driven by the “Deep Convection Scheme” %

8. GFS T62 Normalized Precip (% fraction) and Diurnal departures of 925 hPa winds

9. NARR Diurnal Cycle of Precip and Low-level Winds (3-Hourly) Normalized precip (% fraction) and diurnal departures of 925 hPa winds

10. Local versus Large-scale forcing to the diurnal convection • Diurnal variations of local convective instability • - measured by CAPE (convective available • potential energy) • Large-scale moisture flux associated with GPLLJ

11. Diurnal Variations of Precipitation and CAPE PRECIP CAPE Observed PRECIP and CAPE “in-phase” in SE, but “out-of-phase” in GP

12. Low-Level Jet simulations LLJ frequency calculated based on Bonner (1968)’s Criteria 1 GFDL NARR NCEP NASA

13. Nighttime minus Daytime moisture flux Fig. 12. The nighttime (06+12 UTC) minus daytime (12+18 UTC) moisture flux differences at 925 hPa level from the NARR and the three models. The unit is (m s-1)*(g kg-1).

14. What drives the NCEP/GFS simulation better in GP? • Consistent simulations in the diurnal variations of CAPE and GPLLJ among the three AGCMs • The model’s convection scheme should be primarily responsible • Convection schemes in the three AGCMs are all based on the Arakawa-Schubert convection scheme (local buoyancy closure) • Differences in the details of the convection scheme seem to be critical in introducing different behaviors among the models • Convection trigger functions in the Simplified Arakawa-Schubert scheme of GFS

15. Deep convection trigger/inhibition of the Simplified Arakawa-Schubert in NCEP/GFS (Grell 1993; Pan and Wu 1994) MB: mass flux at cloud base A : cloud work function (~ CAPE) Ac: critical cloud work function  : relaxation time scale • Convection triggered at A-Ac > 0 • Vertical motion-dependent Ac and ( functions of ω@cloud base) • In the presence of large-scale upward motion, the convection intensity becomes stronger • - Enhance the coupling between “Large-scale Dynamics and Local Convection”

16. Vertical Motions in the NCEP GFS Critical CWF (Ac) South East Convective case (@-0.05 pa/s) Neutral case Suppressed case (@+0.05Pa/s) pressure Great Plains • In the presence of large-scale upward motion, Ac decreases – stronger convection J/kg

17. GFS T62 Diurnal Cycle of Precip and Vertical Winds ( 500mb) (Pa/sec, negative upward)

18. hp (entraining plume) cloudtop CWF(< CAPE) LFC (cloud base) h* h ≤ 150 hPa convection starting level • 2) Convection Starting Level • Defined as the level of maximum moist static energy (h) • The level of free convection (LFC) should be also defined for the deep convection – positive cloud work function (CWF) • LFC should be defined within the 150 hPa depth from the convection starting level (max h from ground)

19. NCEP GFS Sensitivity Experiments • Focusing on the Nocturnal Rainfall in the Great Plains/Midwest • Sensitivity to the modifications in the convection trigger functions in SAS Control: Run with the original SAS EXP1:   Run with the fixed critical CWF (independent to the vertical motion) EXP2:   Run with the fixed relaxation time scale (30 minutes) (originally decreasing at the upward motion) EXP3: Parcel starting level is always at the first model level

20. June-July-August Mean Rainfall (mm/day) CTRL EXP1 EXP2 EXP3

21. Diurnal Cycle of Rainfall (peak time in LST) CTRL EXP1 (fixed Ac) EXP2 (fixed t) EXP3 (1st level convection)

22. obs ensm ens1 ens2 ens3 ens4 ens5 Diurnal Cycle of Rainfall – Ensemble Spread in EXP3

23. Nocturnal Precipitation in GP Largely sensitive to the definition of convection starting level CAPE_day > CAPE_night Precip_day < Precip_night ??

24. GFS Simulations at Great Plains total precip CAPE conv precip night day night night night day day day CWF CAPE cloud base cloud top undefined in the daytime (no trigger) Control Run, ens1, -90W, 35N, Case study(00z Jul 13- 00z Jul 17)

25. Moist Static Energy (h) h (ground) h (850 mb) total precip conv precip precipitation GFS @ Great Plains (-90W, 35N)

26. GFS Nocturnal Precipitation Mechanism (Great Plains) Daytime Nighttime GFS NASA cloud top cloud top CWF CWF h* h h* h LFC LFC ≤ 150 hPa Convection starting level (GFS) > 150 hPa Convection starting level Smaller CWF but trigger Bigger CWF but no convection trigger

27. Buoyancy (hmax-h*) GFS Simulations SE US (-80W, 30N) Great Plains (-90W, 35N) “LFC is higher in Great Plains”

28. Global Diurnal Cycle Simulations (compared with TRMM) • Six years (1998–2003) of TRMM Microwave Instrument (TMI) rain retrievals at 2.5°x2.5° grid box (Tom Bell 2004) • NCEP/GFS T62 5 member ensemble runs with climatological SSTs • Seasonal statistics (e.g., JJA) • Rainfall (R) in each grid point is fit to sinusoidal harmonics

29. Simulated global diurnal cycle of precipitation (NCEP/GFS)

30. NCAR CCSM2 UKMO AGCM Yang and Slingo (2001, MWR) NCAR CCM3 Dai and Trenberth (2004, J.Clim) Collier and Bowman (2004, JGR)

31. Simulated global diurnal cycle of precipitation (NASA/NSIPP) (b) control (c) Exp 3 ( PBL mean strapping + relaxation time increase) (Lee et al. 2006, in preparation)

32. Summary • Simulations of the warm season diurnal cycle of precipitation were compared in the three global climate models. • While the models have basically similar convective schemes (buoyancy closures), they have rather different diurnal cycles (phase) in the land region, particularly over the Great Plains. • NCEP/GFS captures the pronounced nocturnal precipitation signals over the Great Plains reasonably well, which feature is not properly simulated by other global models. • Increased resolution has less of an impact on the simulated diurnal cycle of convection, suggesting the importance of model physics. • The models commonly show reasonable diurnal variations of CAPE (local convective instability) and large-scale moisture influx (associated with GPLLJ). • Source of the differences appears to be in the implementation details of the convection scheme, such as the convection trigger functions.

33. Summary (continued) • Nocturnal precipitation mechanisms in the NCEP/GFS model were further investigated. • Various convection triggers that implemented in the Simplified Arakawa-Schubert scheme were further tested through the model sensitivity experiments. • Model simulations are most sensitive to the convection starting level (maximum h in NCEP/GFS) and the definition of the convective cloud base. • Even though the CAPE (or CWF) is much bigger in the daytime over the Great Plains, the modeled convection can not be triggered by the condition that the cloud base should be defined within 150 hPa from the convection starting level; This triggers convection when the convection starting level locates above the nighttime inversion layer. • In spite of the success in the US Great Plains, the NCEP/GFS model still shows early phase biases in the land diurnal convection. • For the better representation of the diurnal cycle of precipitation, the convection scheme should be revised, particularly in coupling process with the PBL and other dynamical triggers.

34. Thank you very much !!