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The DTC Winter Forecast Experiment (DWFE):

The DTC Winter Forecast Experiment (DWFE):. Analysis of Kinetic Energy Spectra. Dave Dempsey ( San Francisco State University; DTC visitor ) William Skamarock ( NCAR/MMM ). Acknowledgements : Ligia Bernardet ( NOAA/FSL/DTC ) Zavisa Janjic ( NCEP ) Bob Gall ( Director, DTC/NCAR ).

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The DTC Winter Forecast Experiment (DWFE):

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  1. The DTCWinter Forecast Experiment (DWFE): Analysis of Kinetic Energy Spectra Dave Dempsey(San FranciscoState University; DTC visitor) William Skamarock(NCAR/MMM) • Acknowledgements: • Ligia Bernardet (NOAA/FSL/DTC) • Zavisa Janjic (NCEP) • Bob Gall (Director, DTC/NCAR)

  2. Outline: • Motivating observations • mountain waves • DTC Winter Field Experiment (DWFE) • Kinetic energy spectra • background • backstory • accidental DWFE discoveries • ARW and NMM results • sensitivity study: NMM spectra and model configurations • Conclusions and Further Work NCAR/DTC

  3. DTC Winter Forecast ExperimentJan 15 - March 31, 2005 ConUS Domain • Two versions of WRF: • Two different dynamical cores (NMM & ARW) • Two different physics packages (NCEP & NCAR) • 5 km resolution; 37 levels • Initial and boundary conditions from Eta212 (40 km) • 00Z initialization, daily 48-hour forecasts • Explicit convection (no convective parameterization) NCAR/DTC

  4. My Context (a parable of howscience sometimes happens) • Sabbatical leave (half pay), Jan-July ‘05 • Invite self to visit DTC. Flexible agenda—will work for chance to be useful, learn something • Gall says “OK”, provides: • office, computer, office supplies • no promises of anything else • Office shared (1 day/wk) w/Steve Koch (NOAA) • Gall says, “Dempsey: DWFE forecasts. See what you can see.” NCAR/DTC

  5. Mountain WavesPotential Temperature Cross Sections(12 hr forecasts valid 12Z Jan 13, 2005) NMM ARW NCAR/DTC

  6. The Parable (cont’d) • NCAR is big place—lots of printers. Can’t find the printer. Plots sit by printer 3 days. • I print copies of mountain wave plots. • It’s NCAR/MMM’s printer, near Bill Skamarock’s office. • Skamarock notices plots. He sees possible support for his (contentious) work on ARW and NMM KE spectra (work he’d given up). • I finally find plots. Bill happens to be there. Bill revives interest in ARW/NMM spectra. • He computes some. NCAR/DTC

  7. Coming Up Next: • Motivating observations • mountain waves • DTC Winter Field Experiment (DWFE) • Kinetic energy spectra • background • backstory • accidental DWFE discoveries • ARW and NMM results • sensitivity study: NMM spectra and model configurations • Conclusions and Further Work NCAR/DTC

  8. mesoscale Kinetic Energy Spectra • Nastrom and Gage (1985):Spectrum computed from GASP observations(commercial aircraft) • Lindborg (1999):Functional fit to MOZAIC observations (aircraft) NCAR/DTC

  9. Backstory • Skamarock and Baldwin (white paper, Nov. 2003) • NMM’s KE spectra show that NMM has too much dissipation • ARW’s KE spectra are fine • Janjic and Black (NCEP) respond: • Not true, on both counts • Don’t know what mesoscale model spectra should look like, anyway • Skamarock (MWR, 2004) refines white paper arguments; veils references to NMM. NCAR/DTC

  10. The Parable (cont’d) • Skamarock’s NMM and ARW KE spectra of DWFE forecasts (Jan. 2005) resemble previous spectra. Dispute over KE spectra threatens to revive. • DTC idea: have Dempsey act as neutral investigator, recalculate spectra, try to get to bottom of Skamarock/Janjic differences. • Hmmm …. NCAR/DTC

  11. Questions • Do DWFE NMM & ARW model spectra resemble the Nastrom & Gage (1985) observed spectrum? • Should they? • If they should, and if they do, do they for the right (physical) reasons? And how can we tell? • If not, why not? NCAR/DTC

  12. Computing model KE spectra • Interpolate u, v to pressure levels • Detrend u, v along each 1-D transect • Compute discrete Fourier transform of kinetic energy along each E/W (or N/S or diagonal) transect • Average spectra over: • multiple transects, and • pressure layer (100 mb deep) NCAR/DTC

  13. NMM “E” grid: compute KE spectraon transects: • diagonally; • east/west; or • north/south • ARW “C” grid: compute spectra: • east/west, or • north/south NCAR/DTC

  14. No terrain smoothing Smallest-scale terrainsmoothed Accidental DWFE Discoveries 1. Tested rewritten KE spectra code on NMM and ARW DWFE topography ARW (DWFE) NMM (DWFE) NCAR/DTC

  15. Spike in initial energyat L = 2x at all levels • (diagonal spectra only) • Gone by 3 hr forecast Accidental DWFE Discoveries 2. Bug in WRF-NMM-SI initialization Contour plot: Initial u-component(2x pattern due to array index error) NCAR/DTC

  16. DWFE Case: 3/27/05 MSL Pressure &3-hr Precip (24 hr fcst) 500 mb Height & w(24 hr fcst) NCAR/DTC

  17. mesoscale mesoscale Spin-up of Kinetic Energy Spectra • Spectra in 300-200 mb layer • 6 hour intervals, 00 to 48 hrs • Note: get mature spectrum by ~< 24 hours ARW NMM initial initial NCAR/DTC

  18. “Mature” Kinetic Energy Spectra • Average over 24-48 hours. Three tropospheric layers shown • Note: • NMM: less mesoscale energy aloft • NMM: more energy on smallest scales, esp. at lower levels ARW NMM Uppertroposphere Uppertroposphere NCAR/DTC

  19. Questions: • Why did NMM spectrum have less energy at mesoscales aloft than observations and ARW, hence lacking transition to k-5/3 slope? • Why did NMM spectrum have more energy than ARW at smallest scales at low altitudes? • Hypotheses: • NMM has unsmoothed topography, very low horizontaldiffusion, and horizontal-divergence damping Initial Testing Strategy: 1. Rerun NMM with various bug fixes 2. Rerun with smoothed topography 3. Rerun without horizontal-divergence damping 4. Rerun with more horizontal diffusion (2nd order, 4th order) NCAR/DTC

  20. Some results of NMM reruns: • Original DWFE run • 3 levels shown; 24-48 hr fcst avg • Rerun #1: Bug fixes • 3 levels shown; 24-48 hr fcst avg 1. Fixing bugs: little effect on spectra NCAR/DTC

  21. modest (x 0.5) decrease in energy at small scales • Rerun #1: Unsmoothed terrain • 3 levels shown; 24-48 hr fcst avg • Rerun #2: Smoothed terrain • 3 levels shown; 24-48 hr fcst avg NMM, unsmoothed terrain NMM,smoothedterrain ARW,smoothedterrain Some results of NMM reruns: 2. Smoothed terrain: NCAR/DTC

  22. Oops! Some results of NMM reruns: • Rerun #3: No divergence damping • Forecasts: 00, 03, 06, 09, 12 hrs; • 300-200mb layer 3. No divergence damping: NCAR/DTC

  23. External modes! Surface pressure change after 03 hrs (contour interval = 2 mb) ConUSdomain NCAR/DTC

  24. External modes:Vertical Velocity Cross Section • 03 hr forecast NCAR/DTC

  25. Revised Testing Strategy: • Rerun with no horizontal-divergence damping butadd a vertically-integrated horizontal-divergence damper (to damp external modes) • Rerun with more horizontal diffusion (2nd order, then 4th order) NCAR/DTC

  26. Results: • Rerun #4: No divergence damping; external-mode damping added • Rerun #3: No divergence damping • 18 hr forecast • 3 tropospheric levels • 18 hr forecast • 3 tropospheric levels External mode removed NCAR/DTC

  27. Conclusions • ARW in DWFE: • Kinetic energy spectra similar to Nastrom & Gage (1985) observations at mid-mesoscales (~7Dx) and larger • Implicit damping and weak explicit diffusion, plus smoothed topography, reduce small scale energy • Arguably desirable (Skamarock and others); or maybe not (Janjic) • NMM in DWFE: • NMM spectra showed less energy at mesoscales than Nastrom & Gage (1985) observations, especially aloft • Can apparently change this by replacing horizontal-divergence damping with sufficient external-mode damping • Spectra show more energy at smallest scales than ARW (but less than observed) • smoothing topography reduces small-scale energy somewhat • can debate whether need more small-scale damping NCAR/DTC

  28. Further Work (NMM) • Fine-tune external-mode damping • Try 4th-order diffusion to reduce energy selectively on smallest scales • NCEP: Adapt 4th-order diffusion to parallel computing (doesn’t work yet) • Evaluate effect of these alternative filters on meteorological fields and traditional verification statistics • NOAA: Test against T-REX data (mountain waves) and Rapid Refresh runs NCAR/DTC

  29. 24-Hour Precipitation BiasesDWFE: Jan 15-Mar 31 2005 6.0 WRF-NMM WRF-ARW Eta 1.0 NCAR/DTC

  30. Grid-Point Storms? (NMM) • 500 mbverticalvelocity • 24 hr forecast • bull’s eyepatterns NCAR/DTC

  31. Grid pts. A Bull’s Eye Close-up:500 mb Vertical Velocity • Data on native “E”-grid(no interpolation orsmoothing beforecontouring) • Depth of verticalmotion: sfc to 300 mb(max near 300 mb) NCAR/DTC

  32. WRF-NMM 24h Forecast (for 00Z 2/15/05) NCAR/DTC

  33. WRF-NMM, 13 Feb 2005 • 00Z init, 24h forecast • vertical velocity (cint = 0.2 m/s) • model level 10 (~ 900 mb) NCAR/DTC

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