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Topographic Forcing and Front Range Winter Storms

Topographic Forcing and Front Range Winter Storms. 24-25 October 1997 Front Range Blizzard and Blowdown in WY/CO/NE/KS Doug Wesley Winter Weather Workshop, August 2002. OUTLINE. Topographic forcing in the context of a strong Front Range winter storm, 24-26 Oct. 1997

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Topographic Forcing and Front Range Winter Storms

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  1. Topographic Forcing and Front Range Winter Storms 24-25 October 1997 Front Range Blizzard and Blowdown in WY/CO/NE/KS Doug Wesley Winter Weather Workshop, August 2002

  2. OUTLINE • Topographic forcing in the context of a strong Front Range winter storm, 24-26 Oct. 1997 • Eta and Meso-Eta forecasts • RAMS forecasts • Advantages of improved NWP resolution in these types of storms • Influence on precipitation fields of small-scale terrain features and larger-scale wind fields

  3. CYS WFO, evening of 24 Oct. 1997

  4. Cheyenne, WY 25 Oct. 1997

  5. Cheyenne, WY 25 Oct. 1997

  6. I-25 Corridor, 25 Oct. 1997

  7. Terrain reliefj:/shared/for_wesley/canada/NorthAmerica….bmp • Rocky Mountain barrier; Colorado region has probably the highest continuous N-S barrier in North America • Relatively lower “plains” to the north • East-west ridges on the “plains” • Source region of modified arctic air masses

  8. Terrain relief in the western US

  9. Idealized portrayal of effects of terrain on winds • http://meted.ucar.edu/mesoprim/flowtopo/index.htm (pp. 1-2, 4-9)

  10. Lee-side eddies/convergence • “Longmont Anticyclone”

  11. Lee eddies (cont’d) • “Denver convergence-vorticity zone”

  12. Eta and meso-Eta model terrain • AWIPS: 2 grids; 40 and 80 km grid spacing, although model is currently running at 12 km • GARP: 80 km plotted grids utilized for Eta in 1997 (model data computed at 48 km) • Misses many major terrain features • Smoothness of N-S cross section

  13. Eta terrain 1997

  14. West-east cross section, Eta

  15. North-south cross section, Eta

  16. North-south Eta x-section, 2002

  17. Meso-Eta terrain (1997)

  18. 12 km Eta terrain (AWIPS)

  19. Sample omega field 2002, Eta

  20. J. Steenburgh’s findings • http://www.met.utah.edu/jimsteen/imtn2001/imtn_wkshp_2001_files/v3_document.htm

  21. RAMS model terrain • 3 grids: 15, 5, 1.7 km

  22. Geographical grid setup

  23. RAMS grid 1 terrain

  24. RAMS grid 2 terrain

  25. RAMS Grid 3 terrain

  26. RAMS 5km grid S-N cross section, through approx. DEN & CYS

  27. Storm overview/synoptics • Upper levels: deep, intensifying cutoff low pressure system over the four corners region and eastward • Influx of cold low-level air from north • Timing of above 2 factors favorable • Result was intense upslope flow for 12+ hours over the Front Range of the Rockies

  28. 700 mb analysis

  29. Platteville profiler data

  30. Observed snowfall

  31. Satellite analysis of snow coverage

  32. Eta 18hr forecast, valid 0600 25 Oct.

  33. Eta 36 hr. accum. precip. at 00Z 26 Oct.; simulation began at 00Z 24 Oct.

  34. Eta 36 hr. accum. precip. at 00Z 26 Oct.; simulation began 12Z 24 Oct.

  35. Meso-Eta 24 hr. accum precip. at 00Z 26 Oct.; simulation began at 15Z 24 Oct.

  36. CSU RAMS Mesoscale Model Characteristics ·Non-hydrostatic ·Nesting capable ·Terrain-following vertical coordinate ·Microphysics: Mixed-phase; predicts cloud droplets, raindrops, pristine ice crystals, snow, aggregates, graupel, hail. . (Generalized gamma distribution used for each) ·Eta (forecast) boundary conditions for this study ·Grid spacing for this study: 15, 5 and 1.67 km

  37. RAMS QPF • Grid 1 • Grid 2 • Grid 3 • Precip. rates compared with observations at Jeffco… “R1” means full microphysics package

  38. RAMS grid 1 total precip., 36 hr

  39. Grid 2 RAMS total precip, 36 hr.

  40. Grid 3 RAMS total precip., 36 hr.

  41. Point comparison, precip., at Marshall site

  42. Northeast of Steamboat Spgs., CO(sometime after 26 Oct. 1997)

  43. Blowdown regions

  44. Aug. 2002 “Hinman” fire

  45. Wind predictions/verification • Observed: mountain wind gusts to 50 m/s on western slope • Meso-Eta: 25 m/s aloft, no amplifying wave (less than 10 m/s at lowest levels) • RAMS 5 km grid: ~25 m/s @ 50m; amplifying mountain wave • RAMS 1.7 km grid: ~30 m/s @ 50m and > 40 m/s aloft; generally ~33% greater speeds; evidence of rotors and non-linear wave dynamics

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