Chapter 12

1. Chapter 12 Small-Scale Winds

2. Figure CO: Chapter 12, Small-Scale Winds--Fog over Golden Gate Bridge © Andy Dean Photography/ShutterStock, Inc.

3. Small-Scale Winds • Subsynoptic-scale weather • Weather phenomena that develop and change across distances you can see (a few tens of miles or less) • Coriolis force usually not important • Balance of forces between horizontal pressure gradient and friction • Geography and topography are crucial

4. Friction, eddies, and turbulence • Molecular viscosity is friction near the ground • Eddies are viscosity within the atmosphere • Eddies are swirls of air that arise as the wind blows around obstacles • Eddies also arise from daytime heating • The atmosphere itself also produces eddies of all sizes • The eddies are also called turbulent eddies

5. Turbulence • Is the irregular almost random pattern of wind • Is bumpiness due to small-scale changes in the wind • Has no precise definition • At smaller scales, winds are slowed down and made irregular, or turbulent, by the effect of eddies

6. Turbulence • Acts like a brake on the pressure gradient force which sets air in motion from high towards low pressure • At the smallest scales, true molecular friction robs the eddies of the energy they take from the wind

7. Figure 01: The relationship among eddies, turbulence, and wind gusts

8. Clear-Air Turbulence (CAT) • Eddies in the upper troposphere are about the same size as turbulent eddies • Aircraft avoid turbulence they can see: • Microbursts • Lenticular clouds • Parallel lines of clouds near mountains • Clear-air turbulence is usually invisible • Keep your seat belt fastened, CAT can kill

9. Figure B01: Photo of wave clouds breaking © Kay Ekwall, www.mtshastaphotography.com

10. Figure 02: Geographic summary of small-scale winds across the contiguous U.S.

11. Mt. Washington, a windy place • Mt. Washington, NH, is an isolated mountain peak—winds blow over, not around the peak • At a height of 6288 feet, has persistent clouds, heavy snow, cold temperatures and record-setting high winds • Record wind: 231 mph set here in 1934, a record for surface wind • Winds exceed hurricane force on average 104 days per year

12. Coastal Fronts • Common in New England and along the east coast of the US • Cold air near mountains; warmer air offshore can lead to a miniature stationary front • Heavy snow—rain separated by only a few km • Stubborn entrenchment of cold air pinned against high mountains is called cold air damming: accompanied by freezing rain

13. Figure 03: Wind flow Source: SSEC, University of Wisconsin-Madison

14. Gravity waves • Alternating patterns of high and low pressure maintained by gravity • Sometimes form long straight lines of clouds • Form when wind blows over a mountain or a thunderstorm • Wind changes in the jet stream can send out ripples of waves • Are very difficult to forecast

15. Figure 04: Lines of clouds caused by gravity waves in the lee of the Appalachian Mountains Courtesy of SSEC, University of Wisconsin-Madison

16. Figure 05: Water vapor image over Alabama Courtesy of CIRA/Colorado State University and NOAA

17. Figure 06: Automated observations of wind and pressure at Birmingham, AL Source: Bradshaw, John T., et al., The Alabama gravity wave event of February 22, 1998. NOAA, 1998. Retrieved February 28, 2011, from http://www.srh.noaa.gov/bmx/n=research_02221998

18. Figure 07: Gravity wave climatology Adapted from Koppel, L., et al., Monthly Weather Review, January 2000: 58

19. Lake Breezes • Resemble the sea breeze: the water is cold compared to the land and a wind blows from the water to the land • The boundary between the lake breeze and the land air can be a focal point for thunderstorm development

20. Figure 08: Lake breeze Courtesy of SSEC, University of Wisconsin-Madison

21. Derechos • Straight-line winds of up to 150 mph forming an hours long windstorm along a line of severe thunderstorms • Storms typically form along a stationary front in summer • Storms form a bow echo • Responsible for 40% of all thunderstorm injuries and deaths • Cause extensive property and tree damage

22. Figure 09: Radar of derecho Courtesy of NOAA

23. Figure 10: A climatology of derechos Modified from Coniglio, M. C., and D. J. Stensrud, Wea. Forecasting 19 (2004): 595-605

24. Blue Northers • Are fast-moving dry cold fronts that sweep across the plains to Texas • Northerly winds occur behind the front • No clouds accompany the fronts • A sharp temperature drop marks the front

25. Snow fences and windbreaks • Help slow the wind like speed bumps do to traffic on a road • Cause turbulent eddies to develop • Snow fences keep snow from blowing across land and roadways • Windbreaks keep soil from blowing across land and roadways

26. Figure B02: Snow acts as a blanket in winter Courtesy of Steven Ackerman

27. Dust storms and the Dust Bowl • A pressure gradient and dry ground are all that are needed for a dust storm • Dry line thunderstorms with downbursts • Dry fronts like blue northers • The dry slot of an extratropical cyclone • Drought in the 1930s: 14 dust storms in 1932 and 38 in 1933 • Soil conservation efforts, wetter conditions prevent dust storms

28. Figure B03: Dust storm Courtesy of NOAA's National Weather Service (NWS) Collection

29. Heat bursts • Originate as high updrafts • Sinking air warms at DALR as it is compressed • Like a hot microburst, air splashes against the ground an spreads out • Last about 30 minutes, have winds of 41 mph on average, and can cause damage • Temperatures rise and dew point falls • Captured by mesonetworks

30. Figure 11A: Heat burst, 11B: Heat burst, 11C: Heat burst Source: Oklahoma Climatological Survey & The Oklahoma Mesonetwork

31. Figure 12: Temperature and dew point plots for heat burst Source: NOAA

32. Chinooks • Warm dry winds on the downslope side of a mountain range • Air warms at the DALR as it descends • Air arrives at the surface warm and dry • Can raise the air temperature extremely rapidly • Have different names in different parts of the world

33. Mountain/Valley winds and windstorms • Upslope winds during the day when the slopes are warmed • Downslope winds at night when the slopes cool • Usually gentle; when strong are called katabatic winds • Any strong pressure gradient can cause funneling of the wind in passes and cause a windstorm with property damage

34. Figure 13: Mountain/valley breezes

35. Figure 14: Winds in the Boulder, Colorado, windstorm of February 2, 1999 Source: University Corporation for Atmospheric Research

36. Dust devils • Thin, rotating columns of air • Created by solar heating • Unstable air rises and creates a tiny low-pressure center • Form under clear skies • Seldom cause damage

37. Figure 15: Dust devil Courtesy of NASA

38. Lenticular clouds • Formed when moist air rises on the crest of a gravity wave, gets saturated • Look like lenses • Stay in the same place • Are a sign of turbulence nearby and beneath the cloud, in spite of its smooth appearance

39. Figure 16: Lenticular cloud Courtesy of Cynthia Stoneburner

40. Figure 17: Wave cloud diagram

41. Figure 18: Flight turbulence Adapted from Lester, P. Turbulence. Jeppesen, 1994.

42. Santa Ana Winds • Another downslope wind • Caused by pressure gradient of an anticyclone over the Rockies and friction • Forces already dry air down the Coast Range or the San Gabriel mountains and out to the ocean • Most common in autumn • Temperature increases and dew point decreases

43. Santa Ana Winds (continued) • Occur in a heavily populated area • Cause extreme fire danger • Similar winds are observed at other locations in other parts of the world

44. Figure 19: Santa Ana Winds Courtesy of JPL/NASA

45. Von Kármán Vortex Sheet • A long interlocking chain of ripples downwind of a mountain • Caused when wind flows around rather than over a mountain • Air closest to the mountain is slowed; farther away air is deflected • Wind shear causes deflected air to roll up into interlocking pairs of vortices, one cyclonic and one anticyclonic; not dangerous

46. Figure 20: von Kármán vortex Courtesy of NASA/EROS, USGS

47. Figure 21: Global wind distribution