Transcript Chapter 12

Chapter 12
Small-Scale Winds
Figure CO: Chapter 12, Small-Scale Winds--Fog over Golden Gate Bridge
© Andy Dean Photography/ShutterStock, Inc.
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
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
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
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
Figure 01: The relationship among eddies, turbulence, and wind gusts
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
Figure B01: Photo of wave clouds breaking
© Kay Ekwall, www.mtshastaphotography.com
Figure 02: Geographic summary of small-scale winds across the
contiguous U.S.
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 recordsetting 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
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
Figure 03: Wind flow
Source: SSEC, University of Wisconsin-Madison
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
Figure 04: Lines of clouds caused by gravity waves in the lee of the
Appalachian Mountains
Courtesy of SSEC, University of Wisconsin-Madison
Figure 05: Water vapor image over Alabama
Courtesy of CIRA/Colorado State University and NOAA
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
Figure 07: Gravity wave climatology
Adapted from Koppel, L., et al., Monthly Weather Review, January 2000: 58
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
Figure 08: Lake breeze
Courtesy of SSEC, University of Wisconsin-Madison
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
Figure 09: Radar of derecho
Courtesy of NOAA
Figure 10: A climatology of derechos
Modified from Coniglio, M. C., and D. J. Stensrud, Wea. Forecasting 19
(2004): 595-605
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
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
Figure B02: Snow acts as a blanket in winter
Courtesy of Steven Ackerman
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
Figure B03: Dust storm
Courtesy of NOAA's National Weather Service (NWS) Collection
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
Figure 11A: Heat burst, 11B: Heat burst, 11C: Heat burst
Source: Oklahoma Climatological Survey & The Oklahoma Mesonetwork
Figure 12: Temperature and dew point plots for heat burst
Source: NOAA
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
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
Figure 13: Mountain/valley breezes
Figure 14: Winds in the Boulder, Colorado, windstorm of February 2, 1999
Source: University Corporation for Atmospheric Research
Dust devils
• Thin, rotating columns of air
• Created by solar heating
• Unstable air rises and creates a tiny lowpressure center
• Form under clear skies
• Seldom cause damage
Figure 15: Dust devil
Courtesy of NASA
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
Figure 16: Lenticular cloud
Courtesy of Cynthia Stoneburner
Figure 17: Wave cloud diagram
Figure 18: Flight turbulence
Adapted from Lester, P. Turbulence. Jeppesen, 1994.
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
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
Figure 19: Santa Ana Winds
Courtesy of JPL/NASA
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
Figure 20: von Kármán vortex
Courtesy of NASA/EROS, USGS
Figure 21: Global wind distribution