A Temperature Minimum of 56C: How does a Cold Air Pool Form in a Small, HighAltitude Limestone Sinkh

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Local Wind Systems in Mountainous Terrain
Dr. Craig Clements Met 172
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Types of Thermally-Driven Winds found in
Mountainous Regions
1. Plain-Mountain Winds 2. Valley Winds 3.
Slope Winds
Thermally-driven refers to the forcing due
to temperature differences!
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Thermally-Driven Winds Found in Mountains
Whiteman(2000)
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Cross-section of a Mountain Valley
Whiteman(2000)
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Valley Winds
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Observations
Up-Valley and Down-Valley Surface Winds (Measured
in Yosemite National Park)
Date and Time
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Valley Winds
Daytime Air is warmer in the valley than over
the plain Pressure is lower in the valley and
higher over the plain at the same elevation The
pressure gradient force is directed from the
plain to the valley A up-valley wind is produced
that blows from the plain into the
valley. Nighttime Pressure gradient force
reverses direction A down-valley wind occurs
Up-Valley Winds
Down-Valley Winds
Whiteman(2000)
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A SODAR (sound-detection-ranging) is similar to
RADAR
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Observations
Time-Height SODAR Wind Profiles 12 August 2003
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Vertical Structure of Down-Valley Winds
Yosemite National Park, 12 Aug. 2003
Wind minimum
Nose is location of Wind speed maximum
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Conceptual wind models for mountain valleys
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(Whiteman 1982)
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The Volume Effect of Valleys
Whiteman(2000)
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Examples of Valley Shapes
Whiteman(2000)
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Topographic Amplification Factor (TAF)
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Yosemite Valley, Yosemite National Park
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Mass conservation in a valley
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Inversion destruction Models in mountain Valleys
(Whiteman 1982)
Pattern 1
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Inversion destruction Models in mountain Valleys
(Whiteman 1982)
Pattern 2
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Inversion destruction Model in mountain Valleys
(Whiteman 1982)
Pattern 3
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Diurnal Temperature Evolution in Mountain Valleys
(from Stull 1988 adapted from Whiteman 1982)
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A Simplified Heat Budget of the Valley Atmosphere
Term 1 local rate of change of potential
temperature Term 2 convergence of potential
temperature flux by mean wind Term 3 convergence
of radiative flux Term 4 convergence of
turbulent sensible heat flux
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The thermodynamic model developed by Whiteman
and McKee (1982)
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Tethersonde Profiles from Yosemite Valley
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Modeled Inversion destruction
(a)
Inversion breakup according to Eq. 2 with Ao
0.45, (a) ? 0.007 K m-1 and (b) ? 0.015 K
m-1 TAF (?) values are indicated in legend.
(b)
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Slope Winds
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Upslope and Downslope Winds

• Sunlight heats mountain slopes during the day and
  the slopes cool by radiation at night
• Air in contact with the surface is heated/cooled
  in response
• A difference in air density is produced between
  air next to the mountainside and air at the same
  altitude away from the mountain
• Density differences produce upslope (day) or
  downslope (night) winds
• Daily upslope/downslope wind cycle is strongest
  in clear summer weather when prevailing winds are
  light

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Slope winds
Whiteman(2000)

• slope winds form from a temperature gradient
  between air next to the slope and air
  horizontally adjacent to the slope.
• Slope winds are usually in the range of 1-4 m/s
  (2-8 mph) are weaker and more gentle than valley
  winds.
• Peak wind speed occurs a few meters above the
  the slope surface.
• Daytime upslope winds are typically stronger and
  deeper than nighttime downslope winds.

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Whiteman(2000)

• Clouds cause brief periods of shade on slopes
  and thus,
• weaken the strength or even reverse the
  slopewinds

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Consequences of downslope flows
Whiteman(2000)
Downslope winds are often called drainage winds
Downslope winds can produce a cold air pool in a
valley or basin. Some of these cold air pools can
last several days to a week, trapping pollutants
in the valley/basin. Cold air pools are often
associated with dense fog, which is hazardous to
aviation.
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Peter Sinks Experiment
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Peter Sinks Experiment
The goal of the experiment was To determine the
role of the slope flows on cold air pool buildup
in a mountain basin. So, the experiment was
designed to measure wind and temperatures along
the slopes of the Peter Sinks (one of the coldest
spots in the US, record of -56 C).
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Peter Sinks Basin
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Observations Slope Flow Evolution in the Peter
Sinks Basin
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METCRAX 2006 Meteor Crater Experiment
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Slope Flow Evolution during METCRAX 2006
Turbulent entrainment causes slope flow depth to
grow with downslope distance.
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Special cases of slope flows
Whiteman(2000)

• Snow is cold and downslope winds occur over snow
• Tree canopy is warm and heated and upslope winds
  form.

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Glacier Winds
A cold air layer forms over ice surface and flows
downhill.
Whiteman(2000)
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Diurnal Evolution of the Boundary Layer over
Mountains
Whiteman(2000)
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Do valley winds always follow the classic
understanding in all mountain areas?
Do winds always flow up valley during the day and
down valley during the night?
Of course not! One example is the Washoe Zephyr
wind system.
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Surface wind roses at each site
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Frequency distributions of winds from the
southwest to northwest quadrant as a function of
year and hour of day for (a) all seasons.
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Frequency distributions of winds from the
southwest to northwest quadrant as a function of
year and hour of day for (b) the summer season.
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Frequency distributions of westerly winds 5 ms1
at Galena for each hour of the day for all
seasons and for 200305
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(top) Difference of sea level pressure between
Sacramento and Reno, (second from top) 700-mb
wind speed and direction from Reno soundings at
0000 UTC on each day, and surface westerly
downslope wind for each hour of the day for
(third from top) Reno and (bottom) Lee Vining for
summer of 2003.
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RAMS simulated wind speed and (c),(d) potential
temperature on an eastwest cross section through
the center of the domain at (left) 1200 and
(right) 2000 LST.
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Findings Washoe Zephyr is generally not caused
by downward momentum transfer as the deep
afternoon convective boundary layer on the lee
slope of the Sierra Nevada and in the Great Basin
penetrates into the layer of westerlies aloft.
Instead, the difference in elevation between
the elevated, semi-arid Great Basin on the
eastern side and the lower region on the western
side of the Sierra Nevada provides a source of
asymmetric heating across the mountain range. The
asymmetric heating evolves during daytime,
generating a regional pressure gradient that
allows air from the west to cross over the crest
and to flow down the eastern slope in the
afternoon. Although a westerly ambient wind is
not necessary for the development of Washoe
Zephyr, its presence leads to strong Washoe
Zephyr events that start earlier in the afternoon
and that last longer.
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Summary
In mountainous regions, local wind systems
develop in response to local heating of the
terrain. The winds in these areas are usually
classified as Valley winds Slope winds And
there are special cases when the winds blow
differently than Expected.