ASOS Wind Sensor

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ASOS Wind Sensor

• The old ASOS wind sensor, the Belfort 2000, uses
  rotating cups to measure wind speed and a vane to
  measure wind direction.
• Over a two-minute period ASOS uses 24 five-second
  averages to determine the two-minute average wind
  speed and direction.
• The highest 5-second wind speed during the
  previous ten minutes is the gust. Gusts are only
  reported if there is a variation of 10 knots
  between peaks and lulls.
• The highest instantaneous wind speed (gust) since
  the last routine report is the peak wind.

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New

• Old

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The New Wind Sensor

• The new ASOS wind sensor, the Vaisala 425NWS, is
  a sonic anemometer. It has no moving parts and is
  designed to operate better in winter weather
  conditions.
• As with the Belfort sensor, over a two-minute
  period, ASOS uses 24 five-second averages to
  determine the two-minute average wind speed and
  direction. But the highest three-second running
  average speed is stored for gust and peak wind
  processing.
• Installation has started.
• The new sensor will be more responsive to
  short-term gusts. Can expect to see more gusts
  and peak winds reported with the new sensor.

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Wind Observations

• Major issue is representativeness
• Surface winds are highly variable due to varying
  surface characteristics and obstacles.
• Wind varies substantially with height and not all
  sensors are at similar elevations about the
  ground.

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Danger of Using Model Output Directly

• Lack of resolution..means larger scale models
  (e.g., GFS) cant accurately define and predict
  local winds forced by mesoscale featuresterrain,
  diurnal circulations. This is getting better.
• Physics problems and particularly PBL
  parameterization issues. MM5 and most other
  mesoscale models tend to overmix winds in the
  verticalparticularly under stable
  conditions--results in excessive winds. Winds
  generally too geostrophic
• Large scale model errorsfrom poor
  initializations and other causes.

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Momentum mixing 12/17/06
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Gap Flow 101 - Misleading the Next Generation!

• The Venturi Effect is still used in some
  introductory texts to explain gap flow!

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Gap Flow 101 - Basics

• 1-D horizontal momentum Equation
• Assume steady state, neglect Coriolis and
  friction and integrate
• This is simply a form of Bernoullis equation.
  Assuming steady state and no friction

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Gap Flow 101 - Basics

• Provides an upper limit to maximum speed at the
  end of the gap
• Commonly used in work from the early 1980s
• E.g. Walter and Overland (1981), Reed (1981)
• Over simplification.

• Gap winds are a boundary layer phenomena
• Must account for drag (both surface drag and drag
  at the inversion)

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Gap Flow 101 - Basics

• Reintroduce friction (bulk aerodynamic form)
• Shown to produce a much closer correlation to
  observed winds
• E.g. Lackmann and Overland (1989), Mass et al
  (1995), Colle and Mass (1986), Bond and Stabeno
  (1998)

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Gap Exit

• The strongest winds are generally in the gap exit
  region.

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Strait of Juan de Fuca is well known for its
easterly gales in the gap exit region.
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Max Winds, 28 Dec. 1990
Fraser River NE Gap Flow
gt 40 ms-1
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The Columbia River Gorge
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• Near Sea Level Gap
• On Border of WA and OR

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DomainDefinition
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Troutdale
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12 km grid spacing
Pass Height 600 m
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1.33 km grid spacing, Pass Height 150 m
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444.4 m grid spacing, Pass Height 100 m
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T on150 mSurface
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Vertical Structure

• Strongest winds near exit
• Hydraulic effects are important

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Gap Winds in the Real World

• Strongest winds tend to be in exit region because
  of hydraulic collapse and because of larger scale
  pressure gradient.
• There can be some venturi acceleration in narrow
  regionsbut that tends to be secondary.

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Mesoscale Pressure and Wind Perturbations on
Mesoscale Terrain Barriers

• A controlling parameter is the Froude number
• FR U
• hN
• where U is the speed, h is the height of the
  barrier, and N is stability (Brunt-Vaisalla freq)
• Large FR is associated with flow going up and
  over terrain (large vertical excursions), small
  FR with flow being deflected around
  (quasi-horizontal flow)

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Mesoscale Pressure Perturbations
Sea Level Pressure
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February 13 1979 The Hood Canal Storm
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Winds over 100 kts destroyed the Hood Canal
Bridge Cost to replace over 100 million dollars
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Sea Breeze Winds
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2-minute average July-August winds along the
Northwest coast.
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Interactions with larger scale flow
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Southern Oregon Coast Near Brookings
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Sea Breeze Winds Along the Southern Oregon Coast

• Gusts frequently reach 30-35 knots during the
  summer during the afternoon.
• Very painful to stay on the beach!
• Strong pressure gradient normal to the coast
  between the coastal thermal trough and cold
  upwelling water.

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Downslope Windstorms
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Enumclaw, WA
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Mountain Wave 101
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Trapped Lee Waves
Lee waves whose energy does not propagate
vertically because of strong wind shear or low
stability above are said to be "trapped.". These
waves are typically at an altitude within a few
thousand feet of the mountain ridge crest and
turbulence is generally restricted to altitudes
below 25,000 feet, particularly in rotors.
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Vertically Propagating Waves
Vertically-propagating waves frequently become
more amplified and tilt upwind with height.
Tilting, amplified waves can cause aircraft to
experience turbulence at very high altitudes.
Clear air turbulence often occurs near the
tropopause due to vertically-propagating waves.
Such waves have been documented up to 200,000
feet and higher.
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Downslope Windstorms

• Under the proper circumstances (e.g., a critical
  level aloft) the wave can amplify and break,
  resulting in a downslope windstorm

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Froude Number and Mountain Waves

• The Froude number expresses a ratio between the
  kinetic energy (wind speed) and the potential
  energy (stability times mountain height).
• If the Froude number is equal to or slightly
  greater than 1, then there is the likelihood of
  mountain wave activity
• If the Froude number is less than one, then the
  airflow is insufficient to carry the flow over
  the mountain and the flow is blocked
• If Froude number is much more than 1, airflow
  proceeds right over the mountain and down the
  other side, with no significant oscillations

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Trapped vs Vertically Propagating

• A key parameter controlling the nature of
  mountain waves is the Scorer Parameter (l)
• l2 N2 - d2U
• U2 Udz2

• k is the primary wavenumber of the terrain
  2pi/L
• , where is the length scale of the terrain
• k lt l vertically propagating, k gtl trapped

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Critical Levels

• A critical level occurs when the flow normal to
  the mountain barrier reverses.

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Critical Level

• Critical levels may be self-induced by wave
  breaking or result from a mean state condition in
  the overall flow.
• Critical levels do not allow the
  vertically-propagating energy associated with
  mountain waves to continue upwards. Instead, that
  energy is deflected by the critical layer back
  toward the surface. Consequently, critical levels
  can contribute to the development of, and/or the
  strengthening of, downslope windstorms.
• The speed of those winds can be 2-3 times the
  upwind speed at mountaintop height.

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Stable Layer

• A stable layer near crest level with less stable
  air above can act like a critical level.
• Happens relatively frequently.

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What to look for for strong vertically
propagating mountain waves

• Strong winds approaching the barrier (and Froude
  number greater than one so air goes over the
  mountains). Winds should be within 45 degrees of
  normal
• Stable layer near crest level. Lesser stability
  aloft.
• Critical level above the mountain barrier (to
  promote wave breaking).
• The existence of weak vertical wind shear or
  reverse shear (winds decreasing with height) are
  more favorable than forward shear (winds
  increasing with height).
• Strong downslope windstorms are often associated
  with large cross-barrier pressure gradients, but
  it is not clear whether those are cause or effect.

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Maximum Wind Gusts on 24 December 1983
Enumclaw Place of Evil Spirits
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High-Resolution MM5 Simulations Do An Extremely
Good Job of Predicting/Diagnosing Such
Gap/Downslope Windstorm Hybrids
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Extreme Longevity and Sharpness of Gap Flow
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Extreme Mesoscale Winds During Synoptic Windstorms
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Bob Houzes home after a minor windstorm he was
standing outside and had to jump for his life!
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The Most Extreme of the Extreme The Columbus
Day Windstorm of 12 October 1962
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Max Winds (mph) Columbus Day Storm 1962
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Columbus Day 1962 At Cape Blanco there were 150
mph with gusts to 179! Strongest winds on bluffs
and windward slopes of coastal orography
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The Thunderbird
The Indians knew about the mesoscale patterns of
windstorm severity they moved summer camps away
from the coast.
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