The current asos wind sensor, the Belfort 2000, uses rotating cups to measure wind speed and a vane to measure wind direction

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ASOS Wind Sensor

  • The current 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.


  • Old

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.

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.

Danger of Using Model Output Directly

  • Lack of resolution..means larger scale models (e.g., GFS) can’t accurately define and predict local winds forced by mesoscale features…terrain, 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 vertical…particularly under stable conditions--results in excessive winds. Winds generally too geostrophic

  • Large scale model errors…from poor initializations and other causes.

Gap Flow 101 - Misleading the Next Generation!

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

Gap Flow 101 - Basics

  • 1-D horizontal momentum Equation:

  • Assume steady state, neglect Coriolis and friction and integrate:

  • This is simply a form of Bernoulli’s equation. Assuming steady state and no friction:

Gap Flow 101 - Basics

  • Provides an upper limit to maximum speed at the end of the gap

    • Commonly used in work from the early 1980’s
      • E.g. Walter and Overland (1981), Reed (1981)
    • Over simplification.

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)
    • Predicted by Overland’s (1984) scale analysis
  • Assumptions:

    • No elevation change along the gap (though this can be accounted for relatively easily)
    • Flow eventually accelerates so that balance develops between friction and pressure gradient force.

Gap Exit

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

The Columbia River Gorge

Vertical Structure

  • Strongest winds near exit

  • Hydraulic effects are important

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 regions…but that tends to be secondary.

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)

Sea Breeze Winds

Interactions with larger scale flow

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.

Extreme Mesoscale Winds During Synoptic Windstorms

Downslope Windstorms

Mountain Wave 101

Trapped Lee Waves

Vertically Propagating Waves

Downslope Windstorms

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

Trapped vs Vertically Propagating

  • A key parameter controlling the nature of mountain waves is the Scorer Parameter (l)

  • l2 = N2 - d2U

  • U2 Udz2

What to look for

  • Strong winds approaching the barrier (and higher Froude number so air goes over the mountains). Winds should be within 30 degrees of normal

  • Stable layer near crest level. Lesser stability aloft.

  • Critical level above the mountain barrier (to promote 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.

Snow Depth: December 18-19, 1990

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