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. New 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 threesecond running average speed is stored for gust and peak wind processing. • Most sites have switched to the new sensor. • The new sensor isbe 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. Momentum mixing: 12/17/06 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: du 1 p fv ku2 dt x u u 2 1 p fv ku2 t x 2 x • This is simply a form of Bernoulli’s equation. Assuming steady state and no friction: u2 1 p x 2 x integrate u12 u02 2P 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 winds are a boundary layer phenomena – Must account for drag (both surface drag and drag at the inversion) Gap Flow 101 - Basics • Reintroduce friction (bulk aerodynamic form) u u 2 1 p ku2 t x 2 x • integrate p p u12 u02 e 2 kx k k where k 2.8CD H 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) Gap Exit • The strongest winds are generally in the gap exit region. Strait of Juan de Fuca is well known for its easterly gales in the gap exit region. Fraser River NE Gap Flow Max Winds, 28 Dec. 1990 > 40 ms-1 The Columbia River Gorge Near Sea Level Gap On Border of WA and OR Domain Definition Troutdale 36 km grid spacing 12 km grid spacing Portland The Dalles Portland The Dalles Pass Height = 700 m Pass Height = 600 m 4 km grid spacing Portland Cascade Locks The Dalles Pass Height = 400 m 12 km grid spacing Portland The Dalles Pass Height = 600 m 1.33 km grid spacing, Pass Height = 150 m • Portland Cascade Locks Troutdale The Dalles 444.4 m grid spacing, Pass Height = 100 m Portland Troutdale Cascade Locks T on 150 m Surface Portland Troutdale 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) Mesoscale Pressure Perturbations Sea Level Pressure February 13 1979: The Hood Canal Storm Winds over 100 kts destroyed the Hood Canal Bridge Cost to replace: over 100 million dollars Sea Breeze Winds Average Summer Wind Speeds 18 16 14 Wind Speed (knots) 12 10 8 6 Hoquiam 4 Newport North Bend Gold Beach 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hour (LDT) 2-minute average July-August winds along the Northwest coast. Interactions with larger scale flow Southern Oregon Coast Near Brookings 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. Downslope Windstorms Enumclaw, WA Mountain Wave 101 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. 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. Downslope Windstorms Under the proper circumstances (e.g., a critical level aloft) the wave can amplify and break, resulting in a downslope windstorm 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 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 =2*pi/L , where is the length scale of the terrain •k < l: vertically propagating, k >l trapped Critical Levels • A critical level occurs when the flow normal to the mountain barrier reverses. 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. Stable Layer • A stable layer near crest level with less stable air above can act like a critical level. • Happens relatively frequently. 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. Maximum Wind Gusts on 24 December 1983 Enumclaw: “Place of Evil Spirits” High-Resolution MM5 Simulations Do An Extremely Good Job of Predicting/Diagnosing Such Gap/Downslope Windstorm Hybrids Extreme Longevity and Sharpness of Gap Flow Extreme Mesoscale Winds During Synoptic Windstorms Bob Houze’s home after a minor windstorm: he was standing outside and had to jump for his life! The Most Extreme of the Extreme: The Columbus Day Windstorm of 12 October 1962 Max Winds (mph) Columbus Day Storm 1962 Columbus Day 1962: At Cape Blanco there were 150 mph with gusts to 179! Strongest winds on bluffs and windward slopes of coastal orography The Thunderbird The Indians knew about the mesoscale patterns of windstorm severity: they moved summer camps away from the coast.