1. Wind Prediction ATMS 452 Spring 2019

2. NWS/FAA Wind Observations ASOS Sensors Have Been Changed from Three-cup Anemometers to Acoustic/Sonic Units

3. Old ASOS Wind Sensor
The old ASOS wind sensor, the Belfort 2000, used rotating cups to measure wind speed and a vane to measure wind direction.
Over a two-minute period ASOS used 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 was the gust. Gusts were 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 was the peak wind.

4. Problems With Old Wind Sensors:
Icing/riming, start up thresholds

5. New Sonic Wind Sensor (direction and speed)

6. The New Wind Sensor
The new ASOS wind sensor, the Vaisala 425NWS, is a sonic anemometer. 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.
The new sensor is more responsive to short-term gusts. Can expect to see more gusts and peak winds reported with the new sensor.

7. Wind Observations
Major issue is representativeness (e.g., sensor in valley, or sheltered by house/tree)
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. Should be 10-m.

8. Wind Gusts

10. Hood Canal Bridge 2019

11. Wind Gusts Associated with Mixing of Higher Momentum From Aloft

12. Gust Ratio Depends on Wind Speed (~1.35 at higher speeds)

13. Why?
Turbulence is weaker and more intermittent when when speeds are low.
More vertical wind shear when turbulence is weak.
Only occasionally does air mix down momentum from aloft

14. Do Model Winds Include Gusts?

15. NO!

16. Observed Wind Speed at the University of Washington
(Sampled every 5 seconds: reports 1 minute averages and highest 5 second wind gusts each minute )

17. MM5 Output Every Time Step from the 4-km Domain: Much Smoother!
Wind Speed

18. WRF or MM5 4-km output every time step appears to have the temporal variability of approximately 15 minute-average winds
Why? Winds are averaged spatially due to model resolution, grid-box averaging of some terms, and model numeric and explicit diffusion/smoothing. Thus, for verification we should compare model wind speeds to temporally averaged observations (~10-15 min).

19. Gust Guidance for the Forecaster
Gust ratio depends on wind speed and vertical stability
In a neutrally stable environment, forecasters often look for the max wind in the PBL as a measure of the max gust
There is software that does this as well.

20. How do we do this for WRF? Historical ratio used (around 1.4)

21. Danger of Using Model Winds 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 as resolution increases.
Physics problems and particularly PBL parameterization issues. WRF and most other mesoscale models tend to overmix winds in the vertical…particularly under stable conditions--results in excessive winds. Winds generally too geostrophic

27. Speed bias worse for low wind speeds

28. All winds

29. 3 knot and more

30. And, of course…
Large scale model errors…from poor initializations and other causes.

31. Use of Models
Aloft: Today’s synoptic and mesoscale models are sufficiently accurate that it is very hard to beat their wind forecasts aloft. The forecaster’s role is mainly in deciding which model to use and perhaps altering the timing, if phase errors are obvious.
At the surface, the models are getting better, but there are larger biases and other errors.

32. Relationship of SL Pressure and Surface Winds
Traditionally, an important tool of the wind forecaster was to start with the SLP pressure fields and deduce the surface (10-m) winds from it.
Still useful to keep in mind
Lets review the pressure-wind connection.

33. Open Areas and Relatively Low Drag
If low drag and relatively steady state conditions, the geostrophic or gradient wind relationships are useful.
Normally centrifugal force is small near surface, so geostrophic balance is a good place to start.

34. But there is surface drag…
Typically drag causes 20-30° cross isobar angle and 20-30% reduction in speed
But there is a lot of modulation with roughness and stability.
Rougher and more stable gives lower speeds and more cross isobar angle.

35. Varying stability over flat land surfaces (no terrain)
Unstable: V/Vg ~ , f ~ 5-15°
Neutral: V/Vg~.75, f ~ 25°
Stable: V/Vg~..5-75, f = 30+°
Drag over water is low
Most geostrophic is over water and low stability (e.g., behind Pacific front): V/Vg~ .95, f ~ 5-10°

37. Geostrophic
Conditions
Less
Geostrophic

38. Water is aerodynamically smooth
Winds are often 50% to 300% stronger over water than land
Only takes a few km of overwater conditions to facilitate a speed up

41. Stability and Vertical Momentum Mixing
Winds at the surface are sensitive to vertical momentum mixing
Momentum mixing is sensitive to vertical stability
So changes in stability can have a big impact on winds

42. Warm Front Example: winds strengthen after passage
Weaker
Less
Stable-
winds stronger
Stable—winds weaker
Stronger
winds

43. Wind Speed Up After WF Passage

46. The Chanukah Eve Windstorm Stability Changes and Winds

47. Momentum mixing: 12/17/06

50. Occluded Fronts Also Have Stability Contrasts and wind contrasts
Less Stable
Stable

51. Orographic barriers can greatly change the pressure and wind fields
Winds near terrain tend to be highly ageostrophic, with a tendency to blow from high to low pressure
Near terrain, winds tend to flow down the alongbarrier pressure gradient, with downgradient acceleration eventually being balanced by surface drag.

53. Explains why cyclones produce strongest winds in Seattle when low centers are north of Puget Sound

54. How far from the mountains?
For stable flow, the Rossby radius (~ km)
For neutral flow, the spatial scale of the barrier (smaller)

55. For stable flow, the scale of topographic influence upstream is controlled by the Rossby Radius of Deformation
R= NH/f
N is Stability (Brunt Vaisalla Frequency)
H is height of barrier
f is Coriolis parameter
Typically km in stable flow
In neutral flow, terrain influence shrinks to scale of terrain.

57. Stability Influences Topographic Blocking and Acceleration

59. 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)

60. Thus, large Froude numbers produce large pressure perturbations
Stronger windward ridges and lee troughs since more vertical motion

61. High Froude Number Situation

62. Why Strongest Winds Often Near the Central Coast?

63. Mesoscale Pressure Perturbations: Large Influence on Winds
Sea Level Pressure

65. 2.2
.66
.22
.0555

66. 2.2
.66
.055
.22

72. Gap Flow
In gaps through mountainous regions the flow is generally highly ageostrophic and downgradient, moving from high to low pressure.
Historically, forecasters have developed simple relationships between the wind speeds and pressure differences across the gaps in question.
Example: SEA office used UIL-BLI gradient (westerly winds at exit= 10*delta p)

73. Gap Flow 101 - Misleading the Next Generation!
The Venturi Effect is still used in some introductory texts to explain gap flow!

74. It turns out that the strongest winds are generally not in the narrowest parts of mesoscale gaps, but in their exit regions

75. Strait of Juan de Fuca is well known for its easterly gales in the gap exit region.

76. Columbia Gorge Troutdale
Also note increased wind speed near CZK. Possible where the flow becomes supercritical. Also possibly Venturi.
Troutdale

77. Assigment https://www. meted. ucar. edu/training_module. php. id=111#

78. 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:
Bernoulli equation basically states that total pressure = dynamic pressure (0.5*rho*v^2) + static pressure

79. Gap Flow 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)
Oversimplification.
Produces winds that are too strong.
Gap winds are a boundary layer phenomena
Must account for drag (both surface drag and drag at the inversion)

80. Gap Flow 101 – Add Drag 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)
CD=Bulk drag coefficient, H=PBL Height.
Overland: Theoretical scaling study of JDF and Shelikof Strait
Lackmann and Overland/Bond and Stabeno: Shelikof Strait
Overland paper used scale analysis to predict the across gap and down gap balance.
Cross-gap Rossby number, Vl/fL2, down-gap Rossby number, V/fl (V, l, L and f are down-gap wind speed, across gap length scale, along-gap length scale and Coriolis parameter respectively.)
Plugging in numbers for typical gaps => Cross gap << 1, down gap > 1. I.e. Geostrophic adjustment likely across the gap, but flow down the gap is highly ageostrophic

82. Hydraulic Effects
There is another important feature of many gap flow situations: hydraulic effects associated with changes in depth of the cold dense air (analogous to water)

83. Hydraulic Effects Tend to Slow the Wind at the Entrance and Speed Up at the Exits

84. Near Sea Level Gap On Border of WA and OR
Use this slide to introduce where the Gorge is and the nature of the gap flow scenario.
Spend some time pointing out important features like:
The mountain barrier, the Columbia Basin and how it provides a topographically confined air shed and note how the Gorge is the only drain
Then switch gears and point out other gaps on this map (Frasier, JDF, Stampede, Chehalis) and mention important results of each
Frasier - cold outflow, Juan de Fuca - easterly gales, westerly gales, PSCZ, Chehalis - PSCZ, Stampede - downslope windstorms, CRG - Cold outflow, gales, wintry precipitation
So meteorological phenomena driven by gaps are obviously very important, especially in the PNW.

85. Also note increased wind speed near CZK
Also note increased wind speed near CZK. Possible where the flow becomes supercritical. Also possibly Venturi.
Troutdale

86. Vertical Structure Strongest winds near exit
Also note increased wind speed near CZK. Possible where the flow becomes supercritical. Also possibly Venturi.
Strongest winds near exit
Hydraulic effects are important

88. Gap Summary
Strongest winds tend to be in the exit region because of hydraulic collapse and because of large-scale pressure gradient down the gap.
There can be some venturi acceleration in narrow regions…but that tends to be secondary.
High-resolution numerical models can do a very good job with fine-enough grid spacing.

89. Diurnal Winds Sea breeze/land breeze Upslope and downslope winds
And combinations of the above.
Generally larger in the warm season.

90. Sea/Land Breeze

91. 2-minute average July-August winds along the Northwest coast.

93. Fovell (UCLA) Sea Breeze Simulations

94. Summer
Diurnal
Winds

95. Land Breezes are Strongest in Locations with Warm Water
Weak in cold-water areas such as the northwest.
One exception: during arctic air outbreaks.

96. Land Breeze example during a cold period.

97. Interactions with larger scale flow
Strong onshore flow results in weak or non-existent sea breeze
Weak to moderate offshore flow: strong sea breeze and sea breeze front
Strong offshore flow: weak or non-existent sea breeze
Additive to flow parallel to the coast (e.g., southern Oregon)

98. Sea Breeze Winds Along the Southern Oregon Coast: Interaction with synoptic scale flow
Gusts frequently reach 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 has a substantial impact.

99. Southern Oregon Coast Near Brookings
North
South
Southern Oregon Coast Near Brookings

101. Slope Winds

103. Figure 7.11: Valley breezes blow uphill during the day; mountain breezes blow downhill at night. (The L’s and H’s represent pressure, whereas the purple lines represent surfaces of constant pressure.)
Fig. 7-11, p. 178

104. Downslope Windstorms: strong winds on the lee side of mountains, generally associated with high amplitude mountain waves

105. Enumclaw, WA

107. Mountain Wave 101

108. Trapped Mountain or 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. No tilt and weaken with height aloft.

109. Vertically Propagating Waves
Vertically-propagating waves occur when waves 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 in the upper troposphere due to vertically-propagating waves. Such waves have been documented up to 200,000 feet and higher.

113. 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).

114. Froude Number and Mountain Waves
If the Froude number is equal to or slightly greater than 1, then there is the potential for 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. Lower probability of mountain waves.
If Froude number is much more than 1, airflow proceeds right over the mountain and down the other side, with no significant oscillations, but there can one high amplitude wave

115. Trapped vs Vertically Propagating
A key parameter controlling the nature of mountain waves is the Scorer Parameter (l)
l2 = N d2U
U U d z2
k is the primary wavenumber of the terrain =2*pi/L, where L is the length scale of the terrain
k < l: vertically propagating, k >l trapped

116. Implications
Shorter wavelength waves from small scale terrain features tend to be trapped
Longer wavelengths tend to be vertically propagating.

117. Change of Scorer Parameter With Height
Trapped lee waves tend to occur when l2(z) decreases strongly with height.
This is especially true if this decrease occurs suddenly in mid troposphere, dividing the troposphere into two regions, a lower layer of large l2(z) (high stability, low wind speed) and an upper layer of small l2(z) (low stability, high wind speed). 

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

123. To get a downslope windstorm, the energy from a mountain wave must be directed downward
Need some kind of cap or reflecting layer
Critical layers or stable layers can do this.

124. Critical Levels
A critical level occurs when the flow normal to the mountain barrier reverses.

125. Critical Levels: Nature’s Reflector
Critical levels may be self-induced by wave breaking or result from the overall environmental 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 towards the surface.
Consequently, critical levels can contribute to the development of, and/or the strengthening of, downslope windstorms.

126. Stable Layer
A stable layer near crest level with less stable air above can also trap wave energy.
Happens relatively frequently.

127. What to look for for in a downslope windstorm forecast
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 to mountain crest.
Stable layer near crest level. Lesser stability aloft.
Critical level above the mountain barrier (to promote wave breaking).
Strong downslope windstorms are often associated with large cross-barrier pressure gradients, but it is not clear whether those are cause or effect.

128. Numerical Models and Downslope Windstorms
Until ~2005, criteria such as the above, used subjectively by forecasters, were the only approach.
During the past decades, high resolution numerical models (2-10 km grid spacing) have become highly effective tools.
Demonstrated repeatedly here in the NW.

129. “Place of evil spirits”
Enumclaw, Washington
“Place of evil spirits”

130. Enumclaw Windstorm Pressure Pattern
December 1983
December 24, 1983

131. December 24, 1983

132. High-Resolution MM5 Simulations Do An Extremely Good Job of Predicting/Diagnosing Such Gap/Downslope Windstorm Hybrids

133. December 18, 2010

138. Major Wine Country Fires

139. High-Resolution WRF Simulation of the Event
km domains
Initialized at 1200 UTC Oct. 8

140. 12-km domain 9
1200 UTC 8 October

142. Strongest winds in lowest 250 m
8 PM October 8
10 PM October 9

143. 1800 UTC
Oct 8

144. 0000 UTC
Oct 9

145. 0300 UTC
Oct 9

146. 0600 UTC
Oct 9
Terrain-Related
Downslope
Windstorm

147. Isallobaric Wind
Can have highly ageostrophic winds without drag/friction or terrain
Dependent on rapid changes of pressure
Geostrophy assumes little rate of change of winds and forcing

148. Isallobaric Wind

149. Isallobaric wind blows toward max pressure falls

150. Schematic

151. Extreme Winds Associated with Strong Cyclones Over the Ocean

152. Shapiro-Keyser Model of Oceanic Cyclones

153. Warm Seclusion Stage Intense Pressure Gradient

156. Strongest Winds With Back-Bent Warm Front

157. Local Effects (bluffs, barriers) can enhance winds substantially

158. Columbus Day 1962: At Cape Blanco there were 150 mph with gusts to 179
Columbus Day 1962: At Cape Blanco there were 150 mph with gusts to 179! Strongest winds on bluffs and windward slopes of coastal orography

161. Also stronger winds in urban areas (e. g
Also stronger winds in urban areas (e.g., between buildings), wind shadowing

163. Wind Forecast Strategies
Aloft. Models hard to beat
Surface. Generally use high-resolution models—but you have to make sure sufficient resolution for problem at hand.
Make adjustments for known model issues (over mixing, too geostropic at surface).
Use ensembles if available.

164. In the end, we need to move to probabilistic wind forecasts

165. The End