Wind Prediction
NWS/FAA Wind Observations
ASOS Sensors Have Been
Changed from Three-cup to
Acoustic Units
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 wss the peak wind.
Problems With Old Wind Sensors
New Acoustic Wind Sensor
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 is 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 (e.g., in
valley or behind a 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.
Wind Gusts
Wind Gusts Associated with
Mixing of Higher Momentum
From Aloft
Gust Ratio Depends on Wind
Speed
Do Model Winds Include Gusts?
NO!
Wind Speed at the University
of Washingon
(Sampled every 5 seconds: reports 1 minute averages and highest 5 second wind gusts each minute )
MM5
Output
Every Time
Step from
the 4-km
Domain:
Much
Smoother!
Wind Speed
MM5 (or WRF) 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 output to temporally averaged
observations.
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.
How do we do this for WRF?
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. 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
• Large scale model errors…from poor
initializations and other causes.
Typical surface wind errors
All winds
3 knot and more
Use of Models
• Today’s synoptic and mesoscale models are
sufficiently realistic and accurate that 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.
Using SL Pressure For 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.
• Lets review these relationships.
Impact of Surface Drag
Variations
• Winds are often 50% to 300% stronger over
water than land
Stability and Vertical Momentum
Mixing
Wind Speed Up After WF Passage
The Chanukah Eve Windstorm
Stability Changes and Winds
Momentum mixing: 12/17/06
Orographic barriers can greatly
change the pressure and wind
fields
• Examples include mesoscale windward
ridging and lee troughing.
• Can greatly enhance or weaken the winds.
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 (quasihorizontal flow)
Strait of Juan de Fuca During the
Coast Field Experiment
Mesoscale Pressure Perturbations: Large
Influence on Winds
Sea Level Pressure
February
13
1979:
The
Hood
Canal
Storm
Very
High
Froude #
Winds over 100 kts destroyed the Hood Canal Bridge
Cost to replace: over 100 million dollars
Stability Influences Topographic
Blocking and Acceleration
Gap Flow
• In gaps through mountainous regions the
flow is generally NOT geostrophc, but
rather 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 uses UIL-BLI gradient
(westerly winds at exit= 10*delta p)
Gap Flow 101 - Misleading the
Next Generation!
• The Venturi Effect is still
used in some introductory
texts to explain gap flow!
It turns out that the strongest
winds are generally not in the
narrowest parts of mesoscale
gaps, but in their exit regions
Strait of Juan de Fuca is well known for its easterly
gales in the gap exit region.
Columbia Gorge
Troutdale
Most Simple Approach
• 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)
– 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)
Gap Flow 101 – Adding 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)
Hydraulic Effects
• There is another important features of many
gap flow situations: hydraulic effects
associated with changes in depth of the cold
dense air (analogous to water)
Hydraulic Effects Tend to Slow the
Wind at the Entrance and Speed Up
at the Exits
Examples of Gap Flow
The Columbia River Gorge
Near Sea Level Gap
On Border of WA and OR
Troutdale
444.4 m grid spacing, Pass Height = 100 m
Portland
Troutdale
Cascade Locks
Vertical Structure
• Strongest winds near exit
• Hydraulic effects are important
Fraser River NE Gap Flow
Max Winds, 28 Dec. 1990
> 40 ms-1
Summary
• 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.
• High-resolution numerical models can do a
very good job with fine-enough grid
spacing.
Diurnal Winds
•
•
•
•
Sea breeze/land breeze
Upslope and downslope winds
And combinations of the above.
Generally larger in the warm season.
Sea Breeze Winds
2-minute average July-August winds along the Northwest coast.
Summer
Diurnal
Winds
Picture take in Sequim looking N
Strong Regional Sea Breeze
Winds During the Summer
Land Breezes are Strongest in
Locations with Warm Water
• Weak in cold-water areas such as the
northwest.
• One exception: during arctic air outbreaks.
Land Breeze
example during a
cold period.
Interactions with larger scale
flow
• Strong onshore flow results in weak or nonexistent sea breeze
• Weak to moderate offshore flow: strong sea
breeze and sea breeze front
• Strong offshore flow: weak sea breeze
• Additive to flow parallel to the coast (e.g.,
southern Oregon)
Sea Breeze Winds Along the
Southern Oregon Coast: Interaction
with synoptic scale flow
• 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 has a substantial impact.
South
North
Southern Oregon Coast Near Brookings
2-minute average July-August winds along the Northwest coast.
Slope Winds
Fig. 7-11, p. 178
Mesoscale Models With
Sufficent Resolution Can Do A
Good Job on Diurnal Circulations
Fovell (UCLA) Sea Breeze
Simulations
• http://www.atmos.ucla.edu/~fovell/ASother
/mm5/LA_seabreeze.html
Downslope Windstorms: strong winds
on the lee side of mountains, generally
associated with high amplitude mountain
waves
Enumclaw, WA
Mountain Wave 101
Trapped MountainWaves
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.
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.
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. 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
Trapped vs Vertically
Propagating
• A key parameter controlling the nature of
mountain waves is the Scorer Parameter (l)
l2 =
N2 - d2U
U2 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
Critical Levels
• A critical level occurs when the flow normal to the mountain
barrier reverses.
Critical Levels
• 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.
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 in a 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).
• 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.
Numerical Models and
Downslope Windstorms
• Until recently, criteria such as the above,
used subjectively by forecasters, were the
only approach.
• During the past decade it is clear that high
resolution numerical models (2-10 km grid
spacing) are highly effective tools.
• Demonstrated repeatedly here in the NW.
Enumclaw, Washington
“Place of evil spirits”
Enumclaw Windstorm Pressure
Pattern
December 24, 1983
December 24, 1983
High-Resolution MM5
Simulations Do An
Extremely Good Job of
Predicting/Diagnosing
Such Gap/Downslope
Windstorm Hybrids
December 18, 2010
Extreme Winds Associated with
Strong Cyclones Over the Ocean
The Most Extreme of the Extreme: The
Columbus Day Windstorm of 12 October 1962
Strongest Winds With Back-Bent
Warm Front
Max Winds
(mph)
Columbus Day
Storm 1962
Shapiro-Keyser Model of
Oceanic Cyclones
Warm
Seclusion
Stage
Interaction with Terrain
• Terrain-induced ageostrophic effects can be
important
• Thus, in terrain wind speeds can depend on
the track and orientation of cyclones.
Tracks for big
windstorms over
Puget Sound
Strongest winds in Seattle when
storm goes to the north of us
• Ageostrophic acceleration down the
pressure gradient.
• When low west of us, winds are more
geostrophic.
• Ageostrophic winds can be larger with same
pressure gradient.
Terrain can enhance or reduce
pressure gradients in locations
and thus winds
Why Strongest Winds Near the Central Coast?
Mesoscale Pressure Perturbations: Large
Influence on Winds
Sea Level Pressure
Local Effects (bluffs, barriers)
can enhance winds substantially
Columbus Day 1962: At Cape Blanco there were
150 mph with gusts to 179! Strongest winds on
bluffs and windward slopes of coastal orography
Isallobaric Wind: An
ageostrophic wind associated
with large pressure changes
Isallobaric
Wind
Isallobaric wind blows toward max
pressure falls
Isallobaric
wind
Advection
Schematic