Wind Science Flashcards

Terms Definitions


http://coastal.er.usgs.gov/african_dust/documentary/hi-res.html


http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?mm5d1_wssfc+//72/3
Air density
pa= -0.0047(temp.) +1.2976
 
Chinook
Pacific cyclone off west coast
Risingand latent heat release (WALR)
Air descends (DALR)
 
Westerly winds that condense and precipitate their moisture when ascending the Roxkies, and then compressionally warm and dry when descending, are described as chinooks or foehs.
 
Some distinguishing surface characteristics:
-increased air temp
-decreased relative humidity
-increased wind speed
-increased westerly wind component
*all increase wind erosion
Nkemdirim 1996
"Canada's Chinook Belt"
 
-latent heat converted to sensible heat
temperature signal: the difference between the highest temperature attained during a chinook event and the daily maximum.
numerical frequency: number of days in the winter with, at least, 1h of chinook wind.
 
-mean point values of the signal range from 13-25K
-Mean seasonal frequency varies from 43-52 winter days
-the cores for these two statistics do NOT coincide
 
Highest frequencies occurr in the pass
Strongest signals found further east in Brooks area.
Both show strong decadal functions (80s chinook strong, 70s chinook poor)
 
The general macroscale patterns required for a chinook are:
1)the establishment of a strong pressure gradient towards the north-east, enabling esterly flow of maritime air into southern AB
2)a surface presure pattern that includes a high pressure system in NW USA and troughs of low pressure in N BC, AB, SK
3)an upper air disturbance in combination with 1) + 2) which promotes the establishment of a strong zonal flow
 
-on west side, cools at DALR until dewpoint, then cools at around 0.6K/100m
-air gains sensible heat as precipitation removes moisture
-on lee, warms at DALR (1K/100m)
 
Temp Enhancement Factors:
1)altitude of place determines the amount of adiabatic warming realized
2)position of point relative to chinook wave (those positions that intersect the trough will be warmed)
 
RESULTS AND DISCUSSION
1)signal is strongest to south and to east
-weakest on rockies
2)The maximum chinook signal does not appear to be correlated with the distribution of heights at the continental divide
3)Average chinook signal peaks in january
4)Leth is in the max zone of "chinook days per year", and just outside of max # of hours/yr zone
5)In terms of frequency, the chinook belt is more homogeneous than thought originally
-the systems that produce chinooks cover large areas
6)SW AB receives the most chinooks
 
CONCLUSION
-average chinook signal is 6.5K and maximum is 25.3K
-average 45-52 days/winter (small variation)
-freq/sig show similar decadal variability, both quantitatively and spatially
Blowing snow effects
Driving hazards
 
Accessibility (drifts blocking pedestrian mobility)
 
Water damage (build-up next to building - damage from melting/freezing)
 
Protracted snow cover (limiting spring drainage, landscaping)
Planetary Boundary Layer
500-100m
 
Below this frictional effects are important.
However, above the level of surface frictional effects the wind speed increases and becomes more or less geostrophic. When vorticity occurs due to ridging and troughing, the wind is more or less gradient (subgeostrophic or supergeostrophic)
 
Primary cause of wind is PGF, all other controls are modifying factors
streamers
sand or snow.. kind of ribbons
gas flaring
removal of unwanted byproducts associated with oil/gas extraction and refining
Relative speed up
Relative speedup = uz/Uz
Particle transport modes
Creep
(>20% of transport)
-usually involves the largest particles moved by wind (0.7 - 2.5mm)
-no appreciable downwind effect since most grains are trapped a short distance from where they originated by small hollows are furrows
 
Saltation
(50-80% of transport)
-particles are generally too large to mobilize more than 1m above the ground surface (0.1-0.7mm)
 
Suspension (20-50% of transport - considered as air pollution)
-Finest particles (0.001-0.1mm) that travel the longest distance because they can bcarried high in the atmosphere
-Major concern is related to inhalation of these particles (block lung alveoli and can lead to respiratory disease or enhance pre-existing conditions)
 
PM10 PM2.5 PM1.0
 
PM2.5 is less than 2.5 micrometers in diameter
-human hair average 70 micrometer diameter
Wind Metrics
-Turbulence and gusts
-Shear stress and shear velocity
-Drag coefficient
-Wind Direction (wind rose)
Other types of turbine
Horizontal Axis (HAWT)
-propeller-style (blades-canada)
 
Vertical axis (VAHT)
-savonius-type
-darrieus-type
Shear velocity
u* (aka friction velocity)
 
Shear velocity relates the difference of wind velocity at two heights
 
shear velocity = k(Uz2-Uz1)/ln(Z2-Z1)
Were Z1 is below Z2, so that Uz1 is less than Uz2
 
When calculating shear stress, assume density is 1.22kg/m3
 
Shear velocity tends to fluctuate between 0 and 1
Wanglor Fork Sensor
-designed for manufacturing asembly lines to recognize defects
-has laser end and photo sensor on other end
-can measure at least 2000 particles passing through per second
-connect it to a data logger
 
CFD and thermokarst definitions
CFD= computational fluid dynamics
 
thermokarst.. thawing of permafrost
Average sealevel pressure
1000mb (approx)
 
Gradient wind is parallel to geostrohpic wind, but is either faster or slower!!
Paper- Hinkel and Hurd
"Permafrost Destabilization and Thermokarst following Snow Fence Installation"
 
-2.2km/4m snow fence in Alaska
-attracts large drift
-monitored soil temp at 5,30,50cm 6yrs
**soil temperatures beneath drift are 2-14C warmer than control on tundra!!
-snow insulates
-mean soil temperature over 6-yr period has warmed 2-5C, and upper permafrost has thawed
*both direct warming and indirect effects of ponding contribute to thermokarst
 
-on average, snow went from 10-90cm
-at the end of a 5yr period, active layer was 2.5 times(6.5m) deeper than control
 
-after 6 yrs, big difference at 30cm, none at 50cm
-increased warming, each year, due to insulation
-even though snow stays into summer, does not counterbalance and keep cool
-pre-existing ponds became larger and deeper
 
CONCLUSIONS
1)a 4m drift forms each year on the lee side of fence, and a 1.5m forms upwind
 
2)Soil temperatures near top of permafrost range from 2-14 degrees warmer than control in winter
 
3)drift persists 4-8weeks after snow has melted from the open tundra. This delays onset of soil thaw and limits soil warming in summer.
 
4)Increased ponding in summer
Cyclogenesis
Canadian coast and Rocky Mtn ranges lie downstream of Pacific storm track
 
Cyclolysis common on windward side
Cyclogenesis common to lee
 
Lee cyclones can usually be traced to a Pacific trough or cyclone
-very high frequency of cyclogenesis in lethbridge area
-on the mesoscale, cyclones appear to form preferentially to the lee of the highest topography
-most lee cyclones can be traced to an upsream prcursor over the Pacific Ocean (Gulf of Alaska)
-Cyclone development or evolution is "masked" by the topography
 
Vertical Pressure Gradients
Average vertical pressure gradients are usually greater than even extreme examples of horizontal pressure gradients as pressure always decreases with altitude
 
At sea level, p=1000mb
at 10km p=300mb
 
Therefore, gradient = (1000-300)/10km
=70mb/km!!
 
VPG about 6000 times more than HPG in this example
 
SO WHY DON"T WE HAVE HUGE VERTICAL WINDS?
Answer: hydrostatic equilibrium
-the downward force of gravity is balanced by a strong vertical pressure gradient (VPG)-- creates hydrostatic equilibrium (exception is convective storms... up and down drafts)
1997


In 1977 an estimated 209 million hectares in the 10 Great Plains states were eroded by wind with an average loss of 11.9 t/ha. 61 percent occurs on cropland, 38 percent on rangeland. Losses vary from 2.9 to 33.4 t/ha.

One-fifth to 1/3 of the worlds cropland is losing topsoil at a rate that is undermining long term productivity. The inherent productivity of 34 percent of U.S. cropland is falling because of loss of topsoil
Environmental Assessment and mitigation of Oil and Gas developent in Prairie Sand Hills
1)Critical Habitat Mapping
-Remote sensing delineation of sparse-vegetation habitat
 
2)Terrain sensitivity mapping
-Improving the characterization of wind and water erosion risk
 
3)Water Resources
-Sand hill near-surface aquifers: volume, recharge rate, extraction.
-Sand hill surficial aquifers: volume, recharge rate
 
4)Climate Change/Variability
-Adapting to future extremes and trends
-changing terrain sensitivity
-water availability
-shorter drilling season
Shear Stress
stress= force per unit area
 
Wind shear stress is the force (per unit area) exerted by the moving fluid (air) on an object or surface. The equation for shear stress is given by:
 
τ= pau*2
 
Where pz is the density of the fluid (air) in kg/m3, and u* is the shear velocity of the moving fluid, usually measured in m/s
 
We cannot measure the fluid shear stress directly so we must use surrogate measurements to resolve it.
 
Why does shear stress matter in wind science??
-Resolves stresses acting on structures
-Determines sediment/snow transport
-Stresses acting on turbines
 
**THINGS MOVE AND DEFORM IN RESPONSE TO STRESS
Planning Considerations
Planning consideration are factors to minimize downwind and peripheral effects.
 
timing - Initiate land clearing when the probability of negative effects is lowest (ie. summer).
 
site design - Optimize site layout to reduce funnelling and accumulation - ie. streets oriented parallel to prevailing wind should have wind breaks at their open end.
 
building design - how to avoid creating local depositional areas for snowdrift accumulation or throughput areas where wind speed is compressed and accelerated.
 
wind breaks - height, density, number of rows, species composition, length, orientation, and continuity - determines the effectiveness of a windbreak in reducing wind speed.
Upwind and downwind implications, particularly for snowdrifting.
Roughness Sublayer
Vegetation interacts with wind within the roughness sublayer in several ways:
-extracting momentum
-producing turbulece in the form of wakes behind obstacles
-breaking down large-scale turbulent eddies into smaller scale motions
 
Flow in the roughness sublayer is complex - further complcates our ability to assess the potential for sediment transport below the displacement height.
 
The gradient of the velocity profile within the roughness sublayer is related to several roughnes element length scales:
-height
-width
-spacing
-density
-geometry
 
Before considering a collection of plants (roughness elements), let's first consider the flow modifications induced by a single plant.
-wake region (deceleration)
-separation
-acceleration
-stalling
Drag Coefficient
The drag coefficient is a dimensionless quantity which is used to quantify the drag or resistance of an object in a fluid environment such as air or water.
 
Cz = (u*/Uz)2
 
Where Cz - the drag coefficient at height Z
 
Values usually around 0.002-0.010 ish
-not much variation at a given height (shear velocity varies more)
Building design and snow build-up
Interesting building aerodynamics exist
-bubble of low wind speed forms on wind-side of building
-highest speeds on top and around wind-side of building
-slowest speeds behind building
 
Can design wind deflector to limit snow buildup and entrance
 
Hinkel and Hurd- Permafrost Destabilization and Thermokarst following snow fence installation
*Ground warming caused by thick winter snow cover!!
Geostrophic Wind
Is the balance of PGF and coriolis.
 
Observations of upper winds (those far enough from the effects of surface friction) show that wind blows more-or-less orthogonal to PGF.
 
Upper level ATM wind blowing parallel to isobars is known as the geostrophic. This condition, however, is only present where isobars are straight and evenly spaced.
 
In most cases isobars are curved, particularly owing to ridges and troughs that dominate the upper ATM in the N.Hem and due to High and Low pressure centres.
 
This changes the geostrophic winds s that they are no longer geostrophic but are instead in gradient wind balance. They still blow parallel to the isobars, but are no longer balanced by only the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.
Air Masses
Mass of air whose physical properties, expecially temperature, moisture and lapse rate are more or less uniform horizontally for 100s of kilometers
Defining Factors:
1)Source area (from which air mass obtains properties)
2)Direction of movement and changes in properties as it moves over long distances
3)Age of air mass
 
-air masses interact at fronts (boundaries), which themselves move or dissipate over time
 
cA (continental arctic) - very cold and dry
cP (cont. polar) - cold and dry
cT (cont trop) - warm and dry
mT (maritime tropical) - warm and moist
mE (maritime equatorial) - very warm, very moist
mP (maritime polar) - cool and moist
Air masses, frontogenesis and wind
depressions: lows and cyclones
 


Mid-latitude depressions (cyclones) begin life as a wave or kink in the front dividing cool polar air from the warmer tropical air mass.

As the wave grows the pressure at the centre of the depression drops and the system intensifies and begins to rotate.

The depression migrates from west to east and forms a characteristic comma shaped mass of cloud.

Starting at the centre the cold front starts to catch up with the warm front forming an occluded front
 
 
Many mid-continental cyclones track with the jet stream.
SFC winds (around a depression) circulate counter-clockwise in an attempt to overcome pressure gradients.
Cold Front
The cold air moves rapidly against warm air, creating convergence within the baroclinic zone between the tow air masses.
baroclinic - ATM condition in which isbaric and constant-density surfaces are not parallel
 
Convergence forces the warm, moist air to ascend along the frontal surface. The deveoping cloud band is inclined rearward with height
The main zone of cloudiness and precipitation is located behind the surface front.
Extent of clouds and rate of precipitation is determined by properties of Warm air mass.
 
As front emerges, winds shift to northerly or westerly, but most often NWerly in S.AB
 
Dec19 2004: A strong low pressure system moving across northern alberta pushes a cold front across central and southern regions of the province, marking the leading edge of a cooler airmass moving in from the NW
-fatality!!
baroclinic instability

While the Hadley regime is a significant driver of global winds, it is not the only driver (inefficient northward advection of heat)

Cyclone/anticyclone eddying is one important mechanism for northward heat transport (referred to as baroclinic instability)

 
What determines land erosivity (ie probability of threshold exceedance)?
Soil Moisture
Surface Roughness
Particle size and density
Air temperature
Humidity
Wind Turbulence
Vegetation heigh and lateral cover
Soil crusts
Ground temperature
Disturbance
The most common grain-scale formula (Bagnold, 1941, p86)
u*t = A(SQRT(((pp-pa)gD)/pa)
 


where A is an empirical coefficient (0.1 for fluid threshold, 0.08 for impact threshold); pp is the density of quartz sand particles (2.65 × 103 kg m-3); pa is air density (1.22 kg m-3); g is gravitational acceleration (9.81 m s-2); and D is sand grain diameter at study area (0.25 × 10-3 m). These values give an impact threshold of 0.231 m s-1
Dunes and depressions
Dunes and other landforms project into the boundary layer, altering the streamlines and resulting in secondary flow modifications (lee separation, re-circulation, stoss acceleration, etc.)
 
Modification of Wind Speed Within a Snall Hollow/Depression
-expansion and deceleration at the entrance, compression and acceleration at the exit
-non-logarithmic profiles indicate complex flow separation
-ends up faster than entry point at 0.3m and slow at 2m
 
Oblique winds are more effective in eroding and deepening of the hollow. Westerly winds are more effective in evaluating sand owing to flow acceleration
Wind @ Surface
Geostrophic wind blows parallel to the isobars because the Coriolis Forces and PGF are in balance. However it should be realized that the actual wind is not always geostrophic, especially near the surface.
 
Close to the surface (within 500m) a variety of elements slow and deflect wind.
 
Ekman spiral
 
*increasing friction deflects wind towards the PGF (low pressure area). Reduces effect of coriolis force.
Cubic Power Law
Relation between the wind speed and power.
Wind power rises as a power function (no pun) relative to wind speed.
I.E. as wind speed increases, wind power increases faster.
Wind and Topography
Hill
Flow passing over a hill, stream lines come closer together above the hill (speed up flow)
-compressed into this space.
 
Relative speed up graph (of wind going over a hill) looks like a hill, with disance on x-axis, and the top of the relative speed hill being at 0m from distance of hill crest.
 
Therefore, ideal spot for a turbine is at the hill crest.
Turbulence levels in the lee of the hill would have disastrous consequences for turbines, as relative speed is significantly reduces (shooting wind over crest)
 
Altering shape of hill/obstruction changes lee and before hill effect. (Taller the hill, longer recovery time).
 
vertical velocity is greatest directly in lee, and lowest at the crest.
 
Valley
-relative velocity greatest in immediate lee of valley
-lowest in middle of depression
-upwind edge has fractional speed-up ratio of slightly higher than 0 (0.2), though downwind is significantly higher (0.6-0.8).
-Centre is around -0.4
 
-Greatest erosion occurs at upwind edge
-greatest deposition occurs in lee of downwind edge and on the upwind valley slope.
Global Wind Power
The most comprehensive study to date found the potential of wind power on land and near-shore to be 72TW, equivalent to 54000 Million tons of oil equivalent (MToE) per year, or over five times the world's current energy use in all forms.
 
The potential takes into account only locations with mean annual wind speeds greater than 6.9m/s at 80m. It assumes 6 turbines per square km for 77m diameter, 1.5 MW turbines on roughly 12% of the total global land area.
 
Renewable energy sources in Canada consist mainly of wind and hydroelectric.
Interestingly enough, electricity production of these two sources alternate according to seasons:
-hydro usefullness peaks in summer, while wind peaks in winter, and the yearly average is pretty much right in between.
Piorecky et. al 1997
"Effect of Chinook Winds on the Probability of Migraine Headache Occurrence"
 
Objective was to determine if Chinook weather conditions in the Calgary area increase the probability of headache attacks in migraine sufferers.
-looked at diaries of 13 migraine patients
-probability of migraine headache onset was greater on days with Chinook weather (17%) than on non-Chinook days (14.7%) (p=0.042). Older patients appeared more weather sensitive than younger patients.
-For patients over the age 50, p=0.007
 
-in contrast to other studies, targeted those affected by migraines
-increased wind speed, falling barometric pressure, and rising temperature might all play a role
-different patients might have different migraine triggers
Global Driving Forces of Wind
At a fundamental level, global imbalances of heating (radiation) drive pressure variation, and, hence, wind.
 
Vertical and Horizontal Circulation patterns:
Hadley Regime (cells): heat-driven gobal circulation pattern; drives global wind patterns.
 


Air moves under pressure gradients, and is modified by the Coriolis Force and friction, so as to rise, move laterally, and fall depending on its density. This circulation pattern is term a cell, or in some instances a convection cell. A non-rotating Earth would, in principle, experience at the surface a warming of air in the low latitudes and a cooling of air near the poles. The higher pole pressures drive the air towards the equator. There the warm air rises and cools, and then is driven poleward. This sets up an upper atmosphere flow towards the poles, where the air, now further cooled, sinks. That air is once again driven near the surface back towards the equator. This produces a single circulation cell,
 
REGIONS
Equator: Low @ surface; converging/rising air. High aloft; diverging air. Equatorial low (ITCZ) shifts with season.
 
Subtropical Region: Cool air subsides into subtropics (low aloft). High at surface... diverging air.
 
Subtropical Surface Highs (anticyclones) migrate north/south according to seasons.
 
ITCZ draws surface winds that deflect eastward according to coriolis force (trade winds).
Several high pressure centers develop in the subtropical zones (cancer/capricorn).
North/southward divergence of mass @ subtropical high combined with coriolis produces mid-lat westerlies.
Climate and wind
Just as with other aspects of climate, wind statistics are subject to natural variability on a wide range of time scales. Decadal and multi-decadal variability in wind speed statistics currently introduce an element of risk into the decision process for citing new wind power generation facilities.
 
Impacts will affect the stats (max, min, mean, and variance) of all meteorological variables.
 
While GCM predictions of changes in wind speed are readily available, these results are not commonly reported in the climate change literature as wind speed is oly of secondary importance to most affected ecosystems.
CHINOOK IS NOT A WARM FRONT!!!
Diagnostics of a chinook:
-air temp
-wind speed
-relative humidity
-air pressure
-inversion is a good ingredient!
 
*worst chinooks/downslope winds are in Wyoming/Colorado!
 
**troughing on lee side of mountains could be cyclogenesis or a chinooks (low pressure)
 
***Compare and contrast cyclone with a chinook
Vegetation interaction with the boundary layer
Wind speeds increase with elevation above earth's frictional surface, where stronger winds sculpt an denude branches of tres
 
Shelterbelts may protect downwind property, but may also create unwanted turbulent eddies
 
Vegetation acts as a "living" wind shelter, reducing wind speed appreciably for some distance downwind in the sheltered "wake" region. The effectiveness depends on the porosity, lateral extent, and heigh [H] of veg
**flow recovery is about 25-25 times H
 
Surface cover (vegetation) and Wind
Generally limits entrainment of soil and snow by wind- but disturbed surfaces and various land use activities increase the susceptibility to aeolian transport.
 
Offers :
1)Cover and protection
2)Momentum extraction (z0) from air
3)Trapping of soil particles
What is the calculated threshold?
5.6 m/s for bare, dry sand.
 
bridging: interstitial pore water... cohesion
 
Entrainment Threshold
Seasonal and spatial changes in surface conditions (moisture, vegetation, ground freezing) complicate prediction of wind speed required to entrain soil.
 
Determining threshold in the field withprecision instruments. Sometimes thresholds remain constant over time, while some increase/decrease depending on what is exposed. For example, threshold increases as moist sand is exposed.
Aeolian sediment transport threshold
The minimum wind speed to initiate sediment transport by wind (ut or u*t)



It is a critical parameter because it establishes the wind speed above which transport is possible. Due to a number of surface conditions (moisture, temperature, vegetation, bed slope, and crusting) threshold is known to vary over a range of spatial and temporal scales. Much of the focus of threshold research is to resolve the influence of these external controls and thereby increase the accuracy of sediment transport models. Currently, there are a variety of methods and instruments used to measure or derive threshold, but it is now apparent that the application of different methods and instruments imposes ambiguity in the definition of threshold.
Brief History of Turbines
1600s - Windmills in the Neterlands, pumping water and grinding grains
 
1888 - Charles Brush develops first large wind generator producing 12kW
 
Early 1900s - Windmills drive pumps and generators across rural North America
 
1941 - first prototype turbine (Putnam's 1.25 MW turbine) - also demonstrates need for lighter materials
Shear stress partitioning
When the soil surface is obstructed by roughness elements the total force (F0) of the wind is partitioned b/w the roughness elements (Fr) and the intervening bare soil (Fg):
 
F0 = Fr + Fg
 
This is relevant because we must know how much force is extracted by the vegetation, and ow much remains to entrain soil at the surface.
The effect of vegetation an also be examined from the ratio of the impact threshold with and without vegetation - but this must be measured.
Upper ATM and 500mb chart
UPPER ATM
-we often use the spatial distribution of the height of a constant pressure (500mb) to examine upper ATM winds
-upper air pressure gradients are best determined throug the heights of constant pressure due to density considerations
-constant pressure surfaces of cooler air will be lower in altitude than those of warmer air
 


The 500 mb chart represents weather conditions in the mid- troposphere, at a level where approximately half the mass of the atmosphere lies below this level. This level is at an altitude of approximately 5,500 meters. This level is often used to represent upper level flow conditions because the level is well above the effects of topography and friction and the level is below the region in the upper troposphere where the air flow may experience strong accelerations and decelerations when in the vicinity of the upper jet streams. Since many weather systems tend to follow the wind flow at this level, this level is often considered to symbolize the steering level of these systems.
St. George and Wolfe
El Nino stills winter winds across the southern Canadian Prairies


Analysis of long-term terrestrial wind speed (u) records demonstrates that inter-annual variability is a major component of near-surface wind dynamics in the southern Canadian Prairies (SCP). Since the early 1950s, there have been several periods when negative anomalies in regional u persisted for 8 to 13 consecutive months, with anomalies for individual months exceeding -1 m s-1. Calm conditions on the SCP usually coincided with negative u anomalies across much of western Canada, and nearly all low-wind events occurred during a ‘moderate’ or ‘stronger’ El Nino. Wind energy facilities in the SCP have been built during a period of relatively stable wind conditions, and the next El Nino may test their ability to maintain expected energy outputs. El Nino may affect u in other parts of the North American wind corridor and be useful for predicting seasonal or inter-annual changes in regional wind energy production.




Background

The southern Prairies (roughly bounded by 101°W to 114°W and 49°N to 51°N) are one of the windiest parts of Canada, and are the northern limit of the North American wind corridor that begins in Texas and extends northward through the American Great Plains. This region also hosts 12 active wind farms, which have a total installed capacity of 779 megawatts

Goals of study:

Examine inter-annual variability of mean monthly wind speed over the last 5 decades and relate the observations to ENSO
 


Winds on the southern Prairies are strongest during winter. In spring, winds slacken modestly (by roughly 0.2 m/s) and continue to decline as the region warms, reaching a minimum during mid-summer (July and August). Mean u during the shoulder seasons (spring and autumn) are approximately equal. Over the region, the amplitude of the seasonal cycle in u is roughly 1 m/s.
 


Mean monthly u decreased significantly at stations in the southern Canadian Prairies between 1953 and 2006. The mean u of the last decade of observations (1997 to 2006) is roughly 0.3 m/s lower than mean of the first decade of observations (1953 to 1962).
 


Anomaly graph shows strong seasonal signal. Persistent weak winds also occurred in 1982-83 (12 consecutive months of negative anomalies), 1992-93, and 1997-98 (13 consecutive months). Low-wind periods always include very weak winds between December and March
 


Six years with anomalously weak winter winds: 1969, 1978, 1983, 1993, 1995 and 1998. Anomaly maps show that, during low-wind winters, weak winds are not restricted to the southern Prairies but instead extend across much of western Canada.

The low-wind event during the 1997/98 winter was most exceptional. Weak winds were observed at almost every station on the Prairies. Low mean annual u were also reported for 1998 at five tall-tower sites in Minnesota, suggesting that anomalous u conditions prevailed over a large portion of the North American interior



Winds on the southern Prairies appear to be connected to the positive phase of ENSO. With the exception of the 1968-69 event, all low-wind events identified in the last 50 years occurred during a ‘moderate’ or ‘stronger’ El Niño. Mean u on the southern Prairies is roughly 0.5 m/s slower during El Niño winters than during all other winters

****Not all El Ninos are associated with weak winds across the region



Low-wind winters on the Prairies coincide with anomalously weak winds aloft (250 MB level) across most of southern Canada.

Mean winter (DJFM) scalar wind anomalies at the 250 millibar
level during (a) low-wind winters on the southern Canadian Prairies and (b) El Niño events. Anomalies are relative to 1968 – 1996 climatology.

CONCLUSIONS


The strong likelihood of similar events occurring in the future may be viewed as a negative, particularly by the wind energy industry, but the connection to El Niño might also allow their prediction several months in advance.

El Niños may signal an increased risk of low-wind winters affecting the southern Prairies and could lead to u reductions over a large portion of western Canada.

It appears that wind energy facilities in this region have been planned and developed during a period characterized by unusually favorable wind conditions. It is not known if the u decreases associated with these low wind events are large enough to have a major impact on the amount of energy produced from wind farms – effectively, whether they could cause something equivalent to a drought in the wind.
Local Winds Produced by Differential  Surface Heating and Cooling
Changes in air temperature causing warm air to rise and cool air to sink can generate horizontal winds
 
Rising warm air creates a surface low and upper level high. Sinking cool air creates a surface high and upper level low.
 
Land heats more quickly than water, creating land-water temperature differences along a coastline.
 
During the day the land's warm-core thermal low draws a sea breeze, while at night, the warmer sea draws a land breeze
-Table Mountain in Cape Town is a good example
 
___________
Water is smoother than the land surface, permitting increases in wind velocity. These increased speeds mean 1)a greater Coriolis force and deflection, 2)divergence and sinking of air at the upwind water surface with convergence at the downwind end
 
____
Similar to land/sea wind in its diurnal cycle are the valley and mountain winds. Valley winds occur in the day because air along mtn slopes is heated more intensely than air at the same elevation over a valley floor (anabatic winds)
Rapid radiational heat loss in the evening reverses the process to produce a mountain or a down valley wind (katabatic wind).. this is density driven
 
Montain snow cover creates a thin layer of high pressure cold air that rushes into lower valleys
-Elevated plateaus with snow cover may foster development of a thin layer of high pressure cold air
-Pressure gradient winds are triggered due to lower pressure above the adjacent valley, pushing cold air into the lower valley.
Paper - Chinooks/Foehns - Hugenholtz
Names include foegn, ibe, zonda, berg, nor'wester
 
env effects: wind erosion, reduced air quality, vegetation mortality, adverse health-related effects
 
synoptic conditions: surface ridge of high pressure on the windward side, trough of low pressure on lee side
 
-Pincher reported 25.5C increase in 1 hour
Reasons for choosing small turbines

-The local electrical grid may be too weak to handle the electricity output from a large machine. This may be the case in remote parts of the electrical grid with low population density and little electricity consumption in the area.

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-There is less fluctuation in the electricity output from a wind park consisting of a number of smaller machines, since wind fluctuations occur randomly, and therefore tend to cancel out.

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-The cost of using large cranes, and building a road strong enough to carry the turbine components may make smaller machines more economic in some areas.

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-Several smaller machines spread the risk in case of temporary machine failure, e.g. due to lightning strikes.

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-Aesthetical landscape considerations may sometimes dictate the use of smaller machines. Large machines, however, will usually have a much lower rotational speed, which means that one large machine really does not attract as much attention as many small, fast moving rotors.

 
Time series analysis - WHY?
To determine range of conditions (at different temporal scales) that occur at the site (how much variation around the mean?)
 
To determine worst case scenarios (is the site prone to frequent episodes of extremely high winds that might damage the turbine?)
 
To determine how variable the wind is at different scales (is it more variable during peak demand periods than outside these periods?)
 
* need lots of recording sites! Data does not correlate if far away
Owen's Lake, California: Dust-producing engine of the southwest
 
Who Cares? and Control measures.
The federal 24hr standard for PM10 is 150 micrograms/cubic meter.
The "significant harm to health" level is 600 micrograms per cubic meter.
***24hr levels of 3,900 have been measured in Keeler and 12,000 at Dirty Socks.
 
The US EPA AIRS Data for 2002 show that of the 30 highest PM10 days reported nationally, 28 occurred at Owens Lake. Owens Lake's highest day of 7,915 was over 13 times higher than any other location in the USA (590 in El Paso, Texas).
 
How will the dust be controlled? A multi-million (100s) dollar project.
 
In 1998, the District adopted a State Implementation Program (SIP) that required the LADWP to control dust emissions from Owens Lake by the end of 2006.
The 1998 SIP required that, within a 90 sq km envelope, 43 sq km of the lake bed had to be controlled by the end of 2003.
It also required the District to continue to collect data on those areas of the lake bed that needed dust controls and revise the SIP in 2003 to incorporate the latest information.
 
**
Three approved methods of controlling dust that are feasible on a large scale:
1)native vegetation
2)flooding with shallow sheets
3)gravel blanket
 
The Key is....vegetation.
Research indicates that if 50% of a dust producing area consists of live or dead vegetative cover, dust emissions will be reduced by 99%.
 
why?
Vegetation is an imporant surface control because it reducs wind erosion in 3 ways:
1)shelters the soil from the shear stress of the wind
2)Reduces the force of wind near the surface by extracting momentum from wind
3)Traps incoming soil particles - induces deposition becuase of (b).
 
It takes approximately 7 acre-feet of water per acre to reclaim the saline soils and establish vegetation the first year. In subsequent years it takes about 2.5 acre-feed of water per year per acre to maintain the 50% veg cover necessary to control PM10
 
Saltgrass
Is attractive because:
1)it is a locally adapted native species (@ Owens Lake)
2)It is very  salt tolerant
3)It spreads via underground runners and thus new growth is protected from wind damage
4)It creates a surface protecting mat.
 
...however.... it is not particularly drought tolerant.
A combination of factors determine southern alberta's wind erosion risk:
High winds
Sparse and low vegetation cover
Fine-grained soils
Aridity
Variations of pressure and wind velocity with height
Pressure cells (highs and lows) can exhibit different characteristics in the vertical according tothe surface temperature of the cell.
 
When surface is relatively cold, usually low pressure aloft, and either high or low at the ground. Common in winter.
cold cyclone: low aloft, low ground. Cold core intensifies aloft.
cold anticyclone: low aloft, high at ground. Leads to downward contraction of pressure aloft.
 
When surface is relatively warm, usually high pressure aloft, with low or high at ground. Common in summer.
warm cyclone: high aloft, low at ground. Weakens aloft and may be replaced by H-pressure.
warm anticyclone: High above, high below. Causes pressure surfaces to bulge upward.
 
**isobaric surface is depressed where cold air occurs near surface
*isobaric surface is raised where warm air occurs near surface
Lethbridge average monthly wind speed
Trends in mean annual wind speed at Lethbridge... Historical decrease of monthly WS by 0.3 m/s (prairie region)
 
WS anomaly plot
-substract the long-term average for each of the 12 months from the number of records of each month (used to isolate monthly variability)
Remote Sensing used to describe roughness
The LIDAR instrument consists of a system controller and a transmitter and receiver
Scanning Mirror sweeps laser beam across the ground.
Range to target is determined by measuring the time interval b/w transmission and return of reflected laser pulse.
Aircraft position is determined using GPS phase differencing techniques.
 
LIDAR systems can emit pulses at rates greater than 100k pulses per second referred to as pulse repetition frequency. A pulse of laser light travels at c, the speed of light (3x 108 m/s). LIDAR technology is based on the accurate measurement of the laser pulse travel time from the transmitter to the target and back to the receiver. The traveling time of a pulse of light, t, is:
 
R=0.5tc
 
Where R is the range (distance) b/w the LIDAR sensor and the object.
 
LIDAR/Laser Altimetry
-Holland et al. using LiDAR to characterise roughness of urban areas
-extract parameters from the LiDAR data required to calculate roughness. zo = 0.5hLc
-Filters, creation of raster datasets, feature extraction.
 
Good for large roughness elements, ineffective for smaller vegetation types (resolution isn't great enough)
 
 
Optical Techniques
Jansinki and Crago 1999 use canopy area density (total single sided area of all the canoy elements per unit ground area) as a surrogate for the frontal area index in order to determine roughness
-generalizations start to be made about stand characteristics
-used assumed stand averages and assuming stand homogeneity
 
Radar
-determined relationships between zo and backscatter
 
LASER SCANNING
Same principls as LiDAR
Data similar to LiDAR pint cloud data (X,Y,Z)
Currently being used for soil surface roughness (physical roughness not aerodynamic roughness)
-characterizng surface roughness using RMSH
-quantifies the deviation from the mean
-Higher RMSH the rougher the surface
(look at formula for this)
 
Have not came across a study using laser scanning to quantify aerodynamic roughness
 
Benefits of laser scanning:
-very high quality data (point cloud resolutions in the order of mm scale)
-relatively inexpensive as compared to LiDAR data
-no generalization of features
-ability to capture very fine features
 
Drawbacks:
-robust datasets
-area coverage is somewhat limited
-no first and last return data (unlike LiDAR)
Vegetation is an important surface control because it reduces wind erosion in 3 ways:
1)It shelters the soil from the erosive force (shear stress) of the wind
2)It reduces the force of wind near the surface by extracting momentum from wind
3)It traps incoming soil particles - also induces deposition.
 
Once vegetation is removed, soil drifting potential immediately increases, then slowly decreases (finest is blown away, vegetation grows back, etc)
Seasonal competition - Wind vs Vegetation/Drifting sow
Winds capable of eroding the ground surface (>25-30 km/h) occur most frequently when vegetation growth conditions are poorest = prime conditions for blowing soil and snow.
 
Many of the factors that increase southern Alberta's susceptibility to soil erosion by wind also apply to snow drifting (ie. high wind power, low vegetation cover, aridity)
 
Without effective roughness elements (fences, wind breaks, building design, etc) snowdrifting can, at times, exceed soil drift rates by orders of magnitude. The effects, however, are short-lived, but they also recur each year if left unattended.
 
Snow drifting depends on temperature, age of snow, wind speed, and heigh of snow cover above the vegetation roughness elements. As the snow age increases, the drift potential decreases.
 
Vegetation effectivenes decreases as snow depth increases.
Surface Drag - the Drag coefficient
 
(more inertial sublayer)
The drag coefficient (Cz) is a dimensionless quantity which is used to quantify the resistance of an object to airflow.
Cz= (u*/Uz)2
where Cz = the drag coefficient at height z. Often Cz is considered a constant for a given z; however, computations by Hsu and Blanchard (1991) for C10 showed a variation of from 0.0005 to 0.005. In fact, Cz varies with season and wind direction and cannot be considered constant.
Typical Sequence of events during the passage of a warm front:
Temp goes from cool to warming suddenly to warmer then leveling
 
ATM pressure goes from decreasing steadily to leveling off to slight rise followed by a decrease
 
Winds go from south/SE to variable to S/SW
 
Precip goes from showers/snow/sleet/drizzle to light drizzle to none
 
Clouds go from stratuses to clearing
Upper (500mb) vs lower (SLP) pressure maps
Why do we discriminate b/w upper/low level pressure?
...because the uper level systems control the major traxks of lower level systems.
 
In other words: Lower level pressure tells us about winds near the surface - upper levels tell us about the movement of ridges/troughs/fronts responsible for the lower level winds.
Module 1 Summary: Future challenges for land development in the (windy) City of Lethbridge
wind power is high - our research shows that it drops appreciably only in winter during strong El Nino phases (ie 1998) - otherwise dominated by modest interannual variability

wind direction is westerly- future land development will occur on the west side, thus wind erosion challenges will persist for some time in this sector

wind power is greatest when natural surface cover is least effecive (ie. outside of growing season)- emission reduction techniques must bear this in mind

Southern AB's landscape is sensitive to disturbance - removal of vegetation cover during initial phases of land development lead to an immediate increase of soil drifting - the bulk of mitigation strategies must focus on this period

there is a multi-fold effect of our climate that enhances soil and snow drifting - but there is direct correspondence b/w many of the natural factors that control these processes; therefore, mitigation techniques may be complimentary
Drylands and Wind Erosion - SO WHAT?
Mainly an issue of land surface sensitivity
 
Dryland climates are typically associated with thin (ie. grassland) or patchy (ie. creosote) vegetation cover
 
These vegetation types are more readily affected b drought than forests, and more easily disturbed. Thus, drylands have increased potential for direct exposure of sediment to wind= higher susceptibility to wind erosion.
 
Vegetation cover is also more susceptible to natural and anthropogenic disturbances (fire, over-grazing, dought, vehicular traffic, etc.)
 
Dry soils are more readily entrained when exposed by agricultural tilage and land clearing during the initial phases of development.
How do you get electricity from the wind?
Most generators have a long, coiled wire on their shafs surrounded by a giant magnet.
 
When the turbine turns, the shaft and rotor is turned. As the shaft inside the generator turns, an electric current is produced in the wire. The electric generator is converting mechanical, moving energy into electrical energy.
 
The generator is based on the principle of "electromagnetic induction" discovered in 1831 by Michael Faraday, a British scientist. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electric current wil flow of "be induced" in the conductor. So the mechanical energy of the moving wire is converted into the elecric energy of the current that flows in the wire.
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