Download Ch09WindSmallScale

Wind: small scale and local
systems
Chapter 9
Scales of atmospheric motion.
The tiny microscale motions constitute a part of the larger mesoscale motions, which, in
turn, are part of the much larger synoptic scale. Notice that as the scale becomes
larger, motions observed at the smaller scale are no longer visible.
The scales of atmospheric motion with the phenomenon’s average size and life span. (Because the actual size
of certain features may vary, some of the features fall into more than one category.)
• Friction and Turbulence in Boundary Layer
– Friction of fluid flow - viscosity
– Laminar flow - smooth flow
• Slowed by molecular viscosity between air layers
– Irregular turbulent motion
• Eddy viscosity - wind speed and direction varies causing gusts
• Roughness near ground
– Mechanical turbulence creates much greater drag
• Decreases away from earth surface
– Planetary Boundary layer (PBL) – friction layer
– Depth of PBL varies- top usually near 1000m
• Thermal turbulence
Winds flowing past an obstacle :
Small scale eddy
(a) In stable air, light winds produce small eddies and little vertical mixing.
(b) Greater winds in unstable air create deep, vertically mixing eddies that produce
strong, gusty surface winds.
When the air is stable and the
terrain fairly smooth:
(a), vertical mixing is at a
minimum, and the effect of surface
friction only extends upward a
relatively short distance above the
surface.
When the air is unstable and the
terrain rough:
(b), vertical mixing is at a
maximum, and the effect of surface
friction extends upward through a
much greater depth of atmosphere.
Within the region of frictional
influence, vertical mixing increases
the wind speed near the ground
and decreases it aloft. (Wind at the
surface is measured at 10 m above
the surface.)
Big Eddies
Satellite image of eddies
forming on the leeward
(downwind) side of the Cape
Verde Islands during April,
2004. As the air moves past
the islands, it breaks into a
variety of swirls as indicated
by the cloud pattern. (The
islands are situated in the
Atlantic Ocean, off Africa’s
western coast.)
Under stable conditions, air flowing past a mountain range can create eddies many
kilometers downwind of the mountain itself.
Roll eddy – rotors, clear air turbulance
Wind shear – abrupt change in wind speed or direction
Strong winds flowing past an obstruction, such as these hills, can
produce a reverse flow of air that strikes an object from the side
opposite the general wind direction
• Observations: Eddies & Air Pockets
– Eddies on leeward side of solid object
– Roll eddies, mountain wave eddy (clear air
turbulence –CAT )
– Increase wind speed/shear deforms layer
into wave and air pocket.
• Microscale winds blowing over the Earth’s surface
– Wind erosion, desert pavements, sand ripples
– Snow ripples, snow dunes, snow rollers, snow fences
– Windbreak, shelter belt
– Effects on oceans/lakes – wind waves depend on:
• Wind speed
• Time
• Fetch
The shape of this sand dune reveals that the wind
was blowing from left to right when it formed. Note also the shape of
the sand ripples on the dune.
Snow rollers — natural cylindrical rolls of snow
grow larger as the wind blows them down a hillside.
Snow drifts accumulating behind snow fences in Wyoming.
A properly designed shelterbelt can reduce the air flow downwind for a distance of 25
times the height of the belt. The minimum wind flow behind the belt is typically
measured downwind at a distance of about 4 times the belt’s height.
Determining wind speed and
direction
• Wind characterized by direction, speed, and
gustiness
• Wind direction describes the direction from
which it is blowing
An onshore wind blows from water to land; whereas an offshore wind blows from land
to water.
Wind direction can be expressed in
degrees about a circle or as compass
points.
• Influence of Prevailing Winds
– Prevailing most frequently observed
direction during a given time period
– Impact human and natural landscape
– Wind rose
This wind rose represents the percent of time the wind blew from different directions at a
given site during the month of January for the past ten years. The prevailing wind is NW
and the wind direction of least frequency is NE.
Determining wind speed and
direction
• Wind Measurements
– Wind vane
– Pressure plate anemometer
– Cup anemometer
– Aerovane
– Rawinsonde
– Wind soundings
• Lidar
• Wind profiler
• QuickScat
A profile of wind direction and speed above Hillsboro, Kansas, on June 28, 2006.
• Thermal circulation
– Heating and cooling of the atmosphere above the ground
create cold, core high and warm, core low pressure cells.
– Wind travels from high to low and rises until it cools and begins
to sink.
• Sea and Land Breeze
– Uneven heating of
land and water
– Day: land hot, water
cold = sea breeze
– Night: water hot, land
cold = land breeze
– Sea breeze front, sea
breeze convergence
Typically, during the summer over Florida, converging sea breezes in the afternoon
produce uplift that enhances thunderstorm development and rainfall. However, when
westerly surface winds dominate and a ridge of high pressure forms over the area,
thunderstorm activity diminishes, and dry conditions prevail.
The convergence of two lake breezes and their influence on the maximum temperature
during July in upper Michigan.
Sinking air develops where surface winds move offshore, speed up, and diverge.
Rising air develops as surface winds move onshore, slow down, and converge.
Local Winds
• Seasonally Changing Winds: The Monsoon
– Arabic for seasonal
– Winds change direction seasonably causing
extreme dry and wet season
– Eastern and southern Asia, North America
Changing annual wind-flow patterns associated with the
winter and summer Asian monsoon.
Enhanced infrared satellite
image with heavy arrow showing strong monsoonal
circulation.
Moist, southerly winds are
causing showers and
thunderstorms (yellow and red
areas) to form over the
southwestern section of the
United States during July,
2001.
Local Winds
• Mountain and Valley Breeze
– On mountain slopes, warm air rises during the day
creating a valley breeze; during night nocturnal
drainage of cool air creating a mountain breeze
– Associated with cumulus clouds in the afternoon
• Katabatic winds
– Cold wind rushes down elevated slopes, usually 10
kts or less but can reach hurricane strength
Strong katabatic winds can form where
cold winds rush downhill from an elevated
plateau covered with snow.
• Chinook/Foehn Winds
– Dry warm descending on the leeward side of
a orographic barrier
– Eastern slope of Rockies (chinook), Europe
(foehn), Argentina (zonda)
• Santa Anna Winds
– Warm dry that blows from east or northeast down
canyons into S. California
– Very fast, desiccates vegetation, providing fuel for fires
Exceptionally hot, dry local winds that form over North Africa and the Sahara Desert.
Desert winds
Dust storms, sand storms, dust devil, haboob
A large dust storm over the
African Sahara Desert during
February, 2001, sweeps
westward off the coast, then
northward into a mid-latitude
cyclonic storm west of Spain, as
indicated by red arrow.
A large haboob (dust storm) moves through Phoenix, Arizona, on July 5, 2011.
The formation of a dust devil. On a hot, dry day, the atmosphere next to the ground
becomes unstable. As the heated air rises, wind blowing past an obstruction twists the
rising air, forming a rotating air column or dust devil. Air from the sides rushes into the
rising column, lifting sand, dust, leaves, or any other loose material from the surface.