Download Small-scale winds interacting with the environment

Wind: small scale and local
systems
Chapter 9
Figure 9.1
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.
Small-scale winds interacting with the environment
• Friction and Turbulence in Boundary Layer
– Laminar flow smooth flow
• Slowed by molecular viscosity
– Eddy viscosity
– Irregular turbulent motion
• Mechanical turbulence creates much greater drag
• Decreases away from earth surface
– Planetary boundary layer (PBL) – friction layer
– Depth of PBL varies
• Thermal turbulence
Figure 9.3
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.
Figure 9.4
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.)
Figure 9.5 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.)
Figure 9.6
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
• 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.
Small-scale winds interacting
with the environment
• 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 – wind waves depend on:
• Wind speed
• Time
Determining wind speed and
direction
• Wind characterized by direction, speed, and
gustiness
• Wind direction describes the direction from
which it is blowing
• Influence of Prevailing Winds
– Prevailing most frequently observed
direction during a given time period
– Impact human and natural landscape
– Wind rose
Determining wind speed and
direction
• Wind Measurements
– Wind vane
– Pressure plate anemometer
– Cup anemometer
– Aerovane
– Rawinsonde
– Wind soundings
• Lidar
• Wind profiler
• QuickSat
• 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
Figure 9.25
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.
Figure 9.27
The convergence of two lake breezes and their influence
on the maximum temperature during July in upper
Michigan.
Figure 9.29
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
Figure 9.30
Changing annual wind-flow patterns associated with the
winter and summer Asian monsoon.
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
Figure 9.36
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)
Local Winds
• Santa Anna Winds
– Warm dry that blows from east or northeast don
canyons into S. California
– Very fast, desiccates vegetation, providing fuel for
fires
Desert winds
Dust storms, sand storms, dust devil, haboob
Figure 9.41
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.
Figure 9.42
An haboob approaching Phoenix, Arizona.
The dust cloud is rising to a height of about
450 m (1475 ft) above the valley floor.
Figure 9.43
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.