Transcript Scales of Motion Winds and whirling eddies evident at the

Chapter 9
Winds: Small scale and local systems
Scales of motion
• Smallest - microscale (few meters or less)
• Middle - Mesoscale (few to about 100 km)
• Large - Synoptic scale (hundreds to thousands of km)
• Global - Planetary scale (tens of thousands of km)
-sometimes synoptic and planetary are combined
-macroscale
Scales of Motion
Winds and whirling eddies evident at the microscale and
mesocale are no longer observed at the synopitic scale.
Macroscale winds that act at the planetary level include
the Jet Stream.
Friction and turbulence in boundary layer
• friction of fluid flow is called viscosity
• slowing of air due to random motion of gas molecules is
called molecular viscosity
• if molecular viscosity were the only type of friction acting on
moving then the friction would disappear quickly above the
surface
• friction produced by turbulent eddies is called eddy viscosity
• eddies created by air blowing over/around/into obstructions
• eddies created by these obstructions are called mechanical
turbulence
• turbulence that is caused by surface heating and instability is
called thermal turbulence
Turbulence & Eddies
Surface
obstructions
slow surface
winds, which
due to viscosity
creates eddies of
whirling winds
in a much
thicker layer.
The vertical
depth of these
eddies and
gusting winds
thickens with
surface wind
velocity.
Scales of Eddies
Whirling eddies of wind are generated at various scales and
orientations.
Here are horizontal whirls leeward of an obstructing mountain
and vertical rotors, or roll eddies, that can extend for several
kilometers.
Turbulent eddies
• Form aloft as well as near the surface
• Turbulence aloft can occur suddenly and unexpectedly
• Occur where winds changes direction and/or speed abruptly
• This is called wind shear
- shearing produces eddies along the mixing zone
- if it occurs in cloud-free air it is called clear-air turbulence
Shear boundary
Air Flow
Small shear
Increasing
shear
Boundary
deforms
Waves
appear
Turbulent
eddies
break
Friction and Mixing
• friction of air flow is a result of the exchange of air
molecules moving at different speeds
• this exchange is quite small compared to turbulent motions
• frictional effect of the surface on moving air depends mainly
on mechanical and thermal turbulent mixing
• the depth of mixing depends on three factors
- surface heating
- strong winds
- rough/hilly landscape
Wind Profiles
Changes in air temperature causing warm air to rise and cool air to
sink can also 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.
Thermal Circulations
Solar heating and radiational cooling of earth's surface generates
cold-core thermal highs and warm-core thermal lows.
Winds can circulate between these two systems.
Cold & Warm Cores
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.
Sea & Land Breezes
Opposing
breezes may
converge on an
isthmus of land,
and this rising
moist unstable
air will trigger
thunderstorms.
The leading edge
of land-water
breezes can bring
rapid changes in
humidity or
pollutants.
Sea Breeze
Sea Breeze
Water, Friction, & Wind Speed
Seasonal reversal of monsoon winds in southern Asia is explained
by continental temperature shifts.
Summer monsoon depressions of low pressure and rains are
enhanced by insolation, latent heat of condensation, and jet
stream patterns.
Seasonal Wind Changes
Monsoon like winds
develop in July
across the
southwestern U.S.
region.
As the continental
interior heats and
rises, humid Gulf
air sweeps in
creating instability
and thunderstorms.
Mountain and Valley Breezes
Solar heating of hill slopes creates air that is warmer and less dense
than air at the same elevation over the valley, and generates rising
valley breezes.
Nocturnal radiational cooling of the mountain slope creates
relatively denser air that sinks as a mountain breeze, or gravity
wind.
Valley & Mountain Breezes
Daily warming of mountain slopes that trigger valley breezes of
moist air may result in cumulus clouds above mountain
summits.
Katabatic Wind
Mountain 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.
Chinook Winds
Westerly winds that condense and precipitate their moisture when
ascending the Rockies, and then compressionally warm and dry
when descending, are described as chinook winds.
Santa Ana Winds
Easterly winds that descend from southern California's elevated
desert plateau are compressionally heated and lowered in humidity.
Many fires have been triggered by winds of this type.
Desert Winds
Strong desert winds can lift dust particles in the air, creating
sandstorms, or haboobs.
Dust Devils
On a hot, dry day solar radiation generates predominantly sensible
heat and rising thermals of air.