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Chapter 9: Small-scale and
Local Systems
Small- and local-scale Flows
In the previous chapter, we discussed larger-scale
winds.
We found that these winds are driven by the pressure
gradient force.
Aloft, the Coriolis force is in balance with the PGF,
creating geostrophic winds.
At the surface, friction opposes the flow, forcing air to
move toward low pressure and away from high
pressure.
Small- and local-scale Flows
These larger-scale flows occur at what is called the
synoptic scale.
There are other scales of flow in the atmosphere:
Microscale flows occur over a few minutes and
are typically short-lived (e.g., smoke from a chimney)
Mesoscale flows occur over a few kilometers to
about 100 kilometers, including things like sea breezes,
thunderstorms, and small tropical systems
Planetary scale flows cover the entire globe
Small- and local-scale Flows
There are other scales of flow in the atmosphere:
Microscale flows occur over a few minutes and
are typically short-lived (e.g., smoke from a chimney)
Mesoscale flows occur over a few kilometers to
about 100 kilometers, including things like sea breezes,
thunderstorms, and small tropical systems
Planetary scale flows cover the entire globe
Sometimes, the plantery and synoptic scales are
combined into what is called the macroscale.
Small- and local-scale Flows
Turbulence
Turbulence is a disturbed flow of air that can produce
wind gusts and eddies.
There are two common ways in which turbulence can
be induced, both of which are related to friction in a
fluid flow, which we call viscosity.
Viscosity
Molecular viscosity is due to the random motion of the
gas molecules in a fluid.
Consider two layers of air on top of each other,
one moving faster than the other, what happens?
Viscosity
Molecular viscosity is due to the random motion of the
gas molecules in a fluid.
Consider two layers of air on top of each other,
one moving faster than the other, what happens?
Eddy viscosity is related to the internal friction induced
by whirling eddies.
Consider air moving over a landscaped dotted
with trees and/or buildings; it breaks into several
irregular, twisting eddies. What do these eddies do?
Turbulence
In eddies, the wind speed and direction fluctuate very
rapidly, producing irregular air motions that we call
wind gusts.
When eddies are formed via obstructions at the
surface, we call this mechanical turbulence.
NOTE: The effects of mechanical turbulence on air flows
far exceeds the effects of molecular viscosity.
Turbulence
Surface heating and instability cause turbulence to
extend to greater altitudes.
Thermals and convective cells that are formed when
the Earth’s surface warms form vertical motions that
create thermal turbulence.
Thermal turbulence tends to be minimal in the early
morning (no surface heating) and largest during the
hottest part of the day.
Turbulence
Example of mechanical turbulence combined with
thermal turbulence.
Turbulence
This turbulence
changes the wind
profile in the planetary
boundary layer (which
is the region of the
atmosphere
influenced by surface
friction)
Turbulence
It is this same effect
that causes the
temperature at the
surface to be
higher on windy
nights than on
calm nights
Turbulence
The depth of this mixing (and the frictional influence)
depend on three main factors:
1)Surface heating – produces a steep lapse rate and
strong thermal turbulence
2)Strong winds – produces strong mechanical
turbulence
3)Rough/hilly landscape – produces strong
mechanical turbulence
Combined, these effects can cause strong a gusty
winds at the surface (i.e., Laramie)
Eddies
Not all eddies are alike . . .
Eddies
Not all eddies are alike . . .
≠
Eddies
Eddies can be big or small.
The size of the obstacle and the wind speed are the
two primary factors that control the size of eddies.
For example, air moving over a shrub produces a small
eddy, while air moving over a mountain produces
large eddies with much stronger motions.
Eddies
Eddies
Not all eddies are formed at the surface…
Turbulent eddies can also be created when the wind
abruptly changes speed or direction, which we call
wind shear.
The shearing creates eddies along a mixing zone; in
clear air, this is called clear air turbulence.
Eddies
The shearing creates eddies along a mixing zone; in
clear air, this is called clear air turbulence.
Eddies
This can happen in the presence of moisture…
Kelvin-Helmholtz waves
Local Wind Systems
Eddies can result in local wind systems (e.g., sea
breezes).
Typically, these local wind systems are caused by
differential surface heating (i.e., adjacent regions
being warmed/cooled at different rates).
Local Wind Systems
1) Initially, the isobaric
(constant pressure) surfaces
are all parallel to the
surface. What is the wind
like?
Local Wind Systems
1) Initially, the isobaric
(constant pressure) surfaces
are all parallel to the
surface. What is the wind
like?
2) Differential heating (warm
to the south, cool to the
north) lifts the isobars to the
south, creating a PGF aloft.
Now what is the wind like?
Local Wind Systems
1) Initially, the isobaric
(constant pressure) surfaces
are all parallel to the
surface. What is the wind
like?
2) Differential heating (warm
to the south, cool to the
north) lifts the isobars to the
south, creating a PGF aloft.
Now what is the wind like?
3) The surface pressure
decreases to the south as
air is transferred northward
aloft, creating a surface
low. Now what is the wind
like?
Local Wind Systems
These circulations are called
thermal circulations.
The local regions of high and
low pressure are called thermal
highs and thermal lows.
There a several common
examples:
sea breezes
land breezes
lake breezes
mountain breezes
valley breezes
Sea/Land Breezes
During the day, the land warms
faster than the adjacent water,
creating high pressure aloft
over the land. At the surface,
air moves onshore. This is called
a sea breeze.
Sea/Land Breezes
During the day, the land warms
faster than the adjacent water,
creating high pressure aloft
over the land. At the surface,
air moves onshore. This is called
a sea breeze.
At night, the land cools faster
than the adjacent water,
creating a high pressure aloft
over the water. At the surface,
air moves offshore. This is called
a land breeze.
Sea/Land Breezes
Over a peninsula, e.g., Florida, sea breezes can form on both
coasts, moving inland as the day progresses.
The breezes converge, creating uplift to help promote
thunderstorm development.
Lake Breezes
The convergence of coastal breezes is not solely
confined to regions next to oceans.
Large lakes (e.g., the Great Lakes) are also capable of
producing local wind systems, which are called lake
breezes.
Similar to Florida, small land areas that are surrounding
by large lakes can also exhibit converging breezes,
e.g., upper peninsula of Michigan.
Lake Breezes
Think of these
as small-scale
cold fronts.
Recall that
frontal lifting is
a mechanism
to trigger
convection.
Valley/Mountain Breezes
During the day, valley walls warm faster than the air at
the same altitude, creating a thermal low along the
valley wall and rising air.
We call this a valley breeze.
Valley/Mountain Breezes
At night, the valley walls cool faster than the air at the
same altitude, creating a thermal high along the valley
wall and sinking air (cold air drainage).
We call this a mountain breeze.
Valley/Mountain Breezes
If the rising air due to daytime warming along mountain
slopes is strong enough, cumulus clouds can form!
Katabatic Winds
Katabatic winds typically originate over high plateaus
that become snow-covered in winter.
The snow keeps the surface temperature low, creating
a shallow dome of high pressure.
The PGF acts to push air toward low altitudes.
As the air moves downhill, it warms via compressional
warming.
Katabatic Winds
Katabatic winds typically originate over high plateaus
that become snow-covered in winter.
The snow keeps the surface temperature low, creating
a shallow dome of high pressure.
The PGF acts to push air toward low altitudes.
As the air moves downhill, it warms via compressional
warming.
Katabatic Winds
Chinook Winds
Chinook winds are warm, dry, downslope winds that
descend the eastern slope of the Rocky Mountains,
warming at the dry adiabatic lapse rate, i.e., 10 °C/km.
(note, similar situations occur in other regions of the world;
however, we use different names)
When these winds move through, temperatures rise and
relative humidities decrease very rapidly (RH < 5% is not
uncommon).
Chinook winds are enhanced when the air first rises over
the mountains, condensing to form clouds, and then
subsiding down the eastern slope – why does this
enhance the warming?
Chinook Winds
Air moving up the mountains cools; condensation
begins, which limits the cooling.
The air sinks down the eastern side, warming at the dry
adiabatic lapse rate.
Santa Ana Winds
Santa Ana winds are warm, dry winds that blows
downhill from the deserts to the east and northeast of
southern California and into the Los Angeles basin.
Air is typically funneled through the narrow mountain
valleys and can be very strong (~100 mph).
They air is very dry because the air originates over the
high desert.
These winds are synonymous with high pressure forming
over the Great Basin.
Santa Ana Winds
Recall that this high
pressure over the
Great Basin creates
a PGF toward
southern California,
which drives the
strong winds.
These winds are
often associated
with fast-moving
forest fires in
southern California.
Santa Ana Winds
Desert Winds
Winds of all sizes form over desert regions.
These winds often lift dust into the air, creating dust
storms.
In desert areas with loose sand, sandstorms are
common.
If dust/sand is lifted into the air from cold downdrafts
along the leading edge of thunderstorms, a large,
dark dust cloud forms, which is called a haboob.
Desert Winds
Large haboob moving through Phoenix, AZ, on July 5, 2011.
Seasonal Winds
Some thermal circulations extend over much larger
areas than local sea and land breezes, e.g., the
monsoon.
Monsoon wind systems change direction on
seasonal time scales. Such a system is well
developed in eastern and southern Asia.
Seasonal Winds
In winter, the air over the continent becomes colder
than the air over the open water.
A large, shallow high-pressure area forms over the
continent.
Air subsides and blows offshore.
Seasonal Winds
In summer, the air over the continent becomes
warmer than the air over the open water.
A large, shallow low-pressure area forms over the
continent.
Air rises and blows onshore.
Seasonal Winds
The rising onshore flow in
summer is enhanced by
the orographic boundary,
allowing deep,
precipitating clouds to
form.
Therefore, the summer is
the wet season.
Seasonal Winds
A similar seasonal change
in circulation occurs of the
southwestern US.
Recall our conversations
earlier in the semester
regarding the abundant
moisture being transported
into the mountain west.