Transcript Exam 1 Review

Exam 2 Review
AOS 121
November 16 2009
Geostrophic Balance and Geostrophic Winds
Balance between the pressure gradient force and Coriolis force
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Will the wind direction forever turn
to right (left) in Northern
(Southern) Hemisphere? The
answer is no. It will keep turning
until a balance between the
pressure gradient force and
Coriolis force is achieved.
This balance happens when the
wind direction becomes parallel to
the isobars. This is called the
geostrophic balance.
The wind satisfying the
geostrophic balance condition is
called the geostrophic wind .
Geostrophic winds are very closed
to (but not exactly the same as)
the actually observed winds.
Actual Wind Converges toward
Low Pressure, and Diverges
outward from High Pressure due
to Friction.
P. K. Wang
Northern Hemisphere
What happen when air moves up and down?
Remember that the atmosphere is stratified—density and pressure
change with height.
Adiabatic Process – a process during which no heat is withdrawn from or added to the system
involved. This means that the total energy of the system is conserved. So the total energy of the system
undergoing an adiabatic process becomes a zero-sum game. If something becomes more, others have to
become less. When air expands adiabatically, it must do expansion work at the expense of other things.
The most common ‘other thing’ is cooling (so the ‘internal energy’ of the system becomes less).
The “system” we are considering here is an air parcel. The figure to the right shows the adiabatic ascent
and descend of an unsaturated air parcel and the important concept of lapse rate.
Air parcel – An air parcel is a
convenient conceptual system for the
purpose of understanding what will
happen to the atmosphere under
certain conditions. It doesn’t have a
specific size. You can imagine it as a
balloon without a skin. It can expand
or shrink (be compressed) like a
balloon and but, unlike a balloon, it
certainly can mix with the
environmental air to exchange mass
and energy.
Air parcel
Environmental air
P. K. Wang
When the air parcel is saturated
Saturation means that the parcel contains a maximum amount of water
vapor with respect to the temperature—warm air can hold more water
vapor per volume than cold air .
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When an air parcel containing moisture rises adiabatically, it will cool
just like a dry air parcel initially. However, when it rises to a level where
its temperature becomes cold enough to saturate the moisture, its
cooling rate will change to smaller values when the parcel continues to
rise. Why?
Because when the parcel becomes saturated, further cooling will cause
excess moisture to condense to form liquid drops. During
condensation, latent heat is released so as to buffer the expansion
cooling rate. This rate is called the moist adiabatic lapse rate—it is
not a constant (it depends on the amount of condensation) but is
always smaller than the dry adiabatic lapse rate.
The level at which the parcel becomes saturated is called the lifting
condensation level (LCL). Traditionally, LCL is taken as the cloud
base.
Convective Cloud Formation
The formation of a convective cloud can be
approximated as a moist adiabatic process. A
moist air parcel is lifted upward adiabatically. It
expands and cools at the dry adiabatic lapse rate
initially. When it reaches its LCL, condensation
begins and cloud appears. Further ascend of the
parcel continues to cool the parcel but at a
smaller moist adiabatic lapse rate. If there is
enough moisture in the parcel, the cloud will
continue to ‘grow’ upward and, when the
temperature is cold enough, ice may start to
appear in the cloud.
In reality, the ascent of an air parcel is not
adiabatic because mixing with its environmental
air (usually drier and colder) always occurs. This
mixing process is called the entrainment.
Therefore the moist adiabatic process should be
regarded only as a convenient approximation.
P. K. Wang
Example:
Imagine two parcels of air, A and B, both with surface
temperatures of 25oC. Compare the temperatures at 4km
above the ground, if parcel A has an LCL at 2km, but
parcel B remains unsaturated. Assume the dry adiabatic
lapse rate is -10oC/km, and the moist adiabatic lapse rate is
-7oC/km.
a). They will have the same temperature.
b). Parcel A will be warmer. (-9oC)
c). Parcel B will be warmer. (-15oC)
Example:
An Unsaturated parcel of air located 3km above the ground
has a temperature of -13oC. What will its temperature be if
it is brought down adiabatically to the surface. Assume the
dry adiabatic lapse rate is -10oC/km, and the moist adiabatic
lapse rate is -7oC/km.
a).
b).
c).
d).
8oC
20oC
17oC
-43oC
Example:
Imagine a parcel of air with a surface temperature of 25oC,
and an LCL at 2km. Lift the parcel adiabatically to 4km
above the ground. If the surrounding environment at 4km
has a temperature of -5oC, would the air parcel warmer or
cooler than its environment. Assume the dry adiabatic
lapse rate is -10oC/km, and the moist adiabatic lapse rate is
-7oC/km.
a). They will have the same temperature.
b). The air parcel will be warmer.
c). The air parcel will be cooler. (Parcel temp = -9oC at 4km)