Lecture07

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```					                    Lecture 7

Homework Due Friday, October 5, 2012
Microphysics Problem set (at the end of the lecture 6 )
TYU Ch 6: 1,2,5,7,11,14,17,18,20;     TYPSS Ch 6: 2
TYU Ch 7: 1,5,10,11,13,14,15,18,20; TYPSS Ch 7: 1
TYU Ch 8: 1,2,6,7,12,14,15,18,20;     TYPSS Ch 8: 2
What does “stability” mean?
“Air Parcel”
•   Number of molecules remains
constant
•   Expands as it rises
•   Pressure equilibrates with
surroundings
•   Density may or may not equal
surroundings
– Depends on temperature
– Larger temperature than
environment will make
molecules move faster and
the pressure higher
– higher pressure will make
parcel volume expand more,
lowering density relative to
the environment
Basic Principle
• Warm air is less dense than colder air at the same pressure
• Weight of air parcel is proportional to its density (more dense air
weights more for equal volume)
• The decrease in pressure with height creates a net force upward
that balances the force of an air parcels weight downward.
• Consider a given pressure change with height, in balance with air
parcels at some environmental temperature T, then
– A parcel with temperature greater than T, will be less dense than environmental
parcels and so its weight or downward force will be less than than of
environmental parcels, and so there will be a net force upward on than parcel
causing it to rise!
– The opposite is true for a cold parcel
• Bottom line: warm air parcels (relative to air parcels surrounding it at
the same pressure) will rise and cold air will sink!
• A thermodynamic process, where there is
no flux of energy or mass across the walls
of the parcel. Therefore, the parcel can
exchange energy with the surroundings
only through performing work on the
surroundings or vise versa.
• Adiabatic process where no phase
changes of water take place
• Adiabatic process where phase changes
of water take place, causing the internal
energy (temperature) to rise as the vapor
condenses or vise versa
Lapse Rate
• Lapse rate is the rate at which temperature
decreases with height
• Environmental lapse rate is that measured
by a thermometer at different pressures (or
height)
• Parcel lapse rate is that occurring within a
parcel as it rises, expands and cools
–   Pressure change causes air to expand and cool,
conserving its total energy
–   This causes temperature of a dry air parcel to decrease at
the dry lapse rate of 9.8K/km
–   If the parcel is saturated, than latent heat released as
water condenses and so the moist lapse rate is < 9.8K km
–   Actual moist lapse decreases with temperature, but a
typical (in cloud) moist lapse rate is ~ 5K/km
–   Average (tropospheric) environmental lapse rate is
~5.5K/km
–   Typical stratospheric environmental lapse rate is ~0K/km
For stability we compare parcel
lapse rate to environmental lapse
rate
UNSTABLE   STABLE

DRY         DRY
Stability depends on
a parcel’s lapse rate
compared to
environmental lapse
rate

UNSTABLE    STABLE

DRY          MOIST
Conditional
Instability
Stuve Diagram

• Compare parcel lapse
rate to environmental
lapse rate
LCL and LFC

UNSTABLE

Positive area (parcel
warmer than environment)
LFC
Negative area (parcel
colder than environment)

Conditionally
LCL                                     Unstable parcel
(after being forced up to
LFC, parcel becimes
unstable)
Mechanisms
to lift a parcel
to LFC

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