Download Moisture and Atmospheric Stability

Moisture and Atmospheric
Stability
AOS 101 Discussion
Discussion Leader – Val
Contouring Help
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Review
• Turn in hw #3
• Badger forecasts
• Why do you dry off faster in a desert
climate?
The biggest power plant on Earth’s
surface-
http://www.srh.weather.gov/jetstream/atmos/hydro.htm
Water can exist in all three phases in
our atmosphere
• What term do we seem to use to quantify
the amount of water in any given volume
of air at one time?
• Answer: Moisture
Ways to measure the moisture
content of the atmosphere (discussed in lec.)
• Absolute Humidity
• Specific Humidity
• Saturation Mixing Ratio
• Vapor Pressure
• Saturation Vapor Pressure
• Relative Humidity
• Dew Point Temperature
The variables we will refer to most
• Mixing Ratio- mass of water vapor/mass of dry air (does
not change).
• Relative Humidity- Vapor Pressure/ Saturation vapor
pressure.
• Dew Point Temperature- The temperature at which air
with the current amount of vapor in it will become
saturated.
Two ways to saturate the air
(or raise the relative humidity)
Two ways to saturate the air
(or raise the relative humidity)
• 1. Add more water vapor to it
• 2. Decrease the temperature
This is because warm air is capable of
“holding” more water vapor molecules than
cold air.
(Remember the water vapor molecules are moving faster in
warm air and less likely to stick and condense)
Moisture
• An air parcel with a large moisture content
has the potential for that parcel to produce
a great amount of precipitation.
- Air with a mixing ratio of 13 g/kg will likely
rain a greater amount of water than air
with a mixing ratio of 6 g/kg.
Moisture
Two parcels of air:
PARCEL 1: Temperature = 31 oF, Dewpoint = 28 oF
PARCEL 2: Temperature = 89 oF, Dewpoint = 43 oF
Parcel 2 contains more water vapor than Parcel 1, because its
dewpoint is higher.
Parcel 1 has a higher relative humidity, because it wouldn’t take
much cooling for the temperature to equal the dewpoint! Thus,
Parcel 1 is more likely to become saturated. But if it happened that
both parcels became saturated then Parcel 2 would have the potential
for more precipitation.
RH is not simply equal to the dewpoint divided by the temp. but is a
good representation.
The other key component to the
hydrologic cycle- Stability
• What is stability?
• Stability refers to a condition of equilibrium
If we apply some perturbation to a system, how will
that system be affected?
– Stable: System returns to original state
– Unstable: System continues to move away from
original state
– Neutral: System remains steady after perturbed
Stability Example
Stable: Marble returns to its original position
Unstable: Marble rapidly moves away from initial position
Stability
How does a bowl and marble relate to the atmosphere??
• When the atmosphere is stable, a parcel of air that is lifted will want to return
back to its original position:
http://www.chitambo.com/clouds/cloudshtml/humilis.html
Stability Cont.
• When the atmosphere is unstable (with respect to a lifted parcel of air), a
parcel will want to continue to rise if lifted:
http://www.physicalgeography.net/fundamentals/images/cumulonimbus.jpg
What do we mean by an air parcel?
– Imaginary small body of air a few meters wide
• Can expand and contract freely
• Does not break apart
• Only considered with adiabatic processes External air and heat cannot mix with the air inside
the parcel
• Parcel warms or cools purely due to pressure
changes
(ΔU = Q – W)
Buoyancy and Stability
• At same pressure if at same altitude!
• So if ρparcel < ρenv. Parcel is positively buoyant
• In terms of temperature that would mean:
T of parcel > T of environment – buoyant! (unstable)
T of parcel < T of environment – sink! (stable)
T of parcel = T of environment – stays put (neutral)
Atmospheric Stability (Review)
This is all well and good but what about day to day applications…
almost there
Vertical Profile of Atmospheric
Temperature
Lapse Rates
Lapse Rate: The rate at which temperature decreases with height
(Remember the inherent negative wording to it)
Environmental Lapse Rate: Lapse rates associated with an
observed atmospheric sounding (negative for an inversion layer)
Parcel Lapse Rate: Lapse rate of a parcel of air as it rises or falls
(either saturated or not)
MALR - Moist Adiabatic Lapse Rate: Saturated air parcel
DALR - Dry Adiabatic Lapse Rate: Dry air parcel
DALR
• Air in parcel must be unsaturated
• (RH < 100%)
• Rate of adiabatic heating or cooling =
9.8°C for every 1000 meter (1 kilometer)
change in elevation
– Parcel temperature decreases by about 10°
if parcel is raised by 1km, and increases
about 10° if it is lowered by 1km
MALR (or SALR)
• As rising air cools, its RH increases
because the temperature approaches the
dew point temperature, Td
• If T = Td at some elevation, the air in the
parcel will be saturated (RH = 100%)
• If parcel is raised further, condensation will
occur and the temperature of the parcel
will cool at the rate of about 6°C per 1km
in the mid-latitudes
DALR vs. MALR
• The MALR is
less than the
DALR because
of latent heating
– As water vapor
condenses into
liquid water for a
saturated parcel,
LH is released,
lessening the
adiabatic
cooling
Remember no heat exchanged with environment
DALR vs. MALR
Absolute Stability
• The atmosphere is
absolutely stable when
the environmental
lapse rate (ELR) is
less than the MLR
– ELR < MALR < DALR
– A saturated OR
unsaturated parcel will
be cooler than the
surrounding
environment and will
sink, if raised
Absolute Stability
• Inversion layers are
always absolutely
stable
– Temperature
increases with
height
– Warm air above
cold air = very
stable
Absolute Instability
• The atmosphere is
absolutely unstable
when the ELR is
greater than the DALR
– ELR > DALR > MALR
– An unsaturated OR
saturated parcel will
always be warmer than
the surrounding
environment and will
continue to ascend, if
raised
Conditional Instability
• The atmosphere is
conditionally unstable
when the ELR is
greater than the MALR
but less than the DALR
– MALR < ELR < DALR
– An unsaturated parcel
will be cooler and will
sink, if raised
– A saturated parcel will
be warmer and will
continue to ascend, if
raised
Conditional Instability
• Example: parcel at
surface
– T(p) = 30°C, Td(p) =
14°C (unsaturated)
– ELR = 8°C/km for first
8km
• Parcel is forced
upward, following
DALR
• Parcel saturated at
2km, begins to rise at
MALR
• At 4km, T(p) =
T(e)…this is the level
of free convection
(LFC)
Conditional Instability
• Example continued…
– Now, parcel will rise
on its own because
T(p) > T(e) after 4km
– The parcel will freely
rise until T(p) = T(e),
again
• This is the
equilibrium level (EL)
• In this case, this
point is reached at
9km
– Thus, parcel is stable
from 0 – 4km and
unstable from 4 – 9km
LCL
Lifting due to Topography
How does the parcel get a lift?
• Convection
• Convergence
• Topography
Lifting due to Topography
• This type of lifting occurs
when air is confronted by
a sudden increase in the
vertical topography of the
Earth
– When air comes across a
mountain, it is lifted up and
over, cooling as it is rising
• The type of cloud formed
is dependent upon the
moisture content and
stability of the air
Lifting by Convection
• As the earth is heated by
the sun, thermals
(bubbles of hot air) rise
upward from the surface
• The thermal cools as it
rises, losing some of its
buoyancy (its ability to
rise)
• The vertical extent of the
cloud is largely
determined by the
stability of the
environment
Lifting by Convection
• A deep stable layer
restricts continued
vertical growth
• A deep unstable layer
will likely lead to
development of rainproducing clouds
• These clouds are
more vertically
developed than
clouds developed by
convergence lifting
Lifting by Convergence
• Convergence exists
when there is a
horizontal net inflow
into a region
• When air converges
along the surface, it is
forced to rise
Lifting by Convergence
• Large scale convergence can lift air
hundreds of kilometers across
• Vertical motions associated with
convergence are generally much weaker
than ones due to convection
• Generally, clouds developed by
convergence are less vertically developed