# Convection

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Convection and Atmospheric
Stability

AOS 101 Dis. 305
Atmospheric 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.maltaweather.info/cumulus.jpg
Stability

• When the atmosphere is unstable (with respect to a lifted parcel of air), a
parcel will want to continue to rise if lifted:

http://blogs.trb.com/news/weather/weblog/wgnweather/archives/051906_cumulus_clouds.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. The
parcel warms or cools purely due to pressure changes.
• Space occupied by air molecules inside parcel defines
the air density
• Average speed of molecules directly related to air
temperature
• Molecules colliding against parcel walls define the air
pressure inside
Buoyancy and Stability
• Since Pressure is the same, the only other variable changing is
temperature. But remember they are on the same side of the
equation so they are inversely proportional

• So if ρparcel < ρenv. Then the parcel floats or in other words is
positively buoyant

• In terms of temperature that would mean
If
T of parcel > T of environment the parcel is positively buoyant
(less dense and will rise) (unstable) (or less stable)
T of parcel < T of environment the parcel is negatively buoyant
(more dense and will sink) (stable)
T of parcel = T of environment the parcel is neutrally buoyant
(will not rise or sink) (neutral)
Atmospheric Stability
Atmospheric Soundings
• Vertical “profiles” of the atmosphere are taken at 0000 UTC (7 AM CDT) and 1200 UTC
(7 PM CDT) at ~80 stations across the country, and many more around the world.
Sometimes also launched at other times when there is weather of interest in the area.
• Weather balloons rise to over 50,000 feet and take measurements of several

http://www.ua.nws.noaa.gov/n
ws_upper.htm
• Temperature
• Dew point temperature
• Wind Direction and Speed
• Pressure

From these variables we
can find the following:
•Saturation Mixing Ratio
•Mixing Ratio

http://www2.ljworld.com/photos/2006/may/24/98598/
Temp.
Sensor

Balloon Attachment point
Humidity
GPS antenna   Sensor

Unwinder

www.chmi.cz/meteo/ oap/eoap_basic.html

Reciever/Processo
r

Calibration        Transmitter
check box          Antenna
Inflating the balloon

http://www.wrh.noaa.gov/re
v/tour/UA/inflation.php

Sensor Inspection and Battery Materials,         Radiosonde being Baselined and
write down the radiosonde calibration            Acquiring GPS Information
information

http://www.wrh.noaa.gov/rev/tour/UA/baseline.php
Balloon Launch

airport

Reno-Tahoe International Airport
view from Launch Site

Before the launch, obtain
measurements of local T,          Wait to get clearance from Federal Aviation

(http://www.wrh.noaa.gov/rev/tour/UA/launch.php)
Balloon Launch

After release the balloon,
activate the tracking system
and monitor the data during
the ascent.

Pre-launch takes about 30 mins, while
sounding may take about 90 mins.

(http://www.wrh.noaa.gov/rev/tour/UA/launch.php)
Vertical Profile of Atmospheric Temperature,
Allow us to assess Atmospheric Stability

We must compare the parcel's
temerature Tp with the
temperature of the
surrounding environment Te.

Tp > Te The parcel is positively
buoyant, it is less dense and
will rise.
Tp < Te The parcel is negatively
buoyant, it is more dense and
will sink.
Tp = Te The parcel is neutrally
buoyant, it will not rise or
sink.
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 (Relative
Humidity < 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°C if
parcel is raised by 1km, and increases about
10°C if it is lowered by 1km
MALR
• 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
MALR
– 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               LCL
unstable from 4 – 9km          - Lifting
condensation
level
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
development of rain-
producing 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
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 Along Frontal Boundaries
• Front – The transition zone between two air
masses of different densities
• Lifting occurs along two different types of
fronts
– Cold Front
– Warm Front
Lifting Along Cold Fronts

• A colder,denser air mass lifts the warm, moist air ahead of it
• As the air rises, it cools and condenses, producing clouds and
precipitation
• The steep slope of the cold front leads to more vigorous rising motion
• Hence, cold fronts are often associated with thunderstorms
Lifting Along Warm Fronts

• A warmer, less dense air mass rises up and over the cold air ahead
of the warm front
• Air rises, cools and condenses
• Warm fronts have gentler slopes and move slower than cold fronts
Skew T Log P Diagram
• It is called Skew-T Log-P because the
temperature axis (x axis) is skewed and the
pressure axis (the vertical coordinate) is a
logorithmic axis.
• Area α Energy/Work
• There are several set of lines on a Skew-T Log-
P diagram and they will be shown in the next
couple slides.
Mixing Ratio Lines
Temperature Lines (Isotherms)

Isotherms
Lifting Condensation Level (LCL)
Level at which a parcel lifted from the surface
would reach saturation (i.e. level at which
temperature = dewpoint temperature)

LCL will refer to the height above the ground
at which the cloud base is located
Level of Free Convection (LFC)
Level at which a parcel lifted from the surface would
be warmer than its environment

Above this level, air is able to freely convect, or
ascend without resistance to the tropopause

Equilibrium Level (EL)
Level at which a parcel is no longer warmer than its
environment, usually is the tropopause

Above the tropopause, environmental temperature
increases with height in the stratosphere
-Corresponds to cloud top height
12

EL
10

Tparcel
Height (km)

8

Tenv
6
LFC
4

LCL
2

Td

-40          -30       -20   -10     0    10   20   30

Temperature (C)
How to read from the diagram?
•   Each point on the plot
corresponds to a temperature and
pressure value. Temperature scale
in blue can be read from the
bottom of the plot.
•   For example:
• A:                                        A
– Pressure : 250 mb
– Temperature : -20 C
• B:
– Pressure : 600 mb                B
– Temperature : -10 C
• C:                                        C
– Pressure : 900 mb
– Temperature : 25C (Since it is
exactly in between the line
when T =20 and T=30 C)
Environmental Temperature Profile
-80C     -70C -60C -50C -40C   Here are some
measurements:
Pressure   Temp.   Dew.pt

1000       30      10
500        -10     -10
Temperatur    200        -65     -80
e profile
Notice on each
pressure level, there is
a set of temperature
and dew point
correspond to it. Start
Dew point
by plotting the
Temperatur
temperature and dew
e profile
points at that same
pressure level, and
then connect the
points on the plot.
•   Let’s lift a parcel from 1000mb

Parcel Path                      •
up to the top.
And the parcel at 1000mb has
temperature of 30C and dew
point of 10C.
-80C   -70C -60C -50C -40C   •   The parcel is unsaturated at
1000mb, therefore when it
rises, it cools at dry adiabatic
lapse rate.
•   At 1000mb, we could read the
mixing ratio value of the parcel
is about 8 g/kg from the plot
(purple lines) at the parcel’s
dew point (10C).
•   Since water vapor inside the
parcel are not used at all to
form any condensates yet when
the parcel is still unsaturated as
it is lifted up, the mixing ratio of
the parcel stays constant.
LCL                          •   So, from the dew point at
RATIO line upward, until the dry
mixing ratio line.
Parcel Path
-80C   -70C -60C -50C -40C
•   Once the parcel is lifted up and
the same as its dew point
temperature, it reaches
saturation.
•   The pressure level at which the
parcel get saturated is called the
lifting condensation level (LCL).
It also marks the where the
cloud base would be if the
cloud is formed by lifting parcels
from 1000mb in this case.
•   As the SATURATED parcel
continue to rise beyond LCL, it
LCL                              cools at moist adiabatic lapse
rate.
•   The dotted line represents the
parcel path.

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