Lifting Mechanisms

1
Lifting Mechanisms

• We shall consider four different ways air can be
  lifted from the surface.
• Orographic Lifting
• Convergence and Divergence
• Surface Boundaries
• Convection

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Lifting Mechanisms

• Orographic Lifting
• Air is forced upward by topography

Adiabatic Cooling (Windward Side)
Adiabatic Warming (Leeward Side)
Usually wetter on the windward side than on the
leeward side.
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Lifting Mechanisms

• Convergence and Divergence
• Convergence at the surface (Low Pressure)
• Divergence Aloft

Divergence aloft
Rising Motion
Convergence at the surface
L
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Lifting Mechanisms

• Surface Boundaries
• Warm and Cold Fronts
• Outflow Boundaries (Thunderstorms)
• Dry Line

Warm
Cold
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Lifting Mechanisms

• When we lift the air, where will condensation
  occur?
• Depends on the moisture content of the air that
  is being lifted.
• Moist air requires less cooling, hence less
  lifting, to reach the dew point.
• Drier air requires more cooling and more lift to
  reach the dew point.

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Lifting Condensation Level

• The lifting condensation level (LCL) is the
  altitude, usually expressed as a pressure, at
  which the lifted air is cooled dry adiabatically
  to saturation.
• Clouds will form at this level.
• We can find this level on our thermodynamic
  diagram!!!!!

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Lifting Condensation Level

• Assume the air at the surface have the following
  measurements.
• Surface Pressure 1000 mb
• Surface Temperature 20?C
• Surface Dew Point 14?C
• If we lift this air by some method, where will
  the clouds form?

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Lifting Condensation Level

• Recall from our section on humidity that
• Also

w
RH
X 100
ws
We can achieve saturation when the parcel
mixing ratio equals the saturation mixing
ratio We have mixing ratio on our chart!!!!!
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Lifting Condensation Level

• As an air parcel rises and cools, the saturation
  mixing ratio decreases.
• The actual mixing ratio does not change.
• When the parcel cools to the point when the
  parcel mixing ratio and the saturation mixing
  ratio are equal, RH will be 100 and a cloud will
  form.

10
Lifting Condensation Level

• How do we do this on the chart?
• Since we are lifting a parcel dry adiabatically,
  we follow the dry adiabat up from the surface
  temperature.
• Since the actual mixing ratio does not change, we
  follow the mixing ratio line up from the surface
  dew point.
• Where they intersect is the LCL.

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Lifting Condensation Level
Schematically
Mixing Ratio
p
LCL
pLCL
Dry Adiabat
T
Td
T
12
LCL
T
Td
13
Lifting Condensation Level

• If lifting continues, the parcel will rise moist
  adiabatically (making a cloud).

LCL
Td
T
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Lifting Mechanisms

• Lifting by convection
• Lift by heating surface (diabatic)
• Parcels of warm air rise from the surface and mix
  with the ambient air.
• Responsible for cumuliform clouds.
• Must have an environmental lapse rate (ELR) -- an
  actual sounding for the next computation.

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Convection Condensation Level

• The convection condensation level (CCL) is the
  altitude, usually expressed as a pressure, at
  which convectively mixed air reaches saturation.
• Convectively mixed air means that we heat the air
  until it becomes dry adiabatic.

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Convective Condensation Level

• How do we do this on the chart?
• We assume the lowest layer of air is dry
  adiabatic.
• Since the actual mixing ratio does not change, we
  follow the mixing ratio line up from the surface
  dew point.
• Where the mixing ratio line and the sounding
  intersect is the CCL.

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Convective Condensation Level
Schematically
Mixing Ratio
p
pCCL
CCL
Sounding
T
Td
T
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Sounding
CCL
Td
T
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Convective Temperature

• Convective Temperature
• The convective temperature is the temperature the
  surface needs to be heated to in order for
  convection to occur.

Sounding
Mixing Ratio
From the CCL, go down dry adiabat to the surface.
This is the Convective Temperature TC.
Dry Adiabat
CCL
T
Td
TCON
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Norman Sounding(Today)
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Clouds

• A visible manifestation of condensation or
  deposition in the atmosphere.
• How can chance collisions of water vapor
  molecules lead to the formation of cloud droplets
  that will be long-lived?

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Clouds

• Consider a clean atmosphere with water vapor in
  it.
• Given a long enough time, some water vapor
  molecules will run in to one another. Typically
  they wont stay together. Even if the atmosphere
  was saturated, the likelihood that water vapor
  molecules will form a droplet is small.

Dry Atmosphere Water Vapor
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Clouds

• If more water is added such that the atmosphere
  is supersaturated (RH300 ), then water
  molecules can form a stable droplet.
• This process is called homogeneous nucleation.

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Clouds

• We can measure the amount of moisture in the air
  and find that the cloud droplets form when the
  air just reaches saturation..Why?
• Recall that dew and frost form on grass or other
  things.
• The water vapor molecules need a gathering
  place.

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Clouds

• Nearly a century ago it was discovered that the
  atmosphere contains particles that have an
  affinity for water ---These serve as centers for
  condensation.

Cloud Condensation Nuclei (CCN)
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Clouds

• With CCN, we need much smaller supersaturations
  (RHgt100). In nature we find supersaturations on
  the order of 1.5.
• The atmosphere has plenty of CCN

Dust Salt Spray from Oceans Volcanoes Sulfate
Particles from Phytoplankton Forest
Fires Trees Anthropogenic Origins
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Clouds

• CCN are more plentiful near the surface of the
  earth.
• CCN are more plentiful over land rather than the
  ocean.
• The formation of cloud droplets using CCN is
  called

Heterogeneous Nucleation.