Vertical Structure of the Atmospheric Boundary Layer in Trade Winds

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Vertical Structure of the Atmospheric Boundary
Layer in Trade Winds

• Yumin Moon
• MPO 551
• September 26, 2005

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Papers to Present

• Riehl, H., Yeh T. C., Malkus J. S., and La Seur,
  N. E., 1951 The Northeast Trade of the Pacific
  Ocean. Quarterly Journal of the Royal
  Meteorological Society. 77, 598-626.
• Augstein E., Schmidt, H., and Ostapoff, F., 1974
  The Vertical Structure of the Atmospheric
  Planetary Boundary Layer in Undisturbed Trade
  Winds over the Atlantic Ocean. Boundary Layer
  Meteorology, 6, 129-150.

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Riehl 1951

• Analyzed observations in the Northeast Pacific
  Ocean during the dry season (July to October).
• Three weather ships, Pearl Harbor, Hickam Field,
  both in Honolulu, HI.
• Hourly surface observations, two radiosonde
  observations per day.

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Riehl 1951

• Wind Steadiness
• Air above the Inversion Top
• Inversion Layer
• Cloud Layer
• Subcloud layer
• Vm mean speed

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Riehl 1951
Mean Vertical Distribution of Wind Speed
Vertical Cross-Section of Wind Speed
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Riehl 1951
Equation of Continuity, assuming steady state
Vertical Distribution of Divergence
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Riehl 1951

• Whereas the inversion ascends downstream,
  individual columns descend, shrink vertically and
  spread horizontally. Large masses of air, located
  above the inversion at 32N have become a part of
  the cloud layer when they reach Honolulu, HI.

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Riehl 1951

• The air that has been incorporated in the
  inversion layer
• The air that has been incorporated in the cloud
  layer
• The air that has been below the inversion
  throughout the journey from 32N.

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Riehl 1951
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Riehl 1951
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Riehl 1951
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Riehl 1951
Equation of Continuity for Latent Heat in a layer
of unit thickness and cross-section and extending
over the distance ds
Source/Sink
Vertical
Horizontal
Turbulent Exchange

• Steady-state is assumed.
• Lateral mixing is neglected compared to vertical
  mixing.
• The vertical coordinate is attached to the
  trajectory of the mean motion ? w vanishes
  everywhere thus the second term (vertical) is
  dropped out, except at the top where the boundary
  is a horizontal surface.

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Riehl 1951
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Riehl 1951

• Rise of the inversion is accomplished through the
  pickup of latent heat by the trade in the course
  of its passage over the tropical ocean.
• The bases of cumulus clouds are nearly uniform
  height, but the tops are very irregular.
• The tops of the cumulus clouds break off and
  evaporate quickly.
• Moisture is introduced into the lower portions of
  the inversion layer.
• Then the air in the inversion layer becomes
  gradually similar to the characteristics of the
  cloud layer.

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Riehl 1951
Equation of momentum
Integrating over a volume bounded by ds, dz, and
of unit thickness in the direction normal to s
Inflow term upstream, downstream
Inflow term top, bottom
Pressure Term
Turbulent term top, bottom
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Riehl 1951
In units of 108 G CM SEC-2
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Augstein 1974

• Analyzed observations collected during the
  Atlantic Expedition 1965 Sep 12 to Oct 11 and the
  Atlantic Tradewind Experiment (ATEX) 1969 Feb
  6-21.
• Three ships, Planet at the northeast, Discoverer
  at the northwest, and Meteor at the south.
• 8 radiosondes observations per day, radar wind
  measurements.

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Augstein 1974

• Surface Layer adiabatic temperature gradient,
  decrease of specific humidity with height, slight
  statical instability.
• Mixed Layer adiabatic temperature lapse rate,
  nearly constant vertical specific humidity.
• Transition Layer nearly isothermal temperature
  distribution, strong upward decrease of moisture.
• Cloud Layer temperature gradient slightly
  higher than the moist adiabatic lapse rate,
  upward weak decrease of specific humidity,
  conditionally unstable.
• Trade Inversion increasing temperature, steep
  decrease of moisture.

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Augstein 1974
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Augstein 1974

• Increasing cumulus convection causes an increase
  of downward flux of inversion air into the cloud
  layer, thus pushing the inversion upward. This
  process is combined with downward heat flux which
  effects a diabatic warming of the cloud layer.

• Strong convective activity destroys the trade
  inversion the organized cloud circulation then
  transports air parcels with relatively low
  potential temperature upward and with high
  potential temperature downward. This process
  results in a diabatic warming of the lower part
  of the cloud region and in a diabatic cooling of
  the upper part

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Augstein 1974
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Augstein 1974
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Augstein 1974