METO621

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METO621

• Lesson 18

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Thermal Emission in the Atmosphere Treatment of
clouds

• Scattering by cloud particles is usually ignored
  in the longwave spectrum (thermal emission)
• We have already treated the effect of full cloud
  cover when discussing the heating rates.
• A common approximation to account for partial
  clouds is to assume

• Where N is the cloud fraction. Note that this
  equation is only strictly true if the clouds are
  very thin (in dimension) and randomly
  distributed.
• If the clouds are not black, but have
  emissivity e, then N is replaced by eN. Fc is
  still the flux for an opaque cloud.

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Treatment of clouds

• As clouds grow in the vertical, the sides of the
  clouds obscure portions of the sky, and the
  effective cloud fraction grows as an observer
  looks at angles away from the vertical
• For these cases the equation for the fluxes has
  the form.

• In the figures that follow the symbol a is the
  aspect ratio of the cloud, i.e. height/radius.

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Effective cloud fraction for isothermal cylinders
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Effective cloud fraction for very tall
non-isothermal cylinders
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Intensity Calculations for the Thermal Infra-red

• The next 12 figures show the calculated spectrum
  of intensity, fluxes and heating rates by
  Ellingson and Serafino (1984).
• The model employs the Goody random band model.
• Because of the quasi-isotropic nature of the
  radiation field (the source is blackbody
  radiation) the spectra shown apply equally well
  to the flux.
• The next figure shows the intensity seen at the
  top of the atmosphere. The two large holes
  between 500-800 cm-1 and 1000-1100 cm-1correspond
  to the 15m band of CO2 and the 9.6 m band of O3.
• At the center of the CO2 bands, the atmosphere is
  opaque up to about 5 km, hence the emission is
  from the troposphere.
• At the very center of the CO2 and O3 bands the
  intensity increases because of emission from the
  stratosphere.

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Intensity Calculations for the Thermal Infra-red

• The 800-1200 cm-1 region is known as the window
  region. The atmosphere absorbs only weakly, and
  the intensity is representative of Blackbody
  radiation from the surface.
• The intensities in the pure rotational water
  vapor band (0-600 cm-1) and the 6.3m
  vibration/rotation band (wavenumbers gt 1200
  cm-1), are representative of temperatures at the
  middle and upper troposphere. The atmosphere is
  opaque to radiation from the surface, and the
  intensity is due to emission from water vapor in
  the troposphere.

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Upwelling intensities at 66 km, clear sky
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Intensity Calculations for the Thermal Infra-red

• Downwelling Radiation
• The region of large upwelling intensity at the
  top of the atmosphere between 800 and 1200 cm-1
  has a very low downwelling intensity at the
  surface.
• The downwelling radiation in the CO2 and water
  vapor bands is characteristic of an altitude of
  about 2 km, due to the opaque nature of the
  absorption.
• The ozone band shows a much lower temperature,
  indicating that the emission is from a higher
  altitude, close to the tropopause.

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Homogeneous clear and black cloud condition
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Homogeneous clear and black cloud condition
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Intensity Calculations for the Thermal Infra-red

• The angular variation of the intensity integrated
  over the intervals 12-20 ( the CO2 band) and
  10-12 micron (800-1000 cm-1) (the window region)
  is shown in the next two slides.
• In these figures a nadir angle of 0-90
  corresponds to upwelling radiation and a nadir
  angle of gt90 corresponds to downwelling.
• The upwelling radiance shows little variation
  with angle, whereas the downwelling radiance
  increases dramatically near the horizon (90
  degrees) because of the longer path length.
• Note that the upwelling radiance is almost
  isotropic (no change with angle). Hence the mean
  angle used in the two stream approximation does
  not need to be specified exactly.

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Longwave radiation as a function of nadir angle
and pressure
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Longwave radiation as a function of nadir angle
and pressure
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Flux and Heating Rate Calculations

• The next two slides present examples of observed
  and calculated profiles of upward and downward
  longwave fluxes for clear and cloudy skies.
• Note that both the upward and downward fluxes
  decrease with increasing, but at different rates.
• The upward flux decrease because the principle
  source of heating is the radiation from the
  ground, and this is attenuated with height.
• The downward radiation fluxes increase towards
  the surface because the increasingly opaque
  atmosphere is emitting at progressively warmer
  temperatures.

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Profiles of clear sky upward and downward fluxes
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Flux and Heating Rate Calculations

• For cloudy conditions, the downward flux
  decreases with increasing altitude, but the
  decrease is slower than for clear skies due to
  the contribution from the nearly black cloud.
• The upward flux decreases rapidly in the lower
  portion of the cloud layer to a value
  approximately equal to the emission from a
  blackbody. That is the cloud absorbs the incident
  radiation, and replaces it with radiation at the
  cloud temperature.
• Near the top of the cloud, the downward flux
  decreases rapidly to the clear sky value, whereas
  the upward flux changes little from the value
  inside the cloud because there is little
  attenuation of the emission by the gases above
  the cloud.

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Profiles for cloudy skies of clear upward and
downward fluxes
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Clear and cloudy sky heating rate profiles
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Spectral contributions to the cooling rate
tropical atmopshere
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Spectral contributions to the longwave cooling
rate
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Vertical profile of total longwave cooling