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Robin Hogan

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f: solar azimuth angle. SHDOM 3D radiation code (Evans 1998) ... The shortwave forcing is strongly dependent on solar azimuth angle ... – PowerPoint PPT presentation

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Title: Robin Hogan


1
The effect of horizontal photon transport on the
radiative forcing of contrails
  • Robin Hogan
  • Amanda Gounou
  • Department of Meteorology, University of
    Reading, UK

2
Motivation
  • IPCC Aviation Special Report (1999)
  • 1992 global contrail coverage 0.1 ? net
    radiative forcing 0.02 W m-2
  • 2050 global contrail coverage 0.5 ? net
    radiative forcing 0.1 W m-2
  • In SE England in winter, forcing currently 0.5 W
    m-2 (Stuber et al., Nature 2006)
  • Although the net effect is quite small, there is
    a large degree of cancellation between the
    shortwave and longwave effects
  • A small change in either of these could have a
    large impact on the net forcing
  • Nearly all previous studies have used the
    independent column approximation (ICA)
  • 3D effect neglected but photon transport through
    contrail sides may be important
  • Here we use the 3D SHDOM radiation code to
    estimate contrail forcing
  • Secondary motivation it can be difficult to
    explain the difference between ICA and 3D
    radiation in clouds
  • Contrails are perfect for visualizing the various
    effects
  • Simple quasi two-dimensional geometry
  • Low optical depth so longwave and shortwave
    effects not yet saturated

3
Experimental configuration
q solar zenith angle f solar azimuth angle
  • SHDOM 3D radiation code (Evans 1998)
  • Periodic in both horizontal directions
  • Contrail infinite in one horizontal direction
  • Compare ICA and 3D runs

4
  • Shortwave downwelling flux
  • Longwave upwelling flux

5
Independent column approximation
Control contrail
Longwave upwelling radiation absorbed emitted at
lower brightness temperature warming effect on
climate, positive forcing
  • Shortwave solar radiation reflected back to
    space cooling effect on climate, negative
    radiative forcing

Radiative forcing difference in mean
top-of-atmosphere upwelling irradiance between
the calculations with and without a contrail,
then scaled up to an equivalent contrail cover of
100
6
3D radiative transfer
Control contrail
  • Radiative forcing difference in mean
    top-of-atmosphere upwelling irradiance between
    the calculations with and without a contrail,
    then scaled up to an equivalent contrail cover of
    100

7
Why is there a 3D shortwave effect?
Photon escape
8
Why is there a 3D shortwave effect?
.
Effect 2
Side illumination
9
Why is there a 3D longwave effect?
Contrail edge absorption
Effect 1
10
Effect of particle type columns
Control contrail
  • For effective radius of 10 mm and wavelength of
    0.55 mm, solid columns have an asymmetry factor g
    of 0.75 (similar for bullet rosettes)

11
Effect of particle type spheres
Sign of net forcing is reversed
Reduced shortwave forcing
  • Spheres have an asymmetry factor g of 0.85 more
    forward scattering

12
Effect of contrail optical depth
d0.4
3D net effect is a factor of 2 or more for all
solar zenith angles
d0.2 (control)
  • Doubling of optical depth
  • Shortwave forcing doubles
  • Less than a doubling for the longwave forcing
    (partial saturation)

13
Effect of contrail aspect ratio
q80
q40
  • As contrails age they tend to spread out
    horizontally
  • We keep thickness constant (400m) and vary width
  • 3D effect tends to vary in proportion to the
    aspect ratio
  • Aged contrails have a lower 3D effect

14
Conclusions
  • Three ways can be identified by which 3D
    transport affects forcing
  • Solar photons can escape through the sides of the
    contrail
  • The sun illuminates contrail sides, lengthening
    the shadow cast by the contrail
  • Upwelling longwave photons can be absorbed by
    contrail sides
  • For solar zenith angle qlt70, inclusion of 3D
    transport
  • Increases the longwave warming effect of the
    contrail on climate
  • Reduces the shortwave cooling effect of the
    contrail on climate
  • This results in a substantial net warming effect
  • For 70ltqlt90
  • The shortwave forcing is strongly dependent on
    solar azimuth angle
  • Net forcing can be doubled or its sign can be
    reversed!
  • There is a need to re-evaluate the global impact
    of contrails on climate to account for the
    effects of 3D transport
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