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

Contrail thickness 400 m
Shortwave optical properties Yang et al. (2000)
Longwave optical properties Mie theory
Contrail shape Elliptical (cos dependence of IWC on dist. from center)
Solar azimuth angle 0 (perpendicular) and 90 (parallel)
Contrail height 10 km (-50C in US Standard Atmosphere)
Control Experiments
Mean optical depth at 0.55 mm 0.2 0.2-0.6
Particle type Solid columns Spheres bullet rosettes
Effective radius 10 ?m 5-25 ?m
Contrail width 800 m 400-1200 m
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