1. Department of Meteorology, University of Reading, UK
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 (-50°C in US Standard Atmosphere)
Control
Experiments
Mean optical depth at 0.55 mm
0.2
Particle type
Solid columns
Spheres & bullet rosettes
Effective radius
10 m
5-25 m
Contrail width
800 m 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 Modest 3D longwave effect
Modest 3D shortwave effect
Modest 3D longwave effect
Net effect is doubled!
Sign of net effect is reversed!
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?
Effect 1: Photon escape
Consider contrail with rectangular cross-section:
Independent column approximation
Clear & cloudy regions treated separately and as horizontally infinite (as in previous studies). Forward-scattered photons stay in contrail and may be scattered back out to space.
3D transport
Forward-scattered photons have chance to leave contrail through sides rather than be backscattered, so forcing is reduced. This is a small effect as it only affects multiple scattering.
Photon escape

8. Why is there a 3D shortwave effect?.
Effect 2: Side illumination / shadow length
For large solar zenith angle and small solar azimuth angle, the sun illuminates the contrail from the side as well as the top, so incoming photons have a greater chance of intercepting the contrail and being scattered, increasing the forcing.
The effect is dependent on the difference in the length of the shadow cast (s) in each case:
ICA: s = x; D:
Effect 2
Side illumination

9. Why is there a 3D longwave effect?
Contrail edge absorption
Effect 3: Contrail edge absorption
Independent column approximation
The fraction of upwelling photons at cloud base that enter contrail is equal to the fractional contral cover.
3D transport
Some upwelling photons enter contrail through the sides, have chance of being absorbed, increasing the radiative forcing.
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
3D net effect is a factor of 2 or more for all solar zenith angles
d=0.2 (control)
Doubling of optical depth:
Shortwave forcing doubles
Less than a doubling for the longwave forcing (partial saturation)

13. Effect of contrail aspect ratio
q=40°
q=80°
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 q<70°, 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 70°<q<90°…
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