The effect of horizontal photon transport on the radiative forcing of contrails

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 Shortwavesolar radiation reflected back to space: cooling effect on climate, negative radiative forcing Longwaveupwelling radiation absorbed; emitted at lower brightness temperature: warming effect on climate, positive 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 Modest 3D longwave effect Net effect is doubled! Sign of net effect is reversed! Modest 3D shortwave effect 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? 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; 3D: . Effect 2 Side illumination

9. Why is there a 3D longwave effect? 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. 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 • Spheres have an asymmetry factor g of 0.85: more forward scattering Sign of net forcing is reversed Reduced shortwave forcing

12. Effect of contrail optical depth d=0.4 • Doubling of optical depth: • Shortwave forcing doubles • Less than a doubling for the longwave forcing (partial saturation) 3D net effect is a factor of 2 or more for all solar zenith angles d=0.2 (control)

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