Robin HoganRobin HoganAmanda GounouAmanda Gounou

Department of Meteorology, Department of Meteorology, University of Reading, UKUniversity of Reading, UK

The effect of horizontal The effect of horizontal photon transport on photon transport on

the radiative forcing of the radiative forcing of contrailscontrails

MotivationMotivation• 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

Experimental Experimental configurationconfiguration

Control Experiments

Mean optical depth at 0.55 m

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

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)

: solar zenith angle

: 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

• Shortwave downwelling flux • Longwave upwelling flux

Independent column Independent column approximationapproximation

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%

Longwave upwelling radiation absorbed; emitted at lower brightness temperature: warming effect on climate, positive forcing

Control contrailControl contrail

3D radiative transfer3D radiative transfer

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%

Modest 3D shortwave

effect

Modest 3D longwave

effectNet effect is doubled!

Sign of net effect is reversed!

Control contrailControl contrail

Why is there a 3D shortwave Why is there a 3D shortwave effect?effect?

Effect 1: Photon escape

Consider contrail with rectangular cross-section:

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

Why is there a 3D shortwave Why is there a 3D shortwave effect?effect?

Side illumination

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 / 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: 2/122 tan1 rxs

Why is there a 3D Why is there a 3D longwavelongwave effect?effect?

Effect 3: Contrail edge absorptionEffect 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 of particle type: Effect of particle type: columnscolumns

• For effective radius of 10 m and wavelength of 0.55 m, solid columns have an asymmetry factor g of 0.75 (similar for bullet rosettes)

Control contrailControl contrail

Effect of particle type: Effect of particle type: spheresspheres

• Spheres have an asymmetry factor g of 0.85: more forward scattering

Reduced shortwave forcing

Sign of net forcing is reversed

Effect of Effect of contrail contrail optical depthoptical depth=0.2 (control)

=0.4

3D net effect is a factor of 2 or more for all solar zenith angles

• Doubling of optical depth:– Shortwave forcing doubles– Less than a doubling for the

longwave forcing (partial saturation)

Effect of contrail aspect ratioEffect of contrail aspect ratio

• 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

=80°=40°

ConclusionsConclusions• 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 <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°<<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