|
|
|
|
|
JOURNAL OF THE ATMOSPHERIC SCIENCES, 64, 1706-1716, 2007.
A sensitivity study of the effect of horizontal photon transport on the radiative forcing of contrails.
A. Gounou, and R. J. Hogan
Abstract
With the rapid growth in air travel, there is concern over the radiative impact of contrails and aircraft-
induced cirrus on climate. Previous radiation calculations on contrails have almost all used the independent
column approximation, which neglects the transport of photons through the sides of the contrail, but in this
study the 3D effects are quantified using the Spherical Harmonic Discrete Ordinate Method (SHDOM).
The authors have investigated the dependence of shortwave and longwave radiative forcing on contrail
aspect ratio, optical depth, solar zenith angle, solar azimuth angle relative to contrail orientation, particle
size, particle habit, surface albedo, and surface temperature. It is found that inclusion of 3D transport results
in an increase in the positive longwave radiative forcing of the contrail and either an increase or a decrease
in the magnitude of the negative shortwave radiative forcing depending on the orientation of the contrail
with respect to the sun. Although these two effects are individually quite modest (of order 10%), the fact
that the total shortwave and longwave forcings largely cancel during the day means that the relative change
in the net radiative forcing due to the 3D effect is substantial; in some cases this results in a doubling of the
net forcing of the contrail, in other cases changing its sign. On a more general note, the relatively simple
geometry of contrail cirrus provides an ideal test case for explaining the various mechanisms by which 3D
photon transport can change the radiative effect of clouds, which can be rather difficult to visualize for more
complex cloud scenarios.
| |
|
