A Sensitivity Study of the Effect of Horizontal Photon Transport on the Radiative Forcing of Contrails

Abstract

With the rapid growth in air travel, there is concern over the radiative impact of contrails and aircraftinduced 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.

Multidimensional Longwave Forcing of Boundary Layer Cloud Systems

“…Future efforts should also explore the possibility of multidimensional effects for other radiatively driven cloud systems. We previously mentioned altocumulus, which are predominantly driven by radiative forcing, but recent studies of cirrus cloud inhomogeneity (Fu et al 2000) and horizontal photon transport in contrails (Gounou and Hogan 2007) suggest multidimensional effects may play a role in cloud dynamics for these radiatively driven cloud systems as well.…”

Effects of optical depth variability on contrail radiative forcing

“…It causes CRF biases of 10−20% which are of similar magnitude to effects caused by other factors determining the optical response of contrails, e.g. optical depth of low-level clouds or ground albedo, altitude or ice water content of contrails, ice crystal habit, or 3D effects (Meerkötter et al, 1999;Gounou and Hogan, 2007;Yang et al, 2010;Markowicz and Witek, 2011;Schumann et al, 2011). We note that differences in the SW and LW CRF among various optical models-in which ice crystal radiative properties are treated by different methods-have been found to reach 44% and 23%, respectively, relative to the mean model value (Markowicz and Witek, 2011).…”

Three‐dimensional radiative transfer in midlatitude cirrus clouds

“…This can be explained by the fact that in 3D, upwelling thermal radiation from lower in the atmosphere has a greater chance to enter the cloud and be absorbed than in ICA because it can enter a cloud through the sides. This mechanism was used by Gounou and Hogan (2007) to explain why aircraft contrails have a larger radiative forcing in the long-wave region when 3D transport is included. In the more complex cloud scenes considered here, there is often not so much of a division between cloudy and clear air, but rather between optically thick and optically thin regions.…”

“…To a limited extent, the short-wave differences between ICA and 3D in the complex geometry of fall-streak cirrus can be explained in terms of a combination of two effects that occur only in a 3D scenario: escape of photons from the sides of clouds, and shadowing. These effects were used by Gounou and Hogan (2007) to explain differences in the much simpler geometry of an aircraft contrail, but the same mechanisms should apply in more complex scenarios. For example, Figure 12(b) shows that at large solar zenith angles, the ICA reflectance is much lower than 3D in case 1 (the most optically thin case).…”

“…For horizontal cloud scales below 5-10 km, the ICA bias became significant, and within a scale of 2 km, the ICA bias in flux-derived radiative fields could exceed 5%. For cirrus studies, 3D radiation has been used with contrails (Gounou and Hogan, 2007) but only ICA has been used with realistic 3D cirrus clouds. Here we present the first use of 3D radiative transfer with realistic cirrus structures.…”