Contrails and Induced Cirrus

C loud radiative forcing, the difference between the top-of-atmosphere radiative fluxes for all sky and clear skies, is a straightforward metric to gauge the climate impacts of contrails and contrail cirrus, which constitute a potentially serious long-term environmental issue associated with aviation (Minnis et al. 2004; Wuebbles et al. 2010). The Intergovernmental Panel on Climate Change (IPCC) Special Report on Aviation and the Global Atmosphere (Penner et al. 1999) reported a best estimate of the global mean contrail radiative forcing of approximately 0.02 W m−2 based on the study of Minnis et al. (1999), albeit a highly uncertain value. Even if the global mean radiative forcing is that small, regional effects may be much larger. Studies of the radiative effects of contrails have made use of various datasets, models, and methods and have examined satellite images of both contrails and contrail clusters. Duda et al. (2001) used satellite data to investigate the evolution of radiative properties of contrail clusters over the United States and suggested that ice crystal shape may exert important effects on contrail radiative forcing. To better estimate the radiative forcings of contrails and contrail cirrus, it is necessary to improve our knowledge about their fundamental optical and radiative properties. This study, based on two subject-specific white papers (Ou and Liou 2008; Yang et al. 2008) for the Aviation Climate Change Research Initiative undertaken by the U.S. Federal Aviation Administration, assesses the present state of knowledge and identifies areas of uncertainty with recommendations for future action. Contrails and contrail cirrus consist mainly of nonspherical ice crystals. The aircraft-generated ice crystal habits (i.e., shapes) may be similar to those in natural cirrus, but their distributions (i.e., the percentage of various habits) may be different from those in natural ice clouds. For example, in situ samples of contrail ice crystals taken at −61°C revealed several ice habits: hexagonal plates (75%), columns (20%), and a few triangular plates (<5%; see Goodman et al. 1998), whereas quite a different habit distribution, inferred from in situ observations, has been reported for natural ice clouds (Baum et al. 2005) that have been assumed to consist of droxtals (100%) for Dmax < 60 μm, where Dmax denotes the maximum dimensions of the ice particles; 3D bullet rosettes (15%), solid columns (50%), and hexagonal plates (35%) for 60 < Dmax< 1000 μm; hollow columns (45%), solid columns (45%), and aggregates (10%) for 1000 < Dmax < 2000 μm; and 3D bullet rosettes (97%) and aggregates (3%) for Dmax > 2000 μm. The nonsphericity of CONTRAILS AND INDUCED CIRRUS Optics and Radiation