This paper summarizes the assessment of the current state of knowledge, areas of uncertainties, and recommendations for future efforts, regarding the optical and radiative properties of contrails and contrail cirrus, which have been reported in two detailed subject-specific white papers for the Aviation Climate Change Research Initiative undertaken by the U.S. Federal Aviation Administration. To better estimate the radiative forcing of aircraft-induced cloudiness, there is a pressing need to improve the present understanding of the optical properties of nonspherical ice crystals within contrails and contrail cirrus, and to enhance the global satellite detection and retrieval of these clouds. It is also critical to develop appropriate parameterizations of ice crystal bulk optical properties for climate models on the basis of state-of-the-art scattering simulations and available in situ measurements of ice crystal size and habit distributions within contrails and contrail cirrus. More accurate methods are needed to retrieve the bulk radiative properties of contrails and contrail cirrus to separate natural from anthropogenic ice cloud effects. Such refined techniques should be applied to past and future satellite imagery to develop a contrail climatology that would serve to evaluate contrail radiative forcing more accurately, to determine trends in contrail cirrus, and to guide and validate parameterizations of contrails in numerical weather and climate models. To point the way forward, we recommend four near-term and three long-term research priorities.

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Department of Atmospheric Sciences, Texas A&M University, College Station, Texas

Department of Atmospheric and Oceanic Sciences, and Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, Los Angeles, California

NASA Langley Research Center, Hampton, Virginia

Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, Indiana