Monday, 11 January 2016
The constant presence of high-altitude air traffic across the globe influences Earth's climate through the formation of jet contrails that grow given the right atmospheric conditions. The contrail radiative forcing (CRF) is used to assess this impact on the Earth-Atmosphere system. It's definition is CRF=Fcf - Fc where Fcf is the upward flux for contrail-free conditions and Fc is the corresponding flux for the atmospheric conditions with the contrail. In this study, satellite data from Terra and Aqua MODIS is used to compute the CRF associated with linear contrails for four seasonal months in the years 2006 and 2012. The cloud properties are retrieved using the NASA-Langley infrared-only Clouds and the Earth's Radiant Energy System (CERES) algorithm. For each pixel classified as having a linear contrail from a sensitive contrail mask, the cloud property retrievals, surface properties, and MERRA atmospheric profiles are used in the 2-stream Fu-Liou radiative transfer model. The CRF is then derived from the model's flux output. The background scene below the contrail is considered to be either clear or filled with single layer liquid or ice clouds. The shortwave, longwave, and net CRF components are computed on a 1x1o grid for both daytime and nighttime conditions and for different background types. The CRF is derived in two ways. For the first method, radiative forcing is normalized to 100% contrail coverage to provide a measure of sensitivity to the presence of contrails. In the second method, the contrail fractions are used to get the total net forcing which indicates the actual effect on climate. By using actual observations of contrails in their environments, this study indicates the extent to which they affect Earth's radiation balance. Contrails that become nearly indistinguishable from the background cirrus cloud shield, otherwise known as contrail cirrus, aren't considered here.
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