Abstract
Abstract. The dependence of the radiative properties of persistent linear contrails on the variability of their ice water path is assessed in a two-stream radiative transfer model. It is assumed that the ice water content and the effective size of ice crystals in aged contrails do not differ from those observed in natural cirrus; the parameterization of these two variables, based on a correlation with ambient temperature derived from in situ observations, allows a more realistic representation than the common assumption of fixed values for the contrail optical depth and ice crystal effective radius. The results show that the large variability in ice water content that aged contrails may share with natural cirrus, together with an assumed contrail vertical thickness between 220 and 1000 m, translate into a wider range of radiative forcings from linear contrails [1 to 66 m Wm−2] than that reported in previous studies, including IPCC's [3 to 30 m Wm−2]. Further field and modelling studies of the temporal evolution of contrail properties will thus be needed to reduce the uncertainties associated with the values assumed in large scale contrail studies.
Highlights
Contrail radiative studies generally assume a fixed particle size distribution (PSD) in the ice cloud (e.g., Strauss et al, 1997; Meerkotter et al, 1999; Minnis et al, 1999) and a fixed optical depth (OD) (e.g., Minnis et al, 2004; Stuber and Forster, 2007; Radel and Shine, 2008)
Recent measurements, such as those from the mid-latitude field campaign CIRRUS III (Schauble et al, 2009) have shown that the ice water contents in cirrus and aged contrails are similar under the same atmospheric conditions, while ground-based observations of an aged contrail made by Atlas and Wang (2010) have confirmed that the IWC of linear contrails can reach values close to the upper limit for mid-latitude cirrus
More extensive contrail field measurements are needed but, given the small differences between the IWC in natural cirrus and in aged contrails at the same temperature in nonconvective regions and the fact that the radiative forcing (RF) of ice clouds is mainly determined by their ice water path, it seems reasonable to use a characterization based on cirrus measurements to represent the developed stages of contrails, as has been done previously (e.g., Wyser and Strom, 1998; Ponater et al, 2002)
Summary
Contrail radiative studies generally assume a fixed particle size distribution (PSD) in the ice cloud (e.g., Strauss et al, 1997; Meerkotter et al, 1999; Minnis et al, 1999) and a fixed optical depth (OD) (e.g., Minnis et al, 2004; Stuber and Forster, 2007; Radel and Shine, 2008) This simplification is made in response to the large uncertainties in the characterization of ice cloud PSDs (e.g., McFarquhar et al., 2007; Jensen et al, 2009) and the large variability found in ice cloud micro- and macro-physical properties, which must be taken into account when deriving representative values for large scale contrail simulations. We estimate the effect of this layer on the local and global radiation fields, allowing its ice water content and effective ice crystal size to vary as a function of ambient temperature
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