Joint Poster Session JP2.4 Sensitivity study of the global contrails radiative forcing and crystals optical properties due to particle shape

Wednesday, 30 June 2010
Exhibit Hall (DoubleTree by Hilton Portland)
Krzysztof M. Markowicz, University of Warsaw, Warsaw, Poland; and M. Witek

Handout (505.2 kB)

This study contains a comparative analysis of contrails optical properties and radiative forcing. The ice crystal optical properties such as the extinction coefficient, the single scattering albedo, and the asymmetry parameter are obtained from different computational techniques and databases. Optical properties in shortwave and longwave range are simulated from a Ray Tracing Geometric technique and Discrete Dipole Approximation method respectively. The global contrails radiative forcing are calculated by model which include interface between Fu-Liou radiative transfer model and databases which describe clouds optical properties (based on Satellite Cloud Climatology Project), atmospheric thermodynamic conditions, and surface optical properties. The global contrails fractions are taken from AERO2K database. We compare optical properties defined following simple crystals habits randomly oriented: hexagonal plates, hexagonal columns with different aspect ratio, and spherical. In addition we used optical proprieties, defined shortwave and longwave, taken from Yang databases for droxtal and hexagonal plates, hexagonal columns particles. We found that in spite of the fact that crystals are randomly oriented the variation of the single scattering albedo and the asymmetry parameter due to different crystal shapes is large. In case of the single scattering albedo and the asymmetry parameter differences between models can be up to 0.1. Both crystal geometry type and it aspect ratio influence optical properties of randomly oriented crystals. We found that different parameterizations of contrails crystal shapes lead to large uncertainties in the radiative forcing. Simulations performed by the Fu-Liou model show significant variability of the shortwave, longwave, and net radiative forcing with crystal shape. The nonspherical crystals have smaller net forcing in contrary to spherical particles. The differences in net radiative forcing between optical models reach up to 50%. The global annual mean of contrails radiative forcing averaged over 10 crystal models are -9.9 mW/m2, 20.9 mW/m2, and 11.0 mW/m2 for shortwave, longwave, and for net respectively. Ratio of the standard deviation to mean value is about 0.2 for shortwave, 0.14 for longwave, and 0.23 for net. The main source of contrails radiative forcing uncertainties is a their optical thickness. Due to limited number of observation of the contrails optical thickness this parameter is often assumed. In this study it was fixed at the level of 0.3 (550 nm). The mean global contrails radiative forcing uncertainties related to contrails optical thickness error slightly vary with crystal model. The mean radiative forcing uncertainties are: 2.7%, 13.5%, and 27% for contrails optical thickness errors ±0.01, ±0.05, and ±0.1 respectively.
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