Optical property calculations and radiation parameterizations in support of CERES Science Team

Abstract

Ice clouds and snow on the surface play a major role in the Earth–atmosphere energy budget through their interactions with shortwave and longwave radiation. The single-­?scattering properties of ice crystals are fundamental to radiative transfer simulations and remote sensing implementations concerning the microphysical and optical properties ice clouds and snow. Generally speaking, ice crystals in clouds and particles in snow on the surface are almost exclusively nonspherical particles with the single-­?scattering properties deviating substantially from the counterparts based on the “equivalent” ice spheres. Therefore, it is necessary to use appropriate optical properties of ice crystals and snow particles. In support of the CERES (Clouds and the Earth’s Radiant Energy System) Science Team, our research group at Texas A&M University has developed a two-­?habit model (THM) for ice cloud optical properties. Figure 1 shows the ice particle shape distribution assumed for the THM. The optical properties associated with the THM leads to improvements in downstream applications to satellite remote sensing. For example, Figure 2 shows the polarized reflectivities simulated based on the CERES Edition 2 and the THM optical properties in comparison with the observations made by the Polarization and Directionality of the Earth’s Reflectance (POLDER). Overall, the observed polarized reflectivity decreases with increasing scattering angle over a range of 100°–170°. The simulations based on the THM closely match with those from the observations except at the scattering angles with 100–120° because of the weak polarized side-­?scattering by the THM, whereas the simulations based on the CERES Edition 2 substantially deviate from the observations. It is obvious that THM represents a significant improvement compared the CERES Edition 2 ice cloud optical model.