PNNL

Abstract

Knowledge of cloud phase (liquid or ice) and crystal habit are of fundamental importance to both remote sensing and climate simulations. Using water droplets instead of ice crystals in retrieving cloud properties from satellite data can lead to errors in the retrieval of cloud height, optical thickness, and microphysical properties. Satellite retrievals of microphysical properties are also influenced by the crystal habit used in the retrieval, either indirectly via an assumed phase function or directly via assumed profiles of ice crystal habits. Realistic treatment of ice cloud radiative and microphysical properties, which depend on crystal habit, is important in climate simulations, especially in tropical anvil regions. In this work, we present a method for retrieving cloud phase and the dominant ice crystal habit from radiances measured by the Multi-angle Imaging Spectro-Radiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS). The shape of an atmospheric particle affects the scattering of light such that water droplets and ice crystals of various habits have somewhat different phase functions. Consequently the radiances measured by the MISR instrument, which observes light scattered from the same cloud at nine different viewing angles, are functions of the crystal shape. In principle, the measured angular radiance pattern can be used to infer the crystal shape. In this work, we present initial results from a cloud phase and ice crystal habit retrieval based on combining the MISR multi-angular visible wavelength measurements with MODIS shortwave infrared measurements. The nine angular measurements provided by the MISR cameras allow a wide range of scattering angles to be viewed in a single scene, which provides sensitivity to particle habit. The presence of the MODIS instrument on the same satellite allows additional information on particle size to be incorporated into the retrievals. Results of the retrieval method are presented for several case studies over the continental United States. Cloud phase can be determined from the MISR angular measurements alone, due to the large differences in the phase functions of water droplets and ice crystals. By combining the MISR and MODIS measurements, crystal habit, effective radius, and optical depth can be inferred simultaneously for ice clouds. Comparisons with ground-based retrieval methods and semi-coincident in situ data illustrate that the retrieved crystal habits and sizes are reasonable.