Estimation of the cirrus cloud scattering phase function from satellite observations

Abstract

Optical and microphysical properties for optically thin ice clouds are retrieved from one year of collocated Aqua/MODIS and CALIPSO/CALIOP measurements in 2008. The values of optical thickness τ and effective particle size Deff are inferred from MODIS measurements at three infrared (IR) bands located at 8.5, 11, and 12µm in conjunction with collocated CALIOP cloud boundary altitudes and the MERRA atmospheric profile datasets. The τ values inferred from MODIS IR window measurements are insensitive to the pre-assumed particle and habit distributions. Based on near-IR measurements at 1.38μm and the IR-based τ, a new method is developed to infer the scattering phase functions over both ocean and land. A comparison between theoretically calculated phase functions and the retrieved counterparts demonstrates that roughened solid columns provide the best match for cirrus clouds over ocean, whereas droxtals may exist in optically thin cirrus clouds. The best-fitted phase functions are generated using appropriate habit mixtures to match the inferred phase functions. The phase function resulting from a mixture of 55% severely roughened solid columns, 35% severely roughened droxtals, and 10% smooth aggregates almost perfectly matches the mean phase function value retrieved over ocean. The asymmetry factor based on the oceanic best-fitted phase functions is 0.778 at a wavelength of 0.65μm. However, it is difficult to find an appropriate habit recipe to fit the inferred phase function over land. This may be caused by the relatively large uncertainties associated with τ retrievals over land. The retrieval of Deff shows that optically thin cirrus clouds consist of smaller ice particles in comparison with optically thicker ice clouds. The mean Deff values of optically thin ice clouds over land and ocean are 41μm and 48μm, respectively.