Radiative and microphysical properties of cirrus cloud inferred from the MODIS infrared split-window measurements

An optimal estimation-based algorithm is developed for retrieval of radiative and microphysical properties of cirrus cloud from the measurements made by the Moderate Resolution Imaging Spectroradiometer (MODIS) at three infrared (IR) split-window bands with center wavelengths at 8.5, 11 and 12 μm. Prior information of cloud top and underlying surface temperatures are from the MODIS operational products. A fast forward model is based on semi-analytical equations for the brightness temperature assuming a single-layer homogeneous ice cloud with prescribed particle habit and size distributions. Modeling errors in the brightness temperature from the present approximate treatment of radiative transfer are insignificant, but relatively more substantial errors occur due to the uncertainties in model parameters including surface emissivity, precipitable water, and cloud bottom temperature. The total measurement-model errors are well correlated for the three bands, which is considered properly in the optimal estimation framework. Retrieval errors of cloud optical thickness and effective particle radius are mainly from uncertainties in a priori cloud top and surface temperatures and model parameters. The three-band IR method is suitable for retrieving optical thickness and effective particle radius for opaque and moderately thick cirrus clouds (with cloud optical thicknesses within a range of 0.5-6). The efficient retrieval algorithm enables global-scale remote sensing at a 1-km2 resolution. A tropical region case study demonstrates advantages of the method; particularly, the ability to be applied to more pixels in optically-thin cirrus in comparison with a solar-reflection based method, and the ability of the optimal estimation framework to produce useful diagnostics of retrieval uncertainties and the retrieval cost that denote the quantitative consistency between measurement and model calculation with several assumptions. The IR retrieval shows smaller optical thickness and larger particle sizes than those from the solar reflection method assuming smooth ice particles. Collocated comparisons with ground-based lidar measurements and remote sensing by spaceborne radar and lidar show reasonable consistency with respect to retrieved particle size and cloud top temperature for single-layer cirrus.