Influence of cloud geometrical thickness on the estimate of ice cloud optical thickness and effective particle size using satellite multispectral measurements

We investigate the influence of ice cloud geometrical thickness on the inference of cloud optical thickness and effective particle size. Radiances are simulated for three infrared bands centered at 8.5, 11.0, and 12.0 µm and three solar bands centered at 0.65, 0.86, and 1.64 µm. Similar bands are available on the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the NASA Terra and Aqua platforms. Radiances are simulated with a combination of the discrete ordinate transfer model (DISORT) for multiple scattering and the Moderate Resolution Transmittance (MODTRAN) code for atmospheric gas absorption. A database of ice cloud bulk scattering properties is used for the forward radiative transfer simulations involved in this sensitivity study. The ice cloud calculations assume a geometrical thickness varying from 0.5 to 5 km for two cloud-top heights at 12 and 15 km and an effective radius that is constant with height. The simulated radiances are then used to infer ice cloud optical thickness and effective particle size on the basis of the pre-calculated lookup tables. An increase of cloud physical thickness results in an underestimation of cloud optical thickness and an overestimation of effective particle size. The influence of cloud physical thickness on a retrieval algorithm based on the infrared bands is greater than for a retrieval algorithm based on the solar bands. These results can be tested using data from an active sensor such as a lidar where an extinction profile through the cloud can be derived.