Passive remote sensing of overlapping clouds

The goal of this dissertation is to understand the effects overlapping clouds have on Moderate Resolution Imaging Spectroradiometer (MODIS) cloud effective radius and optical thickness retrievals. An overlapping cloud detection technique was developed specifically to detect thin cirrus overlaying a lower level water cloud and is applicable only during the daytime. It utilizes the operational MODIS cloud mask, a near infrared reflectance (at 2.1 or 1.6 mum), and the brightness temperatures at 8.5 and 11 mum. The performance of the algorithm is evaluated for two MODIS case studies and results are compared with coincident Cloud Physics Lidar data. In both cases, the overlapping cloud detection algorithm results appear reasonable in comparison with the CPL data.; Analysis of eight days of Aqua MODIS data from 1--8 April 2003 show that the algorithm classifies 12% of global cloudy pixels as overlapping. Zonally, over all surfaces, the overlap frequency ranges between 10% and 20% between 45° S and 60° N. The occurrence of overlapping clouds is correlated with the retrievals of cloud optical thickness and effective radius. The net effect of including overlapping clouds in global effective radius retrieval statistics is a decrease in ice cloud effective radius from 29.1 to 28.2 mum and a slight increase in water cloud effective radius from 14.8 to 15.0 mum. Including overlapping clouds in optical thickness retrieval statistics leads to a decrease in mean ice cloud optical thickness from 16.2 to 15.3 while the effect on global mean water cloud optical thickness retrievals is minimal.; The feasibility of using high-spectral resolution infrared data to retrieve cirrus cloud optical properties when a lower level water cloud is known to be present is also investigated. Analysis of High Resolution Interferometer Sounder (HIS) data during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) field experiment shows that sensitivities to cloud layer separation, and especially to accurate a priori knowledge of the heights of the cirrus and water clouds precludes accurate cirrus retrievals unless ancillary cloud height data is available.