Cirrus cloud properties inferred from multispectral imager and lidar measurements

Wednesday, 17 December 2014
Hironobu Iwabuchi and Shuichiro Katagiri, Tohoku University, Sendai, Japan
The present understanding of cirrus cloud microphysics and their radiative effects is limited, which is an important key for improving climate models. An algorithm using three infrared (IR) bands of the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) to retrieve cirrus radiative and microphysical properties has been applied to global-scale remote sensing at a 1-km horizontal resolution. Results present geospatial and seasonal variation of single-layer high clouds. Global mean of effective particle radius of single-layer cirrus cloud is about 30 µm with mean optical thickness of 1.6, whereas the optical thickness exhibits large dispersion with nearly flat frequency distribution over a range between 0.1 and 10. Tropical cirrus clouds have smaller particle sizes, compared to midlatitude cirrus. The effective particle size generally increases at warmer temperature with lower depolarization ratio obtained from collocated CALIPSO lidar observations. Optically thick cirrus clouds are geometrically thick and composed of larger particles. Using these results, useful parameterizations of effective particle radius are proposed for use in general circulation models. The analysis using only the IR measurements reveals that there is no significant day–night differences in the statistics of microphysical and optical properties when averaged over the globe, whereas diurnal variations are significant in cloud amount and cloud-top pressure. Radiative effects of cirrus cloud should be reproduced in models by including the characteristics obtained.