Tropical ice cloud optical depth, ice water path, and frequency fields inferred from the MODIS level-3 data

The 1.375-μm water vapor absorption band included on the space-borne Moderate resolution Imaging Spectroradiometer (MODIS) provides an unprecedented view of ice clouds (e.g., cirrus) on a global scale. In this study the spatial and temporal patterns of ice clouds over the tropics (i.e., the region between ± 30° latitude) are analyzed. Three successive years (July 2002–August 2005) of data acquired from the MODIS instrument aboard NASA's Aqua satellite are used in the analyses. Specifically, the frequency of occurrence of ice clouds and its zonal trends are investigated. We compare the ice cloud frequency results from this study with those obtained from the High Resolution Infrared Radiation Sounders (HIRS). In addition to the HIRS-type frequency of occurrence of ice clouds, we also investigate the average cloud optical depth and ice water path of these clouds, as well as the zonal trends of these parameters. The present analyses reveal that ice clouds are observed quite frequently in the tropics (approximately 67% of MODIS level-3 data points), and have a higher frequency of occurrence over land, 43%, than over the oceans, 32%. Ice cloud optical depth and ice water path have distributions highly skewed toward small values, indicating a large number of thin cirrus observations. Approximately 70% of the tropics has mean ice cloud optical depth below 2.0, and 80% has mean ice water path less than 40 g m− 2. Moreover, spatial patterns, as well as seasonal cloud migrations, can be clearly seen.