The Spatial and Temporal Variations of High Clouds based on the MODIS 1.375-Micron Channel Measurements

The Moderate Resolution Imaging SpectroRadiometer (MODIS) instruments on the Terra and Aqua spacecrafts are both equipped with a channel near 1.38 mum for remote sensing of cirrus clouds from space. We have developed an operational algorithm for quantitative retrieval of cirrus reflectances in the visible from scatter plots of the MODIS 1.38-micron channel versus 0.66-micron channel. At present, the daily `pixel-based' cirrus reflectance product, which is a standard MODIS Level 2 data product, at the 1-km spatial resolution of the MODIS instrument, and the daily, 8-day, and monthly cirrus reflectance products, which are standard MODIS Level 3 products, at a 1deg by 1deg latitude-longitude grid globally are operationally produced at a NASA computing facility. Through analysis of time series of Terra and Aqua MODIS cirrus reflectance data products, we have observed interesting spatial and temporal variation patterns of cirrus clouds over different geographical regions. For examples, cirrus reflectances over Tibet, tropical Pacific, and tropical Africa Continent have large seasonal variations. Over the western tropic Pacific, cirrus patterns are very different between an El Nino year and a normal year. By comparing the Aqua MODIS data with the Terra MODIS data, we have found that the afternoon Aqua MODIS detects significantly more cirrus clouds over North America and South America in certain seasons than the Terra MODIS. The MODIS cirrus reflectance data products will certainly allow improved understanding of global cirrus climatology.