The Distribution of Tropical Thin Cirrus Clouds Inferred from Terra MODIS Data

Abstract

Thin cirrus clouds (with optical depths τ ? 1) play a potentially important role in the earth's atmosphere. However, their tenuous nature makes them difficult to detect, and as a result there are few quantitative, global analyses of them. The Moderate Resolution Imaging Spectrometer (MODIS) on board the Terra satellite has a channel at 1.375 ?m that is specifically designed to detect these clouds, and can measure optical depths as low as 0.02 with an uncertainty factor of 2. During two 3-day periods from December 2000 and June 2001, about one-third of the pixels flagged as cloud free by the MODIS cloud mask are shown to contain detectible thin cirrus. These thin cirrus generally have optical depths below ?0.05 and appear with greater frequency and optical depth near deep convection.