Selection of the 1.375-µm MODIS Channel for Remote Sensing of Cirrus Clouds and Stratospheric Aerosols from Space

Abstract

Using spectral imaging data acquired with the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) from an ER-2 aircraft at 20-km altitude during the NASA FIRE Phase II Cirrus field program, it was found that narrow channels near the center of the strong 1.38-µm water vapor band are very effective in detecting thin cirrus clouds. The mechanisms for the cirrus detection are straightforward. In the absence of cirrus clouds, AVIRIS channels near 1.38 µm receive little scattered solar radiance by the surface and the low-level water clouds because of the total absorption of solar radiation by atmospheric water vapor located above them. When cirrus clouds are present, however, these channels receive solar radiance scattered by the cirrus clouds that contrasts well on the black background. A near-IR channel centered at 1.375 µm with a width of 30 nm has been selected for the Moderate Resolution Imaging Spectrometer (MODIS) for remote sensing of cirrus clouds from space. MODIS is one of the sensors on the Earth Observing System, planned to be launched in 1998. The sensitivity of the new channel for detecting thin cirrus clouds during the daytime is expected to be one to two orders of magnitude better than the infrared emission techniques. This channel can also complement measurements of stratospheric aerosols with solar occultation techniques when the stratospheric aerosol optical depths at 0.55 µm are 0.01 or larger.