SAGE III Cloud Determination: Method and Examples

The Stratospheric Aerosol and Gas Experiment (SAGE) III is the latest in a series of solar occultation satellite instruments designed for the measurement of aerosol and gases. SAGE III extinction data obtained at three wavelengths (525, 1020 and 1550 nm) is used to determine whether cloud is present along the optical path from the sun to the satellite instrument. The algorithm used differs from that previously used to detect cloud using the SAGE II instrument, where data was not available at 1550 nm. Due to the long optical path through the atmosphere, both instruments are extremely sensitive to low values of extinction. In the troposphere, cloud data is divided into two classes: non-opaque, which is mainly subvisual, and opaque. SAGE III is also able to detect the presence of polar stratospheric cloud. Unlike SAGE II where cloud presence was a research product, cloud presence is a standard data product for SAGE III. SAGE III cloud data from May, 2001 onwards, at altitudes between 6 and 30 km, is currently being made available for general use. The theoretical background to the SAGE III algorithm is described and contrasted with that used with data from the SAGE II instrument. Examples showing how the algorithm is applied to the data are presented for a cloud-free atmosphere, for non-opaque stratospheric and tropospheric clouds, and for opaque clouds. Under some circumstances the signature of thin cloud in the data set can be confused with that of dense aerosol, produced for example as a result of volcanic activity or by lofting of dust from the surface of the earth. This potential confusion necessitates a quality control procedure for the data; this procedure is explained, together with the changes that this operation produces on the output data format and timing. In the interest of the long-term continuity of the SAGE II/ SAGE III cloud data set some of the SAGE III data has been processed using both the current SAGE III algorithm and the older SAGE II algorithm. Preliminary results of this intercomparison are briefly described.