Monday, 12 January 2009
Examination of viewing zenith angle dependence on cloud heights derived from thermal infrared satellite observations
Hall 5 (Phoenix Convention Center)
Cloud effective radiating heights (zeff) are typically derived from passive satellite imagery by matching the estimated cloud effective radiating temperature (Teff) to a vertical temperature sounding and taking the corresponding altitude as the cloud height. However, with the advent of active satellite instruments such as CALIOP, which are more sensitive to particles in the upper layers of clouds, physical cloud tops can be more accurately observed than in the past. It is now known that the physical tops of optically thick ice-phase clouds are found at heights approximately 1-2 km above zeff. A previous study used the cloud top heights observed by CALIOP to estimate the true physical cloud top for satellite observations near nadir. However, the sensor zenith angle can have a dramatic impact on cloud heights derived from satellite observations. As the viewing angle increases, satellite sensors observe more IR radiation from the upper, colder portions of clouds than the lower, warmer layers, resulting in a lower Teff and thus, a greater zeff. At viewing angles of ~60°, which are not uncommon for most imagers, zeff may be a poor characterization of the true cloud top. This study expands on the work from the previous study. Coincident GOES-11 and GOES-12 data over the continental United States are used to develop a correction for cloud tops viewed at off-nadir angles, and a method to estimate ice water content (IWC) from dual-angle satellite observations is introduced.