For many years, ground-based lidar observations have been among the primary "validation" sources for satellite cloud detections and cloud height retrievals. Additionally, lidar can provide valuable information on cloud physical thickness and optical depth. Due to their sensitivity, lidar observations are ideal for studying the characteristics of optically thin cirrus clouds (visible optical depth less than 0.3), and such observations have been used by a number of researchers to develop climatologies of thin cirrus over the lidar sites.

One limitation of these studies, however, is that ground-based lidar measurements are not able to produce global cirrus cloud climatologies. Continents and especially the world's oceans are poorly sampled by these data. A detailed, global climatology is important because cirrus clouds have been shown to affect the planetary radiation budget, and they also respond to and perturb the large-scale atmospheric circulation. In addition, undetected, optically thin cirrus can lead to biases in satellite retrievals of aerosol and surface properties. Unfortunately, due to their tenuous nature, cirrus clouds are difficult to detect using nadir-viewing, high-horizontal-resolution satellite instruments. For this reason they have been studied primarily with limb-sounding instruments, which do not provide sufficient horizontal detail, and infrared channels that are not very sensitive to thin, and possibly more common, cirrus.

One possible solution to this problem is to exploit the capabilities of the Multi-angle Imaging SpectroRadiometer (MISR) instrument on the Terra satellite for detecting cirrus with visible optical depths less than about 0.3, as part of the operational MISR aerosol product. Although the lower optical depth limit of typical satellite cloud detection algorithms is no better than 0.1, it has been demonstrated that MISR is sensitive to aerosol optical depths approximately 0.05 and larger. MISR is sensitive to the presence of thin cirrus because it observes the atmosphere at air mass factors ranging from one to three. It also observes scattering angles ranging from about 60 to 160 degrees in the mid-latitudes, providing considerable information about the scattering phase functions of atmospheric particles, including particle non-sphericity. To test MISR's ability to detect optically thin cirrus and potentially to retrieve mid-visible optical depths, we compare MISR retrievals with coincident observations from ground-based lidar located at a number of Atmospheric Radiation Measurement (ARM) and Micro-pulse Lidar Network (MPLNET) sites. Our preliminary results using a single cirrus particle model suggest that MISR is capable of distinguishing cirrus in the presence of other aerosols if the total column aerosol optical depth is greater than 0.2 and the cirrus contributes more than approximately 20% to this total.