The Nature of Cirrus as Revealed by Lidar (Invited Presentation)

Lidar came along just as climate research was gaining significance and cloud feedbacks to climate change were being recognized as a major puzzle. In particular, the basic macrophysical and microphysical properties, and radiative impacts of the component of the planets high cirrus clouds were largely unknown because of their inaccessibility and lack of effective research tools for studying them. It was an important coincidence.

The full range of developing lidar techniques were soon applied to the study of cirrus, including polarization, high spectral resolution, Raman, and micropulse approaches. Developments moved from the laboratory, field, airborne, to pioneering satellite laser measurements. Cirrus ice clouds are by definition uniquely amenable to lidar probing due to their relatively low optical thickness, a property that limited the application of microwave radar and passive satellite approaches but made them transparent to lidars. Since field lidars generally sense in the solar spectrum, their measurements corresponded directly to the long history of the visual character of cirrus and the radiative transfer of sunlight through them.

As useful as lidar data were, however, their significance was enhanced when combined with active and passive sensing techniques in other portions of the electromagnetic spectrum. As a result, the multiple remote sensor approach became a standard fixture of modern research projects into high clouds, and the basis of sustained ground based research programs to improve our understanding of the state of current climate (e.g., the Facility for Atmospheric Remote Sensing in Utah, and the various Atmospheric Radiation Measurement Program field sites). More recently, coordinated polarization lidar, millimeter wave radar, and optical passive probes on the A-train of polar orbiting satellites have led to a truly comprehensive global view of the distribution and radiative properties of the Earths cirrus clouds. How we got from the first lidar shots into cirrus from out of the laboratory window in the 19960s to today’s expansive view from space is the subject of this review.