Integral Earth-System Properties Observed from Space: Introducing the "Lidar Albedo"

Radiometric observations from space inherently integrate over all atmospheric and surface phenomena within the view volume. This has disadvantages in terms of diagnostic capability. On the other hand, a high-leverage use of satellite observations is to characterize the variation of Earth-system properties, thereby providing integral constraints to be explained by theoretical models in conjunction with more detailed, sub-orbital measurement programs. Spaceborne lidar observations from LITE (1994), GLAS (2003-present), and now CALIPSO (launched April, 2006) offer the prospect of global-scale assessments of the vertical distribution of aerosols and thin clouds. We suggest that these lidar data sets provide, as well, a diagnostic proxy for variations in local planetary albedo (A) associated with these phenomena. We dub this proxy - previously known as integrated attentuated backscatter - the "lidar albedo" (AL). AL is a natural lidar product that represents the fraction of laser energy returned to the receiving telescope, out to a given range. It has units of (1/sr) and can be thought of as a single vector in the middle of the upscatter phase function characterizing the Earth's atmosphere at a given location and time. Thus, it is reasonable to propose that variations in AL will be closely correlated with variations in A. Factors that may cause a non-linear relationship between A and AL include the following: (i) different types of features (gases, aerosol, droplets, crystals) have different scattering phase functions; (ii) a portion of 180º reflectance arises from multiple scattering, much of which will fall outside the field of view of the receiving telescope, and (iii) AL is expected to saturate at lower optical depths than A. Despite these weaknesses, measurements of AL by nadir-pointing lidars offer these advantages: (i) better spatial resolution than broadband flux radiometers, (ii) a simple retrieval with well known accuracy of about 2 percent, and (iii) vertical resolution, which means the ability to diagnose the atmospheric causes of albedo variations without interference from the underlying surface.