FIGURE 4.5 The top panel shows net cloud radiative forcing, annually averaged as observed by the ERBE. Negative values (red colors) indicate that clouds reduced the energy balance of Earth by reflecting more solar radiation than the amount by which they reduced the escaping infrared radiation. The bottom panel shows the fractional area coverage by low clouds as measured by the International Satellite Cloud Climatology Project (ISCCP). Note the close correspondence between low stratocumulus clouds over the ocean and strongly negative cloud radiative forcing. SOURCE: Graphic by D. Hartmann and M. Michelsen, University of Washington.

give unprecedented detail on vertical cloud structure. Cloud radar in space can provide good vertical resolution of reasonably thick clouds, including the tops and bottoms of layered clouds (Stephens et al. 2002). Lidar in space can provide very sensitive measurements of thin layers of clouds or aerosols (Winker 1997). These data have provided an unprecedented view of cloud structure, particularly in showing how clouds are layered vertically, which was not possible with visible, infrared, or microwave passive instruments.

The CloudSat radar and the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar provide a new dimension in observing the atmosphere. Rather than providing horizontal distributions of cloud and aerosol features typical of more conventional satellite sensors, these new nadir-pointing active sensors measure

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