Determination by spaceborne backscatter lidar of the structural parameters of atmospheric scattering layers.

Spaceborne active lidar systems are under development to give new insight into the vertical distribution of clouds and aerosols in the atmosphere and to provide new information on variables required for improvement of forecast models and for understanding the radiative and dynamic processes that are linked to the dynamics of climate change. However, when they are operated from space, lidar systems are limited by atmospheric backscattered signals that have low signal-to-noise ratios (SNRs) on optically thin targets. Therefore specific methods of analysis have to be developed to ensure accurate determination of the geometric and optical properties of scattering layers in the atmosphere. A first approach to retrieving the geometric properties of semitransparent cloud and aerosol layers is presented as a function of false-alarm and no-detection probabilities for a given SNR. Simulations show that the geometric properties of thin cirrus clouds and the altitude of the top of the unstable atmospheric boundary layer can be retrieved with standard deviations smaller than 150 m for a vertical resolution of the lidar system in the 50-100-m range and a SNR of 3. The altitudes of the top of dense clouds are retrieved with a precision in altitude of better than 50 m, as this retrieval corresponds to a higher SNR value. Such methods have an important potential application to future spaceborne lidar missions.