Due to the short wavelength of lidar systems used to probe the atmosphere, the range is limited by features with high optical thickness such as clouds, rain and fog. Therefore, two important questions are often raised by the user community: What will be the expected coverage of space borne lidar systems ? How will the global coverage of the lidar data compare with results achieved with passive remote sensing systems ?
We are addressing these questions by an analysis of global cloud statistics using the ISCCP (International Satellite Cloud Climatology Project) D2 data set. Since July 1983 monthly means of coverage as well as visible optical thickness (at 600 nm) of nine different clouds types have been collected (among other cloud related data). The resolution of the data set is 2.5 x 2.5 degrees which corresponds to approximately 250 km. This data set is used to yield quantitative results concerning the extension of coverage by data availability above optically thick clouds as well as by penetration of optically thin clouds.
From the definition of the cloud types in the ISCCP data set it can be concluded that cirrus is the only cloud type that can be penetrated by a space borne lidar system. However, the analysis shows that for cirrus clouds large regions of the globe, optical thicknesses lower than 2 allow an observation of the levels below from a space borne lidar system. This leads to improved coverages in comparison to passive remote sensing systems. Largest improvements are observed in the tropics and over land areas.
Further investigations with the other ISCCP data sets will be carried out. ISCCP D1 provides global cloud data every 3 hours, clearly improving the statistical significance of the results found with the D2 data set. Additionally ISCCP DX provides data with 30 km horizontal resolution every 3 hours, allowing the investigation of the changes in coverage for single synoptic systems and therefore an assessment of the improving effects of space borne lidar systems on numerical weather forecasts.