Before the launch of ICESat (GLAS) and CALIPSO, the Doppler lidar community relied upon airborne missions to conceptualize the global distribution of backscatter. In addition, cloud statistics, with a space-based lidar perspective, had been limited to < 40 hours of LITE (Lidar In-space Technology Experiment) data obtained during a 14 day shuttle mission in 1994. Most currently available cloud statistics are based upon data from imagers or passive sounders (for example, ISCCP). Global cloud coverage varies from ~ 65% to >80% depending upon the threshold of optical depth chosen for defining a cloud and the “pixel” size. However, these satellite based cloud coverage statistics are not appropriate for laser beams with diameters of a few meters. The general conclusions of the limited LITE data analyses of cloud porosity for lasers were that the532nm beam provided a ground return more often (~60 - 65%) than the current cloud climatologies based upon passive imagers suggested (~30-40%) and that the porosity of the clouds to the LITE beams was on the order of 50%.

More recently, very preliminary analysis of GLAS cloud data have shown that:

1. 70 - 80% of the GLAS lidar samples involved some return from clouds (assumed that “no cloud/no ground returns” intercepted thick layers of optically thin clouds)

2. 75 - 80% of the GLAS lidar samples detected the earth's surface (adjusted for smooth water returns)

3. When clouds were present, 25 – 40% of the time at least two layers were detected.

Following the work we have done with GLAS/ICESat, we are developing a set of cloud free line-of-sight (CFLOS) and cloud penetration statistics and atmospheric optical properties for space-based Doppler Wind Lidar (DWL) design using both LITE and CALIPSO data, and also using a Doppler Lidar Simulation Model (DLSM) to conduct basic trade studies that relate directly to DWL lidar design and scanning options. The primary LOS statistics are: probability of CFLOS penetrations to specific levels (1 km resolution) in the atmosphere; probability of multiple level interceptions of clouds of low to modest optical depths (< .5); and probabilities of contiguous CFLOSs for various durations of beam stares and shot integration.

The impacts that these cloud statistics have on a future space-based Doppler Wind Lidar will be discussed and simulated coverage illustrated.