The presence or absence and vertical location of cloud layers impose powerful constraints on the radiative properties of an atmospheric column and, thereby, modulate strongly the radiative heating rate of the column. Comprehensive information on the vertical distribution of cloud layers have been largely missing from analyses of conventional passive-sensor satellite radiometers that observe only the emitted and reflected radiance from the atmosphere and surface. Conventional satellite data have only allowed us to crudely estimate the location and vertical extent of clouds. Active remote sensors such as CloudSat, on the other hand, are uniquely adapted to observe the vertical location of hydrometeor layers. Therefore, a basic requirement of the CloudSat project is to use data from the Cloud Profiling Radar (CPR) to identify those levels in the vertical column sampled by CloudSat that contain significant radar echo from hydrometeors and to produce an estimate of the radar reflectivity factor for each volume deemed to contain significant echo (Stephens et al, 2001).
The CloudSat orbit will follow closely the orbit of the EOS PM1 satellite (Aqua) on which a number of advanced passive remote sensors will observe the earth. This synergistic association can add significantly to our understanding of the CloudSat geometrical profile and we will use this data source to our advantage. In particular, the Moderate-Resolution Imaging Spectroradiometer (MODIS) will provide information regarding the occurrence and horizontal distribution of clouds within the CloudSat footprint. The MODIS cloud mask product (Ackerman et al., 1998) will be available for use in the operational CloudSat processing stream. The MODIS cloud mask contains not only an estimate of the likelihood of clouds in 1km MODIS pixels, but also contains the results of a number tests with MODIS-observed spectral radiances that will provide limited information on the nature of the clouds in the vertical column. We will use the results of these tests to help us evaluate the characteristics of the CPR hydrometeor returns by performing comparisons of the CPR hydrometeor mask and the MODIS spectral tests.