A First Look on Cloud Homogeneity from CALIPSO

With the launch of the CALIPSO satellite mission in 2006, considerable insight has been gained on the vertical distribution and occurrence of clouds over the globe that has helped advanced our understanding of cloud processes, forcings, and feedbacks. Most cloud studies using CALIPSO observations have accumulated statistics by treating measurements as samples over fixed intervals to represent average or median properties. Fewer studies, on the other hand, have examined cloud correlation length scales or homogeneity indices from the CALIPSO record, which can shed light on the spatial structure of air mass properties or organized circulations.

This study reports on cloud spatial homogeneity statistics revealed by CALIPSO's lidar (CALIOP) and its visible Wide Field-of-view Camera (WFC). CALIOP operates at wavelengths of 532 and 1064 nm and has a narrow field of view (100 m) with profile measurements separated by 333 m. The WFC is co-aligned with the lidar and operates continuously at a wavelength of 645 nm (similar to channel 1 for MODIS). Within a distance of 2.5 km of the lidar ground track, WFC pixels have a horizontal resolution of 125 m. For the portion of the WFC swath that extends beyond the near region to a distance of 30 km, pixel resolution is reduced to 1 km. The combination of these two instruments permits a detailed assessment of the cloud field along and slightly off the lidar track. For this study, marine stratus fields are examined because they are often bright and easily detected by the WFC (and CALIOP for background measurements) and have organized cell structures. A comparison with MODIS (Aqua) is made to illuminate similarities and differences revealed from the CALIOP measurement system.