Satellite Remote Sensing of Cloud Optical and Microphysical Properties: Historical Perspective, Insights, and Future Capabilities

Space-based cloud retrievals support a number of important applications, including the development of climatologies and assessing climate change, cloud parameterization and physical processes studies, and model validation and data assimilation. Substantial progress has been made in satellite cloud remote sensing capabilities over the last two decades. This development has been due to significant improvements in instrument capability and calibration, retrieval algorithm advances, and lessons learned from earlier efforts coupled with better physical models. Retrieved parameters of primary interest have been cloud detection, cloud-top properties, and optical and microphysical properties (optical thickness and effective particle size). These parameters are now operationally retrieved from a variety of instruments/platforms and algorithms. An overview of the historical development will be presented, including advances in instrument capability and recent results. The emphasis will be on optical and microphysical retrievals using the solar reflectance part of the spectrum measured with imaging instruments (e.g., AVHRR and MODIS), though these retrievals rely critically on cloud detection as well cloud-top information. Remaining cloud retrieval difficulties as well as cloud quantities that have yet to be adequately retrieved will also be discussed. Some of these outstanding issues can be rectified, or at least better understood, with observations from the active sensors on the NASA Earth System Science Pathfinder (ESSP) missions scheduled for launch in autumn 2005. Future cloud-related missions and capabilities will be briefly summarized.