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    History of the International Satellite Cloud Climatology Project
    (2022) Rossow, William B.
    The International Satellite Cloud Climatology Project (ISCCP) was formally established as the first project of the World Climate Research Programme (WCRP) in August 1982 to collect and analyze global satellite observations of Earth’s clouds for climate research. This 40-year history emphasizes the evolution of ideas about the purposes of the project and how that evolution shaped the characteristics of the data products. The history first covers a period before ISCCP, the planning workshops, the project initiation and the development in the first project phase, followed by a discussion of the evolution of the project concept to articulate more specifically the tasks required to quantify cloud effects on radiation exchanges in climate. The history continues with the production of the first version of the cloud data products in the late 1980s and early 1990s. Significant achievements at this stage were: (1) establishment and release of the first absolute radiance calibrations for the global constellation of weather satellite imaging instruments, (2) development and testing of a cloud detection procedure from quantitative evaluations of available ideas, (3) production of usefully accurate determinations of cloud radiative effects by employing radiative transfer models both for retrieval and flux calculations with consistent cloud microphysics and (4) provision of globally uniform depictions of diurnal, synoptic, and seasonal cloud variations. The interruption of satellite radiance calibration monitoring by the aerosols injected into the stratosphere by the Mt. Pinatubo volcano led to a period of evaluation based on an international set of supporting field experiments. Also, the reorganization of ISCCP within the Global Energy and Water Cycle Experiment (GEWEX) under WCRP shifted the project focus to include clouds and precipitation along with clouds and radiation. These events led to a second version of the data products produced from the 1990s into the early 2010s. This new version included improved polar cloud and cirrus detection, identification and treatment of ice clouds and release of higher resolution products for cloud process studies. The subsequent use of the new products led to better understanding of cloud types and their vertical structures, which allowed determination of radiation flux profiles. Analysis of patterns in mesoscale cloud property distributions helped advance understanding of cloud processes, including precipitation, in different meteorological situations. The advent of more advanced satellite cloud measurements in the late 1990s and 2000s supported a second revision that enhanced the usefulness of the ISCCP products for cloud process studies. In the 2010s, a growing emphasis on extending the length of record for climate studies led to the decision to transition the project to a fully operational organization to provide long-term context for field and other satellite measurements. The evolution of the project concept finally encompassed elucidating the complete role of clouds in weather and climate variations. On-going studies using ISCCP products include diagnosis of exchanges of radiative and latent energy by clouds, evolution of cloud properties over the lifecycle of tropical and extratropical storms, and estimates of cloud feedbacks on weather systems. The final sections summarize the accomplishments of ISCCP, discuss the status of knowledge about clouds and cloud processes as of 2022, and briefly outline of the next measurements and analyses.