Foundational Contributions to CALIPSO Data Quality from European Correlative Measurements

After 17 years of highly successful operations, the CALIPSO satellite mission, a partnership between NASA and CNES, ended in August 2023, leaving behind a long and very stable measurement record that has helped to transform our understanding of climate, weather, and air quality. The CALIPSO instrument suite included a two-wavelength (532 and 1064 nm), depolarization-sensitive profiling lidar for clouds and aerosols, an imaging infrared radiometer optimized for better understanding cirrus optical and microphysical properties, and a wide field of view camera to provide meteorological context. Thus far, over 4300 peer-reviewed publications have been produced from the CALIPSO data set. CALIPSO was truly a pathfinder mission, not only with its unprecedented ability to probe the vertical structure of the Earths atmosphere, but also in the development of sophisticated new algorithms for retrieving the spatial and optical properties of aerosols and clouds. Throughout the mission development and operational phases, the CALIPSO team relied heavily on aerosol and cloud measurements from ground-based and airborne sensors to aid in the design and revision of fully automated retrieval algorithms, the assessment of data products, as well as with the evaluation of instrument performance and calibration. This presentation highlights the impressive breadth of validation support the CALIPSO team received from the European field measurements community across all data processing levels. CALIPSO Level 1 (L1) data report geolocated and fully calibrated lidar backscatter profiles. Because L1 is the wellspring for all subsequent data products, rigorous calibration validation is absolutely essential. While NASAs high spectral resolution lidar validated the CALIPSO calibration over North America (Rogers et al., 2011), earlier studies conducted by European research institutes (e.g., Mamouri et al., 2009; Mona et al., 2009; Pappalardo et al., 2010) were critically important in establishing the global reliability of the CALIPSO L1 data. Similarly, European investigators were at the forefront in assessing the geophysical variables reported in the CALIPSO Level 2 (L2) products. Extensive Raman lidar measurements from SAMUN-2 and SALTRACE (Wandinger et al., 2010; Tesche et al., 2013; Haarig et al., 2017) sparked years of vigorous, highly rewarding discussion about regional variations in CALIPSOs dust lidar ratio assignments. For clouds, combined in-situ and airborne lidar measurements of cirrus provided key insights into the effects of ice crystal morphology on CALIPSO retrievals of cirrus extinction coefficients (Mioche et al., 2010). The literature is replete with additional, highly compelling L2 assessments of aerosols, clouds, and the algorithms used to derive their properties. Finally, Level 3 (L3) delivers a climatological rather than instantaneous data representation by aggregating L2 variables onto uniform space-time grids. Leveraging years of EARLINET observations, Papagiannopoulos et al. (2016) conducted a comprehensive assessment of the monthly mean vertical distributions of aerosol occurrence frequency and optical properties reported in CALIPSOs L3 tropospheric aerosol product.