Abstract
Abstract. At CNR-IMAA, an aerosol lidar system has operated since May 2000 in the framework of EARLINET (European Aerosol Research Lidar Network), the first lidar network for tropospheric aerosol study on a continental scale. High quality multi-wavelength measurements make this system a reference point for the validation of data products provided by CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), the first satellite-borne lidar specifically designed for aerosol and cloud study. Since 14 June 2006, dedicated measurements have been performed at CNR-IMAA in coincidence with CALIPSO overpasses. For the first time, results on 1-year comparisons between ground-based multi-wavelength Raman lidar measurements and corresponding CALIPSO lidar Level 1 profiles are presented. A methodology for the comparison is presented and discussed in detail. Night-time cases are considered to take advantage from Raman capability of the ground based lidar. Cases with the detection of cirrus clouds in CALIPSO data are separately analysed for taking into account multiple scattering effects. For cirrus cloud cases, few cases are available to draw any conclusions. For clear sky conditions, the comparison shows good performances of the CALIPSO on-board lidar: the mean relative difference between the ground-based and CALIPSO Level 1 measurements is always within its standard deviation at all altitudes, with a mean difference in the 3–8 km altitude range of (−2±12)%. At altitude ranges corresponding to the typical PBL height observed at CNR-IMAA, a mean difference of (−24±20)% is observed in CALIPSO data, probably due to the difference in the aerosol content at the location of PEARL and CALIPSO ground-track location. Finally, the mean differences are on average lower at all altitude ranges for the closest overpasses (at about 40 km) respect to the 80-km overpasses.
Highlights
Tropospheric aerosols, and in particular anthropogenic aerosols, are one of the most uncertain elements in the estimation of radiation budget
In order to increase and validate the accuracy of aerosol optical properties retrieved from the CALIPSO pure backscatter lidar, comparisons with ground-based elastic/Raman lidar and/or High Spectral Resolution Lidar (HSRL) measurements are strongly necessary, since these techniques allow the characterization of atmospheric aerosols in terms of vertical profiles of extinction and backscatter coefficients without any assumptions on the aerosol type and composition (Ansmann et al, 1990, 1992; Hair et al, 2008)
In order to reduce the noise in the CALIPSO signal, profiles are averaged on an horizontal scale of 5 km, corresponding to the horizontal resolution of CALIPSO Level 2 Layer Aerosol products (Vaughan et al, 2005)
Summary
Tropospheric aerosols, and in particular anthropogenic aerosols, are one of the most uncertain elements in the estimation of radiation budget. In order to increase and validate the accuracy of aerosol optical properties retrieved from the CALIPSO pure backscatter lidar, comparisons with ground-based elastic/Raman lidar and/or High Spectral Resolution Lidar (HSRL) measurements are strongly necessary, since these techniques allow the characterization of atmospheric aerosols in terms of vertical profiles of extinction and backscatter coefficients without any assumptions on the aerosol type and composition (Ansmann et al, 1990, 1992; Hair et al, 2008). In cases of high temporal variability shown by the PEARL acquired range corrected signal, a shorter temporal window for optical properties retrieval allows the reduction of systematic errors due to variability in aerosol and cloud fields (Ansmann et al, 1992) With these resolutions, in night time conditions, typical statistical errors due to the signals detection in the PBL are below 5% for the aerosol backscatter coefficients at 355 and 532 nm, and below 10% for the extinction coefficients at 355 nm and 532 nm. Both the system and the algorithms used have been quality checked and are the object of continuous checks within the EARLINET Quality Assurance program (Matthias et al, 2004; Bockmann et al, 2004; Pappalardo et al, 2004; Amodeo et al, 2007)
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