Abstract
Abstract. Since the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite first began probing the Earth's atmosphere on 13 June 2006, several research groups dedicated to investigating the atmosphere's optical properties have conducted measurement campaigns to validate the CALIPSO data products. Recently, in order to address the lack of CALIPSO validation studies in the Southern Hemisphere, and especially the South American continent, the Lasers Environmental Applications Research Group at Brazil's Nuclear and Energy Research Institute (IPEN) initiated efforts to assess CALIPSO's aerosol lidar ratio estimates using the AERONET sun photometers installed at five different locations in Brazil. In this study we develop a validation methodology to evaluate the accuracy of the modeled values of the lidar ratios used by the CALIPSO extinction algorithms. We recognize that the quality of any comparisons between satellite and ground-based measurements depends on the degree to which the instruments are collocated, and that even selecting the best spatial and temporal matches does not provide an unequivocal guarantee that both instruments are measuring the same air mass. The validation methodology presented in this study therefore applies backward and forward air mass trajectories in order to obtain the best possible match between the air masses sampled by the satellite and the ground-based instruments, and thus reduces the uncertainties associated with aerosol air mass variations. Quantitative comparisons of lidar ratios determined from the combination of AERONET optical depth measurements and CALIOP integrated attenuated backscatter measurements show good agreement with the model values assigned by the CALIOP algorithm. These comparisons yield a mean percentage difference of −1.5% ± 24%. This result confirms the accuracy in the lidar ratio estimates provided by the CALIOP algorithms over Brazil to within an uncertainty range of no more than 30%.
Highlights
Aerosols and clouds play an important role in the Earth’s radiation budget since their physical and optical properties affect the scattering and absorption processes of solar radiation (Solomon et al, 2007)
The core of our validation methodology is to define “coincidence” not in terms of the proximity of measurements with respect to one another in time and/or space but instead in terms of the proximity of the measurements with respect to a single air mass whose location may be spatially and temporally varying. We accomplish this by using transport models to generate backward or forward trajectories, as required, to ensure that the air/aerosol parcel measured at the validation site is, to the best of our ability, the same the air/aerosol parcel measured by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar, similar to the approach taken by Wandinger et al (2010)
The question we address here is whether the lidar ratios assigned by this classification scheme are in good agreement with the actual lidar ratios of the aerosol layers being measured
Summary
Aerosols and clouds play an important role in the Earth’s radiation budget since their physical and optical properties affect the scattering and absorption processes of solar radiation (Solomon et al, 2007). Clouds act on atmospheric radiation processes by reflecting incoming sunlight back into space and by trapping thermal radiation emitted from the Earth’s surface. Aerosols can act to either cool or warm the atmosphere. Cooling occurs when aerosols scatter incoming sun radiation back into space, whereas warming occurs due to the absorption of the incoming sunlight. Aerosols affect climate processes on both local and global scales, representing a large source of uncertainties in the prediction of climate changes, mainly due to their spatial and temporal variability (Anderson et al, 2005). Aerosol optical and physical properties are highly complex, and vary considerably due to differences in their composition, distribution, sources (natural or anthropogenic) and local meteorology. One of the main challenges in the atmospheric sciences lies in acquiring more accurate knowledge about aerosol and cloud properties and how their interactions can affect climate model
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