EARLINET evaluation of the CATS Level 2 aerosol backscatter coefficient product

Abstract

Abstract. We present the evaluation activity of the European Aerosol Research Lidar Network (EARLINET) for the quantitative assessment of the Level 2 aerosol backscatter coefficient product derived by the Cloud-Aerosol Transport System (CATS) aboard the International Space Station (ISS; Rodier et al., 2015). The study employs correlative CATS and EARLINET backscatter measurements within a 50 km distance between the ground station and the ISS overpass and as close in time as possible, typically with the starting time or stopping time of the EARLINET performed measurement time window within 90 min of the ISS overpass, for the period from February 2015 to September 2016. The results demonstrate the good agreement of the CATS Level 2 backscatter coefficient and EARLINET. Three ISS overpasses close to the EARLINET stations of Leipzig, Germany; Évora, Portugal; and Dushanbe, Tajikistan, are analyzed here to demonstrate the performance of the CATS lidar system under different conditions. The results show that under cloud-free, relative homogeneous aerosol conditions, CATS is in good agreement with EARLINET, independent of daytime and nighttime conditions. CATS low negative biases are observed, partially attributed to the deficiency of lidar systems to detect tenuous aerosol layers of backscatter signal below the minimum detection thresholds; these are biases which may lead to systematic deviations and slight underestimations of the total aerosol optical depth (AOD) in climate studies. In addition, CATS misclassification of aerosol layers as clouds, and vice versa, in cases of coexistent and/or adjacent aerosol and cloud features, occasionally leads to non-representative, unrealistic, and cloud-contaminated aerosol profiles. Regarding solar illumination conditions, low negative biases in CATS backscatter coefficient profiles, of the order of 6.1 %, indicate the good nighttime performance of CATS. During daytime, a reduced signal-to-noise ratio by solar background illumination prevents retrievals of weakly scattering atmospheric layers that would otherwise be detectable during nighttime, leading to higher negative biases, of the order of 22.3 %.

Cloud-Aerosol Transport System (CATS) 1064 nm calibration and validation

“…The results shown in this paper are critical to understanding the uncertainties in CATS 1064 nm Level 2 data products, as the calibration uncertainties from backscatter lidars generally impose a lower bound on the uncertainties in cloud/aerosol extinction and optical depth retrievals from such instruments (Young et al, 2013(Young et al, , 2016. To date, the CATS cloud and aerosol top/base heights have been used for various applications, including volcanic plume transport (Hughes et al, 2016), above-cloud aerosol properties (Rajapakshe et al, 2017), pyrocumulonimbus smoke heights (Christian et al, 2019), and cloud diurnal variability (Noel et al, 2018). More recently, CATS cloud and aerosol optical properties (e.g., extinction, optical depth, ice water content) from Level 2 V-3.00 data have been compared to EARLINET aerosol products (Proestakis et al, 2019), used to estimate thin cirrus radiative forcing (Dolinar et al, 2019), and demonstrated the diurnal variability of aerosol properties (Lee et al, 2019).…”

“…where α par is the particulate extinction and z s is the CATS altitude. The particulate backscatter was computed from the Polly XT 1064 and 607 nm signals through the methodology described in Proestakis et al (2019). The uncertainty in the backscatter coefficient retrieval is estimated to be between 5 % and 20 % (Ansmann et al, 1992;Whiteman et al, 2003;Povey et al, 2014).…”

“…Previous studies have investigated the validity of using EARLINET for spaceborne lidar validation (Mamouri et al, 2009;Papagiannopoulos et al, 2016;Proestakis et al, 2019) and have found it is a useful method for lidar validation. A major source of the variability between the ground-based and spaceborne measurement results was found to be the variances in the atmospheric scene observed due to the spatial and temporal differences in the measurements.…”

“…Lidar plays a crucial role in observing the Earth's atmosphere as it enhances our understanding of the roles clouds and aerosols play in the climate system by providing vertical profiles of backscatter coefficient and other optical properties. Lidars have been utilized to study the vertical distribution and injection heights of smoke plumes (e.g., McGill et al, 2003;Wang et al, 2013;Rajapakshe et al, 2017), properties and transport of mineral dust aerosols (e.g., Papayannis et al, 2009;Yang et al, 2013;Haarig et al, 2017), and layer and optical properties of clouds (e.g., Yorks et al, 2011;Avery at al., 2012;Haarig et al, 2016;Noel et al, 2018). Lidar, particularly from a spaceborne platform, has the capability to provide these vertical profiles of cloud and aerosol optical properties globally.…”

Interrelations between surface, boundary layer, and columnar aerosol properties derived in summer and early autumn over a continental urban site in Warsaw, Poland

“…Schaap et al, 2009;Stachlewska et al, 2017bStachlewska et al, , 2018, and PM 1 (e.g. Qin et al, 2018) concentrations can improve our knowledge of the atmospheric environment.…”