Comparison of CALIPSO aerosol optical depth retrievals to AERONET measurements, and a climatology for the lidar ratio of dust

Abstract

Abstract. We compared CALIPSO column aerosol optical depths at 0.532 µm to measurements at 147 AERONET sites, synchronized to within 30 min of satellite overpass times during a 3-yr period. We found 677 suitable overpasses, and a CALIPSO bias of −13 % relative to AERONET for the entire data set; the corresponding absolute bias is −0.029, and the standard deviation of the mean (SDOM) is 0.014. Consequently, the null hypothesis is rejected at the 97 % confidence level, indicating a statistically significant difference between the datasets. However, if we omit CALIPSO columns that contain dust from our analysis, the relative and absolute biases are reduced to −3 % and −0.005 with a standard error of 0.016 for 449 overpasses, and the statistical confidence level for the null hypothesis rejection is reduced to 27 %. We also analyzed the results according to the six CALIPSO aerosol subtypes and found relative and absolute biases of −29 % and −0.1 for atmospheric columns that contain the dust subtype exclusively, but with a relatively high correlation coefficient of R = 0.58; this indicates the possibility that the assumed lidar ratio (40 sr) for the CALIPSO dust retrievals is too low. Hence, we used the AERONET size distributions, refractive indices, percent spheres, and forward optics code for spheres and spheroids to compute a lidar ratio climatology for AERONET sites located in the dust belt. The highest lidar ratios of our analysis occur in the non-Sahel regions of Northern Africa, where the median lidar ratio at 0.532 µm is 55.4 sr for 229 retrievals. Lidar ratios are somewhat lower in the African Sahel (49.7 sr for 929 retrievals), the Middle East (42.6 sr for 489 retrievals), and Kanpur, India (43.8 sr for 67 retrievals). We attribute this regional variability in the lidar ratio to the regional variability of the real refractive index of dust, as these two parameters are highly anti-correlated (correlation coefficients range from −0.51 to −0.85 for the various regions). The AERONET refractive index variability is consistent with the variability of illite concentration in dust across the dust belt.

Middle East versus Saharan dust extinction-to-backscatter ratios

“…Recent Aerosol Robotic Network (AERONET) sunphotometer-based studies of Schuster et al (2012) and combined observations with polarization lidar and sun/sky photometer by Mamouri et al (2013) suggest that lidar ratios of Middle East dust are significantly lower (35-45 sr) than the ones for Saharan dust (45-60 sr). The reason seems to be that the illite concentration in dust particles decreases from values around 80 % in western Saharan regions to less than 5 % in the dust particles in eastern Saharan and Middle East desert regions (Schuster et al, 2012). As a consequence the real part of the refractive index decreases from 1.55 for Arabian dust More studies are needed to corroborate these findings.…”

Improving retrieval accuracy for aerosol optical depth by fusion of MODIS and CALIOP data

“…This instrument estimates the aerosol layer optical depths and total column optical depth by backscattering lidar signals [17÷20]. Validation studies show that CALIOP achieved an average AOD retrieval bias of -13% relative to AERONET over 147 global sites from June 2006 to May 2009 [9]. The results show that CALIOP is biased 20% and 12% below the AERONET AODs in the Northern African sites and Middle Eastern region, respectively.…”

“…CALIOP level 2 version 3 cloud-free aerosol layer products are used in this study at a 5-km resolution around AERONET sites, with the following two additional settings: CALIOP cloud-aerosol discrimination (CAD) score <−20 and extinction quality control flag QC=0 for all layers [9,19]. The two settings require the absence of clouds in the CALIOP data columns and consider only quality-controlled aerosol data.…”

Saharan and Arabian Dust Aerosols: A Comparative Case Study of Lidar Ratio

“…These preliminary results are in agreement with other LR values reported for dust in previous works [i.e., 6-8; 11] dependent on the desert dust source region. In addition, that observed disparity can be based on the singular composition of the suspended dust aerosols arriving from different desert areas, as reported in [6]. In that work an inverse relationship was found between LR and real refractive index, based on AERONET LR retrievals; in particular, Saharan dust presented a lower value of the real refractive index than that found for dust from the Arabian Middle East area; correspondingly, they obtained a higher LR (55 sr) for the Saharan dust than for Arabian dust (43 sr) [6].…”

“…In addition, that observed disparity can be based on the singular composition of the suspended dust aerosols arriving from different desert areas, as reported in [6]. In that work an inverse relationship was found between LR and real refractive index, based on AERONET LR retrievals; in particular, Saharan dust presented a lower value of the real refractive index than that found for dust from the Arabian Middle East area; correspondingly, they obtained a higher LR (55 sr) for the Saharan dust than for Arabian dust (43 sr) [6]. Similar LR values were also derived by lidar measurements performed in Cyprus region for Arabian dust particles [8].…”