Abstract

Abstract. With its height-resolved measurements and near global coverage, the CALIOP lidar onboard the CALIPSO satellite offers a new capability for aerosol retrievals in cloudy skies. Validation of these retrievals is difficult, however, as independent, collocated and co-temporal data sets are generally not available. In this paper, we evaluate CALIOP aerosol products above opaque water clouds by applying multiple retrieval techniques to CALIOP Level 1 profile data and comparing the results. This approach allows us to both characterize the accuracy of the CALIOP above-cloud aerosol optical depth (AOD) and develop an error budget that quantifies the relative contributions of different error sources. We focus on two spatial domains: the African dust transport pathway over the tropical North Atlantic and the African smoke transport pathway over the southeastern Atlantic. Six years of CALIOP observations (2007–2012) from the northern hemisphere summer and early fall are analyzed. The analysis is limited to cases where aerosol layers are located above opaque water clouds so that a constrained retrieval technique can be used to directly retrieve 532 nm aerosol optical depth and lidar ratio. For the moderately dense Sahara dust layers detected in the CALIOP data used in this study, the mean/median values of the lidar ratios derived from a constrained opaque water cloud (OWC) technique are 45.1/44.4 ± 8.8 sr, which are somewhat larger than the value of 40 ± 20 sr used in the CALIOP Level 2 (L2) data products. Comparisons of CALIOP L2 AOD with the OWC-retrieved AOD reveal that for nighttime conditions the L2 AOD in the dust region is underestimated on average by ~26% (0.183 vs. 0.247). Examination of the error sources indicates that errors in the L2 dust AOD are primarily due to using a lidar ratio that is somewhat too small. The mean/median lidar ratio retrieved for smoke is 70.8/70.4 ± 16.2 sr, which is consistent with the modeled value of 70 ± 28 sr used in the CALIOP L2 retrieval. Smoke AOD is found to be underestimated, on average, by ~39% (0.191 vs. 0.311). The primary cause of AOD differences in the smoke transport region is the tendency of the CALIOP layer detection scheme to prematurely assign layer base altitudes and thus underestimate the geometric thickness of smoke layers.

Highlights

  • Beginning with the first Intergovernmental Panel on Climate Change (IPCC) assessment, tremendous progress has been made in modeling the global impacts of aerosols on the Earth’s climate

  • Six years (2007–2012) of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data from the two regions indicated in Fig. 1 have been analyzed using the opaque water cloud (OWC) constrained technique

  • Because accurate knowledge of γWC, SS, no aerosol (NA) is very important in the derivation of aerosol optical depth (AOD) using the OWC technique, in this subsection we examine the spatial variability of γWC, SS, NA and its potential impact on the retrieved AODs

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Summary

Introduction

Beginning with the first Intergovernmental Panel on Climate Change (IPCC) assessment, tremendous progress has been made in modeling the global impacts of aerosols on the Earth’s climate. As summarized in the most recent 5th assessment report (Stocker et al, 2013), significant uncertainties remain. Recent model intercomparisons have shown a large diversity in the vertical distribution of aerosols (Kinne et al, 2006; Textor et al, 2006; Huneeus et al, 2011) which can be attributed more to uncertainties in the simulation of aerosol processes than in the realism of the aerosol precursor emissions used by the models. Errors in modeling the vertical distribution of aerosols cause errors in the aerosol atmospheric lifetime and global distribution. While comparisons with observations are clearly necessary to evaluate and improve model performance, until recently global measurements of aerosol vertical distribution were notably lacking, largely because previous generations of space-based passive sensors had only limited abilities to retrieve aerosol

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