Abstract

Abstract. To help satellite retrieval of aerosols and studies of their radiative effects, we demonstrate that daytime aerosol optical depth over low-level clouds is similar to that in neighboring clear skies at the same heights. Based on recent airborne lidar and sun photometer observations above the southeast Atlantic, the mean aerosol optical depth (AOD) difference at 532 nm is between 0 and −0.01, when comparing the cloudy and clear sides, each up to 20 km wide, of cloud edges. The difference is not statistically significant according to a paired t test. Systematic differences in the wavelength dependence of AOD and in situ single scattering albedo are also minuscule. These results hold regardless of the vertical distance between cloud top and aerosol layer bottom. AOD aggregated over ∼2∘ grid boxes for each of September 2016, August 2017 and October 2018 also shows little correlation with the presence of low-level clouds. We posit that a satellite retrieval artifact is entirely responsible for a previous finding of generally smaller AOD over clouds (Chung et al., 2016), at least for the region and time of our study. Our results also suggest that the same values can be assumed for the intensive properties of free-tropospheric biomass-burning aerosol regardless of whether clouds are present below.

Highlights

  • A significant amount of atmospheric particles are transported above liquid water clouds on the global scale (Waquet et al, 2013)

  • Estimates of the direct aerosol radiative effect alone see large intermodel spread for areas with large aerosol optical depth (AOD) over widespread clouds (Stier et al, 2013; Zuidema et al, 2016)

  • Among them is the paper by Chung et al (2016) which used the level 2 products of Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) (Winker et al, 2009) to calculate the AOD above the maximum low-cloud-top height in each grid cell in clear sky as well as the AOD above low clouds on a global 2◦ × 5◦ latitude–longitude grid

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Summary

Introduction

A significant amount of atmospheric particles are transported above liquid water clouds on the global scale (Waquet et al, 2013). Among them is the paper by Chung et al (2016) which used the level 2 products of Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) (Winker et al, 2009) to calculate the AOD above the maximum low-cloud-top height in each grid cell in clear sky as well as the AOD above low clouds on a global 2◦ × 5◦ latitude–longitude grid. The detection threshold varies with the atmospheric features (e.g., aerosols, high-altitude cirrus or boundary layer clouds), the horizontal averaging required by CALIOP for detection, and (importantly) the background lighting conditions (see Fig. 4 by Winker et al, 2009). 2 along with sampling and statistical hypothesis testing methods This is followed by comparisons of AOD and other aerosol properties above the height of cloud top between cloudy and clear skies

Instrumentation
Sampling
Mesoscale monthly-mean sampling
Local-scale near-synchronous sampling
Statistical hypothesis testing
Results
Discussion and conclusions
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