New Global View of Above-Cloud Absorbing Aerosol Distribution Based on CALIPSO Measurements

Wenzhong Zhang,Nana Liu,Tao Luo,Wenyue Zhu,Yinbo Huang,Shumei Deng,Yang Wu,Xuebin Li

doi:10.3390/rs11202396

Abstract

Above-low-level-cloud aerosols (ACAs) have gradually gained more interest in recent years; however, the combined aerosol–cloud radiation effects are not well understood. The uncertainty about the radiative effects of aerosols above cloud mainly stems from the lack of comprehensive and accurate retrieval of aerosols and clouds for ACA scenes. In this study, an improved ACA identification and retrieval methodology was developed to provide a new global view of the ACA distribution by combining three-channel CALIOP (The Cloud–Aerosol Lidar with Orthogonal Polarization) observations. The new method can reliably identify and retrieve both thin and dense ACA layers, providing consistent results between the day- and night-time retrieval of ACAs. Then, new four-year (2007 to 2010) global ACA datasets were built, and new seasonal mean views of global ACA occurrence, optical depth, and geometrical thickness were presented and analyzed. Further discussion on the relative position of ACAs to low clouds showed that the mean distance between the ACA layer and the low cloud deck over the tropical Atlantic region is less than 0.2 km. This indicates that the ACAs over this region are more likely to be mixed with low-level clouds, thereby possibly influencing the cloud microphysics over this region, contrary to findings reported from previous studies. The results not only help us better understand global aerosol transportation and aerosol–cloud interactions but also provide useful information for model evaluation and improvements.

Highlights

The long-range transport of aerosols plays an important role in several regions of the world, having a potential impact on aerosol–cloud interactions, atmospheric chemistry, and air quality [1,2,3,4,5,6,7,8]
The results indicate that the mean liquid cloud optical thickness can be increased by roughly 6%, and the mean liquid effective radius can be increased by roughly 2.6% after correcting for the effect of the above-low-level cloud aerosols (ACAs) [22,28,29,30,31,32]
Other than these major aerosol source regions, our results indicate some weak aerosol source regions, such as South America, Central America, and Timor Sea, where ACA frequently occurs in some seasons, but this has rarely been studied in previous studies

Summary

Introduction

The long-range transport of aerosols plays an important role in several regions of the world, having a potential impact on aerosol–cloud interactions, atmospheric chemistry, and air quality [1,2,3,4,5,6,7,8]. Aerosols often overlay lower level clouds [9], for example, biomass burning aerosols and wind-blown dust overlay low-level cloud deck over the Atlantic. The above-low-level cloud aerosols (ACAs) occupy about 25% of the mean aerosol optical depth (fine mode) at a global scale [9], and this fraction could be much higher regionally and seasonally [10]. Current models experience significant inter-model discrepancies in aerosol forcing assessments, especially over the aerosol–cloud overlap regions [11], that result from inter-model differences in both aerosol and cloud properties [11,12]. A recent evaluation showed that most models cannot reproduce the observed large aerosol load episodes [13]. Improved observations are needed to better understand ACA–cloud interactions and to constrain the aerosol–cloud radiative processes in models

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Methods

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Discussion

Conclusion