Lateral Boundary of Cirrus Cloud from CALIPSO Observations

Yunfei Fu,Fang Qin,Guosheng Liu,Tao Xian,Aoqi Zhang,Lu Yu,Xiangdong Zhang,Rui Li,Yilun Chen

doi:10.1038/s41598-017-14665-6

Abstract

AbsractDue to the thinness and small scale of cirrus clouds, its lateral boundary may be missed by conventional passive remote-sensing techniques and climate models. Here, using satellite observations in June–August from 2006 to 2011, a global dataset for the cirrus cloud lateral boundary (CCLB) was established. The results indicate that the optical properties, such as the lidar backscatter, the depolarization ratio and the optical depth, sharply decrease from cloudy regions to clear-sky regions. There are significant regional differences in optical properties and height and thickness of the CCLB. Based on a quantitative estimation, the strongest longwave warming effects (>0.3 W m−2) are found near the Equator and over tropical continents. The global average longwave warming effect of the CCLB is at least 0.07 W m−2, which is much larger than some of the radiative forcings considered in the Intergovernmental Panel on Climate Change (IPCC) reports. Specifically, the CCLB in traditional “clear-sky” region may be totally missed by current models and IPCC reports, which contributes 28.25% (~0.02 W m−2) of the whole CCLB radiative effect, twice greater than contrail effect. It is recommended that the CCLB effect should be taken account in future climate models and the next IPCC reports.

Highlights

Cirrus clouds regularly cover approximately 20% of the globe, providing significant radiative forcing (RF) on the Earth climate system[1,2,3]
Using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements, Li et al (2014) analyzed the lateral boundary layer of cirrus clouds in the east and south of China[22] and estimated that the corresponding longwave RF is about 10 W m−2. Based on their conservative estimation, the RF induced by this cirrus cloud lateral boundary (CCLB) is at least 0.0047 W m−2 globally, which is not taken account in any global atmospheric circulations models, because there is no global model considers the lateral boundary of clouds
Note that the thickness in our study is thinner than other reports, such as Sassen et al.[27], because we only focus on the thickness at the lateral boundary rather than the whole cirrus cloud

Summary

Introduction

Cirrus clouds regularly cover approximately 20% of the globe, providing significant radiative forcing (RF) on the Earth climate system[1,2,3]. Using CALIOP measurements, Li et al (2014) analyzed the lateral boundary layer of cirrus clouds in the east and south of China[22] and estimated that the corresponding longwave RF is about 10 W m−2 Based on their conservative estimation, the RF induced by this cirrus cloud lateral boundary (CCLB) is at least 0.0047 W m−2 globally, which is not taken account in any global atmospheric circulations models, because there is no global model considers the lateral boundary of clouds. Their RF result was calculated based on the assumption that coverage, width and RF of the CCLB are all fixed values. Because of the scan method (nadir-looking) of CALIOP, the transition zone is the wide regions around cirrus boundaries, as opposed to the exact 3D boundaries

Objectives

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Results

Conclusion