Cloud Longwave Scattering Effect and Its Impact on Climate Simulation

Wenjie Zhao,Yiran Peng,Jiangnan Li,Bin Wang

doi:10.3390/atmos9040153

Wenjie Zhao, Yiran Peng + Show 2 more

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https://doi.org/10.3390/atmos9040153

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Abstract

The cloud longwave (LW) scattering effect has been ignored in most current climate models. To investigate its climate impact, we apply an eight-stream DIScrete Ordinates Radiative Transfer (DISORT) scheme to include the cloud LW scattering in the General circulation model version of the LongWave Rapid Radiative Transfer Model (RRTMG_LW) and the Community Atmospheric Model Version 5 (CAM5). Results from the standalone RRTMG_LW and from diagnostic runs of CAM5 (no climate feedback) show that the cloud LW scattering reduces the upward flux at the top of the atmosphere and leads to an extra warming effect in the atmosphere. In the interactive runs with climate feedback included in CAM5, the cloud LW scattering effect is amplified by the water vapor-temperature feedback in a warmer atmosphere and has substantial influences on cloud fraction and specific humidity. The thermodynamic feedbacks are more significant in the northern hemisphere and the resulting meridional temperature gradient is different between the two hemispheres, which strengthens the southern branch of Hadley circulation, and modulates the westerly jet near 50° S and the upper part of Walker circulation. Our study concludes that the cloud LW scattering effect could have complex impacts on the global energy budget and shall be properly treated in future climate models.

Highlights

Longwave (LW) radiation plays an indispensable role in modulating the global radiative budget and maintaining the Earth climate system
To study the cloud LW scattering effect, we introduce an 8-stream discrete ordinates radiative transfer (DISORT) solver [29] into RRTMG_LW as a benchmark scheme for the LW radiation calculation
In this subsection we present the variation of cloud optical properties in LW spectral bands, providing a base to demonstrate the longwave scattering ability of water/ice cloud particles

Summary

Introduction

Longwave (LW) radiation plays an indispensable role in modulating the global radiative budget and maintaining the Earth climate system. Increases in water vapor and greenhouse gases trap more LW radiation in the earth-atmosphere system and generate more LW emission from the atmosphere to the surface, which results in a warming condition in climate [1]. The cloud effect in LW is similar to the greenhouse effect, especially for high and thin ice clouds on trapping terrestrial radiation [2,3]. Since temperature is decreased with height in the troposphere and LW emission is proportional to the fourth power of temperature, the outgoing LW radiation (OLR) at the top of the atmosphere (TOA) is much less than that would have emitted from the earth surface, keeping energy in the earth-atmosphere system. Calculation of the LW cloud effect involves absorbing, scattering, and emitting

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