Title: Low-Cloud Feedback in CAM5-CLUBB: Physical Mechanisms and Parameter Sensitivity Analysis

Abstract

The physical mechanism of low–cloud feedbacks is examined by using perturbed–parameter ensemble experiments in a unified scheme of boundary layer turbulence and shallow convection, named Cloud Layers Unified by Binormals (CLUBB) coupled to Community Atmosphere Model version 5 (CAM5). The shortwave cloud feedbacks in CAM5–CLUBB are positive in the most stable tropical regime, which is related to the weaker turbulence in the planetary boundary layer (PBL) in a warmer climate that is possibly triggered by the strengthened stability of the cloud layer. The positive feedback between low cloud cover (LCC), cloud top radiative cooling, and PBL turbulent mixing may further enhance the decrease in LCC. The stronger inversion stability of PBL partly counters the decrease in LCC, and a recently developed index, the estimated cloud–top entrainment index, is a better predictor for LCC changes than conventional stability indices. The relative strength of shallow convection increases in the warmer climate, but its effect on low–cloud feedback is complicated by the unified treatment of shallow convection and PBL turbulence in CLUBB. Stronger shallow convection means more convective drying but also less PBL turbulence and less LCC in the present climate, which leads to less reduction in LCC. The parameters related to dynamicmore » turbulent structure and double Gaussian closure in CLUBB are the most influential parameters on low–cloud feedbacks. Lastly, our results suggest that a unified treatment of shallow convection and turbulence may give rise to the predominate role of the PBL turbulent mixing in determining low–cloud feedback.« less

Authors:

Zhang, Haipeng  ^[1];

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Wang, Minghuai  ^[1];

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Guo, Zhun ^[2]; Zhou, Chen ^[1]; Zhou, Tianjun  ^[2];

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Qian, Yun  ^[3];

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Larson, Vincent E.  ^[4];

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Ghan, Steven  ^[3];

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Ovchinnikov, Mikhail  ^[3];

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Bogenschutz, Peter A. ^[5]; Gettelman, Andrew  ^[6]

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+ Show Author Affiliations

1. School of Atmospheric Sciences, Nanjing University, Nanjing China, Joint International Research Laboratory of Atmospheric and Earth System Sciences & Institute for Climate and Global Change Research, Nanjing University, Nanjing China
2. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing China
3. Pacific Northwest National Laboratory, Richland WA USA
4. Department of Mathematical Sciences, University of Wisconsin-Milwaukee, Milwaukee WI USA
5. Lawrence Livermore National Laboratory, Livermore CA USA
6. National Center for Atmospheric Research, Boulder CO USA

Publication Date:

2018-11-13

Research Org.:

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1482127

Alternate Identifier(s):

OSTI ID: 1482128; OSTI ID: 1483422

Report Number(s):

PNNL-SA-139280
Journal ID: ISSN 1942-2466

Grant/Contract Number:  

AC06-76RL01830; AC05-76RL01830; 2017YFA0604002; 41575073; 41621005; 41405103; 41775102

Resource Type:

Published Article

Journal Name:

Journal of Advances in Modeling Earth Systems

Additional Journal Information:

Journal Name: Journal of Advances in Modeling Earth Systems; Journal ID: ISSN 1942-2466

Publisher:

American Geophysical Union (AGU)

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; low‐cloud feedback; CLUBB; shallow convection; turbulence mixing; inversion stability; convective drying