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Title: On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs

In this study, it is shown that CMIP5 global climate models (GCMs) that convert supercooled water to ice at relatively warm temperatures tend to have a greater mean-state cloud fraction and more negative cloud feedback in the middle and high latitude Southern Hemisphere. We investigate possible reasons for these relationships by analyzing the mixed-phase parameterizations in 26 GCMs. The atmospheric temperature where ice and liquid are equally prevalent (T5050) is used to characterize the mixed-phase parameterization in each GCM. Liquid clouds have a higher albedo than ice clouds, so, all else being equal, models with more supercooled liquid water would also have a higher planetary albedo. The lower cloud fraction in these models compensates the higher cloud reflectivity and results in clouds that reflect shortwave radiation (SW) in reasonable agreement with observations, but gives clouds that are too bright and too few. The temperature at which supercooled liquid can remain unfrozen is strongly anti-correlated with cloud fraction in the climate mean state across the model ensemble, but we know of no robust physical mechanism to explain this behavior, especially because this anti-correlation extends through the subtropics. A set of perturbed physics simulations with the Community Atmospheric Model Version 4 (CAM4)more » shows that, if its temperature-dependent phase partitioning is varied and the critical relative humidity for cloud formation in each model run is also tuned to bring reflected SW into agreement with observations, then cloud fraction increases and liquid water path (LWP) decreases with T5050, as in the CMIP5 ensemble.« less
Authors:
 [1] ;  [2] ;  [1] ;  [3] ;  [2]
  1. University of Washington, Seattle, WA (United States). Atmospheric Sciences Department
  2. Yale Univ., New Haven, CT (United States). Geology and Geophysics Department
  3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Program for Climate Model Diagnosis and Intercomparison
Publication Date:
Report Number(s):
LLNL-JRNL-736460
Journal ID: ISSN 1942-2466
Grant/Contract Number:
AC52-07NA27344; SC0012580
Type:
Published Article
Journal Name:
Journal of Advances in Modeling Earth Systems
Additional Journal Information:
Journal Volume: 8; Journal Issue: 2; Journal ID: ISSN 1942-2466
Publisher:
American Geophysical Union (AGU)
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES
OSTI Identifier:
1251713
Alternate Identifier(s):
OSTI ID: 1251715; OSTI ID: 1409979

McCoy, Daniel T., Tan, Ivy, Hartmann, Dennis L., Zelinka, Mark D., and Storelvmo, Trude. On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs. United States: N. p., Web. doi:10.1002/2015MS000589.
McCoy, Daniel T., Tan, Ivy, Hartmann, Dennis L., Zelinka, Mark D., & Storelvmo, Trude. On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs. United States. doi:10.1002/2015MS000589.
McCoy, Daniel T., Tan, Ivy, Hartmann, Dennis L., Zelinka, Mark D., and Storelvmo, Trude. 2016. "On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs". United States. doi:10.1002/2015MS000589.
@article{osti_1251713,
title = {On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs},
author = {McCoy, Daniel T. and Tan, Ivy and Hartmann, Dennis L. and Zelinka, Mark D. and Storelvmo, Trude},
abstractNote = {In this study, it is shown that CMIP5 global climate models (GCMs) that convert supercooled water to ice at relatively warm temperatures tend to have a greater mean-state cloud fraction and more negative cloud feedback in the middle and high latitude Southern Hemisphere. We investigate possible reasons for these relationships by analyzing the mixed-phase parameterizations in 26 GCMs. The atmospheric temperature where ice and liquid are equally prevalent (T5050) is used to characterize the mixed-phase parameterization in each GCM. Liquid clouds have a higher albedo than ice clouds, so, all else being equal, models with more supercooled liquid water would also have a higher planetary albedo. The lower cloud fraction in these models compensates the higher cloud reflectivity and results in clouds that reflect shortwave radiation (SW) in reasonable agreement with observations, but gives clouds that are too bright and too few. The temperature at which supercooled liquid can remain unfrozen is strongly anti-correlated with cloud fraction in the climate mean state across the model ensemble, but we know of no robust physical mechanism to explain this behavior, especially because this anti-correlation extends through the subtropics. A set of perturbed physics simulations with the Community Atmospheric Model Version 4 (CAM4) shows that, if its temperature-dependent phase partitioning is varied and the critical relative humidity for cloud formation in each model run is also tuned to bring reflected SW into agreement with observations, then cloud fraction increases and liquid water path (LWP) decreases with T5050, as in the CMIP5 ensemble.},
doi = {10.1002/2015MS000589},
journal = {Journal of Advances in Modeling Earth Systems},
number = 2,
volume = 8,
place = {United States},
year = {2016},
month = {5}
}