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Title: Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations

A common feature of the stratocumulus-to-cumulus transition (SCT) is the presence of layers in which the concentration of particles larger than 0.1 μm is below 10 cm –3. These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF–NCAR Gulfstream V (G-V) aircraft between California and Hawaii. UCLs are commonly located in the upper part of decoupled boundary layers, with coverage increasing from less than 5% within 500 km of the California coast to ~30%–60% west of 130°W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 μm. Many UCL clouds are optically thin and do not fully attenuate the G-V lidar and yet are frequently detected with a 94-GHz radar with a sensitivity of around –30 dBZ. Satellite data indicate that UCL clouds have visible reflectances of ~0.1–0.2 and are often quasi laminar, giving them a veil-like appearance. These optically thin veil clouds exist for 1–3 h or more, are associated with mesoscale cumulus clusters, and likely grow by spreading under strong inversions. Active updrafts in cumulus (Cu) clouds have droplet concentrations of ~25–50 cm –3. Collision–coalescence in the Cu and later sedimentation inmore » the thinner UCL clouds are likely the key processes that remove droplets in UCL clouds. UCLs are relatively quiescent, and a lack of mixing with dry air above and below the cloud may help to explain their longevity. In conclusion, the very low and highly variable droplet concentrations in UCL clouds, together with their low geometrical and optical thickness, make these clouds particularly challenging to represent in large-scale models.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [2] ;  [3] ;  [3] ;  [4] ;  [5] ;  [5] ;  [6] ;  [7]
  1. Univ. of Washington, Seattle, WA (United States)
  2. Univ. of Miami, Miami, FL (United States)
  3. Argonne National Lab. (ANL), Lemont, IL (United States)
  4. Univ. of Wisconsin, Madison, WI (United States)
  5. Michigan Technological Univ., Houghton, MI (United States)
  6. Max Planck Institute for Chemistry, Mainz (Germany)
  7. NASA Langley Research Center, Hampton, VA (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Journal of the Atmospheric Sciences
Additional Journal Information:
Journal Volume: 75; Journal Issue: 5; Journal ID: ISSN 0022-4928
Publisher:
American Meteorological Society
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
National Science Foundation (NSF); USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23). Atmospheric System Research
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; clouds; aerosols; cloud cover; cloud droplets; cloud microphysics; cloud radiative effects
OSTI Identifier:
1459946

Wood, Robert, O, Kuan -Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce. A., Zuidema, Paquita, Ghate, Virendra, Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond A., Fugal, Jacob, and Minnis, Patrick. Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations. United States: N. p., Web. doi:10.1175/JAS-D-17-0213.1.
Wood, Robert, O, Kuan -Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce. A., Zuidema, Paquita, Ghate, Virendra, Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond A., Fugal, Jacob, & Minnis, Patrick. Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations. United States. doi:10.1175/JAS-D-17-0213.1.
Wood, Robert, O, Kuan -Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce. A., Zuidema, Paquita, Ghate, Virendra, Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond A., Fugal, Jacob, and Minnis, Patrick. 2018. "Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations". United States. doi:10.1175/JAS-D-17-0213.1.
@article{osti_1459946,
title = {Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations},
author = {Wood, Robert and O, Kuan -Ting and Bretherton, Christopher S. and Mohrmann, Johannes and Albrecht, Bruce. A. and Zuidema, Paquita and Ghate, Virendra and Schwartz, Chris and Eloranta, Ed and Glienke, Susanne and Shaw, Raymond A. and Fugal, Jacob and Minnis, Patrick},
abstractNote = {A common feature of the stratocumulus-to-cumulus transition (SCT) is the presence of layers in which the concentration of particles larger than 0.1 μm is below 10 cm–3. These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF–NCAR Gulfstream V (G-V) aircraft between California and Hawaii. UCLs are commonly located in the upper part of decoupled boundary layers, with coverage increasing from less than 5% within 500 km of the California coast to ~30%–60% west of 130°W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 μm. Many UCL clouds are optically thin and do not fully attenuate the G-V lidar and yet are frequently detected with a 94-GHz radar with a sensitivity of around –30 dBZ. Satellite data indicate that UCL clouds have visible reflectances of ~0.1–0.2 and are often quasi laminar, giving them a veil-like appearance. These optically thin veil clouds exist for 1–3 h or more, are associated with mesoscale cumulus clusters, and likely grow by spreading under strong inversions. Active updrafts in cumulus (Cu) clouds have droplet concentrations of ~25–50 cm–3. Collision–coalescence in the Cu and later sedimentation in the thinner UCL clouds are likely the key processes that remove droplets in UCL clouds. UCLs are relatively quiescent, and a lack of mixing with dry air above and below the cloud may help to explain their longevity. In conclusion, the very low and highly variable droplet concentrations in UCL clouds, together with their low geometrical and optical thickness, make these clouds particularly challenging to represent in large-scale models.},
doi = {10.1175/JAS-D-17-0213.1},
journal = {Journal of the Atmospheric Sciences},
number = 5,
volume = 75,
place = {United States},
year = {2018},
month = {5}
}