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

Journal Article · · Journal of the Atmospheric Sciences
 [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)
+ Show Author Affiliations

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.

Research Organization:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Organization:
National Science Foundation (NSF); USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23). Atmospheric System Research
Grant/Contract Number:
AC02-06CH11357
OSTI ID:
1459946
Journal Information:
Journal of the Atmospheric Sciences, Vol. 75, Issue 5; ISSN 0022-4928
Publisher:
American Meteorological SocietyCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 37 works
Citation information provided by
Web of Science

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Cited By (8)

The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2 journal January 2018
Influence of deepening and mesoscale organization of shallow convection on stratiform cloudiness in the downstream trades
  • Vogel, Raphaela; Nuijens, Louise; Stevens, Bjorn
  • Quarterly Journal of the Royal Meteorological Society, Vol. 146, Issue 726 https://doi.org/10.1002/qj.3664
journal November 2019
The efficacy of aerosol–cloud radiative perturbations from near-surface emissions in deep open-cell stratocumuli journal January 2018
Subgrid variations of the cloud water and droplet number concentration over the tropical ocean: satellite observations and implications for warm rain simulations in climate models journal January 2019
The Tropical Atlantic Observing System journal May 2019
Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic journal January 2020
The Interaction Between Boundary Layer and Convection Schemes in a WRF Simulation of Post Cold Frontal Clouds Over the ARM East North Atlantic Site journal April 2019
Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic journal July 2019

Figures / Tables (13)

FIG. 1(p. 47)figure FIG. 1
FIG. 2(p. 48)figure FIG. 2
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FIG. 11(p. 57)figure FIG. 11
FIG. 12(p. 58)figure FIG. 12
FIG. 13(p. 59)figure FIG. 13

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54 ENVIRONMENTAL SCIENCES
clouds
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