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

Abstract

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 mu m is below 10 cm(-3). These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF-NCARGulfstreamV(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; similar to 30%-60% west of 1308W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 mm. 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; similar to 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; similar to 25-50 cm(-3). Collision-coalescence in the Cu and latermore » 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. 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:
; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
National Science Foundation (NSF); USDOE Office of Science - Office of Biological and Environmental Research - Atmospheric System Research
OSTI Identifier:
1459946
DOE Contract Number:
AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the Atmospheric Sciences; Journal Volume: 75
Country of Publication:
United States
Language:
English
Subject:
clouds, aerosols

Citation Formats

Wood, Robert, O, Kuang-Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce A., Zuidema, Paquita, Ghate, Virendra P., Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond, Frugal, Jacob, and Minnis, Patrick. Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations. United States: N. p., 2018. Web. doi:10.1175/JAS-D-17-0213.1.
Wood, Robert, O, Kuang-Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce A., Zuidema, Paquita, Ghate, Virendra P., Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond, Frugal, 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, Kuang-Ting, Bretherton, Christopher S., Mohrmann, Johannes, Albrecht, Bruce A., Zuidema, Paquita, Ghate, Virendra P., Schwartz, Chris, Eloranta, Ed, Glienke, Susanne, Shaw, Raymond, Frugal, Jacob, and Minnis, Patrick. Tue . "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, Kuang-Ting and Bretherton, Christopher S. and Mohrmann, Johannes and Albrecht, Bruce A. and Zuidema, Paquita and Ghate, Virendra P. and Schwartz, Chris and Eloranta, Ed and Glienke, Susanne and Shaw, Raymond and Frugal, 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 mu m is below 10 cm(-3). These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF-NCARGulfstreamV(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; similar to 30%-60% west of 1308W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 mm. 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; similar to 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; similar to 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. 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 = ,
volume = 75,
place = {United States},
year = {Tue May 01 00:00:00 EDT 2018},
month = {Tue May 01 00:00:00 EDT 2018}
}