Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land
<p>The mass flux of air lifted within the updraughts (updraft in American English) of shallow convection is usually thought to be compensated outside the cloud through either large-scale subsidence or stronger downdraughts in a thin shell surrounding the cloud. Subsiding shells were postulated...
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doaj-caf399e4351a45fa8a640ecb00e935932020-11-24T21:21:38ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242019-08-01199769978610.5194/acp-19-9769-2019Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over landC. Mallaun0A. Giez1G. J. Mayr2M. W. Rotach3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Flight Experiments, Oberpfaffenhofen, GermanyDeutsches Zentrum für Luft- und Raumfahrt (DLR), Flight Experiments, Oberpfaffenhofen, GermanyUniversity of Innsbruck, Institute of Atmospheric and Cryospheric Sciences, Innsbruck, AustriaUniversity of Innsbruck, Institute of Atmospheric and Cryospheric Sciences, Innsbruck, Austria<p>The mass flux of air lifted within the updraughts (updraft in American English) of shallow convection is usually thought to be compensated outside the cloud through either large-scale subsidence or stronger downdraughts in a thin shell surrounding the cloud. Subsiding shells were postulated based on large eddy simulation and are experimentally tested in this study for shallow convection over land. Isolated cumulus clouds were probed with a small research aircraft over flat land and mountainous terrain, in different wind situations and at different levels of the clouds. The average of the 191 cloud transects shows the subsiding shell as a narrow downdraught region outside the cloud boundaries. The ensemble-mean subsiding shell is narrower on the upwind side of the cloud, while it is at least half a cloud diameter wide and more humid on the downwind side. At least half of the upward mass transport in the cloud is compensated within a distance of <span class="inline-formula">20</span> % of the cloud diameter. However, this shell is not uniform. Distinct regions of downdraughts and updraughts with high variability in the vertical wind are frequent and randomly distributed in the vicinity and also within the cloud. The median diameter of the draughts directly at the cloud boundary is at least 4 times as large as inside the clouds and in the environment. Downdraughts at the cloud boundary are twice as frequent as updraughts. In contrast to the updraughts the major part of the downdraughts is situated outside of the cloud. The subsiding shell results from the distribution of these up- and downdraughts.</p>https://www.atmos-chem-phys.net/19/9769/2019/acp-19-9769-2019.pdf |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
C. Mallaun A. Giez G. J. Mayr M. W. Rotach |
spellingShingle |
C. Mallaun A. Giez G. J. Mayr M. W. Rotach Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land Atmospheric Chemistry and Physics |
author_facet |
C. Mallaun A. Giez G. J. Mayr M. W. Rotach |
author_sort |
C. Mallaun |
title |
Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
title_short |
Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
title_full |
Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
title_fullStr |
Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
title_full_unstemmed |
Subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
title_sort |
subsiding shells and the distribution of up- and downdraughts in warm cumulus clouds over land |
publisher |
Copernicus Publications |
series |
Atmospheric Chemistry and Physics |
issn |
1680-7316 1680-7324 |
publishDate |
2019-08-01 |
description |
<p>The mass flux of air lifted within the updraughts (updraft in American English) of shallow
convection is usually thought to be compensated outside the cloud through
either large-scale subsidence or stronger downdraughts in a thin shell
surrounding the cloud. Subsiding shells were postulated based on large eddy
simulation and are experimentally tested in this study for shallow convection
over land. Isolated cumulus clouds were probed with a small research aircraft
over flat land and mountainous terrain, in different wind situations and at
different levels of the clouds. The average of the 191 cloud transects shows
the subsiding shell as a narrow downdraught region outside the cloud
boundaries. The ensemble-mean subsiding shell is narrower on the upwind side
of the cloud, while it is at least half a cloud diameter wide and more humid
on the downwind side. At least half of the upward mass transport in the cloud
is compensated within a distance of <span class="inline-formula">20</span> % of the cloud diameter.
However, this shell is not uniform. Distinct regions of downdraughts and
updraughts with high variability in the vertical wind are frequent and randomly
distributed in the vicinity and also within the cloud. The median diameter of
the draughts directly at the cloud boundary is at least 4 times as large as
inside the clouds and in the environment. Downdraughts at the cloud boundary
are twice as frequent as updraughts. In contrast to the updraughts the major part
of the downdraughts is situated outside of the cloud. The subsiding shell
results from the distribution of these up- and downdraughts.</p> |
url |
https://www.atmos-chem-phys.net/19/9769/2019/acp-19-9769-2019.pdf |
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