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| Titel |
On the reversibility of transitions between closed and open cellular convection |
| VerfasserIn |
G. Feingold, I. Koren, T. Yamaguchi, J. Kazil |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 13 ; Nr. 15, no. 13 (2015-07-08), S.7351-7367 |
| Datensatznummer |
250119875
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| Publikation (Nr.) |
 copernicus.org/acp-15-7351-2015.pdf |
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| Zusammenfassung |
| The two-way transition between closed and open cellular
convection is addressed in an idealized cloud-resolving modeling framework.
A series of cloud-resolving simulations shows that the transition between
closed and open cellular states is asymmetrical and characterized by a rapid
("runaway") transition from the closed- to the open-cell state but slower
recovery to the closed-cell state. Given that precipitation initiates the
closed–open cell transition and that the recovery requires a suppression of
the precipitation, we apply an ad hoc time-varying drop concentration to
initiate and suppress precipitation. We show that the asymmetry in the
two-way transition occurs even for very rapid drop concentration
replenishment. The primary barrier to recovery is the loss in turbulence
kinetic energy (TKE) associated with the loss in cloud water (and associated
radiative cooling) and the vertical stratification of the boundary layer
during the open-cell period. In transitioning from the open to the closed
state, the system faces the task of replenishing cloud water fast enough to
counter precipitation losses, such that it can generate radiative cooling and
TKE. It is hampered by a stable layer below cloud base that has to be
overcome before water vapor can be transported more efficiently into the
cloud layer. Recovery to the closed-cell state is slower when radiative
cooling is inefficient such as in the presence of free tropospheric clouds or
after sunrise, when it is hampered by the absorption of shortwave radiation.
Tests suggest that recovery to the closed-cell state is faster when the
drizzle is smaller in amount and of shorter duration, i.e., when the
precipitation causes less boundary layer stratification. Cloud-resolving
model results on recovery rates are supported by simulations with a simple
predator–prey dynamical system analogue. It is suggested that the observed
closing of open cells by ship effluent likely occurs when aerosol intrusions
are large, when contact comes prior to the heaviest drizzle in the early
morning hours, and when the free troposphere is cloud free. |
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