Open AccessWhat determines altocumulus dissipation time?
Author(s) -
Larson Vincent E.,
Smith Adam J.,
Falk Michael J.,
Kotenberg Kurt E.,
Golaz JeanChristophe
Publication year - 2006
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2005jd007002
Subject(s) - overcast , subsidence , cloud computing , environmental science , dissipation , turbulence , liquid water content , radiative transfer , sensitivity (control systems) , atmospheric sciences , cloud feedback , mechanics , meteorology , physics , thermodynamics , climate change , climate model , geology , computer science , climate sensitivity , optics , sky , operating system , paleontology , oceanography , engineering , structural basin , electronic engineering
This paper asks what factors influence the dissipation time of altocumulus clouds. The question is addressed using three‐dimensional, large‐eddy simulations of a thin, midlevel cloud that was observed by aircraft. The cloud might be aptly described as “altostratocumulus” because it was overcast and contained radiatively driven turbulence. The simulations are used to construct a budget equation of cloud water. This equation allows one to directly compare the four processes that diminish liquid: diffusional growth of ice crystals, large‐scale subsidence, radiative heating, and turbulent mixing of dry air into the cloud. Various sensitivity studies are used to find the “equivalent sensitivity” of cloud decay time to changes in various parameters. A change from no sunlight to direct overhead sunlight decreases the lifetime of our simulated cloud as much as increasing subsidence by 1.2 cm s −1 , increasing ice number concentration by 780 m −3 , or decreasing above‐cloud total water mixing ratio by 0.60 g kg −1 . Finally, interactions among the terms in the cloud water budget are summarized in a “budget term feedback matrix.” It is able to diagnose, for instance, that in our particular simulations, the diffusional growth of ice is a negative feedback.