Nonprecipitating Shallow Cumulus Convection Is Intrinsically Unstable to Length Scale Growth

Condensation in cumulus clouds plays a key role in structuring the mean, nonprecipitating trade wind boundary layer. Here, we summarize how this role also explains the spontaneous growth of mesoscale [> O (10) km] fluctuations in clouds and moisture around the mean state in a minimal-physics, lar...

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Published inJournal of the atmospheric sciences Vol. 80; no. 3; pp. 849 - 870
Main Authors Janssens, Martin, de Arellano, Jordi Vilà-Guerau, van Heerwaarden, Chiel C., de Roode, Stephan R., Siebesma, A. Pier, Glassmeier, Franziska
Format Journal Article
LanguageEnglish
Published Boston American Meteorological Society 01.03.2023
Subjects
Anomalies
Boundary layers
Clouds
Condensation
Convection
Cumulus clouds
Fluctuations
Growth
Large eddy simulation
Large eddy simulations
Mesoscale phenomena
Moisture
Moisture effects
Oceanic eddies
Physics
Positive feedback
Precipitation
Radiation
Simulation
Trade winds
Vertical profiles
Vertical velocities
Wind
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Summary:Condensation in cumulus clouds plays a key role in structuring the mean, nonprecipitating trade wind boundary layer. Here, we summarize how this role also explains the spontaneous growth of mesoscale [> O (10) km] fluctuations in clouds and moisture around the mean state in a minimal-physics, large-eddy simulation of the undisturbed period during BOMEX on a large [ O (100) km] domain. Small, spatial anomalies in condensation in cumulus clouds, which form on top of small moisture fluctuations, power circulations that transport moisture, but not heat, from dry to moist regions, and thus reinforce the condensation anomaly. We frame this positive feedback as a linear instability in mesoscale moisture fluctuations, whose time scale depends only on (i) a vertical velocity scale and (ii) the mean environment’s vertical structure. In our minimal-physics setting, we show both ingredients are provided by the shallow cumulus convection itself: it is intrinsically unstable to length scale growth. The upshot is that energy released by clouds at kilometer scales may play a more profound and direct role in shaping the mesoscale trade wind environment than is generally appreciated, motivating further research into the mechanism’s relevance.
ISSN:0022-4928
1520-0469
DOI:10.1175/JAS-D-22-0111.1