TY - JOUR
T1 - Nonprecipitating Shallow Cumulus Convection Is Intrinsically Unstable to Length Scale Growth
AU - Janssens, Martin
AU - Vilà-Guerau De Arellano, Jordi
AU - Van Heerwaarden, Chiel C.
AU - De Roode, Stephan R.
AU - Siebesma, A. Pier
AU - Glassmeier, Franziska
PY - 2023
Y1 - 2023
N2 - 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.
AB - 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.
KW - Convection
KW - Cumulus clouds
KW - Instability
UR - http://www.scopus.com/inward/record.url?scp=85147094432&partnerID=8YFLogxK
U2 - 10.1175/JAS-D-22-0111.1
DO - 10.1175/JAS-D-22-0111.1
M3 - Article
AN - SCOPUS:85147094432
SN - 0022-4928
VL - 80
SP - 849
EP - 870
JO - Journal of the Atmospheric Sciences
JF - Journal of the Atmospheric Sciences
IS - 3
ER -