Publication Date
4-1-2020
Document Type
Article
Department
Meteorology and Climate Science
Publication Title
Journal of the Atmospheric Sciences
Volume
77
Issue
4
DOI
10.1175/JAS-D-19-0301.1
First Page
1313
Last Page
1328
Abstract
This is the Part II of a two-part study that seeks a theoretical understanding of an empirical relationship for shallow cumulus clouds: subcloud updraft velocity covaries linearly with the cloud-base height. This work focuses on continental cumulus clouds that are more strongly forced by surface fluxes and more deviated from equilibrium than those over oceans (Part I). We use a simple analytical model for shallow cumulus that is well tested against a high-resolution (25 m in the horizontal) large-eddy simulation model. Consistent with a conventional idea, we find that surface Bowen ratio is the key variable that regulates the covariability of both parameters: under the same solar insolation, a drier surface allows for stronger buoyancy flux, triggering stronger convection that deepens the subcloud layer. We find that the slope of the Bowen-ratio-regulated relationship between the two parameters (defined as l) is dependent on both the local time and the stability of the lower free atmosphere. The value of l decreases with time exponentially from sunrise to early afternoon and linearly from early afternoon to sunset. The value of l is larger in a more stable atmosphere. In addition, continental l in the early afternoon more than doubles the oceanic l. Validation of the theoretical results against ground observations over the Southern Great Plains shows a reasonable agreement. Physical mechanisms underlying the findings are explained from the perspective of different time scales at which updrafts and cloud-base height respond to a surface flux forcing.
Funding Number
DE-SC0018996
Funding Sponsor
U.S. Department of Energy
Recommended Citation
Youtong Zheng, Mirjana Sakradzija, Seoung Soo Lee, and Zhanqing Li. "Theoretical understanding of the linear relationship between convective updrafts and cloud-base height for shallow cumulus clouds. Part II: Continental conditions" Journal of the Atmospheric Sciences (2020): 1313-1328. https://doi.org/10.1175/JAS-D-19-0301.1
Comments
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