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Theoretical understanding of the linear relationship between convective updrafts and cloud-base height for shallow cumulus clouds. Part II: Continental conditions

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Sakradzija,  Mirjana
Precipitating Convection, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Zheng, Y., Sakradzija, M., Lee, S.-S., & Li, Z. (2020). 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, 77, 1313-1328. doi:10.1175/JAS-D-19-0301.1.


Cite as: https://hdl.handle.net/21.11116/0000-0006-0F78-4
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 λ) is dependent on both the local time and the stability of the lower free atmosphere. The value of λ decreases with time exponentially from sunrise to early afternoon and linearly from early afternoon to sunset. The value of λ is larger in a more stable atmosphere. In addition, continental λ in the early afternoon more than doubles the oceanic λ. 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.