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

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