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Assessing the mechanisms governing the daytime evolution of marine stratocumulus using large‐eddy simulation
Author(s) -
McMichael Lucas A.,
Mechem David B.,
Wang Shouping,
Wang Qing,
Kogan Yefim L.,
Teixeira João
Publication year - 2019
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.3469
Subject(s) - liquid water path , environmental science , atmospheric sciences , cloud base , liquid water content , entrainment (biomusicology) , marine stratocumulus , radiative cooling , daytime , cloud top , large eddy simulation , planetary boundary layer , mixed layer , radiative transfer , cloud forcing , meteorology , radiative forcing , boundary layer , climatology , geology , mechanics , cloud computing , physics , turbulence , precipitation , aerosol , computer science , rhythm , acoustics , operating system , quantum mechanics
Large‐eddy simulation (LES) output for a case of thin stratocumulus off the coast of California is examined in a mixed‐layer analysis framework to identify the specific mechanisms responsible for governing the evolution of the cloud system. An equation for cloud‐base height tendency isolates the individual cloud‐modulating mechanisms that control the evolution of boundary‐layer liquid‐water static energy ( S l ) and total water mixing ratio ( q T ). With a suitable spin‐up procedure, the control simulation performs admirably compared with observed estimates of liquid water content, vertical velocity variance, and radiative fluxes sampled during an aircraft field campaign. Investigation of the cloud response to various environmental forcing scenarios was addressed through a suite of sensitivity simulations, including variations in subsidence velocity, surface fluxes, wind shear near the inversion, and radiative forcing. In the control simulation, rising cloud‐base tendencies are associated with entrainment warming/drying and short‐wave absorption, whereas lowering cloud‐base tendencies are driven by long‐wave cooling. Even in the presence of substantial afternoon solar heating, entrainment fluxes remained active. The thin cloud demonstrated unexpected resiliency, with mixed‐layer analysis indicating that, as the short‐wave flux decreases later in the afternoon, the relative contribution of long‐wave cooling often becomes large enough to offset entrainment warming/drying and result in a reversal of cloud‐base tendency. The evolution of cloud‐base tendency is found to be insensitive to the net radiative flux divergence for most of the simulations (liquid water path ranging from ~10–50 g/m 2 ). Error analysis in comparison with LES S l and q T budgets suggests that our method of entrainment flux calculation could be improved by a more complete understanding of entrainment‐layer physics.