Publication:
Assessing the mechanisms governing the daytime evolution of marine stratocumulus using large‐eddy simulation

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Authors
McMichael, Lucas A.
Mechem, David B.
Wang, Shouping
Wang, Qing
Kogan, Yefim L.
Teixeira, João
Subjects
afternoon
large-eddy simulation
marine
mixed-layer model
stratocumulus
Advisors
Date of Issue
2019-02
Date
Publisher
Royal Meteorological Society
Language
Abstract
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 (Sl) and total water mixing ratio (qT). 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/m2). Error analysis in comparison with LES Sl and qT budgets suggests that our method of entrainment flux calculation could be improved by a more complete understanding of entrainment‐layer physics.
Type
Article
Description
The article of record as published may be found at http://dx.doi.org/10.1002/qj.3469
Series/Report No
Department
Meteorology
Organization
Naval Postgraduate School (U.S.)
Identifiers
NPS Report Number
Sponsors
California Institute of Technology
Funder
U.S. Department of Energy Atmospheric Systems Research, DE-SC0016522
Office of Naval Research (ONR) Departmental Research Initiative (DRI), N00014-11-1-0518
N00014-11-1-0439
N00014-16-1-2487
N0001411IP20087
N0001411IP20069
Format
22 p.
Citation
McMichael, Lucas A., et al. "Assessing the mechanisms governing the daytime evolution of marine stratocumulus using large‐eddy simulation." Quarterly Journal of the Royal Meteorological Society 145.719 (2019): 845-866.
Distribution Statement
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.
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