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Earth Science Seminar

Turbulence Structure in a Stratocumulus-topped Boundary Layer from Very High-resolution Large-eddy Simulations
Presented by Georgios Matheou
University of Connecticut

Tuesday, January 15, 2019
10:00 A.M. in 233-305E

Abstract
A series of numerical experiments where both physical and numerical-model parameters are varied with respect to a reference setup is used to investigate the physics of a stratocumulus cloud and the performance of a large-eddy simulation (LES) model. Strong feedback between cloud liquid, cloud top radiative cooling, and turbulence leads to slow grid convergence of the turbulent fluxes. For a methodology that diagnoses cloud liquid from conserved variables, small errors in the total water amount result in large liquid-water errors, which are amplified by the cloud top radiative cooling leading to large variations of buoyancy forcing. In contrast, when the liquid–radiation–buoyancy feedback is not present in simulations without radiation, the turbulence structure of the boundary layer remains essentially identical for grid resolutions between 20 and 1.25 m. The entrainment rate does not depend on grid resolution but shows strong dependence on physical processes. Even though cloud top radiative cooling is regarded as a key attribute of stratocumulus, the present simulations show that surface fluxes, radiative cooling, and surface shear each contribute about equally to the total turbulent kinetic energy. The liquid water structure in the cloud is studied using a fine-scale simulation utilizing 1.25-m grid resolution and 5.12 km horizontal domain extent. The simulation captures the observed cloud morphology, including elongated regions of low liquid water path, cloud holes, and pockets of clear air within the cloud. The cloud can be partitioned into two broad layers with respect to the maximum mean liquid. The lower layer resembles a convective turbulent structure with classical inertial range scaling of the velocity and scalar energy spectra. The top and shallower layer is directly influenced by the cloud top radiative cooling and the entrainment process.

JPL Contact: Marcin Witek (4-0250)

About the Speaker
Georgios Matheou an Assistant Professor in the Department of Mechanical Engineering at the University of Connecticut. Before joining UConn he was a research scientist at Jet Propulsion Laboratory and a Visiting Associate in Aerospace at the California Institute of Technology. He received his Diploma in Mechanical Engineering (2002) from the National Technical University of Athens and Ph.D. in Aeronautics from Caltech (2008). Dr. Matheou's research interests include fluid dynamics and turbulence, modeling of multi-scale multi-physics flows, numerical methods, and high performance computing. Dr. Matheou received the American Physical Society's Milton Van Dyke Award in 2011 and Galley of Fluid Motion Award in 2016, and NASA's Early Career Public Achievement Metal in 2016.


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