Universität zu Köln

Rauterkus, Robert ORCID: 0000-0001-8288-3203 (2021). Large-Eddy Simulation of Arctic Stratocumulus: Process Representation and Surface Heterogeneity. PhD thesis, Universität zu Köln.

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Abstract

Small-scale processes are crucial for the evolution of Stratocumulus and act on scales reaching down to less than one meter. Most large-eddy simulation studies still apply a horizontal resolution of tens of meters, limiting the ability to resolve cloud-driving processes. I investigate such small-scale processes in a reference case that is—based on the recent field campaigns ACLOUD and PASCAL—defined within this thesis to represent a mixed-phase Stratocumulus during Arctic spring. I apply large-eddy simulations with horizontal resolutions of 35m, 10m, 3.5m, and 3m and a vertical resolution of about 3m. My analysis focuses on the resolution sensitivity of cloud-top entrainment processes and the effects of surface heterogeneity structure on the atmospheric boundary layer. First, I find that for a horizontal grid spacing larger than 10m, the effects of small-scale microphysical cooling and turbulent engulfment on cloud-top entrainment are only represented sufficiently for the atmospheric boundary layer bulk profiles but not on a process level. The stratification-limited size of energy-containing eddies violates the assumptions underlying many sub-grid scale models of turbulent mixing. Second, I observe a decrease in cloud-top entrainment for a horizontal resolution coarser than 10m, which results in 15% more cloud water after six hours of simulation and a corresponding optical thickening of the Stratocumulus. Third, I find that structuring surface heterogeneity does not affect zero- and first-order bulk quantities outside the surface layer. A notable sensibility in higher altitudes is only observed for higher-order quantities, which show increased values over structured surface heterogeneity. Fourth, I observe structured surface heterogeneity to form a streamwise elongated, roll-like, secondary circulation perpendicular to the mean wind. Its formation is neither captured by traditional Arctic lead theory nor by the theory of surface heterogeneity effects on cloud-free atmospheric boundary layers. It turns out that the streamwise elongated structure evolves due to streamwise "smudging" of the surface signals at the lower cloud boundary. This "smudging" is a consequence of weak vertical motion and cloud-induced turbulence—a unique feature compared to other studies investigating the effects of surface heterogeneity structure.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Rauterkus, Robert r.rauterkus@posteo.net orcid.org/0000-0001-8288-3203 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-539280
Date:	2021
Place of Publication:	Köln
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Arctic English Turbulence English Boundary layer English Clouds English Large eddy simulations English Microscale processes English
Date of oral exam:	14 January 2021
Referee:	Name Academic Title Shao, Yaping Prof. Dr. Löhnert, Ulrich Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/53928