Universität zu Köln

Kostelecky, Jonathan Andrej ORCID: 0000-0002-9983-9310 (2025). Rough Surface Conditions for the Turbulent Atmospheric Boundary Layer. PhD thesis, Universität zu Köln.

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Abstract

The thesis investigates the effect of surface roughness on the atmospheric boundary layer (ABL) - the lower portion of the atmosphere where humans live. The Earth's surface is rough at multiple scales (e.g., ice, mud, plant cover). Surface roughness is a ubiquitous feature of the ABL, which modifies the mixing and transport properties of the flow and enhances the drag compared to aerodynamically smooth surfaces. In the ABL, dynamical processes are governed by atmospheric turbulence, the critical agent in surface-atmosphere coupling. At the same time, the state of the ABL is primarily influenced by radiative processes. In the absence of solar irradiation, the surface cools, and the stably stratified boundary layer (SBL) forms, characterized by reduced turbulence intensity. In large-scale climate or numerical weather prediction (NWP) models, unresolved processes, such as turbulence or the effect of surface roughness, are parameterized. In particular, the very stable regime remains poorly understood and challenges these models with drastic implications for weather forecasting. Here, process-level insight is gained through direct numerical simulation (DNS) of turbulent Ekman flow subjected to small-scale surface roughness (ratio of roughness height to boundary layer depth scale is O(1%)). In DNS, turbulent motions and three-dimensional roughness elements are explicitly, fully resolved. Performing DNS at scales relevant to geophysical problems requires a highly optimized numerical framework. This is prepared as part of the thesis by implementing and validating an immersed boundary method (IBM) along with a pressure treatment to avoid artificial oscillations. In Study I, the effect of surface roughness under neutral conditions on the bulk properties of the flow is investigated. The bottom of the domain is covered with 56x56 homogeneously distributed cuboids of varying mean height. The cases range from the aerodynamically smooth to the verge of the fully rough regime. The total drag increases with roughness height, along with the friction of velocity and scalar, and is measured using an integration approach of the budget equations. The enhanced turbulence intensity results in a deeper logarithmic layer as the friction Reynolds number significantly increases, and an accurate collapse of data onto the rough wall scaling is observed. Further, a pronounced veering of the wind with height from the ground is observed, which considerably outweighs the typical decrease in veering angle with increasing Reynolds number under smooth surface conditions. In Study II, a rough case located at the verge of the fully rough regime is subjected to an incrementally increasing strength of stable density stratification. Roughness efficiently counteracts buoyancy-induced suppression of turbulence. It extends the stability regime, where turbulence is in a continuous state, by inducing flow instabilities and producing detached eddies from sharp edges of roughness elements. Despite maintaining turbulence at much larger stability, global intermittency is observed once stratification becomes strong enough. With increasing stability, an over-veering of the wind appears, with a strong veering already within the roughness. Veering of the wind in the SBL challenges classical atmospheric surface layer (ASL) theory. However, it holds within the known limits and agrees with semi-empirical fits from field observations. The results demonstrate that-based on modern HPC-an extension of the well-established modelling framework for DNS to rough configurations is possible and allows unveiling dynamics in the rough surface layer based on first principles.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Kostelecky, Jonathan Andrej jkostele@uni-koeln.de orcid.org/0000-0002-9983-9310 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-753782
Date:	2025
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 Earth sciences
Uncontrolled Keywords:	Keywords Language Direct Numerical Simulation English Surface roughness English Surface heterogeneity English Turbulence English Meteorology English Monin–Obukhov Similarity theory English Stratified flows English Wall-bounded flow English Surface layer similarity English
Date of oral exam:	19 February 2025
Referee:	Name Academic Title Ansorge, Cedrick Dr. Vercauteren, Nikki Prof.in Dr.in
Projects:	ERC 2019 starting grant: "Turbulence resolving approaches to the intermittent atmospheric boundary layer" (851374)
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75378