Numerical investigations of turbulent flow through forests and urban canopies

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Bannister, Edward James ORCID: https://orcid.org/0000-0003-1410-6204 (2022). Numerical investigations of turbulent flow through forests and urban canopies. University of Birmingham. Ph.D.

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Abstract

Most of the terrestrial living world, humanity included, spends nearly all of its time in the atmospheric boundary layer (the ABL). This thesis addresses some of the limitations in our knowledge of turbulent exchange in the ABL. It is relevant to anyone wishing to understand the details of the weather, climate, and air quality around cities, forests, and crops. The work pays particular attention to the transport of scalar quantities—e.g., pollutants, CO\(_2\), or bioaerosols—in patchy, inhomogeneous landscapes, especially those comprising canopies of large obstacles, such as forests and cities. This thesis comprises three main research sections. The work is presented in order of ‘increasing realism’, with the studies becoming less idealised and more realistic and one moves through the thesis.

The first section investigates pollution in an idealised model of a patchy urban area. It uses conceptual arguments and large-eddy simulation to identify and diagnose two urban flow regimes based on the size of patches of different density. This work: (i) identifies possible locations of pollution hot spots; (ii) quantifies what is loosely referred to as the ‘urban background’ in an air-pollution context; and (iii) provides a conceptual basis for further research into neighbourhood-scale air-pollution problems and the transport of fluid constituents in other porous media.

The second section reviews, synthesises, and discusses current understanding of forest-atmosphere exchange around patchy real-world forests. The overarching goal of this broad chapter is to improve numerical models of forest-atmosphere exchange by making the model forests less idealised, and more like the real world. For example, the transport equations can be modified efficiently to account for patchy forests and moving plant elements. The chapter provides rules-of-thumb for forest areas in which edge effects dominate, a minimum size constituting a gap from an aerodynamic perspective, and recommendations for using computing resources effectively when faced with the dilemma of improving a model’s scale versus its resolution.

The third section investigates air-parcel residence times in a mature forest, calculated from observations at a free-air carbon dioxide enrichment facility. It shows that median daytime residence times in the tree crowns are twice as long when the trees are in leaf versus when they are not. Residence times increase with greater atmospheric stability, as does the variability around their central values. Robust parametrisations of air-parcel residence times can be obtained using gamma-like distributions, with the parameters estimated from widely measured flow variables. Large volumes of pooled air are sometimes sporadically and unpredictably vented during calm evenings. Under certain circumstances, simple models can be used to model the passage of passive scalar quantities on the mean flow.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
MacKenzie, Rob	UNSPECIFIED	orcid.org/0000-0002-8227-742X
Cai, Xiaoming	UNSPECIFIED	orcid.org/0000-0002-5934-9800
Jesson, Michael	UNSPECIFIED	orcid.org/0000-0002-6798-1371