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Rising from the ground: Distributed drag parameterization of urban environments for numerical weather prediction

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Title: Rising from the ground: Distributed drag parameterization of urban environments for numerical weather prediction
Authors: Sutzl, Birgit S.
Item Type: Thesis or dissertation
Abstract: Urban environments in numerical weather prediction models are currently parameterised as part of the atmosphere-surface exchange at ground level. The vertical structure of buildings is represented by the average height, which does not account for heterogeneous building forms at the subgrid level. This thesis investigates aerodynamic effects of subgrid heterogeneity and develops a distributed drag parameterization that represents buildings at their real height. Urban flow at neighbourhood scale is studied using large-eddy simulations of idealised, heterogeneous urban morphologies with identical building plan area index and frontal area index. Large differences in estimated roughness parameters and total canopy drag, which are strongly correlated to the maximum height and height variability of the buildings, mean-wind profiles and vertical momentum transport suggest that subgrid heterogeneity is inadequately represented by current models. A height-dependent frontal area function is introduced to capture a vertical urban morphology profile with full height extent and variability. The morphology profiles correlate to the distinct distributed-drag profiles of the simulations, and a drag parameterization was derived by a third-order polynomial function of the morphology profiles. Morphology profiles were calculated for Greater London and parameterised by an exponential distribution with the ratio of maximum to mean building height as parameter. A case study with the high-resolution London Model and the new drag parameterization appears to capture more realistic features of the urban boundary layer compared to the standard parameterization. The simulation showed increased horizontal spatial variability in total surface stress, identifying a broad range of morphology features (densely built-up areas, high-rise building clusters, parks and the river). Vertical effects include heterogeneous wind profiles, extended building wakes, and internal boundary layers. This thesis demonstrates the potential of height-distributed urban parameterizations to improve urban weather forecasting, with further research into distribution of heat- and moisture-exchange necessary.
Content Version: Open Access
Issue Date: Jan-2021
Date Awarded: May-2021
URI: http://hdl.handle.net/10044/1/95185
DOI: https://doi.org/10.25560/95185
Copyright Statement: Creative Commons Attribution Licence
Supervisor: van Reeuwijk, Maarten
Cotter, Colin
Sponsor/Funder: Engineering and Physical Sciences Research Council
Funder's Grant Number: EP/L016613/1
Department: Civil and Environmental Engineering
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Civil and Environmental Engineering PhD theses



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