Rising from the ground: Distributed drag parameterization of urban environments for numerical weather prediction
File(s)
Author(s)
Sutzl, Birgit S.
Type
Thesis
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.
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.
Version
Open Access
Date Issued
2021-01
Date Awarded
2021-05
Copyright Statement
Creative Commons Attribution Licence
License URL
Advisor
van Reeuwijk, Maarten
Cotter, Colin
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/L016613/1
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)