Abstract
Abstract. Conventional footprint models cannot account for the heterogeneity of the urban landscape imposing a pronounced uncertainty on the spatial interpretation of eddy-covariance (EC) flux measurements in urban studies. This work introduces a computational methodology that enables the generation of detailed footprints in arbitrarily complex urban flux measurements sites. The methodology is based on conducting high-resolution large-eddy simulation (LES) and Lagrangian stochastic (LS) particle analysis on a model that features a detailed topographic description of a real urban environment. The approach utilizes an arbitrarily sized target volume set around the sensor in the LES domain, to collect a dataset of LS particles which are seeded from the potential source area of the measurement and captured at the sensor site. The urban footprint is generated from this dataset through a piecewise postprocessing procedure, which divides the footprint evaluation into multiple independent processes that each yield an intermediate result. These results are ultimately selectively combined to produce the final footprint. The strategy reduces the computational cost of the LES–LS simulation and incorporates techniques to account for the complications that arise when the EC sensor is mounted on a building instead of a conventional flux tower. The presented computational framework also introduces a result assessment strategy which utilizes the obtained urban footprint together with a detailed land cover type dataset to estimate the potential error that may arise if analytically derived footprint models were employed instead. The methodology is demonstrated with a case study that concentrates on generating the footprint for a building-mounted EC measurement station in downtown Helsinki, Finland, under the neutrally stratified atmospheric boundary layer.
Highlights
Micrometeorological measurements in densely built city environments pose an antipodal problem: they are essential in establishing the fundamental basis for the study of urban microclimates, but these measurements are endowed with pronounced uncertainties, which mainly originate from the topographic and elemental complexity of the urban landscape
In situations where the heterogeneity of the surface becomes relevant, like for urban landscapes, and the structures surrounding the measurement site can no longer be considered as a homogeneous layer of roughness elements, the use of analytical footprint models becomes highly suspect
This methodology is based on high-resolution large-eddy simulation (LES)–Lagrangian stochastic (LS) analysis where the simulation domain features a detailed description of the urban topography and accounts for the entire vertical extent of the atmospheric boundary layer
Summary
Micrometeorological measurements in densely built city environments pose an antipodal problem: they are essential in establishing the fundamental basis for the study of urban microclimates, but these measurements are endowed with pronounced uncertainties, which mainly originate from the topographic and elemental complexity of the urban landscape. The presented contribution places special emphasis on the issue of composing footprints for flux measurement sites that are surrounded by arbitrarily heterogeneous topography and may be compromised by the fact that they are mounted on top of actual buildings instead of conventional radio-mast-like towers Such a complex urban setting requires a new mechanism for constructing footprints, which is accompanied by a requirement that the associated LES–LS simulation is capable of resolving the relevant turbulent structures ranging from the street-canyonscale phenomena within the roughness sublayer to the larger ABL structures, while accounting for the interaction between them (Anderson, 2016)
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