Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas

Abstract

Nine methods to determine local-scale aerodynamic roughness length (z 0 ) and zero-plane displacement (z d ) are compared at three sites (within 60 m of each other) in London, UK. Methods include three anemometric (single-level high frequency observations), six morphometric (surface geometry) and one reference-based approach (look-up tables). A footprint model is used with the morphometric methods in an iterative procedure. The results are insensitive to the initial z d and z 0 estimates. Across the three sites, z d varies between 5 and 45 m depending upon the method used. Morphometric methods that incorporate roughness-element height variability agree better with anemometric methods, indicating z d is consistently greater than the local mean building height. Depending upon method and wind direction, z 0 varies between 0.1 and 5 m with morphometric z 0 consistently being 2-3 m larger than the anemometric z 0 . No morphometric method consistently resembles the anemometric methods. Wind-speed profiles observed with Doppler lidar provide additional data with which to assess the methods. Locally determined roughness parameters are used to extrapolate wind-speed profiles to a height roughly 200 m above the canopy. Wind-speed profiles extrapolated based on morphometric methods that account for roughness-element height variability are most similar to observations. The extent of the modelled source area for measurements varies by up to a factor of three, depending upon the morphometric method used to determine z d and z 0 .

Impact of atmospheric conditions and levels of urbanization on the relationship between nocturnal surface and urban canopy heat islands

“…The turbulent source area of T a cannot be defined confidently using a traditional approach (e.g. a footprint model: Kent et al ., ) because the measurement height (3 m AGL in this study) is within the urban canopy layer. Although the turbulence source area can be local, the selection of the sites considered their representativeness (as best as can be achieved in the heterogenous urban area) of microclimatic environments of the area on a neighbourhood scale (≲ 1 km), which is about the spatial resolution of the satellite data of T s .…”

Attribution and mitigation of heat wave-induced urban heat storage change

“…Note that for the vegetation cover scenario, the effects of surface roughness change are excluded in order to focus on the role of surface wetness in moderating rQ . The impacts of surface roughness (including vegetation) on wind profiles can be found in Kent et al (2017b).…”

Interactive comment on “CALIOPE-Urban v1.0: Coupling R-LINE with a mesoscale air quality modelling system for urban air quality forecasts over Barcelona city (Spain)” by Jaime Benavides et al.

“…In the research literature, the influence of atmospheric stability on vertical mixing within a street canyon has been demonstrated using experimental measurements (Rotach, 1995) and wind tunnel experiments (Salizzoni et al, 2009), and it has been implemented in some dispersion models (e.g. Soulhac et al, 2011;Kim et al, 2018). The ratio of wind speeds at surface and rooftop levels (ws sfc /ws bh ) estimated by R-LINE using similarity theory (Monin and Obukhov, 1954) is used as a proxy for the vertical mixing.…”

“…In order to overcome these limitations, coupling the regional and urban scales offline by downscaling the regional model using a dispersion kernel has been successfully applied in some cities Moussafir et al, 2014;Isakov et al, 2014;Jensen et al, 2017;Maiheu et al, 2017;Kim et al, 2018;Hood et al, 2018;Fagerli et al, 2019). For instance, Hood et al (2018) coupled a regional climate-chemistry model with 5 km horizontal resolution (EMEP4UK) with the fine-scale model ADMS-URBAN to simulate air quality over London in 2012.…”