Abstract
Knowledge of the vertical wind-speed profile in cities is important for the construction and insurance industries, wind energy predictions, and simulations of pollutant and toxic gas release. Here, five methods to estimate the spatially- and temporally-averaged wind-speed profile are compared in London: the logarithmic wind law (LOG); the Deaves and Harris equilibrium (DHe) and non-equilibrium (DHv) models; an adaptation of the power law (PL) and the Gryning et al. (GR) profile. Using measurements at 2.5 times the average building height, a source area model is used to determine aerodynamic roughness parameters using two morphometric methods, which assume homogeneous and variable roughness-element heights, respectively. Hourly-averaged wind speeds are extrapolated to 200 m above the canopy during strong wind conditions, and compared to wind speeds observed with Doppler lidar. Wind speeds are consistently underestimated if roughness-element height variability is not considered during aerodynamic parameter determination. Considering height variability, the resulting estimations with the DHe and GR profiles are marginally more similar to observations than the DHv profile, which is more accurate than the LOG and PL methods. An exception is in directions with more homogeneous fetch and a gradual reduction in upwind roughness, where the LOG and PL profiles are more appropriate.
Highlights
Modelling the wind-speed profile in the lowest few hundred metres of the urban boundary layer (UBL) is becoming increasingly important
In London, wind speeds observed with Doppler lidar have been used to assess how accurately wind speeds can be: translated from a ‘rural’ airport site to central London (Drew et al, 2013); and, estimated using the logarithmic wind law extrapolated from observations at approximately 2.5 times the canopy height, using a range of methods to determine zd and z0 (Kent et al, 2017a)
Comparison of the assessed wind-speed profile methods during similar conditions demonstrates their operation in the lower boundary layer
Summary
Modelling the wind-speed profile in the lowest few hundred metres of the urban boundary layer (UBL) is becoming increasingly important. The urban canopy layer (UCL) is where surface roughness elements such as buildings are located (Oke, 2007) and is associated with highly variable flow. Between the RSL and approximately 10% of the boundary layer depth is the inertial sublayer (ISL), where the flow becomes free of the wakes associated with individual roughness elements. If the airflow is fully adapted to upwind roughness elements in the ISL, a horizontally homogeneous flow is observed (Barlow, 2014) and it is possible to determine a spatially- and temporally-averaged wind-speed profile. Analysis is undertaken for neutral conditions, to allow the accuracy of extrapolated profiles during ‘ideal’ conditions to be understood first, without the additional uncertainties associated with thermal effects (e.g. Ho€gstro€m, 1996)
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