Observations of Tall-Building Wakes Using a Scanning Doppler Lidar

Abstract. High-rise buildings, increasingly a feature of many large cities, impact local atmospheric flow conditions. Tall building wakes affect air quality downstream due to turbulent mixing and require parametrization in dispersion models. Previous studies using numerical or physical modelling have been idealised and under neutral conditions. There has been a lack of data available in real urban environments due to the difficulty in deploying traditional wind sensors. Doppler wind lidars (DWLs) have been used frequently for studying wind turbine wakes but never building wakes. This study is a year-long deployment of a DWL in a complex urban environment studying tall building wakes under atmospheric conditions. A HALO Photonic Streamline DWL was deployed in a low- and mid-rise densely packed area in central London. From its roof-top position (33.5 m agl compared to mean building height 12.5 m), Velocity Azimuth Display (VAD) scans at zero-degree elevation intersected with two, taller nearby buildings of 90 and 40 m agl. Using an ensemble averaging approach, wake dimensions were investigated in terms of wind direction, stability and wind speed. Boundary-layer stability categories were defined using eddy covariance observations from the BT Tower (191 m) and mixing height estimations from vertical stare scans. A method for calculating normalised velocity deficit from VAD scans is presented. For neutral conditions, wake dimensions around both buildings for the prevailing wind direction were compared with the ADMS-Build wake model for a single, isolated cube. The model underpredicts wakes dimensions, confirming previous wind tunnel findings for the same area. Under varying stability, unstable and deep boundary layers were shown to produce shorter, narrower wakes. Typical observed wake lengths were 120–300 m and widths were 80–150 m and were reduced by 50–100 m downwind. Stable and shallow boundary layers were less frequent and produced an insignificant difference in wake dimensions to neutral conditions. The sensitivity to stability was weakened by enhanced turbulence upstream (i.e., due to other building wakes). Weakened stability dependence was confirmed if there were more obstacles upstream as the wind direction incident on the buildings changed. The results highlight the potential for future wake studies using multiple DWLs deploying both vertical and horizontal scan patterns. Dispersion models should incorporate the effect of a complex urban canopy within which tall buildings are embedded.