Observations of tall-building wakes using a scanning Doppler lidar

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 parameterisation in dispersion models. Previous studies using numerical or physical modelling have been performed under idealised and 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 presents a year-long deployment of a DWL in a complex urban environment, studying tall-building wakes under atmospheric conditions. A HALO Photonics StreamLine DWL was deployed in a low- and mid-rise densely packed area in central London. From its rooftop position (33.5 m a.g.l. compared to mean building height of 12.5 m), velocity azimuth display (VAD) scans at 0° elevation intersected with two taller nearby buildings of 90 and 40 m a.g.l. 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 were made 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 wake 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. Typically observed wake lengths were 120–300 m, and widths were 80–150 m and reduced by 50–100 m downwind. Stable and shallow boundary layers were less frequent and produced an insignificant difference in wake dimensions compared 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.