Wind tunnel experiments were conducted to understand flow and dispersion characteristics of tall building clusters surrounded by different surface roughness using simultaneous 3D laser Doppler anemometry and fast flame ionisation detector measurements for velocity and pollutant concentration measurements, respectively. Two different surface roughnesses, Suburban roughness elements (height equal to 20 mm), and Urban roughness (height of 70 mm) were considered to mimic two different urban canopy depths. Wake velocity measurements show a higher streamwise and wall-normal velocity component for the Urban case due to enhanced channelling effects

between buildings. The wake recovery downstream of the cluster is influenced by the vertical as well as the lateral shear layer it generates. When the cluster is surrounded by the Urban blocks, a strong upwash is observed, which brings near-wall low-momentum fluid upward, leading to the delay in the wake recovery in the near-wake regime compared to the Suburban roughness case. This phenomenon contributes to stretching the near-, transition and far-wake regions of the tall building clusters defined by Mishra et al. (2023). The strong vertical motion significantly influences the pollutant dispersion characteristics, with the cluster wake immersed in the deeper canopy witnessing a higher vertical spread of the plume than the Suburban case.