A field study into the role of large gusts and coherent turbulent structures in momentum, heat and particle transport in an urban street canyon

Ultrasonic anemometers have been deployed to measure turbulence in a downtown street canyon with busy traffic in Manchester, UK. Data has been recorded at a height of 5 m at the canyon mid-length. The data was recorded as part of an eddy correlation system, providing synchronised measurements of fine aerosol number concentration. In this study conditional sampling and quadrant analysis are applied to identify the contribution of large gusts and coherent turbulent structures (ejections and sweeps) to the transport of momentum, sensible heat and particles within the canyon. It was found that larger than average gusts were infrequent, but transported more than half the flux of momentum, heat and particles. Sweeps and ejections were found to dominate heat and particle fluxes. An approach flow greater than 40 ° to the canyon sometimes led to a recirculating flow within the canyon, during which large sweeps were found to transport particles downwards towards the canyon floor.