Turbulent Transport of Scalars Over a Complex Urban Terrain

The complexities in the turbulent transport of scalars during the growing season over an urban area are investigated here. Data from a 40-m tall flux tower located in the suburban Salt Lake Valley, Utah, USA is used in this analysis. The tower had instrumentation at five different levels and monitored the fluxes of heat, water vapor and carbon dioxide (CO2), in addition to other meteorological variables such as air temperature, pressure, relative humidity, wind speed and wind direction. The study found that the transport of scalars in the urban inertial sub-layer is very much influenced by non-local effects and conflicting mechanisms that resulted in multi-scale transport of fluxes at disparate scales. For example, during the daytime period when the site acted as a net sink of CO2, positive fluxes were observed at the low frequency end of the vertical velocity-CO2 cospectrum. This behavior is in stark contrast to the transport of momentum. Integral statistics of momentum reasonably conformed to surface layer similarity. The study also found that unlike forest and crop ecosystems, the urban CO2 (co)spectra (power spectra and cospectra) are characterized by multiple peaks and long plateaus. The analysis also found large-scale advective processes to influence the transport of CO2 during the stable periods.