Comparison of atmospheric turbulence characteristics and turbulent fluxes from two urban sites in Essen, Germany

From September 2006 to September 2007, the intersite variability of turbulence characteristics and turbulent heat fluxes was analysed at two urban stations in Essen, Germany. One site was situated within an urban residential setting while the other was located at the border of an urban park and suburban/urban residential housing. Therefore, the surroundings at both sites contributing to surface–atmosphere exchange differed in terms of surface cover and surface morphology. During the 1-year measurement period, 19% of data were characterised by stable atmospheric stratification. Since observations of urban turbulence characteristics under stable stratification are scarce, so far, this work adds additional input to this discussion. Turbulence characteristics, i.e. normalised standard deviations of wind components, were in agreement to empirical fits from other urban observations under both instable and stable atmospheric stratification. However, differences in magnitude of turbulence characteristics between sites were observable. Comparison of turbulent heat fluxes indicated typical urban features in the site located in the urban setting with increased surface heating and higher surface heat fluxes by about 30%. Also the temporal evolution of heat fluxes on the diurnal course was affected. Differences in momentum flux were of minor magnitude with about 6% variation on average between sites. Findings indicate that multiple urban flux measurements within one city may be characterised by general similarities in terms of turbulent characteristics but are still significantly influenced by differences in the surface cover of the flux footprint.