TURBULENCE CHARACTERISTICS AND FLUXES OF HEAT AND MOMENTUM: INTER-SITE COMPARISON OF TWO URBAN SITES

From September 2006 to September 2007 the inter-site variability of turbulence characteristics and turbulent heat fluxes was analysed at two urban stations in Essen, Germany. One site was installed 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. The structure of turbulent flow turned out to be rather comparable at both sites, e.g. only 6 % difference on average in momentum flux between sites occured. Turbulence characteristics, i.e. normalised standard deviations of wind components, were in agreement to empirical fits from other urban observations. Small deviations in inter-site comparison were attributed to differences in roughness and surface cover around the sites. During the one-year measurement period 20 % of data was characterised by stable atmospheric stratification. Generally, observations of urban turbulence characteristics in stable stratification are scarce so far. 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