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Turbulent transfer characteristics over a suburban surface Roth, Matthias


The main motive for studying turbulent flow in an urban environment is to understand the processes governing momentum, heat and mass exchange between the atmosphere and a very inhomogeneous and aerodynamically rough surface. This exchange regulates the microclimate wherein about 40% of the current world population lives. An understanding of its mechanisms is essential for a variety of reasons and applications. The structure of the atmosphere close to this irregular surface is not homogeneous and there is reason for concern that traditional micrometeorological theories are inadequate to describe the turbulent transfer in this environment. The main objective of the present study is to investigate the turbulent transfer mechanism and the applicability of the Monin Obukhov similarity framework in an unstable suburban atmosphere. In addition the first full set of energy balance data including longer term directly-measured sensible and latent heat fluxes is presented. The results suggest that the (co)spectra in respect to shape and location of the peaks are relatively insensitive to surface features. They generally agree well with homogeneous surface layer data with the exceptions of u, T, uw and possibly q which all exhibit slight anomalies which may be attributed to particular surface features. The non-dimensional dissipation functions and most of the integral statistics results follow the trends predicted by similarity theory (i.e. they are a function of stability), however, the magnitudes are often smaller. Analysis of the correlation coefficients shows that under near neutral and slightly unstable conditions the transfers of momentum and heat are most efficient (and enhanced compared to the homogeneous surface layer) whereas the transfer efficiency of moisture is generally least efficient. This results in a dissimilar behaviour of heat and moisture. It is shown that the humidity statistics not only depend on surface boundary conditions but are also influenced by the entire PBL. Observational support in this respect is obtained from a time series analysis of humidity signals which shows the sporadic occurrence of strong, dry downdrafts (under mainly cloudy conditions) which result in positive contributions to the moisture flux. There is evidence that the present observation levels are sometimes within the roughness sub-layer. At around noon and in the early afternoon the Bowen ratio measured using the gradient approach was often larger than the Bowen ratio obtained from directly measured fluxes. This affects the turbulent fluxes derived from the Bowen ratio-energy balance approach. It is suggested that beside the inequality of the transfer efficiencies sampling problems affect the gradient measurements. The average diurnal energy balance is in general agreement with previous summertime observations from the same site. The results indicate that the storage heat flux, obtained as the energy balance residual using directly measured turbulent fluxes, peaks slightly earlier than predicted by the objective hysteresis model.

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