Observational Comparison of Rural and Urban Boundary Layer Turbulence

Abstract

A unique set of data was collected during a large-scale field study of diffusion about an urban complex at Fort Wayne, Ind. Observations of turbulent wind fluctuations were obtained on two towers, one in a typical rural setting, the other in the heart of the downtown area of this medium size city. These turbulence data allow us to compare the state of the atmospheric boundary layer (to a height of 60 m) in its normal rural condition with its state after modification by the city. Predictions of the effect of the city have been verified and quantized. The increased roughness of the city had a drag effect that changed the shape of the wind speed profile. The city ?heat island? decreased the stability of atmosphere most noticeably in the lowest layers and much less with increasing height. This effect was more noticeable when the rural atmosphere was slightly stable. Turbulence was more intense in the rougher and less stable urban environment, but the ratios of normalized intensifies in dynamic similarity coordinates were quite comparable, indicating the universality of the similarity methods. The shapes of the spectral curves were different in the urban and rural regions and peaks in the plots were shifted toward higher frequencies in the urban atmosphere. Secondary high-frequency peaks were found only in high-wind-speed cakes; lower wind speeds resulted merely in a flattening and shifting of the spectra. The most interesting observation was the persistence of the turbulence, in both intensity and wavelength. While the city caused the turbulence intensity to increase at low levels, and shifted the wavelength where the maximum intensity was found, the order between trials established in the rural area was maintained in the city. Therefore, the prediction of turbulence intensity and related parameters in the city from wind observations in nearby rural areas appears feasible.