In the constant flux layer over flat and homogeneous surfaces with low roughness (ideal terrain), the vertical profiles of turbulence statistics such as the drag coefficient and the turbulence intensities (TI) can be described for neutral conditions within the framework of a the logarithmic wind profile. There is considerable interest to assess the applicability of these functions over cities, where the structure of the atmosphere is modified by the high roughness of the surface. Urban observations show that these statistics normalized as a function of non-dimensional height (effective height to roughness length), are well organized and follow the theoretical predictions derived over ideal terrain. This is surprising for small non-dimensional heights and suggests equilibrium between the local flux of momentum and the gradient of the mean wind speed even close to the canopy. The normalized standard deviations of wind (NS) are similar to the values observed over ideal terrain and roughly constant with height, however variability between studies is large.
The effect of stratification is investigated using the Monin-Obukhov similarity (MOS) framework. The urban statistics generally follow the power laws established over ideal terrain, however, the magnitudes are higher for IT and lower for NS, especially under near neutral and slightly unstable conditions. The normalized temperature statistics on the other hand agree well with reference data.
The results discussed are based on a comprehensive review of urban observations from the past 50 years. Only few studies passed the stringent selection criteria required to obtain high-quality data. In particular there is a need for more observational research at heights close to the urban canopy and within to be able to better understand the turbulence transfer in this important region of momentum absorption.