European Cooperation in the Field of Scientific and Technical Research Mixing Height and Inversions in Urban Areas European Commission Cost Action 715 Mixing Height and Inversions in Urban Areas Introduction the Mixing Height in Urban Areas -a Review

The urban boundary layer (UBL), in comparison with ‘rural’ homogeneous PBLs, is characterised by greatly enhanced mixing, resulting from both the large surface roughness and increased surface heating, and by horizontal inhomogeneity of the mixing height (MH) and other meteorological fields due to variations in surface roughness and heating from rural to central city areas. So, it is reasonable to consider the UBL as a specific case of the atmospheric boundary layer over a non-homogeneous terrain. Most of the parameterisations of MH were developed for the conditions of a homogeneous terrain, so their applicability for urban conditions should be verified. Just a few authors suggested specific methods for MH determination in urban areas. Some authors tested the applicability of MH methods for specific urban sites, but a comprehensive analysis of the applicability does not exist yet. Proceeding from analysis of different methods of the MH estimation for urban areas, the following preliminary suggestions are discussed. For the estimation of the daytime MH, the applicability of common methods is more acceptable than for the nocturnal MH. For the convective UBL the simple slab models were found to perform quite well. The formation of the nocturnal UBL occurs in a counteraction with the negative ‘non-urban’ surface heat fluxes and positive anthropogenic/urban heat fluxes, so the applicability of the common methods for the SBL estimation is less promising. Applicability of some newly developed methods for the SBL height estimation, including diagnostic and prognostic equations and an improved Rimethod are also discussed.