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The height of the atmospheric boundary layer (ABL) or the mixing height (MH) is a fundamental parameter characterising the structure of the lower troposphere. Two basic possibilities for the practical determination of the MH are its derivation from profile data (measurements or numerical model output) and its parameterisation using simple equations or models (which only need a few measured input values). Different methods suggested in the literature are reviewed in this paper. The most important methods have been tested on data sets from three different sites in Europe (Cabauw – NL, Payerne – CH, Melpitz – D). Parcel and Richardson number methods applied to radiosonde profiles and the analysis of sodar and wind profiler data have been investigated. Modules for MH determination implemented in five currently used meteorological preprocessors for dispersion models have been tested, too. Parcel methods using a revised coefficient for the excess temperature and Richardson number methods using a surface excess temperature worked well under convective conditions. Under stable conditions, the inherent difficulties call for a combination of several methods (e.g., mast and sodar). All the tested parameterisation schemes showed deficiencies under certain conditions, thus requiring more flexible algorithms able to take into account changing and non-classical conditions. Recommendations are formulated regarding both the analysis of profile measurements and the use of parameterisations and simple models, and suggestions for the preprocessor development and for future research activities are presented.

The role of the land surface in controlling climate is still underestimated and access to information from the boreal-forest zone is instrumental to improve this situation. This motivated the organ ...

A method to determine the regional integrated heat flux from measurements on the evolution of the mixed-layer is presented. The necessary input for the method can be deduced from successive radiosoundings. The regional heat flux over the NOPEX experimental area, an inhomogeneous region with patches of forest, agricultural fields, mires and lakes within the boreal zone, was determined for 3 days of the campaign in 1994. It was found to be lower than the heat flux over forest and higher than the heat flux over agricultural fields. The regional heat flux estimated by the mixed-layer evolution method was compared to a land-use-weighted average heat flux. The two independent estimates of the regional heat flux were found to be in general agreement.

The state of the boundary layer at the release site during the two tracer experiments is described, based on measurements of atmospheric turbulence carried out by a sonic anemometer, profiles of horizontal and vertical wind by a SODAR as well as frequent radiosonde releases. The boundary layer height is derived from radiosoundings, modelled and discussed. The study is meant as background for discussions of the tracer behaviour near the release site, and possible influence of the local meteorology at the release site on the long-range dispersion. The difference in the initial meso-meteorological conditions between the two experiments is discussed and some of its consequences for the dispersion of the released tracer are clarified. It is concluded that the first experiment is well suited for validation of long-range transport and dispersion models, whereas the second experiment calls for further model development. It is concluded that the micrometeorological measurements are very important as background information for the understanding of plume behaviour on small as well as long scales.

This paper presents an analysis of wind field and atmospheric turbulence in Athens, Greece covering a typical late-summer period with prevailing northerly wind. The wind climatology study shows a distinct difference between the northern and southern parts of the urban area. In the northern part of Athens the northerly flows (315°–45°) occur more than 60% of the time and the winds from all other directions are equally frequent (about 5% per 30° sector). In the southern part of Athens the northerly flows are more than 50%, but southerly flows (165°–225°) occur about 25% of the time indicating sea breeze development. The variability of the flow characteristics in the whole area was classified into three categories: northerly winds in the whole Athens basin; sea breeze throughout the whole Athens basin; and changing sea breeze (comprising partial penetration of the sea breeze over the city and/or changing wind direction throughout the day). This classification was used as basis for the investigation of the atmospheric turbulence and vertical profiles of meteorological parameters over the area. A model of the internal boundary layer height over an area with irregular coastline ( Gryning and Batchvarova (1996) Boundary Layer Meteorology 78, 405–413) is applied for an Athens site and the results are in good agreement with the measurements.

A simple model is described that relates the dispersion of material from ground-level sources at two areas, taking into account dissimilarities in the surface roughness parameter, zo, and the atmospheric stability characterized by the Monin-Obukhov length, L. The geostrophic wind speed is assumed constant and equal at the two areas, and is used to interrelate the wind conditions. The ratio of the crosswind-integrated concentrations at a given downwind distance turned out to depend on only four dimensionless parameters, namely zo/L and z/L for the two areas in question. Here z is the mean height of particles that have travelled a distance x; $ z/L can be found when zo/Landx/zo are known. The model is reduced to three dimensionless parameters by merging z /L for the two areas into a composite parameter. Dimensionless results from the model are illustrated for discrete values of this composite parameter.

Five atmospheric dispersion experiments, all assigned Pasquill stability class D, were performed at Riso National Laboratory. The tracer sulphurhexafluoride was released at a height of 60 m from the Riso meteorological tower, situated on a peninsula in the Roskilde Fjord, Denmark, and was sampled at ground level about 5 km downwind. The experiments were all performed with westerly winds. The dispersion was influenced by the internal boundary layer that develops downwind from the inhomogeneity between the Roskilde Fjord and the Riso site. During the experiments, turbulent wind velocity fluctuations were measured at the height of release with a three-dimensional ultrasonic anemometer. The lateral dispersion parameter, σy, was estimated according to three methods based on quantitative descriptions of the flow, and compared to the value of σy, derived from the measured tracer concentrations. The methods described in Hay and Pasquill (1959), and Draxler (1976) were found to work well in predicting σy in the actual experiments. Equivalent comparisons of the vertical dispersion parameter, σz, were troublesome, as the sampling unit set-up only allowed for an indirect estimate of σz, based on continuity considerations. But the method described in Draxler (1976) was found to predict the indirect measured σz reasonably well when taking the effect of the developing internal boundary layer into account. The ability of the various methods to predict the maximum ground level tracer concentration was investigated. The agreement between the estimated maximum ground level concentrations derived from Draxler (1976) and the concentrations actually measured was found to be better in most cases than the absolute calibration uncertainty in the tracer analysis.

A method for calculating the dispersion of plumes in the atmospheric boundary layer is presented. The method is easy to use on a routine basis. The inputs to the method are fundamental meteorological parameters, which act as distinct scaling parameters for the turbulence. The atmospheric boundary layer is divided into a number of regimes. For each scaling regime we suggest models for the dispersion in the vertical direction. The models directly give the crosswind-integrated concentrations at the ground, x y , for nonbuoyant releases from a continuous point source. Generally the vertical concentration profile is proposed to be other than Gaussian. The lateral concentration profile is always assumed to be Gaussian, and models for determining the lateral spread σ y are proposed. The method is limited to horizontally homogeneous conditions and travel distances less than 10km. The method is evaluated against independent tracer experiments over land. The overall agreement between measurements and predictions is very good and better than that found with the traditional Gaussian plume model.

A crosswind integrated K-model with wind- and K-profiles described by Monin-Obukhov similarity expressions is solved for a continuous surface release to yield the vertical spread of the plume as a function of the surface roughness z 0 and the Monin-Obukhov length L for a given downwind distance. The vertical spread of the plume is translated into σ z , and lines were traced in a ( z 0 , L − ) plane for which the σ z of the K-model matched the corresponding σ z of Pasquill's system. By this technique a new classification scheme is constructed. Knowing z 0 and L , the scheme tells which σ z curve in the Pasquill system should be used to describe the dispersion. This dispersion classification scheme is used to organize 3 years of data from two meteorological masts, one placed directly at a shoreline and the other roughly 1 km inland. Differences in the dispersion climatology over land and water are studied by averaging the data selectively. The large differences for water and land surfaces between the seasonal and diurnal variation of the dispersion class distributions are illustrated. It is found that the water surface influences the dispersion climatology as far as 20 km inland.