Günter Gross

On the applicability of numerical mass-consistent wind field models

A three-dimensional nonhydrostatic model is used to calculate the complex wind field in a coastal and a hilly region. These results are used to test the applicability of a mass-consistent wind model for different numbers of input wind data. While for the coastal area the mass consistent model calculates reasonable results even for a small number of input wind data, this number increases significantly in complex terrain.

The wake of a wind turbine and its influence on sound propagation

The wake of a single wind turbine was measured with a Doppler wind lidar system during a night with a well developed low-level jet. Two meteorological microscale models with different model assumptions were applied to the same situation to generate consistent three-dimensional fields of wind components, turbulent kinetic energy and temperature. A three-dimensional ray-based sound particle model was used to simulate the propagation of the wind turbine noise into the downwind area. Two-dimensional sound propagation simulations were performed on the basis of the lidar measurements and three-dimensional simulations were based on the results of the meteorological models. Both meteorological models are capable of reproducing the main features of the measured wake flow. However, the results differ in many details from each other as well as from the lidar measurements. The acoustical model results show that the wake flow favours the sound propagation from the upper sources (aerodynamic noise at the wing tips near the crest of the rotor plane) towards the ground. The acoustical simulations also suggest a high sensitivity of the noise impact near the ground to differences between the simulated meteorological fields.

Effects of different vegetation on temperature in an urban building environment. Micro-scale numerical experiments

A three-dimensional micro-scale model is used to study the effects of various greenery on temperature in a built-up environment. Green design elements like roofs and facades, lawns in courtyards and single trees are studied individually as well as in various combinations. Measures for comparison are temperatures at 2 m height and mean temperatures for the urban atmosphere up to the building height. Different types of greenery can reduce local temperatures up to 15 K during specif c daytime hours. However, this extraordinary effect is restricted to a short time and especially to the direct surroundings, while an impact over larger distances is small. Roof and facade greenery have hardly any inf uence on temperature at the 2 m level but reduce daytime heating of the urban atmosphere to a minor degree, while the relevance of trees is more or less limited to the shadow effect. A signif cant decrease in urban temperatures can be achieved only with a large number of very different individual green elements. The largest effect on the urban atmosphere was simulated for a change in albedo resulting in a temperature decrease of some degrees around noon.

On Modeling Dry Deposition of Long-Lived and Chemically Reactive Species over Heterogeneous Terrain

An explicit multi-layer subgrid-scheme was developed for a meso-γ/β-scale model to consider subgrid-scale surface heterogeneity, dry deposition, biogenic and anthropogenic emission of trace gases. Since dry deposition measurements of highly reactive trace species are scarce we try to evaluate this scheme by heuristic principles. The results of simulations conducted for a 5 × 5 km2 resolution with and without this scheme are evaluated by using results of a model run with 1 × 1 km2 resolution, which is taken as a ‘grand thruth’ and which has the same resolution as the subgrid. The explict multi-layer subgrid scheme provides a similar distribution of dry deposition fluxes as the much more computationally expensive simulation with the 1 × 1 km2 resolution.

On the Parametrization of Urban Land Use in Mesoscale Models

The effects of urban structures on the distribution of meteorological variables can be included in mesoscale models by an appropriate parametrization. The different approaches are conventionally tested against wind profiles in the centre of the urban area while flow distortions around are not considered. In this study, the quality of different parametrizations in capturing the main wind-field modifications in, as well as around, a complex obstacle is investigated. The method applied consists of a building resolved microscale model and a mesoscale model including a suitable parametrization. The results demonstrate that a drag or a porosity approach can reproduce very satisfactorily the main characteristics of the airflow completely, while a simpler roughness length concept in general approximates the mean flow unsatisfactorily.

Numerical simulations to the diurnal variation of wakes behind wind turbines

A micro-scale numerical model was used to study the wind distribution in the vicinity of a wind turbine. Numerical results show, that the wake behind the turbine strongly depends on the time of day with a moderate horizontal extension during neutral daytime conditions and an elongated wake during stable thermal stratification. A downshift of the maximum velocity deficit with increasing turbine distances has been simulated for neutral conditions, while an upshift was found during night-time situations. Due to a strong wind shear in the lower planetary boundary layer at night, larger values for turbulence intensity have been calculated than for daytime conditions. These modifications of the wind may cause a reduction of the energy yield of a wind turbine located in the wake of up to 50 %. The results of the numerical simulations show an overall qualitatively good agreement with available observations.

Estimation of annual odor load with a concentration fluctuation model

A concentration fluctuation model is presented which is suitable to simulate the dispersion of odorous substances in the atmosphere. This tool has been combined with a meteorological model for the planetary boundary layer in order to provide the relevant meteorological variables wind and turbulence. The model system has been validated against field observations and the results show a good agreement in the spatial distribution of mean concentration as well as for concentration variance and other statistical measures. The validated model was used to simulate the long-term dispersion of odors for one complete year. The calculated time series has been analysed with respect to threshold crossing rates and annual odor hours. A comparison of the results of different methods for calculating the odor hours demonstrates the deficiencies of simple approaches.

Numerical simulation of future low-level jet characteristics

In this paper, results of numerical simulations of low-level jet (LLJ) characteristics are presented. By forcing a boundary layer model with the results of a regional climate model, long time series have been calculated which are analyzed with special regard to the night-time wind regime. The simulated LLJ statistics are compared qualitatively with available observations and show overall good agreement. The frequency distributions for jet height and jet speed at the end of this century show light changes with a tendency to an increased jet height.

On the applicability of a numerical model system for calculating new European air pollution measures

A coupled meteorology-dispersion model system is used to study the dispersion of air pollutants over one continuous year. While wind and turbulence are calculated with a one-dimensional time dependent model, the time evolution of air pollutants is calculated three-dimensional. The results of the long term run of the meteorological model have been verified against surface observations and show a very good agreement in the frequency distributions as well as in time evolution. Several dispersion calculations over one year with different source characteristics and model assumptions have been carried out with the following main findings. For a realistic simulation of limit values and alert thresholds the knowledge of the time dependency of the emission is essential. Steady state dispersion calculations for specific times instead of the one year continuous run results in large differences in concentration patterns especially for elevated sources.

Dispersion of traffic exhausts emitted from a stationary line source versus individual moving cars – a numerical comparison

A three-dimensional microscale model was used to study the effects of moving vehicles on air pollution in the close vicinity of a road. The numerical results are compared to general findings from wind tunnel experiments and field observations. It was found that the model is suitable to capture the main flow characteristics within an urban street canyon, in particular the modifications relating to running traffic. A comparison of the results for a stationary line source approach and for multiple single moving sources demonstrates significant differences. For a street in a flat terrain, the near-road concentrations are underestimated by up to a factor of two if the emissions are approximated by a stationary line source. This underestimation decreases with increasing distance, and becomes negligible 30–50 m away from the road. For an urban canyon situation, the line source assumption is a conservative approximation for the concentrations at the leeside of the street, while on the opposite pavement and wall, a systematic underestimation was found. Also, the effects of different traffic situations have been studied and discussed.