Modeling of wind conditions in Warsaw

Airflow is one of the most important weather parameters in a city. It is important for the air quality, the city's heat balance, pedestrian comfort and the safety of high-rise buildings. Local flow at the scale of streets and districts is difficult or impossible to capture in regional weather models. Computational Fluid Dynamic models are the solution. In this paper, the OpenFoam model was used to model wind direction and speed in specific meteorological situations. The results were compared with measurement stations in Warsaw, and the model was improved on their basis. An averaged Navier Stokes turbulence model was used under steady-stable flow conditions. The Darcy-Forchheimer model was used to take vegetation into account. The poster presents the first results of analyzes related to the spatial distribution of wind direction and speed, delineates areas at risk of low air quality and compares it with the results from measuring stations. In addition to the basic model, a model containing ground thermals was also created to study the extent and intensity of the urban heat island and to study the phenomenon of smog during temperature inversion in selected meteorological conditions. A comparative analysis of both models was made. The first results show that it is possible quite accurately to map airflow in a city. It also indicates that some existing ventilation channels of the city have been blocked or limited due to new investments. The most important ventilation channel is the Vistula valley, which is 500-600 m wide in Warsaw. However, due to the terrain, its most important role is not fulfilled during prevailing westerly winds, and then the air quality decreases, especially at low wind speeds. In most cases, the northern districts are also generally better ventilated (spatial distribution of buildings, higher wind speeds) than the southern districts, but this is not always visible when assessing air quality. The immediate vicinity also influences the aspects of mechanical ventilation of the city and the way buildings are heated. Districts that theoretically should have better conditions for air exchange are often areas of single-family houses and independent boiler rooms. The city centre, despite tighter development, is heated by the municipal heating plant, and they are not direct emitters of pollution. Another aspect is vehicle traffic. In the city centre, more vehicle traffic is another pollutant emitter. For this reason, pollutants specific to heating and traffic were analysed separately. The general problem in high-resolution city-scale modelling is the use of adequate computational power. This initially precludes using CFD models in meteorological nowcasting and short-term modelling.