## 1 Introduction
For environmental reasons, outdoor exercise yards of pig buildings are often partly or totally roofed to minimise the impact of rain to remoisten the soiled areas. Additionally, the roof of a building plays an important role in air separation, airflow pattern within and around the building [23; 24] and airflow characteristics in the wake of the building [25], and thus is expected to affect the pollutant dispersion from the building. The effect of building roof on air pollutant transport has been investigated inside and over urban street canyons [26]. It was found that the roof shape [27; 28] and the roof slope [28] significantly affected the air vortex within the canyon and the pollutant concentration and dispersion. The influences of the roof type (open-ridge, semi-monitor and mono-slope) and roof slope on air movement for a naturally ventilated dairy house were studied in a wind tunnel, and the dispersion properties were predicted from the airflow measurement results [18]. The authors found that the open-ridge roof type tended to increase dispersion downwind from the house, and the roof slope largely affected the air movement and contaminant dispersion [18]. However, their work only measured air velocities at the vertical symmetry plane of the building, and detailed airflow field information downwind the building was limited. Moreover, the configurations of NVPBs with roofed outdoor exercise yards are very different from street canyons and dairy houses. There is lack of airflow information about this type of buildings with roofed outdoor exercise areas. Therefore, it is required to understand in detail airflow characteristics including both mean (time-averaged) velocity and turbulent fluctuations downwind NVPBs with outdoor access. This knowledge can contribute to a better understanding of the transport and dispersion processes of airborne pollutants and of an optimised roof design. Wind tunnel and scaled model experiments are widely applied in aerodynamics studies because of their advantages of allowing fully controlled boundary conditions, working with real airflow [29], providing a large amount of data in a short time [30], and the flexibility in the experimental setup [11]. Because of the scaled-down model, similarity criteria e.g., geometric similarity, boundary similarity, Reynolds number similarity have to be met in order to make the wind tunnel experimental results be comparable to the results from full-scaled buildings [31]. By carefully checking the similarity criteria, the wind tunnel tests method was therefore used in this paper in order to conduct the airflow characteristics research. Moreover, the data obtained from the wind tunnel measurements can also be used to validate accuracy and reliability of computational fluid dynamics (CFD) models and therefore to investigate more complex flow (e.g., airflows above emitting surfaces) and dispersion phenomena, to perform comprehensive parametric studies, and finally to mitigate the production of emissions from livestock buildings. The objectives of this study were to investigate mean and turbulent characteristics of airflows downwind of a NVPB with an outdoor exercise yard covered with roofs with different roof slopes, to predict air pollutant distribution and dispersion properties, and to provide valuable experimental data for CFD validation. The novelties of this study are as follows:  
* It is the first to provide detailed airflow information around a naturally ventilated livestock building combined with a roofed outdoor area;
* It predicts the potential distribution of gaseous pollutants from the building using airflow measurement results;
* It provides a large amount of experimental data of both mean velocity and turbulent fluctuations downwind the building with a high resolution that can be used for CFD validation.