"AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings" were recently published, which were mainly targeted for strong wind problems around high-rise buildings. However, urban heat island phenomenon and air pollution problem become serious in weak wind regions such as behind buildings and within the street canyons. Urban ventilation is now broadly recognized as one of the effective countermeasures to tackle these problems, thus it is becoming extremely important to ensure adequate air ventilation in weak wind regions. In order to apply CFD techniques to estimate the air ventilation, thermal and pollutant dispersion in urban areas, it is important to assess the performance of turbulence models that to be adopted for simulating these phenomena. Thus, a new working group was organized by the Architectural Institute of Japan (AIJ) to implement this validation. As the first step of the work, wind tunnel experiments and CFD simulations for gas and thermal dispersion behind a high-rise building in unstable non-isothermal turbulent flow were carried out. Standard k-ε model and"two-equation model for heat transfer" were used as the investigated turbulence model in this study. The calculated results of these two models showed that there was little difference between these models, and both models overestimated the size of the recirculation region behind the building and underestimated the lateral dispersion of the gas.