The atmospheric boundary layer dynamics is significantly affected by buoyancy. A stratification created by nocturnal ground cooling creates a medium for gravity waves propagation, increase intermittency and nonlinear effects on turbulence. In the daytime a convective instability promotes vertical heat flux and creates large-scale vortex structures (cells, rolls, etc.). These processes are even more complicated in an urban environment, where the ground temperature distribution is usually non-uniform due to different albedo in different parts of the city or the industrial activities. Buoyant convection and stratification affect the pollution distribution in urban atmosphere. In the upcoming talk we will discuss these subjects from the numerical modeling point of view and describe the possible ways of making the fine-scale urban simulations more affordable with the growing amount of available computational power.