For walls as external boundaries, the symmetry condition is the simplest option. In this case, all gradients run parallel to the wall, and the flow is frictionless. This "natural" boundary condition is automatically applied to all surfaces without specific boundary conditions in some methods. It is suitable for lateral walls parallel to the wind direction. However, for the upper boundary of an atmospheric boundary layer, it is advisable to specify a shear stress or velocity according to the formulas in Section 6.3.1 to better represent real atmospheric conditions.