Predicting velocities and turbulent momentum exchange in isolated street canyons

► We derive a semi-analytical model to predict velocities in ideal street canyons. ► We parameterize the turbulent momentum exchange using plume entrainment theory. ► Aspect ratio, façade roughness and orientation to the wind affect these values. ► The model is evaluated using computational fluid dynamics in a generic way. ► The exchange velocity can be used to calculate heat and species fluxes.