Ratio of τ/τ_b as a function of convective stability from a solution of the non-local Reynolds stress model as presented in Kupka & Montgomery (2002) assuming the downgradient approximation for third order moments. The time scale τ_b is computed from where the absolute value of β is taken. Sign changes are hence indicated by spikes. Both the overshooting zones below and above the lower and the upper convectively unstable zone show the same increase of τ/τ_b from 0 to more than 10 (the finite grid resolution prevents τ/τ_b from becoming actually zero).