Diffusion coefficient for the 7 M_⊙ (a) Cepheid. Heavy black lines indicate the convective boundaries of both the convective shell and the outer convection zone. The thin black dotted line is the diffusion coefficient of the convective shell based on the mixture model proposed in Eq. (18), and the red dashed line is the diffusion coefficient based on the EVT analysis (Eq. (17)). A gray line extrapolates our model to show a possible diffusion coefficient for the outer convection zone. The solid yellow line is a model for mixing in the radiative zone that experiences overshooting from both above and below, based on Eq. (20). In this example, D_MLT^envelope = 0.8 D_MLT.