An important challenge in LES of the atmospheric boundary layer is the parameterization of the non-resolved scales using a subgrid-scale (SGS) model. Numerical simulations and field experiments were performed to address open issues in SGS modeling. We present a new scale-dependent dynamic model to parameterize the SGS fluxes in the LES filtered heat equation. This model is analogous to that proposed by Porté-Agel et al (J. Fluid Mech., 2000) for the eddy-viscosity parameterization of the SGS stresses in the momentum equation. The model coefficient is optimized at every time step and position in the flow based on the resolved velocity and temperature fields. The dependence of the computed coefficient on distance from the ground, grid scale and atmospheric stability are studied and compared with results obtained from a-priori field studies. The new scale-dependent dynamic model does not require any parameter specification and yields more realistic simulations (better able to reproduce Monin-Obukhov similarity theory) than the traditional eddy-diffusion and scale-invariant dynamic models.