A one-dimensional model with detailed radiative transfer, a second-order closure scheme for turbulence, and bulk cloud microphysics has been employed to simulate the convective atmosphere above a wide open lead. The development of a thermal internal boundary layer (TIBL) associated with the atmospheric convection above a wide open lead is modeled for several different temperature profiles characteristic of the Arctic winter. Large differences in the predicted top of the TIBL are observed between two simple diagnostic models and the time-dependent model introduced here. The modeled thermodynamic processes (i.e., radiative transfer and condensation/evaporation) occurring within the TIBL above an open lead are shown to account for as much as 20% of the predicted TIBL top height. Radiative heating dominates the heat budget in the lowest model level while sensible and latent heating occur throughout the TIBL. The production of cloud water affects the modeled TIBL top height via enhanced radiative cooling near the top of the TIBL and by latent heat released during condensation and freezing. Under stable conditions the TIBL top height above an open lead increases with time as a decaying exponential. The initial temperature of a surface-based isothermal layer can account for up to 50% of the variations in TIBL top height. The depth of the surface-based inversion layer and the stability of the atmosphere above the inversion are both important controlling factors in TIBL development while the temperature difference across the inversion layer is slightly less important. ¿ American Geophysical Union 1995