A one-dimensional, second-order turbulence model with bulk cloud microphysics and detailed radiative transfer is used to simulate the evolution of a thermal internal boundary layer (TIBL) which develops above a wide, open lead. A mixed-phase cloud, originally based at the surface, is produced within the TIBL. The cloud initially fills the entire TIBL but is later elevated above the surface with its top coincident with the top of the TIBL. Model-derived cloud ice and cloud liquid water mixing ratios exceed 0.06 g kg-1 directly above the open lead, with a secondary maximum near the top of the TIBL. In addition, precipitating ice particles or snow fills the TIBL with a maximum snow mixing ratio of about 0.05 g kg-1. Radiative flux divergence results in strong cooling at cloud top (which contributes to the local maxima in cloud water mixing ratio at this level) and warming near the surface. The lead-induced cloud increases the downwelling long-wave irradiance received at the surface by up to 70 W m-2 (reducing the surface radiative cooling by over 40%) during the baseline case. This value is quite sensitive to the assumed particle size and cloud particle concentration. The vertical structure and composition of the lead-induced cloud is shown to strongly depend on the rate of snow production and the cloud water partitioning. ¿ American Geophysical Union 1995