A second-order modeling technique for stratified turbulent flows with application to buoyancy driven mixed layers is presented. The models of pressure terms, dissipation and destruction of temperature variance are to a great extent based on invariant techniques. The unknown constants in these models were determined empirically. The turbulent transport model, based on solutions to the approximated rate equations for the third-order turbulent moments, incorporates buoyancy effects. The buoyancy contributions to the third moments were found to be crucial to modeling the dynamics of entrainment. The numerical results presented illustrate the capability of the model to realistically predict the structure of the entraining mixed layer including the vertical flux of turbulence energy and the downward heat flux at the mixed layer/inversion interface.