We present a non-local second moment closure model for modeling turbulent mixing in the oceanic mixed layer and the atmospheric PBL. The model uses a prognostic equation for the turbulence length scale and incorporates counter-gradient terms for scalar fluxes to better simulate mixing under convective conditions. The results are tested against LES data on convective mixing. We conclude that while the presence of counter-gradient terms in scalar fluxes is a highly desirable feature, it is necessary to reexamine the modeling of third moments.