This paper presents a simple model of the atmospheric boundary layer designed for large-scale models and applications where high vertical resolution is not feasible. The model is formulated to handle cases where the boundary-layer structure and capping inversion are not fully resolved. Surface fluxes are represented using similarity theory, while turbulent diffusivities above the surface layer are derived from bulk similarity considerations and matching conditions at the top of the surface layer. The boundary-layer depth is expressed using a modified bulk Richardson number to account for thermals. The model predicts the growth of the convectively mixed layer accurately and performs well in scenarios with weak surface heat flux and transitions between stable and unstable conditions. The study also examines the impact of different ratios of surface evaporation to potential evaporation on the boundary-layer depth, finding that variations in surface evaporation significantly affect the boundary-layer depth more than the choice of the boundary-layer depth formulation. The model is less specialized than traditional mixed-layer growth models but does not consider boundary-layer clouds.This paper presents a simple model of the atmospheric boundary layer designed for large-scale models and applications where high vertical resolution is not feasible. The model is formulated to handle cases where the boundary-layer structure and capping inversion are not fully resolved. Surface fluxes are represented using similarity theory, while turbulent diffusivities above the surface layer are derived from bulk similarity considerations and matching conditions at the top of the surface layer. The boundary-layer depth is expressed using a modified bulk Richardson number to account for thermals. The model predicts the growth of the convectively mixed layer accurately and performs well in scenarios with weak surface heat flux and transitions between stable and unstable conditions. The study also examines the impact of different ratios of surface evaporation to potential evaporation on the boundary-layer depth, finding that variations in surface evaporation significantly affect the boundary-layer depth more than the choice of the boundary-layer depth formulation. The model is less specialized than traditional mixed-layer growth models but does not consider boundary-layer clouds.