A simple model of the atmospheric boundary layer is presented, suitable for use in large-scale models where simplicity is required. The model uses similarity theory to represent surface fluxes and bulk similarity considerations to formulate turbulent diffusivities. The boundary-layer depth is expressed in terms of a modified bulk Richardson number that includes the influence of thermals. The model focuses on the relationship between boundary-layer growth, turbulent diffusivity profile, countergradient heat flux, and truncation errors. The model predicts the growth of the convectively mixed layer reasonably well and performs well in cases of weak surface heat flux and transitions between stable and unstable conditions. The evolution of the boundary layer is studied for different ratios of surface evaporation to potential evaporation, showing that surface evaporation has a greater effect on boundary-layer depth than the choice of depth formulation. The model is developed for applications where high vertical resolution is not possible, such as studying interactions between the atmospheric boundary layer and soil moisture transport. Due to the crude nature of surface evaporation representation, a high-resolution model is not justified. The model focuses on developing a boundary-layer depth formulation that does not require resolution of the capping inversion, allows for a continuous transition between stable and unstable boundary layers, describes the near-neutral case where surface heat flux is unimportant, and removes inconsistencies between surface similarity theory and countergradient flux correction. Several low-resolution boundary-layer models have been proposed for use in large-scale models. Some models use bulk effects by interpolating variables from the large-scale model without resolving boundary-layer structure. Others use turbulent diffusivities expressed in terms of local gradients of mean profiles. However, these formulations are sensitive to small changes in mean profiles and are not practical for large-scale models due to computational requirements. The present model uses a prescribed profile shape for turbulent diffusivities as a function of z/h and scale parameters derived from similarity arguments, reducing resolution requirements and offering more flexibility than purely bulk models. The model differs from previous approaches in both profile formulations and boundary-layer height determination. It is less specialized than usual mixed-layer growth models but does not consider boundary-layer clouds.A simple model of the atmospheric boundary layer is presented, suitable for use in large-scale models where simplicity is required. The model uses similarity theory to represent surface fluxes and bulk similarity considerations to formulate turbulent diffusivities. The boundary-layer depth is expressed in terms of a modified bulk Richardson number that includes the influence of thermals. The model focuses on the relationship between boundary-layer growth, turbulent diffusivity profile, countergradient heat flux, and truncation errors. The model predicts the growth of the convectively mixed layer reasonably well and performs well in cases of weak surface heat flux and transitions between stable and unstable conditions. The evolution of the boundary layer is studied for different ratios of surface evaporation to potential evaporation, showing that surface evaporation has a greater effect on boundary-layer depth than the choice of depth formulation. The model is developed for applications where high vertical resolution is not possible, such as studying interactions between the atmospheric boundary layer and soil moisture transport. Due to the crude nature of surface evaporation representation, a high-resolution model is not justified. The model focuses on developing a boundary-layer depth formulation that does not require resolution of the capping inversion, allows for a continuous transition between stable and unstable boundary layers, describes the near-neutral case where surface heat flux is unimportant, and removes inconsistencies between surface similarity theory and countergradient flux correction. Several low-resolution boundary-layer models have been proposed for use in large-scale models. Some models use bulk effects by interpolating variables from the large-scale model without resolving boundary-layer structure. Others use turbulent diffusivities expressed in terms of local gradients of mean profiles. However, these formulations are sensitive to small changes in mean profiles and are not practical for large-scale models due to computational requirements. The present model uses a prescribed profile shape for turbulent diffusivities as a function of z/h and scale parameters derived from similarity arguments, reducing resolution requirements and offering more flexibility than purely bulk models. The model differs from previous approaches in both profile formulations and boundary-layer height determination. It is less specialized than usual mixed-layer growth models but does not consider boundary-layer clouds.