OCELAND: A Conceptual Model to Explain the Partitioning of Precipitation between Land and Ocean

The spatial distribution of precipitation is often misrepresented by General Circulation Models. In particular, precipitation tends to be underestimated over land and overestimated over ocean.

In this study, we investigate whether large-scale constraints on the partitioning of precipitation between land and ocean exist by using a conceptual box model based on water balance equations. With a small number of empirical but physically motivated parametrizations of the water balance components, we construct a set of coupled ordinary differential equations which describe the dynamical behavior of the water vapor content of land and ocean atmospheres as well as the soil moisture content of land. We compute the equilibrium solution of this system and analyze the sensitivity of the equilibrium state to model parameter choices.

Our results show that the precipitation ratio between land and ocean is primarily controlled by the land fraction, a scale-dependent atmospheric moisture transport parameter and the permanent wilting point of the soil. We demonstrate how the proposed model can be adapted for applications on both global and local scales, e.g. to model island precipitation enhancement. For a global scale model configuration with one ocean and one land domain, we show that the precipitation ratio is constrained to a range between zero and one and are able to explain this behavior based on the underlying equations and the fundamental property of land to lose water through runoff.