An efficient numerical terrestrial scheme (ENTS) for Earth system modelling

Abstract

We present a minimal spatial model of vegetation carbon, soil carbon and soil water storage and the exchange of energy, water and carbon with the atmosphere. The efficient numerical terrestrial scheme (ENTS) is designed for long time period simulations and large ensemble studies in Earth system models of intermediate complexity (EMICs). ENTS includes new parameterisations of vegetation fractional cover and roughness length as functions of vegetation carbon, and a relationship between soil carbon storage and soil water holding capacity. We make and justify the approximation that when the solar forcing is a diurnal average, as in our EMIC, the land radiation balance equilibrates with the atmosphere within a few days. This allows us to solve directly for equilibrium land temperature, making ENTS very computationally efficient and avoiding problems of numerical instability that beset many land surface schemes. We tune the carbon cycle parameters towards observed values of global carbon storage in vegetation and soil and estimated global fluxes of net photosynthesis, vegetation respiration, leaf litter and soil respiration. When the model is forced with long term monthly mean fields of NCEP reanalysis climate data, we find ENTS yields broadly accurate patterns of vegetation and soil carbon storage, vegetation fraction, surface albedo, land temperature and evaporation.