The role of the land surface for surface climate: results from a stepwise land–atmosphere coupling experiment

Abstract Given the important role of the land surface for climate, this study aims at (1) to evaluate the quality of the simulation of surface climate by the land-surface component of the EC-Earth3 ESM and (2) to assess the role of the coupling of the land surface with the atmosphere for the simulation of the surface climate in EC-Earth3. It is based on three simulations with different configurations of the EC-Earth3 ESM: an offline simulation with the land-surface component, a partially coupled simulation with the atmospheric component and a fully coupled simulation with the atmospheric component of EC-Earth3. The land-surface component of EC-Earth3 shows a characteristic geographical distribution of the biases for the different variables describing surface climate. As for the land-surface temperature, the model is characterized by warm biases in the tropics and the mid- and high latitudes of the Northern Hemisphere and a cold bias in the subtropics. For surface soil moisture, on the other hand, the model shows wet biases in the tropics and the mid- and high latitudes of the Northern Hemisphere and a dry bias in the subtropics. As for the surface energy fluxes, net radiation and sensible heat flux are underestimated in the tropics and the mid- and high latitudes of the Northern Hemisphere and overestimated in the subtropics, and the opposite behaviour is found for latent heat flux. The land-surface component of EC-Earth3 is characterised by an overall cold bias and a general underestimation of net radiation and sensible heat flux. Neither the coupling with the atmosphere nor the feedback between the land surface and the atmosphere affect the geographical distribution of the biases in surface climate characterising the offline simulation with the land-surface component of EC-Earth3 but have impacts on the magnitude of the local biases and regional details. The coupling with the atmosphere decreases land-surface temperature in the tropics and the mid- and high latitudes of the Northern Hemisphere and increases land-surface temperature in the subtropics, resulting in colder land-surface temperature and, thus, amplifying the overall cold bias found in the offline simulation. The feedback between the land surface and the atmosphere, on the other hand, increases land-surface temperature in the tropics and the mid-latitudes of the Northern Hemisphere and decreases land-surface temperature in the subtropics and the high latitudes of the Northern Hemisphere. The overall effect of the feedback between the land surface and the atmosphere is notably smaller than the effect of the coupling with the atmosphere for land-surface temperature and net radiation, similar for the fluxes of sensible and latent heat and stronger for surface soil moisture. The results of this study emphasize the need to improve the quality of the land-surface component of EC-Earth3 parallel with other components of the ESM.