Abstract
Abstract. The Community Atmosphere Biosphere Land Exchange (CABLE) model has been coupled to the UK Met Office Unified Model (UM) within the existing framework of the Australian Community Climate and Earth System Simulator (ACCESS), replacing the Met Office Surface Exchange Scheme (MOSES). Here we investigate how features of the CABLE model impact on present-day surface climate using ACCESS atmosphere-only simulations. The main differences attributed to CABLE include a warmer winter and a cooler summer in the Northern Hemisphere (NH), earlier NH spring runoff from snowmelt, and smaller seasonal and diurnal temperature ranges. The cooler NH summer temperatures in canopy-covered regions are more consistent with observations and are attributed to two factors. Firstly, CABLE accounts for aerodynamic and radiative interactions between the canopy and the ground below; this placement of the canopy above the ground eliminates the need for a separate bare ground tile in canopy-covered areas. Secondly, CABLE simulates larger evapotranspiration fluxes and a slightly larger daytime cloud cover fraction. Warmer NH winter temperatures result from the parameterization of cold climate processes in CABLE in snow-covered areas. In particular, prognostic snow density increases through the winter and lowers the diurnally resolved snow albedo; variable snow thermal conductivity prevents early winter heat loss but allows more heat to enter the ground as the snow season progresses; liquid precipitation freezing within the snowpack delays the building of the snowpack in autumn and accelerates snow melting in spring. Overall we find that the ACCESS simulation of surface air temperature benefits from the specific representation of the turbulent transport within and just above the canopy in the roughness sublayer as well as the more complex snow scheme in CABLE relative to MOSES.
Highlights
One of the main issues in climate modelling is understanding the dependence of climate on the interaction between clouds, radiation, precipitation, and the land surface processes
Kowalczyk et al (2013) highlighted differences in the present-day land surface climatology of the two ACCESS submissions to CMIP5, but the impact of the different land surface models used in each simulation was difficult to determine due to other differences in atmospheric settings
Differences found in K2013 that we can largely attribute to the land surface processes and model configuration include smaller seasonal temperature amplitude manifested by a warmer winter and a cooler summer, and an earlier runoff from snowmelt in the Northern Hemisphere in ACCESS1.1 (CABLE)
Summary
One of the main issues in climate modelling is understanding the dependence of climate on the interaction between clouds, radiation, precipitation, and the land surface processes. The LSM includes a representation of the turbulent transport of momentum, heat, and water between the land surface, canopy, and the atmospheric boundary layer, as well as descriptions of thermal and hydrological processes in the soil and snow. We explore the impact on the simulated climate by changing the LSM in an atmospheric model (the UM) from the original scheme that was developed with the model (MOSES) to an alternate LSM (CABLE). Simulation of the phase and amplitude of the diurnal cycle of the near-surface variables allows the testing of the model representation of the interaction between the surface, the boundary layer, and the atmosphere above. A focus on summer (Sect. 4.2) and winter (Sect. 4.3) separately highlights the different processes that are important in different seasons
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