Implementation of a canopy air space scheme into the Community Land Model

Abstract:

A single-layer Canopy Air Space Scheme (CASS) is implemented into the Community Land Surface Model version 4.5 (CLM4.5) in this study. It considers the canopy storages for heat, water, and trace gases that are generally neglected in the CLM4.5 surface flux calculation algorithm. Moreover, the CASS introduces prognostic equations for the surface variables and eliminates the CLM4.5 Crank-Nicolson iterative solution for computing surface skin temperature, which usually brings residual errors into the model and causes numerical instability. Two off-line simulations (one with the CASS and the other with the origin CLM4.5 scheme) were conducted and their results were compared with the FLUXNET observations. Preliminary results show that compared with the origin CLM4.5 scheme, the CASS has similar or better skills in representing land surface exchanges for heat, water and carbon under several biome types. The implementation of the CASS into the CLM4.5 not only improves the land model skills, but also provides a modeling framework to incorporate more complex canopy processes into the land surface model, such as bi-directional emission schemes for various trace gases and multi-layer canopy energy balance models.