Optimal Coupling Height of the Atmosphere and Land Surface—An Earth System Modeling Perspective

In Earth system modeling, the land surface is coupled with the atmosphere through surface turbulent fluxes. These fluxes are computed using mean meteorological variables between the surface and a reference height in the atmosphere. However, the dependence of flux computation on the reference height, which is usually set as the lowest level in the atmosphere in Earth system models, has not received much attention. Based on high-resolution large-eddy simulation (LES) data under unstable conditions, we find the setting of reference height is not trivial within the framework of current surface layer theory. With a reasonable prescription of aerodynamic roughness length (following the setting in LESs), reference heights near the top of the surface layer tend to provide the best estimate of surface fluxes, especially for the momentum flux. Furthermore, this conclusion for the sensible heat flux is insensitive to the ratio of roughness length for momentum versus heat. These results are robust, whether using the classical or revised surface layer theory. They provide a potential guide for setting the proper reference heights for Earth system modeling and can be further tested in the near future using observational data from land–atmosphere feedback observatories.