On the Representation of Heterogeneity in Land-Surface–Atmosphere Coupling

Abstract

A realistic representation of processes that are not resolved by the model grid is one of the key challenges in Earth-system modelling. In particular, the non-linear nature of processes involved makes a representation of the link between the atmosphere and the land surface difficult. This is especially so when the land surface is horizontally strongly heterogeneous. In the majority of present day Earth system models two strategies are pursued to couple the land surface and the atmosphere. In the first approach, surface heterogeneity is not explicitly accounted for, instead effective parameters are used to represent the entirety of the land surface in a model’s grid box (parameter-aggregation). In the second approach, subgrid-scale variability at the surface is explicitly represented, but it is assumed that the blending height is located below the lowest atmospheric model level (simple flux-aggregation). Thus, in both approaches the state of the atmosphere is treated as being horizontally homogeneous within a given grid box. Based upon the blending height concept, an approach is proposed that allows for a land-surface–atmosphere coupling in which horizontal heterogeneity is considered not only at the surface, but also within the lowest layers of the atmosphere (the VERTEX scheme). Below the blending height, the scheme refines the turbulent mixing process with respect to atmospheric subgrid fractions, which correspond to different surface features. These subgrid fractions are not treated independently of each other, since an explicit horizontal component is integrated into the turbulent mixing process. The scheme was implemented into the JSBACH model, the land component of the Max Planck Institute for Meteorology’s Earth-system model, when coupled to the atmospheric general circulation model ECHAM. The single-column version of the Earth system model is used in two example cases in order to demonstrate how the effects of surface heterogeneity are transferred into the atmosphere, influencing local stability and the turbulent mixing process. Furthermore, a simple flux-aggregation scheme was implemented into the JSBACH model. By comparing single-column simulations utilizing the VERTEX scheme and the simple flux-aggregation scheme, it can be shown that the horizontal disaggregation of the turbulent mixing process has a substantial impact on the simulated mean state of a grid box. Here, the differences between simulations with the two schemes may, in certain cases, be even larger than the differences between simulations with the simple flux-aggregation scheme and simulations in which surface heterogeneity is not explicitly accounted for (i.e., a parameter-aggregation scheme).