Impact of surface heterogeneities on land surface water and energy fluxes

in English Land surface characteristics, such as land cover, soil properties and hydrological characteristics, as well as important processes on the land surface often vary on scales that are not resolved by the horizontal structure of present day Earth System Models (ESMs). The surface energy and moisture fluxes depend non-linearly on these spatially heterogeneous characteristics and processes. The present Phd thesis investigates this interrelation between spatial sub-grid scale (SSGS) heterogeneity at the surface as well as within the lowest layers of the atmosphere and the surface energy and moisture fluxes. The work completed in the course of this dissertation divides into model development, namely the implementation of the newly developed VERTEX scheme in the Max Planck Institute for Meteorology’s Earth System Model (MPI-ESM), and into a series of investigations in which the VERTEX scheme is used for the coupling of the land surface and the atmosphere. The VERTEX scheme is the first coupling scheme designed for the application in ESMs that explicitly represents atmospheric SSGS heterogeneity and allows for an interaction of homogeneous sub-areas in the atmosphere. In the scheme, atmospheric SSGS heterogeneity is modelled based on the blending height concept. This theory provides a framework for relating atmospheric SSGS heterogeneity to the causative SSGS heterogeneity at the surface and the reference height above ground. In a first study, the MPI-ESM’s single column mode is used to demonstrate how an explicit representation of SSGS heterogeneity affects near surface processes and how surface and atmospheric SSGS heterogeneity relate to the vertical turbulent transport of moisture and dry static energy. Furthermore, it can be shown that the explicit representation of different surface types not only has an impact on the individual states of the respective surface types, but that an impact is evident in the grid box mean state. As findings are limited to the specific location the single column model is applied to, the results can only provide a limited insight into certain processes and allow for qualitative conclusions. For more general conclusions, the MPI-ESM’s atmospheric model ECHAM6 coupled to the land surface component JSBACH (ECHAM/JSBACH) is used in simulations on the global scale. Here, large scale atmospheric effects often make it difficult to investigate impacts that occur on the sub-grid scale. Therefore, the impact of different representations of SSGS heterogeneity on the near surface processes is studied using a model configuration that prevents atmospheric feedbacks. This study also allows to identify the characteristics which have the strongest influence on the partitioning of the available energy at the surface into latent and sensible heat flux. Subsequently, ensembles of simulations are produced in which the land surface model is coupled to the atmospheric model without further restrictions but with prescribed sea-surface temperature and sea-ice extent. With the surface-atmosphere feedback accounted for, these ensemble simulations allow to estimate the impact of an explicit representation of SSGS heterogeneity on the simulated state of surface and atmosphere as well as on the surface fluxes. Finally, the thesis is focused on the influence of irrigation on simulated global climate. Irrigation as a SSGS feature exhibits some unique characteristics with respect to the