Abstract
Abstract. The influence of land processes and in particular of diffuse/direct radiation partitioning on surface fluxes and associated regional-scale climate feedbacks is investigated using ERA-40 driven simulations over Europe performed with the COSMO-CLM2 Regional Climate Model (RCM). Two alternative Land Surface Models (LSMs), a 2nd generation LSM (TERRA_ML) and a more advanced 3rd generation LSM (Community Land Model version 3.5), and two versions of the atmospheric component are tested, as well as a revised coupling procedure allowing for variations in diffuse/direct light partitioning at the surface, and their accounting by the land surface component. Overall, the RCM performance for various variables (e.g., surface fluxes, temperature and precipitation) is improved when using the more advanced 3rd generation LSM. These improvements are of the same order of magnitude as those arising from a new version of the atmospheric component, demonstrating the benefit of using a realistic representation of land surface processes for regional climate simulations. Taking into account the variability in diffuse/direct light partitioning at the surface further improves the model performance in terms of summer temperature variability at the monthly and daily time scales. Comparisons with observations show that the RCM realistically captures temporal variations in diffuse/direct light partitioning as well as the evapotranspiration sensitivity to these variations. Our results suggest that a modest but consistent fraction (up to 3 %) of the overall variability in summer temperature can be explained by variations in the diffuse to direct ratio.
Highlights
The physical, chemical and biological processes acting at the interface between the land surface and the atmosphere influence the Earth’s climate at various spatiotemporal scales (e.g., Bonan, 2008; Seneviratne et al, 2010; Arneth et al, 2010)
The important role of land-atmosphere interactions was already explicitly recognized during the design of the first climate models, which conceptualized the exchanges of radiation, heat and water between the land and the atmosphere through relatively simple land surface parametrization (e.g., Manabe, 1969)
By testing two alternative Land Surface Models (LSMs) within a Regional Climate Model (RCM), this study quantitatively addresses the role of land processes in simulating regional climate over Europe
Summary
The physical, chemical and biological processes acting at the interface between the land surface and the atmosphere influence the Earth’s climate at various spatiotemporal scales (e.g., Bonan, 2008; Seneviratne et al, 2010; Arneth et al, 2010). Land Surface Models (LSMs) have been continuously improved, to a point where they can represent the linkages between energy, water and nutrients cycles within the terrestrial biosphere. The progress towards global Earth System Models (ESMs) explicitly representing climate-carbon cycle interactions have motivated the use of advanced, biogeochemistry-enabled LSMs within global climate models (Friedlingstein et al, 2006), while regional climate modelling studies have been comparatively more focused on atmospheric processes and often relied on much simpler LSMs (Giorgi, 2006). Recent efforts have pointed out the potential added value of integrating more comprehensive LSMs into RCMs. The coupling between RAMS (Regional Atmospheric Modelling System) and the CENTURY biogeochemistry model showed that the phenological cycle of vegetation can significantly affect water and energy fluxes and regional climate (Lu et al, 2001).
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