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Axel Kleidon

Abstract

Two inverse methods are applied to a land surface model to infer global patterns of the hydrologically active depth of the vegetation's rooting zone. The first method is based on the assumption that vegetation is optimally adapted to its environment, resulting in a maximization of net carbon uptake [net primary production (NPP)]. This method is implemented by adjusting the depth such that the simulated NPP of the model is at a maximum. The second method assumes that water availability directly affects the leaf area of the vegetation, and therefore the amount of absorbed photosynthetically active radiation (APAR). Rooting depth in the model is adjusted such that the mismatch between simulated and satellite-derived APAR is at a minimum. The inferred patterns of rooting zone depth from both methods correspond well and reproduce the broad patterns of rooting depth derived from observations. Comparison to rooting depth estimates from root biomass distributions point out that these may underestimate the hydrological significance of deep rooted vegetation in the Tropics with potential consequences for large-scale land surface and climate model simulations.

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Klaus Fraedrich, Axel Kleidon, and Frank Lunkeit

Abstract

The effect of vegetation extremes on the general circulation is estimated by two atmospheric GCM simulations using global desert and forest boundary conditions over land. The difference between the climates of a “green planet” and a “desert world” is dominated by the changes of the hydrological cycle, which is intensified substantially. Enhanced evapotranspiration over land reduces the near-surface temperatures; enhanced precipitation leads to a warmer mid- and upper troposphere extending from the subtropics (induced by ITCZ, monsoon, and Hadley cell dynamics) to the midlatitudes (over the cyclogenesis area of Northern Hemisphere storm tracks). These regional changes of the surface water and energy balances, and of the atmospheric circulation, have potential impact on the ocean and the atmospheric greenhouse.

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Maik Renner, Axel Kleidon, Martyn Clark, Bart Nijssen, Marvin Heidkamp, Martin Best, and Gab Abramowitz

Abstract

The diurnal cycle of solar radiation represents the strongest energetic forcing and dominates the exchange of heat and mass of the land surface with the atmosphere. This diurnal heat redistribution represents a core of land–atmosphere coupling that should be accurately represented in land surface models (LSMs), which are critical parts of weather and climate models. We employ a diagnostic model evaluation approach using a signature-based metric that describes the diurnal variation of heat fluxes. The metric is obtained by decomposing the diurnal variation of surface heat fluxes into their direct response and the phase lag to incoming solar radiation. We employ the output of 13 different LSMs driven with meteorological forcing of 20 FLUXNET sites (PLUMBER dataset). All LSMs show a poor representation of the evaporative fraction and thus the diurnal magnitude of the sensible and latent heat flux under cloud-free conditions. In addition, we find that the diurnal phase of both heat fluxes is poorly represented. The best performing model only reproduces 33% of the evaluated evaporative conditions across the sites. The poor performance of the diurnal cycle of turbulent heat exchange appears to be linked to how models solve for the surface energy balance and redistribute heat into the subsurface. We conclude that a systematic evaluation of diurnal signatures is likely to help to improve the simulated diurnal cycle, better represent land–atmosphere interactions, and therefore improve simulations of the near-surface climate.

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Reinder A. Feddes, Holger Hoff, Michael Bruen, Todd Dawson, Patricia de Rosnay, Paul Dirmeyer, Robert B. Jackson, Pavel Kabat, Axel Kleidon, Allan Lilly, and Andrew J. Pitman

From 30 September to 2 October 1999 a workshop was held in Gif-sur-Yvette, France, with the central objective to develop a research strategy for the next 3–5 years, aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data.

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