Using atmospheric forcing data generated from a general circulation climate model, sixteen land surface schemes participating in the Project for the Intercomparison of Land-surface Parametrization Schemes (PILPS) were run o!-line to equilibrium using forcing data from a GCM representative of a tropical forest and a mid-latitude grassland grid point. The values for each land surface parameter (roughness length, minimum stomatal resistance, soil depth etc.) were provided. Results were quality controlled and analyzed, focusing on the scatter simulated amongst the models. There were large di!erences in how the models' partitioned available energy between sensible and latent heat. Annually averaged, simulations for the tropical forest ranged by 79 W m\ for the sensible heat #ux and 80 W m\ for the latent heat #ux. For the grassland, simulations ranged by 34 W m\ for the sensible heat #ux and 27 W m\ for the latent heat #ux. Similarly large di!erences were found for simulated runo! and soil moisture and at the monthly time scale. The models' simulation of annually averaged e!ective radiative temperature varied with a range, between all the models, of 1.4 K for tropical forest and 2.2 K for the grassland. The simulation of latent and sensible heat #uxes by a standard &bucket' models was anomalous although this could be corrected by an additional resistance term. These results imply that the current land surface models do not agree on the land surface climate when the atmospheric forcing and surface parameters are prescribed. The nature of the experimental design, it being o%ine and with arti"cial forcing, generally precludes judgements concerning the relative quality of any speci"c model. Although these results were produced de-coupled from a host model, they do cast doubt on the reliability of land surface schemes. It is therefore a priority to resolve the disparity in the simulations, understand the reasons behind the scatter and to determine whether this lack of agreement in decoupled tests is reproduced in coupled experiments.
In the PILPS Phase 2a experiment, 23 land-surface schemes were compared in an off-line control experiment using observed meteorological data from Cabauw, the Netherlands. Two simple sensitivity experiments were also undertaken in which the observed surface air temperature was artificially increased or decreased by 2 K while all other factors remained as observed. On the annual timescale, all schemes show similar responses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatures and latent heat fluxes all increase while net radiation, sensible heat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes and, especially, the changes in the latent heat fluxes are not linearly related to the change of temperature. Theoretically, the nonlinear relationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturation vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonlinear relationship is shown in the form that the increase of the latent heat flux for a 2-K temperature increase is larger than its decrease for a 2-K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the ϩ2-K experiment is smaller than the decrease of the latent heat flux in the Ϫ2-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theoretical relationship and the Cabauw sensitivity experiments results (or the asymmetry) is due to (i) the involvement of the  g formulation, which is a function of a series stress factors that limited the evaporation and whose values change in the Ϯ2-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of the drag coefficient induced by the changes in stratification due to the imposed air temperature changes (Ϯ2 K) in parameterizations of latent heat flux common in current land-surface schemes. Among all stress factors involved in the  g formulation, the soil moisture stress in the ϩ2-K experiment induced by the increased evaporation is the main factor that contributes to the asymmetry.
Le modèle biosphère haute résolution (HRBM) : état des travaux, validation, résultats Dans le cadre du projet ESCOBA, le Modèle de Biosphère Haute Résolution (HRBM) a été utilisé afin d'atteindre trois objectifs. Premièrement, des expériences de validation ont été menées sur plusieurs parties de la structure modulaire du HRBM. Des données ponctuelles de productivité et de phytomasse ont été utilisées pour valider le module de productivité. Un test de sensibilité montre que la productivité du HRBM est principalement limitée par la disponibilité en eau (précipitations). Une étude régionale sur la distribution du carbone organique dans le sol a été effectuée en collaboration avec la station de recherches de l'ITE de Merlewood (Hoffstadt et al., dans ce volume). Un deuxième objectif consistait à étudier l'impact du facteur anthropique sur le cycle du carbone. Le HRBM considère les changements d'utilisation du sol ainsi que les incendies de la végétation. Dans le module d'incendies, les probabilités de feux sont fonction de variables climatiques et des types de biomes. Ce module a été validé par comparaison avec les observations.de feux saisonniers en Afrique de l'Ouest (Cahon et al., 1992 ; Menaut et al., 1991). Pour 1980, le HRBM estime une quantité maximum de 1,6 Pg С provenant de la combustion des déchets agricoles. Le besoin général d'un module approprié de cycle de l'eau nous a conduit à choisir un schéma de surface existant (SiB2, Sellers et al., 1992). Nous avons testé ce modèle avec succès en utilisant des données micrométéorologiques acquises sur une longue période. Les résultats ont suggéré quelques modifications dans le couplage de SiB2 et du HRBM. Le cycle de l'eau et les processus de productivité sont liés du fait de leur dépendance commune à la conductance foliaire pour l'eau et le CO₂. La troisième partie de nos activités a été consacrée à l'amélioration de notre compréhension de la conductance stomatique pour différentes conditions climatiques et concentrations en CO₂. Nous avons donc mené des expériences en laboratoire conjointement à des campagnes sur le terrain en Italie du Nord et au Paraguay. Nous avons confirmé l'existence de classes fonctionnelles se distinguant par la réponse de leur conductance à la concentration interne en CO₂. Nous avons répertorié les distributions de ces classes pour des espèces dominantes le long de gradients d'humidité.
The Project for Intercomparison of Land‐Surface Parameterization Schemes (PILPS) is a World Climate Research Programme project operating under the auspices of the Working Group on Numerical Experimentation (WGNE) and the Global Energy and Water Cycle Experiment (GEWEX). Its goal is to improve our understanding of how the modeling of land‐surface processes affects atmospheric conditions in climate and weather forecast models. PILPS will assess the range of responses of surface parameterizations to atmospheric forcing and detect the source of these differences. This effort is needed to improve the capabilities of climate and weather prediction models when surface processes strongly influence the movement of the atmosphere. Since it began in 1992 [Henderson‐Sellers et al., 1992], PILPS has initiated a number of complementary sensitivity studies designed by the land‐surface scientific community.
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