Calculations of river-runoff in the giss GCM - impact of a new land-surface paramatrization and runoff routing model on the hydrology of the Amazon river

This study examines the impact of a new land-surface parameterization and a river routing scheme on the hydrology of the Amazon basin, as depicted by the NASA/Goddard Institute of Space Studies (GISS) global climate model (GCM). The more physically realistic land surface scheme introduces a vegetation canopy resistance and a six-layer soil system. The new routing scheme allows runoff to travel from a river's headwater to its mouth according to topography and other channel characteristics and improves the timing of the peak flow. River runoff is examined near the mouth of the Amazon and for all of its sub-basins. With the new land-surface parameterization, river runoff increases significantly and is consistent with that observed in most basins and at the mouth. The representation of the river hydrology in small basins is not as satisfactory as in larger basins. One positive impact of the new land-surface parameterization is that it produces more realistic evaporation over the Amazon basin, which was too high in the previous version of the GCM. The realistic depiction of evaporation also affects the thermal regime in the lower atmosphere in the Amazon. In fact, the lower evaporation in some portions of the basin reduces the cloudiness, increases the solar radiation reaching the ground, increases the net radiation at the surface, and warms the surface as compared to observations. Further GCM improvement is needed to obtain a better representation of rainfall processes.Pages: 349-36

Key results and implications from phase 1(c) of the Project for Intercomparison of Land-surface Parametrization Schemes

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 off-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 differences in how the models' partitioned available energy between sensible and latent heat. Annually averaged, simulations for the tropical forest ranged by 79 W m-2 for the sensible heat flux and 80 W m-2 for the latent heat flux. For the grassland, simulations ranged by 34 W m-2 for the sensible heat flux and 27 W m-2 for the latent heat flux. Similarly large differences were found for simulated runoff and soil moisture and at the monthly time scale. The models' simulation of annually averaged effective 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 fluxes 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 offline and with artificial forcing, generally precludes judgements concerning the relative quality of any specific 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

Sensitivity of latent heat flux from PILPS land-surface schemes to perturbations of surface air temperature.

ABSTRACT 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