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Sixteen land-surface schemes participating in the project for the Intercomparison of Land-surface Schemes (PILPS) Phase 2(c) were run using 10 years (1979-1988) of forcing data for the Red-Arkansas River basins in the Southern Great Plains region of the United States. Forcing data (precipitation, incoming radiation and surface meteorology) and land-surface characteristics (soil and vegetation parameters) were provided to each of the participating schemes. Two groups of runs are presented. (1) Calibration-validation runs, using data from six small catchments distributed across the modeling domain. These runs were designed to test the ability of the schemes to transfer information about model parameters to other catchments and to the computational grid boxes. (2) Base-runs, using data for 1979-1988, designed to evaluate the ability of the schemes to reproduce measured energy and water fluxes over multiple seasonal cycles across a climatically diverse, continental-scale basin. All schemes completed the base-runs but five schemes chose not to calibrate. Observational data (from 1980-1986) including daily river flows and monthly basin total evaporation estimated through an atmospheric budget analysis, were used to evaluate model performance. In general, the results are consistent with earlier PILPS experiments in terms of differences among models in predicted water and energy fluxes. The mean annual net radiation varied between 80 and 105 W m-2 (excluding one model). The mean annual Bowen ratio varied from 0.52 to 1.73 (also excluding one model) as compared to the data-estimated value of 0.92. The run-off ratios varied from a low of 0.02 to a high of 0.41, as compared to an observed value of 0.15. In general, those schemes that did not calibrate performed worse, not only on the validation catchments, but also at the scale of the entire modeling domain. This suggests that further PILPS experiments on the value of calibration need to be carried out.

The energy components of sixteen Soil-Vegetation Atmospheric Transfer (SVAT) schemes were analyzed and intercompared using 10 years of surface meteorological and radiative forcing data from the Red-Arkansas River basin in the Southern Great Plains of the United States. Comparisons of simulated surface energy fluxes among models showed that the net radiation and surface temperature generally had the best agreement among the schemes. On an average (annual and monthly) basis, the estimated latent heat fluxes agreed (to within approximate estimation errors) with the latent heat fluxes derived from a radiosonde-based atmospheric budget method for slightly more than half of the schemes. The sensible heat fluxes had larger differences among the schemes than did the latent heat fluxes, and the model-simulated ground heat fluxes had large variations among the schemes. The spatial patterns of the model-computed net radiation and surface temperature were generally similar among the schemes, and appear reasonable and consistent with observations of related variables, such as surface air temperature. The spatial mean patterns of latent and sensible heat fluxes were less similar than for net radiation, and the spatial patterns of the ground heat flux vary greatly among the 16 schemes. Generally, there is less similarity among the models in the temporal (interannual) variability of surface fluxes and temperature than there is in the mean fields, even for schemes with similar mean fields.

The water-balance components of 16 Soil-Vegetation Atmospheric Transfer (SVAT) schemes were evaluated by comparing predicted and observed streamflow, predicted evapotranspiration and evapotranspiration inferred from an atmospheric moisture budget analysis, and soil moisture storage changes for a seven-year period (1980-1986) using data from the Red-Arkansas River basins of the Southern Great Plains of the USA. The evaluations support the following suggestions: (a) The mean annual runoff of all models follows, at least generally, the strong climatic East-West gradient of precipitation, although most models predict too much runoff in the dry part of the basin. (b) The mean monthly storage change tends to be underestimated, even though all models capture reasonably well the seasonality of the evapotranspiration. (c) The wide range of conceptualizations used for generation of surface and subsurface runoff strongly affect runoff generation on seasonal, and shorter, time scales. Model responses to summer precipitation ranged from almost no summer runoff (one model) to the (more common) situation of persistent overprediction of summer runoff, especially in the driest part of the basin. (d) All models tended to underpredict evapotranspiration in summer and overpredict in winter. (e) Model-derived mean seasonal cycles of changes in soil moisture storage are qualitatively similar to those inferred from observations, but most models do not predict the decrease in April soil moisture storage and the increase in October that is inferred from observations.

The aim of this paper is to propose a general interface for coupling general circulation models (GCMs) to land surface schemes (LSS) in order to achieve a plug compatibility between these complex models. As surface parameterizations include more processes, they have moved from being subroutines of GCMs to independent schemes which can also be applied for other purposes. This evolution has raised the problem within climate modeling groups of coupling these schemes to GCMs in a simple and flexible way. As LSS reaches a larger independence, a general interface is needed to enable exchange within the community. This paper discusses the tasks LSS have to fulfill when coupled to a GCM after a review of the current state of the art and the likely future evolutions of both components. The numerical schemes used for the processes which couple the land surfaces to the atmosphere are reviewed to ensure that the interface can be applied to all LSS and GCMs after only minor changes.

The theoretical framework of the vertical discretization of a ground column for calculating Earth’s skin temperature is presented. The suggested discretization is derived from the evenly heat-content discretization with the optimal effective thickness for layer-temperature simulation. For the same level number, the suggested discretization is more accurate in skin temperature as well as surface ground heat flux simulations than those used in some state-of-the-art models. A proposed scheme (“op(3,2,0)”) can reduce the normalized root–mean–square error (or RMSE/STD ratio) of the calculated surface ground heat flux of a cropland site significantly to 2% (or 0.9 W m−2), from 11% (or 5 W m−2) by a 5-layer scheme used in ECMWF, from 19% (or 8 W m−2) by a 5-layer scheme used in ECHAM, and from 74% (or 32 W m−2) by a single-layer scheme used in the UCLA GCM. Better accuracy can be achieved by including more layers to the vertical discretization. Similar improvements are expected for other locations with different land types since the numerical error is inherited into the models for all the land types. The proposed scheme can be easily implemented into state-of-the-art climate models for the temperature simulation of snow, ice and soil. [ABSTRACT FROM AUTHOR]

Abstract: Phase 1 of isotopes in the Project for Intercomparison of Land-surface Parameterization Schemes (iPILPS) compares the simulation of two stable water isotopologues (1H2 18O and 1H2H16O) at the land–atmosphere interface. The simulations are offline, with forcing from an isotopically enabled regional model for three locations selected to offer contrasting climates and ecotypes: an evergreen tropical forest, a sclerophyll eucalypt forest and a mixed deciduous wood. Here, we report on the experimental framework, the quality control undertaken on the simulation results and the method of intercomparisons employed. The small number of available isotopically enabled land-surface schemes (ILSSs) limits the drawing of strong conclusions, but, despite this, there is shown to be benefit in undertaking this type of isotopic intercomparison. Although validation of isotopic simulations at the land surface must await more and much more complete, observational campaigns, we find that the empirically based Craig-Gordon parameterization (of isotopic fractionation during evaporation) gives adequately realistic isotopic simulations when incorporated in a wide range of land-surface codes. By introducing two new tools for understanding isotopic variability from the land surface, the isotope transfer function and the iPILPS plot, we show that different hydrological parameterizations cause very different isotopic responses. We show that ILSS-simulated isotopic equilibrium is independent of the total water and energy budget (with respect to both equilibration time and state), but interestingly the partitioning of available energy and water is a function of the models'' complexity. [Copyright &y& Elsevier]

Abstract: Isotope-enabled Global Climate Models (GCMs) have been previously used to investigate water isotope fluxes in present and past climate systems. Water isotope modelling is also important in modern ecosystem studies, for investigating the source and transport of moisture and carbon dioxide fluxes. However, in this modelling, there has only been limited investigation of the effect of parameterisation complexity on stable water isotope partitioning. The present study has two aims. The first is to incorporate a stable water isotope parameterisation into a flexible land surface scheme (LSS): the CHAmeleon Surface Model (CHASM). This scheme offers five different modes, each containing up to four water reservoirs (canopy interception, snowpack, root zone and bare ground storage), and each mode offers the opportunity to modify resistances to evaporative fluxes from these reservoirs. These different modes allow the effect of model complexity on different parameterisations to be explored within a common modelling framework. Two modelling experiments are reported which use the iPILPS Phase 1 forcing: the ‘control’ experiment and one using a simpler mode of CHASM comprising a bucket hydrology plus a fixed stomatal resistance. The additional set of experiments using iCHASM''s variable modes shows, contrary to an earlier experiments with CHASM, that a land surface model with only a bucket hydrology scheme and constant surface resistance cannot reproduce the behaviour of an LSS that has additional functionality (such as bare ground evaporation, and canopy interception and aerodynamic, surface and stomatal resistances). [Copyright &y& Elsevier]

A distributed land-surface model is used to simulate the seasonal cycle of snow cover for a high-latitude basin. In comparison with the distributed model, a simulation using surface parameters and meteorological data averaged over the entire basin overestimates the peak snow accumulation and underestimates the duration of snow cover. Dividing the basin into a small number of elevation bands and performing separate simulations for each band greatly improves the results. The improvement is less marked if information on the elevation dependence of vegetation cover and meteorological conditions is not used. Distributed simulations with different models produce a wider range of results than distributed and aggregated simulations with the same model. [Copyright &y& Elsevier]

This paper describes the impact of changes in aerodynamic roughness length for snow-covered surfaces in a land-surface scheme (LSS) on simulated runoff and evapotranspiration. The study was undertaken as the LSS in question produced widely divergent results in runoff, depending on whether it was used in uncoupled one-dimensional simulations forced by observations from the PILPS2e project, or in three-dimensional simulations coupled to an atmospheric model. The LSS was applied in two versions (LSS1 and LSS2) for both uncoupled and coupled simulations, where the only difference between the two versions was in the roughness length of latent heat used over snow-covered surfaces. The results show that feedback mechanisms in temperature and humidity in the coupled simulations were able to compensate for deficiencies in parameterizations and therefore, LSS1 and LSS2 yielded similar runoff results in this case. Since such feedback mechanisms are absent in uncoupled simulations, the two LSS versions produced very different runoff results in the uncoupled case. However, the magnitude of these feedback mechanisms is small compared to normal variability in temperature and humidity and cannot, by themselves, reveal any deficiencies in a parameterization. The conclusion we obtained is that the magnitude of the aerodynamic resistance is important to correctly simulate fluxes and runoff, but feedback mechanisms in a coupled model can partly compensate for errors. [Copyright &y& Elsevier]

Improved representations of snow processes in vegetation canopies and snow hydrology are implemented in the Met Office Surface Exchange Scheme 2 (MOSES 2) land-surface model. Snow falling on forests is partitioned between interception and throughfall; the intercepted snow is removed by sublimation, melt and unloading. A reduction in sublimation of wind-blown snow through trapping by shrubs is simply represented by reducing the scalar roughness length for shrubs with snow cover. Melt water may be retained and refrozen within snow packs. By reducing the amount of snow lost through sublimation and delaying the runoff of melt water, these modifications are found to improve the simulations of runoff for two Swedish catchments used in the PILPS 2e intercomparison project. [Copyright &y& Elsevier]

Present-day land surface schemes used in weather prediction and climate models include parameterizations of physical processes whose complex nonlinear interactions can lead to models of unknown spatial and temporal characteristics. This paper describes the timescales of the evapotranspiration response of 16 land surface schemes which participated in the Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) Phase 2(c) Red-Arkansas River experiment. The basins were represented by 61, 1°×1° grid boxes. Ten years of hourly meteorological data were used to force 16 land surface schemes off line. The evapotranspiration responses of the models are characterized by an impulse response function (or unit kernel) which is described by a two-parameter model, representing the fast response of evaporation from the canopy surface or bare soil and a slower one due to transpiration. The analysis of the results shows significant differences among the various LSS in their characteristic timescales across the basins. [Copyright &y& Elsevier]