C. E. Desborough

Cabauw Experimental Results from the Project for Intercomparison of Land-Surface Parameterization Schemes

In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m22 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (6 10 Wm 2 2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of

The Representation of Snow in Land Surface Schemes: Results from PILPS 2(d)

Twenty-one land surface schemes (LSSs) performed simulations forced by 18 yr of observed meteorological data from a grassland catchment at Valdai, Russia, as part of the Project for the Intercomparison of Land-Surface Parameterization Schemes (PILPS) Phase 2(d). In this paper the authors examine the simulation of snow. In comparison with observations, the models are able to capture the broad features of the snow regime on both an intra- and interannual basis. However, weaknesses in the simulations exist, and early season ablation events are a significant source of model scatter. Over the 18-yr simulation, systematic differences between the models’ snow simulations are evident and reveal specific aspects of snow model parameterization and design as being responsible. Vapor exchange at the snow surface varies widely among the models, ranging from a large net loss to a small net source for the snow season. Snow albedo, fractional snow cover, and their interplay have a large effect on energy available for ablation, with differences among models most evident at low snow depths. The incorporation of the snowpack within an LSS structure affects the method by which snow accesses, as well as utilizes, available energy for ablation. The sensitivity of some models to longwave radiation, the dominant winter radiative flux, is partly due to a stability-induced feedback and the differing abilities of models to exchange turbulent energy with the atmosphere. Results presented in this paper suggest where weaknesses in macroscale snow modeling lie and where both theoretical and observational work should be focused to address these weaknesses.

The project for intercomparison of land-surface parameterization schemes (PILPS) phase 2(c) Red-Arkansas River basin experiment: 3. Spatial and temporal analysis of water fluxes

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.

Simulations of a Boreal Grassland Hydrology at Valdai, Russia: PILPS Phase 2(d)

The Project for the Intercomparison of Land-Surface Parameterization Schemes (PILPS) aims to improve understanding and modeling of land surface processes. PILPS phase 2(d) uses a set of meteorological and hydrological data spanning 18 yr (1966‐83) from a grassland catchment at the Valdai water-balance research site in Russia. A suite of stand-alone simulations is performed by 21 land surface schemes (LSSs) to explore the LSSs’ sensitivity to downward longwave radiative forcing, timescales of simulated hydrologic variability, and biases resulting from single-year simulations that use recursive spinup. These simulations are the first in PILPS to investigate the performance of LSSs at a site with a well-defined seasonal snow cover and frozen soil. Considerable model scatter for the control simulations exists. However, nearly all the LSS scatter in simulated root-zone soil moisture is contained within the spatial variability observed inside the catchment. In addition, all models show a considerable sensitivity to longwave forcing for the simulation of the snowpack, which during the spring melt affects runoff, meltwater infiltration, and subsequent evapotranspiration. A greater sensitivity of the ablation, compared to the accumulation, of the winter snowpack to the choice of snow parameterization is found. Sensitivity simulations starting at prescribed conditions with no spinup demonstrate that the treatment of frozen soil (moisture) processes can affect the long-term variability of the models. The single-year recursive runs show large biases, compared to the corresponding year of the control run, that can persist through the entire year and underscore the importance of performing multiyear simulations.

Simulation of freeze‐thaw cycles in a general circulation model land surface scheme

Using observed meteorological data collected from four sites in the former Soviet Union, a land surface scheme designed for use in climate models was integrated for 6 years. Using observed available soil moisture and freezing front data, the model simulation of freeze-thaw cycles over the 6 years for each station was validated. The model was able to simulate the general characteristics of the seasonal freeze-thaw cycles for all stations for all years. There were problems in the simulation of the spring period by Best Approximation of Surface Exchanges (BASE): the model simulated excess accumulation of liquid soil moisture in spring and failed to match the observed timing of total freeze and total thaw. Most problems could be linked to the parameterization of hydraulic conductivity, and when this was modified, the model was able to simulate the available soil moisture more accurately. Moreover, it was shown that the inclusion of a frozen soil parameterization is necessary in order to capture the broad seasonal cycle of soil moisture and that the impact of not accounting for frozen soil can extend beyond the spring thaw period. To improve the simulations further, it may be necessary to model the effects of cryoturbation (including macropores), freezing fronts, ice-lensing and other periglacial processes. Given the spatial resolution of climate models, the vertical resolution of land surface schemes (1–5 layers), and the lack of suitable observed data, even if these processes could be parameterized, the quality of the simulations are unlikely to increase enough to warrant the additional computational expense.

The Impact of Root Weighting on the Response of Transpiration to Moisture Stress in Land Surface Schemes

Abstract Land surface schemes (LSSs) for large-scale climate models use a variety of different methods to represent the influence of soil moisture on transpiration. One area in which they differ is in the treatment of vertical soil moisture distribution. While some schemes use root weighting to integrate moisture stress throughout the root zone, others use only the bulk root-zone soil moisture. The sensitivity of transpiration to surface root fraction is examined in a simple off-line soil moisture model and a complex Deardorff-type LSS. The seasonal cycle of transpiration is found to be quite sensitive to surface root fraction, with transpiration becoming progressively more susceptible to moisture stress as the surface root fraction is increased. Varying the surface (10 cm) root fraction between 10% and 90% produces local transpiration difference of up to 80 W m−2 and relative annual transpiration differences of up to 28%. The sensitivity of root-weighted methods to surface root fraction is found to be pa...

The BASE land surface model

Abstract The Best Approximation of Surface Exchanges (BASE) land surface model is described in detail. BASE is designed for large-scale climate modelling and for investigation of inter-model parameterisation differences. It has a tiled surface structure with an explicit canopy layer through which the foliage, underlying ground and atmosphere interact. A three-layer diffusion scheme is used to model soil temperature, moisture and ice content. A slab model is used to represent snow.

Uncertainty in the simulation of runoff due to the parameterization of frozen soil moisture using the Global Soil Wetness Project methodology

Four simulations of the region 30°N–90°N are performed using the Global Soil Wetness Project methodology and a single land surface scheme. Four methods are used to represent soil ice: an explicit representation of the thermal and hydrological effects of soil ice; two implicit methods (which only account for the hydrological effects); and finally the simplest approach where soil ice is not accounted for. Substantial impacts on total runoff, evaporation and temperature result from the choice of parameterization. The partitioning of total runoff between drainage and surface runoff is also changed. The impacts on temperature are large enough to cause problems for “fingerprinting” of global change while the change in the runoff generation process, and the timing of maximum runoff are large enough to concern ocean modelers. Evidence presented here and elsewhere indicates that land surface schemes should include the thermal effects of soil ice melting and freezing. However, the hydrological effects of soil ice suppressing infiltration and encouraging surface runoff may be based on observations taken at a scale inappropriate to climate model parameterization. We show that for one basin, the Mackenzie, a land surface model which ignores soil ice entirely simulates runoff better than the other methodologies tested here. We therefore hypothesize that it may be preferable to not include soil ice in the runoff formulations used in land surface models until we have more observations at an appropriate spatial scale. Testing of frozen soil moisture parameterizations in other catchments with high-quality observed runoff data should be conducted to test this hypothesis.

Limited-Area Model Sensitivity to the Complexity of Representation of the Land Surface Energy Balance

Abstract By coupling a multimode land surface scheme with a regional climate model, three scientific issues are addressed in this paper: (i) the regional models sensitivity to the different levels of complexity presented by the land surface parameterization, (ii) relative model sensitivity to the land surface parameterization as compared with that to other model physical representations, and, (iii) following offline calibration, whether different complexity in the land surface representation leads to different model performance in the coupled experiments. In this study, a version of a regional model [Division of Atmospheric Research Limited Area Model (DARLAM)] is coupled with the Chameleon Surface Model (CHASM). Three sets of experiments are analyzed in this paper, employing six different complexity modes of CHASM. Model results from these coupled experiments show that the regional model is sensitive overall to different complexities represented in the CHASM modes. Moreover, these model sensitivities ar...

Brief description of bare essentials of surface transfer and results from simulations with the HAPEX-MOBILHY data

Abstract The Bare Essentials of Surface Transfer (BEST) land surface scheme is briefly described and the key physical parameterisations discussed. Results are then presented to illustrate how the model performs, with forcing data for HAPEX-MOBILHY, compared to a series of other schemes in the simulation of evaporation and sensible heat. The implications of the models performance, and some indications of the future development of the scheme are provided. The basic version of BEST was found to overestimate evaporation for the HAPEX-MOBILHY data, simulating 816 mm yr−1 compared to a range of 550 to 816 mm yr−1 for all models. A modification to the β parameterisation reduced the evaporation to 759 mm yr−1 which, although an improvement, is still probably too high.