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lmd_Laval1997_abstracts.html

1997 .

(2 publications)

P. Peylin, J. Polcher, G. Bonan, D. L. Williamson, and K. Laval. Comparison of two complex land surface schemes coupled to the National Center for Atmospheric Research general circulation model. Journal of Geophysical Research, 102:19413, August 1997. [ bib | DOI | ADS link ]

Two climate simulations with the National Center for Atmospheric Research general circulation model (version CCM2) coupled either to the Biosphere Atmosphere Transfer Scheme (BATS) or to Sechiba land surface scheme are compared. Both parameterizations of surface-atmosphere exchanges may be considered as complex but represent the soil hydrology and the role of vegetation in very different ways. The global impact of the change in land surface scheme on the simulated climate appears to be small. Changes are smaller than those obtained when comparing either one of these schemes to the fixed hydrology used in the standard CCM2. Nevertheless, at the regional scale, changing the land-surface scheme can have a large impact on the local climate. As one example, wre detail how circulation patterns are modified above the Tibetan plateau during the monsoon season. Elsewhere, mainly over land, changes can also be important. In the tropics, during the dry season, Sechiba produces warmer surface temperatures than does BATS. This warming arises from differences in the soil hydrology, both storage capacity and the dynamics of soil water transport. Over the Tundra biotype, the formulation of the transpiration induces significant differences in the energy balance.

T. H. Chen, A. Henderson-Sellers, P. C. D. Milly, A. J. Pitman, A. C. M. Beljaars, J. Polcher, F. Abramopoulos, A. Boone, S. Chang, F. Chen, Y. Dai, C. E. Desborough, R. E. Dickinson, L. Dümenil, M. Ek, J. R. Garratt, N. Gedney, Y. M. Gusev, J. Kim, R. Koster, E. A. Kowalczyk, K. Laval, J. Lean, D. Lettenmaier, X. Liang, J.-F. Mahfouf, H.-T. Mengelkamp, K. Mitchell, O. N. Nasonova, J. Noilhan, A. Robock, C. Rosenzweig, J. Schaake, C. A. Schlosser, J.-P. Schulz, Y. Shao, A. B. Shmakin, D. L. Verseghy, P. Wetzel, E. F. Wood, Y. Xue, Z.-L. Yang, and Q. Zeng. Cabauw Experimental Results from the Project for Intercomparison of Land-Surface Parameterization Schemes. Journal of Climate, 10:1194-1215, June 1997. [ bib | DOI | ADS link ]

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 m2 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 (10 W m2). 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 of30 W m2 and 25 W m2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m2 for sensible heat flux and 10 W m2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.

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