CABAUW EXPERIMENTAL RESULTS FROM THE PROJECT FOR INTERCOMPARISON OF LAND-SURFACE PARAMETERIZATION SCHEMES

In the Project for Intercomparison of Land-Surface Parameterization Sc hemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux s chemes designed for use in climate and weather models. Schemes were ev aluated by comparing their outputs with long-term measurements of surf ace sensible heat fluxes into the atmosphere and the ground, and of up ward longwave radiation and total net radiative fluxes, and also compa ring them with latent heat fluxes derived from a surface energy balanc e. Tuning of schemes by use of the observed flux data was not permitte d. On an annual basis, the predicted surface radiative temperature exh ibits a range of 2 K across schemes, consistent with the range of abou t 10 W m(-2) in predicted surface net radiation. Most modeled values o f monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (+/-10 W m(-2)). However , modeled radiative surface temperature appears to have a systematic p ositive bias in most schemes; this might be explained by an error in a ssumed emissivity and by models' neglect of canopy thermal heterogenei ty. Annual means of sensible and latent heat fluxes, into which net ra diation is partitioned, have ranges across schemes of 30 W m(-2) and 2 5 W m(-2), respectively. Annual totals of evapotranspiration and runof f, into which the precipitation is partitioned, both have ranges of 31 5 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resis tance under non-water-stressed conditions. Many schemes tend to undere stimate 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 m(-2) for sensible heat flux and 10 W m(-2) for latent heat flux). Actual runoff at the s ite is believed to be dominated by vertical drainage to groundwater, b ut several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of t otal pore volume) in the simulated annual mean root-zone soil moisture . Unfortunately, no measurements of soil moisture were available for m odel evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to expl ain 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.