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  • Author or Editor: A. C. M. Beljaars x
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A. C. M. Beljaars
and
A. A. M. Holtslag

Abstract

In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.

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A. C. M. Beljaars
,
P. Schotanus
, and
F. T. M. Nieuwstadt

Abstract

This paper discusses the results of a surface-layer experiment near the Cabauw meteorological mast. We measured momentum, heat and moisture fluxes at two heights, namely, 3.5 and 22.5 m. The measurements also include the mean wind speed and mean temperature profiles. The purpose was to investigate surface-layer similarity laws under nonideal fetch conditions. We found that under such conditions, the shell stress increases with height because of obstacles upstream. As a consequence flux-profile relationships differ from those over uniform terrain. It is shown that these deviations imply a slow relaxation in the exchange coefficient for heat and momentum over a terrain with changing surface roughness. Furthermore, we found that horizontal velocity fluctuations scale on a friction velocity representative of a large area. On the other hand, vertical velocity fluctuations scale on the local friction velocity.

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B. J. J. M. van den Hurk
and
A. C. M. Beljaars

Abstract

Two simplifying assumptions adopted in the current ECMWF surface scheme are explored: a uniform skin temperature for all grid-box fractions with variable latent heat release and a fixed value of an effective heat conductivity defining the soil heat flux density. This paper proposes relatively simple modifications of the ECMWF scheme with a better physical basis, without large input or computer infrastructure requirements.

A uniform skin temperature overestimates evaporation from relatively wet surface fractions when the other surface components are dry and warm. This is shown to be the case for an evaporating soil after rain and vegetation evaporation in a sparse Mediterranean vineyard canopy. Allowing different temperatures for each surface fraction significantly reduces the overestimations and introduces only little additional computation.

The default effective conductivity value (7 W m−2K−1) employed by the current ECMWF scheme is shown to be too low for the sparse vineyard canopy. By raising the conductivity to 17 W m−2 K−1 for the bare-soil part of the surface, the daytime simulated soil heat flux was improved considerably.

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