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Wilfried Brutsaert

Abstract

An expression is presented for the roughness length of water vapor (or sensible heat, etc.) that is the surface value intercept of the straight line resulting from a semi-logarithmic plot of the mean specific humidity (or potential temperature, etc.) profile in the dynamic sublayer. The derivation is based on the standard assumption of continuity in the mean profile at the interface between the interfacial transfer sublayer and the fully turbulent surface sublayer. It is found that the resulting formulation yields similar results for a number of empirical and theoretical equations for the interfacial transfer that are available in the literature. The roughness length of any scalar admixture depends not only on the nature of the surface but also on the intensity of the surface shear stress and on the molecular diffusivity and the viscosity. In meteorological applications under rough flow conditions this roughness length may be considerably smaller than the aerodynamic roughness length Z 0, whereas under smooth flow conditions it is usually somewhat larger.

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Wilfried Brutsaert

Abstract

No abstract available.

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Wilfried Brutsaert and Michiaki Sugita

Abstract

The scalar roughness for sensible beat can be directly formulated in terms of the surface temperature. Therefore, in the case of an anisothermal vegetation canopy, the concept of a scalar roughness is ill defined and it may vary greatly depending on the method and scale of measurement of the surface temperature. Through an analysis of the turbulent transport phenomena inside the vegetation canopy, it is shown how this uncertainty and variability of the scalar roughness of a vertically anisothermal surface can be reduced by defining a properly weighted average surface temperature. Implications are discussed for the optimal measurement of surface temperature by remote sensing with radiation thermometers.

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Michiaki Sugita and Wilfried Brutsaert

Abstract

A comparison was made between temperature and humidity profiles obtained from radiosonde measurements under unstable conditions over hilly terrain in northeastern Kansas during the summer and fall of 1987. The summer profiles were found to be similar in a layer over the height range given by 65(±45)m≤z;≤124(±54) m where z is the height above the ground surface reference level. Evidence for this similarity was the closeness of the scaled ratio of the temperature and humidity gradients at any given time to the mean Bowen ratio determined at ground-based surface flux stations. Between this layer and the ground surface some similarity was observed in the profiles but it was not manifested as strongly. Above the layer, in the outer region of the atmospheric boundary layer it was found that the profiles of humidity and potential temperature are completely dissimilar in terms of the surface Bowen ratio. The profiles of the fall campaign when soil moisture content was low and vegetation senescent displayed a less pronounced similarity.

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Daoyi Chen and Wilfried Brutsaert

Abstract

Remotely sensed images of vegetation density expressed as the Normalized Difference Vegetation index (NDVI) recorded during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment were mapped on the same coordinate system covering the 20 km × 20 km experimental site. The temporal behavior of frequency distributions and semivariograms of the NDVI exhibited similarity; these distributions were quite symmetric, except near the extremes. Although values at individual locations showed considerable scatter, when averaged over the experimental area, the NDVI values were well correlated with ground-based measurements of the leaf area index. The persistence tendencies of the spatial patterns of NDVI were evaluated through correlation analysis of the images on successive days. The spatial patterns were preserved well during wet periods; they gradually changed by deep drying of the soil but were restored after heavy rainfall.

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GOUR-TSYH YEH and WILFRIED BRUTSAERT

Abstract

In the finite-difference formulation of the two dimensional steady-state turbulent diffusion equation for solving evaporation problems, two difficulties arise caused by the automatic satisfaction of one of the boundary conditions at the surface and by the infinite size of the solution domain. A general numerical scheme is developed to overcome these difficulties by the use of appropriate transformations. The results of some numerical experiments show that the longitudinal diffusion term is usually negligible and that, with suitable parameters for roughness and stability, power laws can be as useful for practical solutions as the more complicated logarithmic law.

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Wilfried Brutsaert and William P. Kustas

Abstract

An analysis was made of vertical profiles of mean specific humidity in the hilly pre-Alpine region in Switzerland under conditions of new neutral atmospheric stability. The profile data were derived from radiosonde observations during the summer season of 1982. A logarithmic layer was found to extend from approximately 2h to 6h [where h(=95 m) is the mean height of the hills]. The zero-plane displacement height for water vapor d appeared to be close to that for momentum, i.e., about 50 m; on the other hand, the water vapor roughness height z for those ascents with a wet surface was found to be around 10 to 15 times smaller than the momentum roughness z 0(=3.8 m). The values of evaporation, deduced from the humidity profiles with the friction velocity of the corresponding velocity profiles, were very close to those measured with a lysimeter.

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Wilfried Brutsaert and William P. Kustas

Abstract

Measurements were made of the profiles of mean wind velocity, of temperature, and of specific humidity in the unstable atmospheric boundary layer over macro-rough terrain; these data were obtained from radiosonde observations in a calibrated watershed of 3.2 km2 in the hilly Pre-Alps of Switzerland during the summer of 1982. The regional evaporation was reasonably well correlated (R = 0.7) with these profile measurements through a logarithmic height dependency between roughly 2h0 (where h0 ≈ 100 m is the mean height of the roughness obstacles) and 0.6h (where h is the height of the boundary layer above the mean valley level). The shapes of the profiles appear to be essentially independent of the Monin–Obukhov parameter (zd0)/L, but they display a dependency on the wind shear aloft and on the value of (z0/h) (where z0 is the roughness height). Over this rugged surface the relative importance of mechanical turbulence, as compared to convective turbulence, is larger than over terrain with smaller (z0/h) for the same degree of instability of the atmosphere.

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Wilfried Brutsaert and Shaw Lei Yu

Abstract

Experimental evaporation data which were obtained with shallow, square pans of different sizes and colors, were studied by means of regression analysis relating evaporation with the difference between the water vapor pressure at the water surface and in the air, pan size, and wind speed at various elevations. These relationships are expressed by different equations for mass transfer whose functional form is widely accepted in the literature, or derivable by means of theoretical diffusion models. A comparison is made of the goodness-of-fit of the different theoretical equations to the experimental data.

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Shaw Lei Yu and Wilfried Brutsaert

Abstract

Evaporation from eight 1-inch deep square pans was studied. The pans were insulated with styrofoam and built in three different sizes. The 1-ft and the 8-ft pans were executed in two colors, black and white, and the 4-ft pans in black, white, green and gray. The measured physical data are tabulated. Some aspects of the energy budget of pan evaporation are analyzed and discussed.

It was found that color has no significant effect on the turbulent vapor diffusion. The value of the constant in Bowen's ratio for the different pans varied between 0.71 and 0.83, which is somewhat larger than the value of the psychrometric constant, 0.65, but which is much smaller than the value, 1.52, for the U.S. Weather Bureau Class A pan. The difference between the evaporation from the black and the white pans and the incoming solar radiation were found to be highly correlated.

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