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Peter H. Schuepp

Abstract

The local convective mass transfer (i.e., the mass transfer as a function of polar angle) of smooth and rough particles has been investigated with an electrochemiluminescence technique at a Schmidt number Sc ≈ 103 for Reynolds numbers 3 × 103 < Re < 105. Comparative qualitative wind tunnel observations were made of the evaporation of naphthalene-coated rough spheres with Sc = 2.4 and, wherever possible, the findings were compared with existing heat transfer measurements in order to minimize their dependence on the molecular properties of the fluid.

Although some uncertainty still exists, it is possible to present estimates of the qualitative and quantitative effect of surface roughness on the local convective heat and mass transfer of hailstones, and especially to show the large increase in local transfer at polar angles around 90° for Re > 104, which may help to explain the growth of big hailstones.

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Behzad Abareshi and Peter H. Schuepp

Abstract

Observations from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) showed that it is difficult to estimate the sensible heat flux from routinely observed environmental parameters. This study, therefore, explores the use of backpropagation neural networks to elucidate the link between sensible heat flux on the one hand and horizontal wind speed, air temperature, radiometric surface temperature, net radiation, and time on the other. Data collected over the FIFE site in 1987 and 1989 were used for network training and validation. Networks trained on part of the data from a narrow range of space–time coordinates performed well over the other part, with error (rms divided by mean of observations) values as low as 0.24. This indicates the potential in neural networks for linking sensible heat flux to routinely measured meteorological variables and variables amenable to remote sensing. When the networks were tested with data from other space–times, performance varied from good to poor (average error values around 1.27), depending on the degree of similarity between the training and validation datasets in terms of parameters not explicitly included in the training set. Poor predictive performance was primarily associated with the lack of input variables parameterizing canopy morphology and soil moisture, indicating that such variables should be incorporated in the design of future networks intended for large-scale applications. Observations also showed that an underparameterized network cannot be made more general by expanding the size of the training dataset. These findings have repercussions on the potential to derive energy and moisture balance estimates from standard meteorological and satellite-based remote sensing observations.

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Peter H. Schuepp and Roland List

Abstract

The convective mass transfer of smooth and rough sphere test particles over a range of Reynolds numbers between 3 × 103and 105 was studied by means of an electrochemical method at a Schmidt number of 2170. The experiments were carried out in a liquid tunnel where the electrolyte is forced past the stationary test particles at variable flow rates. Three distinctly different levels of turbulence intensity of the flow allowed an evaluation of the turbulence effect on the transfer rates.

An extrapolation of the results to the heat and mass transfer coefficients of hailstone models in the atmosphere shows that surface roughness may account for increases in the transfer rates of up to a factor of 2. For relatively large roughness elements (cluster Particles) the increase in the convective transfer coefficients at equal Reynolds numbers (based on equal volume diameters) is approximated by the increase in surface area. Atmospheric turbulence is not expected to change these transfer coefficients significantly since the intensity of fluctuations with suitable time and space scales is expected to be rather small.

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Ya Guo and Peter H. Schuepp

Abstract

The effect of local advection on evaporation and Bowen ratio over alternating crosswind infinite dry-warm and wet-cool surface strips (patches), by redistribution of surface heat, is analysed. The analysis shows that evaporation over the region is enhanced by local advection, as observed experimentally by other researchers.

Results suggest that evaporation can be enhanced more than 10% or 20% by local advection on the area covered by patches smaller than 10t u Ū d m or 5t u Ū d m, respectively, where t u is a unit time and Ū d the mean wind velocity in the internal boundary layer. This means that the traditional first-order approximation of surface energy balance, without considering local advection, might lead to significant error for such small-scale inhomogeneity. It may, however, be applicable to large patches (≫)10t u Ū d m, where energy partition may then be directly approximated from area-weighted averages of properties derived from extended wet and dry surfaces, according to the patchy area fractions involved.

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Peter H. Schuepp and Roland List

Abstract

Heat transfer measurements from a smooth and a rough (lobe structured) sphere to water (Prandtl number Pr = 7) at Reynolds numbers 3 × 103 < Re < 7.5 × 104 are described and compared to mass transfer measurements made previously under identical flow conditions but at Schmidt number Sc = 2170. The comparison shows how the molecular properties of the fluid affect the transfer of smooth and rough particles and, by inference, how the results of transfer measurements in liquids can be applied to air. The new measurements help to establish a safer basis for model experiments. They essentially confirm previously made extrapolations on the roughness effect on the convective heat and mass transfer of spherical hailstones.

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