Model experiments on free-convection heat and mass transfer of leaves and plant elements

Effects of orientation, shape, and surface structure on the free convection mass transfer of plates, leaf- and plant-models were studied in an electrolytic system. The results were extrapolated to the transfer of heat and mass in air, in an effort to predict realistic boundary-layer transfer coefficients for leaves and plant-like surfaces. Flow visualization complemented the investigation.The results indicate that increases in transfer of 50% over that of a vertical plate are possible for non-vertical rough leaves, but that a 25% increase is a more reasonable estimate for an average plant. Evidence is also presented that more reliable caluclations of the transfer properties of pine needles are obtained if the needles are approximated by vertical cylinders, rather than by horizontal ones.     

An evaluation of aircraft flux measurements of CO2, water vapor and sensible heat

Ground-based flux measurements of carbon dioxide and water vapor integrate physiological processes taking place on a field scale. Aircraft flux measurements have recently been undertaken to attempt to widen the scope of applicability of such measurements. However, because of the intermittency of turbulent transfer, flux measurements must be averaged over long periods of time or long distances to give reproducible results. This requirement makes it difficult to relate aircraft flux measurements to local surface processes. Flux measurements of CO₂, latent and sensible heat obtained from repeated passes in four directions and at three elevations over a homogeneous wheat-growing area are compared with ground-based measurements. Averages based on four runs of 4 km in length gave results consistent with ground-based measurements. The largest percentage differences were in the sensible heat flux. Cospectral analyses showed no significant high frequency losses for the data from flight levels of 25 and 50 m, but an underestimation of approximately 10% resulted at 10 m. Flight direction with respect to wind direction was relatively unimportant at 10 and 25 m but some effects were observed at 50 m. It was also shown that at 25 m, over a relatively smooth and homogeneous surface, the means of either three or four runs 4 km in length were similar to the means of 12–16 km runs. This confirms that at this altitude, most of the flux contribution is contained at wavelengths less than 4 km and that the mean of 3 to 4 passes accounts for most of the intermittency of turbulent transfer.     

The effects of small water bodies on the atmospheric heat and water budgets over the MacKenzie River Basin

The effects of small water bodies or lakes on the surface sensible and latent heat fluxes and the transport of heat and water vapour in the atmospheric boundary layer (ABL) over the Mackenzie River Basin (MRB) are studied from two cases, which occurred on 2 and 8 June 1999 during the warm season. The synoptic condition for the cases is representative of about 33% of the synoptic situation over the MRB. The two events are simulated using the Canadian mesoscale compressible community (MC2) model. A one‐way nesting grid approach is employed with the highest resolution of 100 m over a domain of 100 km². Experiments were conducted with (LAKE) and without (NOLAKE) the presence of small water bodies, whose size distribution is obtained through an inversion algorithm using information of their linear dimension determined from aircraft measurement of surface temperature during MAGS (the Mackenzie GEWEX (Global Energy and Water Cycle Experiment) Study) in 1999. The water bodies are assumed to be distributed randomly in space with a fractional area coverage of 10% over the MRB. The results show that, in the presence of lakes, the domain‐averaged surface sensible heat flux on 2 June 1999 (8 June 1999) decreases by 9·3% (6·6%). The surface latent heat flux is enhanced by 18·2% (81·5%). Low‐level temperature advection and the lake surface temperature affect the air–land/lake temperature contrast, which in turn controls the sensible heat flux. In the absence of lakes the surface wind speed impacts the latent heat flux, but in the presence of lakes the moisture availability and the atmospheric surface layer stability control the latent heat flux. The enhancement is smaller on 2 June 1999 as a result of a stable surface layer caused by the presence of colder lake temperatures. The domain‐averaged apparent heat source and moisture sink due to turbulent transports were also computed. The results show that, when lakes are present, heating and drying occur in the lowest 100 m from the surface. Above 100 m and within the ABL, there was apparent cooling. However, the apparent moistening profiles reveal that lakes tend to moisten the ABL through transfer of moisture from the lowest 50–100 m layer. Copyright © 2004 John Wiley & Sons, Ltd.     

Transfer processes in vegetation by electrochemical analog

Electrochemical modelling may provide fast order-of-magnitude estimates of energy and mass transfer in crops and partly fill the gap between rigid, expensive field experiments and oversimplified mathematical modelling. The technique comprises breaking up a flowing electrolyte in a manner analogous — but not necessarily completely similar — to the way the atmospheric flow is broken up by crops and studying the flow of ions in the electrolyte.The effects of variations in free-flow velocity, plant spacing, row spacing and orientation on velocity field, transfer coefficients and eddy diffusivities have been studied in an electrochemical model under conditions relevant to well-ventilated crops with relatively simple canopy structure. A one-dimensional analytical model is developed for the transfer at solid surfaces, with direct proportionality between transfer coefficient and eddy diffusivity. The proportionality constant has values in the model of (3.5±0.5)×10^–3cm^–1 at the ground and (5+-2)×10^–4cm^–1 (for a cylindrical probe) near canopy top. The latter is not too different from the values proposed for foliage surfaces in real canopies, viz., approximately 1×10^–3 cm^–1 (Philip, 1964) and 7×10^–4 cm^–1 (Uchijima, 1966). may therefore become a useful parameter in scaling transfer coefficients from systems with different molecular diffusion properties.A tentative extrapolation of measured data to corresponding values in air is given. The agreement between predicted values and presently available field data is encouraging although more precise field data are required for a final judgment on the validity of the model.This paper is published with the permission of the International Institute of Heat and Mass Transfer; an abbreviated version of the paper is given in the Proceedings, Intl. Seminar on Heat and Mass Transfer in the Environment of Vegetation, Dubrovnik, Aug. 26–30, 1974.     

Electrochemical simulations of mass transfer from isolated wet spots and droplets on realistic fluttering leaves

This paper reports on forced-convection mass transfer from isolated discs on rectangular plates as well as hemispheres on realistic fluttering leaves. An electrochemical method was used where the convective transfer of ions to the test electrode (the droplet or the wet spot) in an electrolytic flow system was measured as a function of flow rates, sizes of discs and hemispheres. Measurements showed that the local transfer coefficient for uniformly transferring plates varied as expected while the transfer from isolated discs on plates was much less a function of the distance from the leading edge. An expression to describe the transfer coefficient for an isolated disc as a function of distance from the leading edge was determined. An expression describing the transfer from hemispherical drops on fluttering leaves was derived and compared with the predictions from transfer theory for a sphere in free space.