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1.
Heat transfer was studied between intact leaves of various sizes and shapes in vivo under free and forced air conditions. Use of a wind tunnel and a microwave transmitter to heat the leaves facilitated measurements of convective, along with radiative and evaporative, heat losses from plant leaves. Knowledge of input energy, analysis of cooling curves, and established formulae, respectively, formed the basis of the steady-state, unsteady-state, and analytical methods for the determination of heat transfer coefficients.Typical values of steady-state free convection coefficients for Peperomia obtusifolia varied from 1.5 × 10–4 to 1.9 × 10–4 cal cm–2 s–1 C–1 as the temperature difference was increased from 5.9 to 9.6°C, whereas the forced convection coefficient was found to be 4.2 × 10–4 cal cm–2 s–1 C–1 at 122 cm s–1 wind velocity. For egg-plant, this value was about 9 × 10–4 cal cm–2 s–1 C–1 at 488 cm s–1 wind velocity. Convection coefficients as determined under steady-state conditions are compared with those of the unsteady-state and with analytical values for a single leaf and leaves of three different plants. In general, experimental values were found to be higher than the analytical ones.  相似文献   

2.
Sea-surface stress measurements were made from a rigid tower in shallow water near San Diego, California, by both the direct covariance and inertial dissipation techniques. Stress estimates from the dissipation technique were generally higher than the directly measured values, with average drag coefficients of 0.99 x 10-3 and 0.77 x 10-3, respectively, for 8-m wind speeds of 5 to 7 m s-1. In the inertial subrange, ratios of vertical to streamwise velocity spectra averaged 1.06 ± 0.16, significantly less than the isotropic value of 4/3 observed over land, suggesting that turbulence over water may be altered by the presence of waves.  相似文献   

3.
The heat and mass transfer coefficients for exchange across the fluid dynamic boundary layer over tree leaves were simultaneously determined in a controlled environment chamber. The mass transfer coefficients were calculated from measured values of evaporation, air specific humidity and a value of leaf specific humidity at leaf temperature. The heat transfer coefficients were calculated from measured values of air temperature, leaf temperature and an estimate of the sensible heat flux density calculated as the measured net radiation at the leaf surfaces minus the latent heat flux density. The experiments described in this paper indicate that the equations based on laminar boundary-layer theory can give reasonable estimates of the transfer coefficients of real tree leaves for the velocities most commonly experienced in plant canopies, if they are adjusted by a constant multiplier greater than one. Calculations of local mass transfer coefficients based on temperature measurements at three locations at different distances from the leading edge of the leaves, indicate that the deviation from theory is probably the result of transition to turbulent boundary-layer flow at some distance from the leading edge.  相似文献   

4.
Moderating effects of Lake Apopka, Florida on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 °C under conditions of moderate winds (~4 m s–1). No substantial temperature effects were observed with surface wind speed less than 1 m s–1. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for non-advective conditions.IFAS Journal Series No. 1006.  相似文献   

5.
A technique was developed that allows the determination of the stable carbon isotope ratio of isoprene in air. The method was used for a limited number of ambient measurements as well as laboratory studies of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis), including the light and temperature dependence. The mean stable carbon isotope ratio ( 13C) of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis) for all our measurements is –27.7 ± 2.0 (standard deviation for 23 data points). Our results indicate a small dependence of the stable carbon isotope ratios on leaf temperature and photosynthetic photon flux density (PPFD). The light dependence is 0.0026 ± 0.0012/( mol of photons m–2 s–1) for the studied range from 400 to 1700 mol of photons m–2 s–1. The temperature dependence is 0.16 ± 0.09/K. On average, the emitted isoprene is 2.6 ± 0.9 lighter than the leaf carbon. An uncertainty analysis of the possibility to use stable carbon isotope ratio measurements of isoprene for estimates of its mean photochemical age suggests that meaningful results can be obtained. This is supported by the results of a small number of measurements of the stable carbon isotope composition of ambient isoprene at different locations. The results range from approximately –29 to –16. They are consistent with vegetation emissions of isoprene that is slightly depleted in 13C relative to the plant material and enrichment of 13C in the atmosphere due to isotope fractionation associated with the reaction with OH-radicals. The stable carbon isotope ratio of ambient isoprene at locations directly influenced by isoprene emissions is very close to the values we found in our emission studies, whereas at sites located remote from isoprene emitting vegetation we find substantial enrichment of 13C. This suggests that stable carbon isotope ratio measurements will be a valuable, quantitative method to determine the extent of photochemical processing of isoprene in ambient air.  相似文献   

6.
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.  相似文献   

7.
Bulk transfer coefficients were evaluated from eddy correlation flux measurements on a fixed pier during onshore winds. The mean values are C D = 1.69 × 10-3, C H = 2.58 × 10-3 and C E = 1.51 × 10-3. The drag coefficient, C D, gradually increases with wind speed but C H and C E are independent of wind speed. According to theory and empirical formulas based on experimental results over flat grassland, the transfer coefficients should gradually increase with increasing instability. This is confirmed experimentally in the stable region in our case. However, the drag coefficient appears to decrease with increasing instability, which is against the theoretical result. A stability dependence is not clearly observed for C H or C E.  相似文献   

8.
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.  相似文献   

9.
Geometric and aerodynamic roughness of sea ice   总被引：2，自引：0，他引：2  
The aerodynamic drag of Arctic sea ice is calculated using surface data, measured by an airborne laser altimeter and a digital camera in the marginal ice zone of Fram Strait. The influence of the surface morphology on the momentum transfer under neutral thermal stratification in the atmospheric boundary layer is derived with the aid of model concepts, based on the partitioning of the surface drag into a form drag and a skin drag. The drag partitioning concept pays attention to the probability density functions of the geometric surface parameters. We found for the marginal ice zone that the form drag, caused by floe edges, can amount to 140% of the skin drag, while the effect of pressure ridges never exceeded 40%. Due to the narrow spacing of obstacles, the skin drag is significantly reduced by shadowing effects on the leeward side of floe edges. For practical purposes, the fractional sea-ice coverage can be used to parameterize the drag coefficientC dn, related to the 10 m-wind. C dnincreases from 1.2 · 10-3 over open water to 2.8 · 10-3 for 55% ice coverage and decreases to 1.5 · 10-3 for 100% ice coverage.Aircraft turbulence measurements are used to compare the model values of C dnwith measureents. The correlation between measured and modelled drag coefficients results in r 2 = 0.91, where r is the correlation coefficient.  相似文献   

10.
Summary Whole-plant transpiration (T) measurements have many applications, but appropriate methods have remained somewhat elusive. A new method using a constant power heat balance gauge, wherein the xylem mass flow rate is calculated from a balance of heat into and out of a stem, has been shown to provide accurate stem flow measurements. To evaluate the applicability of this promising method to field experiments, cotton (Gossypium hirsutum L. GP 3774) stem flow measurements were compared withT measured from a weighing lysimeter. Initially to confirm method accuracy, stem flow values were compared in the glasshouse withT values determined by mass measurements of a potted plant. The root mean square error (RMSE) between the daylight losses from both (n = 16) was 8.6% of the mean measuredT values. In the field, hourly stem flow and lysimeterT values were also similar, but there was a large variation in stem flow values among the different plants. To account for differences in plant size between the plants with gauges and all lysimeter plants, stem flow values were adjusted using a stem area ratio factor, which adjusted values, on the average for the season, by 25%. Before adjustment, daylight stem flow totals were consistently greater than lysimeterT values. After adjustment, the means differed by only 9%, and theRMSE was reduced from 129 to 69 g plant–1 d–1. The coefficient of variation of daylight stem flow totals increased throughout the season. In the glasshouse, method accuracy was comparable (errors < ± 10%) to what has been previously determined. In the field, determining method accuracy was confounded by plant-to-plant variability and, possibly, by errors, unique to the gauge design used in this study, at high flow rates. Thus, this method can provide accurate flow measurements from individual herbaceous plants and is a valuable technique for many applications.With 7 Figures  相似文献   

11.
Air-sea bulk transfer coefficients in diabatic conditions   总被引：13，自引：0，他引：13  
On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u 10>3 m s–1, the coefficient for sensible heat C H is larger by about 10% than that for momentum C D . For u 10<5 m s–1, however, the value of C D exceeds the value of C H , and for u 10=15 m s–1 it is shown that C H 0.8C D . It may be also proposed that 103 C D =1.11 to 1.70, 103 C E =1.18 to 1.30, and 103 C H =1.15 to 1.26 for a range of u 10=4 to 20 m s–1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae.  相似文献   

12.
Below-cloud aerosol scavenging is generally estimated from field measurements using advanced instruments that measure changes in aerosol distributions with respect to rainfall. In this study, we discuss various scavenging mechanisms and scavenging coefficients from past laboratory and field measurements. Scavenging coefficients derived from field measurements (representing natural aerosols scavenging) are two orders higher than that of theoretical ones for smaller particles (Dp < 2 μm). Measured size-resolved scavenging coefficients can be served as a better option to the default scavenging coefficient (e.g. a constant of 10?4 s?1 for all size of aerosols, as used in the CALPUFF model) for representing below-cloud aerosol scavenging. We propose scavenging correction parameter (CR) as an exponential function of size-resolved scavenging coefficients, winds and width in the downwind of the source–receptor system. For a wind speed of 3 m s?1, CR decrease with the width in the downwind for particles of diameters Dp < 0.1 μm but CR does not vary much for particles in the accumulation mode (0.1 < Dp < 2 μm). For a typical urban aerosol distribution, assuming 3 m s?1 air-flow in the source–receptor system, 10 km downwind width, 2.84 mm h?1 of rainfall and using aerosol size dependent scavenging coefficients in the CR, scavenging of aerosols is found to be 16% in number and 24% in volume of total aerosols. Using the default scavenging coefficient (10?4 s?1) in the CALPUFF model, it is found to be 64% in both number and volume of total aerosols.  相似文献   

13.
Although the bulk aerodynamic transfer coefficients for sensible (C H ) and latent (C E ) heat over snow and sea ice surfaces are necessary for accurately modeling the surface energy budget, they have been measured rarely. This paper, therefore, presents a theoretical model that predicts neutral-stability values of C H and C E as functions of the wind speed and a surface roughness parameter. The crux of the model is establishing the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces on the basis of a surface-renewal model in which turbulent eddies continually scour the surface, transferring scalar contaminants across the interface by molecular diffusion. Matching these interfacial sublayer profiles with the semi-logarithmic inertial sublayer profiles yields the roughness lengths for temperature and water vapor. When coupled with a model for the drag coefficient over snow and sea ice based on actual measurements, these roughness lengths lead to the transfer coefficients. C E is always a few percent larger than CH. Both decrease monotonically with increasing wind speed for speeds above 1 m s–1, and both increase at all wind speeds as the surface gets rougher. Both, nevertheless, are almost always between 1.0 × 10–3 and 1.5 × 10–3.  相似文献   

14.
From measurements during the Atlantic Trade Wind Experiment (ATEX) 1969, amplitudes and phases of the diurnal harmonic water-temperature variation between the sea surface and 50-m depth and of the semi-diurnal wind variation between 1 and 8 m were obtained. If the vertical diffusion of heat in the ocean is thought to be constant, a coefficient of K= 320 cm2 s–1 in the equation of diffusion fits best the observed data in the mixed layer. However, the measurements point to a decrease of K with depth.The height variation of the semi-diurnal zonal wind wave is caused by the influence of eddy viscosity. Our data are well fitted by results of the equation of diffusion, using the assumption of Lettau (1974) that the transfer coefficient of vertical transport of momentum is not only a function of height but also depends on time because of the semi-diurnal variation of surface stress.  相似文献   

15.
Geostrophic drag coefficients are obtained from direct measurements of the momentum flux and from an objective analysis of the synoptic pressure field by the method of least squares. At a site in the Kiel Bight, a mean geostrophic drag coefficient c g = 0.0223 was obtained with near neutral/ slightly unstable conditions and a surface Rossby Number of 1.2 × 109.Contribution of the sonderforschungsbereich Meeresforschung Hamburg der Deutschen Forschungsgemeinschaft, Hamburg, F.R.G.  相似文献   

16.
The effect of turbulence on boundary-layer resistances to heat and water vapour transfer from leaves inclined to the mean airflow has been studied using heated square plates in a wind tunnel. Heat and water vapour transfer coefficients increased with streamwise turbulence intensity for all angles of inclination of the plates to the mean flow, and the increase was dependent on the ratio of the longitudinal integral length scale to the plate dimension. This dependence on the turbulence length scale probably results from a resonant interaction between the boundary layer on the plate and the turbulence in the approaching mean flow.The paper also presents results of experiments with heated plates having serrated leading edges and/or a transverse ridge on the surface, conducted in an attempt to understand the aerodynamic importance of morphological irregularities on the leaf surface. The irregularities studied here disturbed the boundary layer on the plate, and greatly increased heat transfer when the angle of inclination of the plates to the mean wind was small, but had little effect when the angle of inclination exceeded 40 °.  相似文献   

17.
Three recent experiments allow evaluation of the bulk transfer coefficients for momentum, water vapour and sensible heat over water bodies of different sizes. As part of a study of evaporation rates from a swamp, measurements of latent and sensible heat fluxes were made over Lake Wyangan in southern N.S.W., Australia. This lake is of several kilometers diameter. In a later experiment, Reynolds stress and sensible heat transfer were measured from a natural-gas platform standing in Bass Strait, south of mainland Australia. The most recent experiment involved the direct measurement of each of these turbulent fluxes from a fixed tower erected in Lake Michigan, U.S.A.Perhaps the most important of the results is the finding that drag coefficients measured over Bass Strait are not significantly different from those over Lake Michigan, despite the obvious differences in depth, fetch, and hence surface wave structure. At both locations, drag coefficients are found to increase slightly with increasing wind speed, while at low wind speeds they are not significantly different from those corresponding to aerodynamic smoothness.Near-neutral bulk transfer coefficients for sensible heat and for water vapour are found to be similar. An average value of about 1.4 × 10–3 is obtained.It is emphasized that stability effects should be considered in any discussion of drag coefficients or bulk transfer coefficients. Large errors can result if near-neutrality is incorrectly assumed.  相似文献   

18.
Four bulk schemes (LKB, FG, D and DB), with the flux-profile relationships of Liuet al. (1979), Francey and Garratt (1981), Dyer (1974), and Dyer and Bradley (1982), are derived from the viscous interfacial-sublayer model of Liuet al. These schemes, with stability-dependent transfer coefficients, are then tested against the eddy-correlation fluxes measured at the 50 m flight level above the western Atlantic Ocean during cold-air outbreaks. The bulk fluxes of momentum (), sensible heat (H), and latent heat (E) are found to increase with various von Kármán constants (k M for k H forH, andk E forE). Except that the LKB scheme overestimates by 28% (46Wm–2), on the average, the fluxes estimated by the four bulk schemes appear to be in fairly good agreement with those of the eddy correlation method (magnitudes of biases within 10% for , 17% forH, and 13% forE). The results suggest that the overall fluxes and surface-layer scaling parameters are best estimated by FG and thatk H <k E . On the average, the FG scheme underestimates by 10% (0.032N m–2) andE by 4% (12Wm–2), and overestimatesH by 0.3% (0.5W m–2). The equivalent neutral transfer coefficients at 10 m height of the FG scheme compare well with some schemes of those tested by Blanc (1985).The relative importance of various von Kármán constants, dimensionless gradients and roughness lengths to the oceanic transfer coefficients is assessed. The dependence of transfer coefficients on wind speeds and roughness lengths is discussed. The transfer coefficients for andE agree excellently between LKB and FG. However, the ratio of the coefficient forH of LKB to that of FG, increasing with decreasing stability, is very sensitive to stability at low winds, but approaches the neutral value of 1.25 at high winds.  相似文献   

19.
Emission rates of biogenic volatile organic compounds emitted by the forests were estimated for five geographical regions as well as for all Switzerland. Monoterpene and isoprene emissions rates were calculated for each main tree species separately using the relevant parameters such as temperature, light intensity and leaf biomass density. Biogenic emissions from the forests were found to be about 23% of the total annual VOC emissions (anthropogenic and biogenic) in Switzerland. The highest emissions are in July and lowest in January. Calculations showed that the coniferous trees are the main sources of the biogenic emissions. The major contribution comes from the Norway spruce (picea abies) forests due to their abundance and high leaf biomass density. Although broad-leaved forests cover 27% of all the forests in Switzerland, their contribution to the biogenic emissions is only 3%. Monoterpenes are the main species emitted, whereas only 3% is released as isoprene. The highest emission rates of biogenic VOC are estimated to be in the region of the Alps which has the largest forest coverage in Switzerland and the major part of these forests consists of Norway spruce. The total annual biogenic VOC emission rate of 87 ktonnes y–1 coming from the forests is significantly higher than those from other studies where calculations were carried out by classifying the forests as deciduous and coniferous. The difference is attributed to the high leaf biomass densities of Norway spruce and fir (abies alba) trees which have a strong effect on the results when speciation of trees is taken into account. Besides the annual rate, emission rates were calculated for a specific period during July 4–6, 1991 when a photochemical smog episode was investigated in the Swiss field experiment POLLUMET. Emission rates estimated for that period agree well with those calculated for July using the average temperatures over the last 10 years.  相似文献   

20.
Based on classic iterative computation results, new equations to calculate the surface turbulent transfer coefficients are proposed, which allow for large ratios of the momentum and heat roughness lengths. Compared to the Launiainen scheme, our proposed scheme generates results closer to classical iterative computations. Under unstable stratification, the relative error in the Launiainen scheme increases linearly with increasing instability, even exceeding 15%, while the relative error of the present scheme is always less than 8.5%. Under stable stratification, the Launiainen scheme uses two equations, one for 0 < Ri B ≤ 0.08 and another for 0.08 < Ri B ≤ 0.2, and does not consider the condition that Ri B > 0.2, while its relative errors in the region 0 < Ri B ≤ 0.2 exceed 31 and 24% for momentum and heat transfer coefficients, respectively. In contrast, the present scheme uses only one equation for 0 < Ri B ≤ 0.2 and another equation for Ri B > 0.2, and the relative error of the present scheme is always less than 14%.  相似文献   

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