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1.
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   

2.
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   

3.
The vertical turbulent fluxes have been determined during the Atlantic Trade Wind Experiment (ATEX) both by direct and profile methods. The drag coefficient obtained from direct measurements was c D = 1.39 × 10–3. A distortion of the wind profile due to wave action could be demonstrated, this produced an increased drag coefficient estimated by the profile method. The dissipation technique using the downwind spectrum gave a lower drag coefficient of 1.26 × 10–3, probably due to non-isotropic conditions (the ratio of vertical to downwind spectrum at high frequencies scattered considerably with an average of 1 instead of 4/3).From direct measurements, the sensible heat flux showed a poor correlation with the bulk parameter product U, contrary to the heat flux obtained from profiles. It is shown that this is due to the higher frequency part of the cospectrum, say above 0.25 Hz, which contributes more than 50 % of the total flux. Determination of the heat flux from temperature fluctuations by the dissipation method would be in agreement with the direct determination only if the corresponding Kolmogoroff constant were 2.1 instead of 0.8.For the vertical flux of water vapor obtained from profiles, the bulk transfer coefficient was 1.28 × 10–3.This work was supported by the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm Meeresforschung and later the Sonderforschungsbereich Meeresforschung Hamburg.  相似文献   

4.
We analyze gust probe measurements obtained in the convective boundary layer over the central equatorial Pacific as part of the Equatorial Pacific Ocean Climate Studies (EPOCS) program. From the lowest level flights, the bulk transfer coefficients are found to be 1.1 × 10-3 and 1.4 × 10-3 for latent and sensible heat fluxes, respectively. Vertical profiles of water vapor density, potential temperature and wind velocity are obtained as are the profiles of the fluxes of latent and sensible heat and momentum. From the extrapolated profiles, we obtain surface fluxes of 120 W m-2 and 13 W m-2 for latent and sensible heat, respectively, and 0.11 N m-2 for momentum. The 10 m drag coefficient is 1.5 × 10-3. Two convergence boxes, north and south of the ITCZ, are analyzed. Enhanced convergence is found in the northern trades relative to the southern trades. The advective acceleration is found to be comparable in magnitude to the other terms in the horizontal equations of motion.  相似文献   

5.
Absolute rate coefficient measurements have been carried out for the reactions of Cl atoms with propene and a series of 3-halopropenes, at room temperature (298 ± 2) K using a newly constructed laser photolysis-resonance fluorescence (PLP-RF) system. The rate coefficients obtained (in units of cm3 molecule–1 s–1) are: propene (1.40± 0.24) ×10–10, 3-fluoropropene (4.92 ± 0.42) ×10–11, 3-chloropropene (7.47 ± 1.50) × 10–11, 3-bromopropene (1.23± 0.14) ×10–10 and 3-iodopropene (1.29± 0.15) ×10–10. In order to test this new system, the reactions of Cl atoms with acetone and isoprene have also been studied and compared with data previously reported. The rate coefficients determined at room temperature for these last two reactions are (2.93 ± 0.20) ×10–12 cm3 molecule–1 s– 1 and (3.64± 0.20)×10–10 cm3 molecule–1 s–1, respectively. The measured values were independent of pressure over the range 20–200 Torr. The influence of the different halogen atoms substituents on the reactivity of these alkenes with Cl atoms as well as the atmospheric implications of these measurements are studied and discussed for the first time in this work and compared with the reactivity with NO3 and OH radicals.  相似文献   

6.
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   

7.
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   

8.
Summary Field observations were carried out in order to determine the transfer coefficeient of sensible heat flux above a melting snow surface at the Moshiri experimental site. The coefficient is calculated as the ratio of the sensible heat flux determined by the eddy correlation method using a sonic anemometer to the product of the wind speed and the temperature difference between the air and snow surface. The sensible heat fluxes are also compared with the result of the precise heat balance observations. The nondimensional transfer coefficienth shows a good correlation with the atmospheric stability,Ri. The value ofh=2.3×10–3 is obtained in the range of 0<Ri<0.1, however, it is smaller and scattered under stronger stable condition (Ri>0.1). The dimensional transfer coefficent for sensible heat flux is calculated, and a linear relationship is obtained as a function of the logarithm of atmospheric stability.With 11 Figures  相似文献   

9.
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   

10.
For the first time, the exchange coefficient of heat CH has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land atJodhpur (26°18' N, 73°04' E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law CH = 0.025U10 -0.7 (where U10 is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient CDhas already been reported in an earlier study. Low winds (<4 m s-1) are associated with both near neutral and strong unstable situations. It is noted that CH increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients,corrected using Monin–Obukhov (M–O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger's formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditionsdiscussed in a coming paper.  相似文献   

11.
The relative rate technique has been used to measure rate constants for the reaction of chlorine atoms with peroxyacetylnitrate (PAN), peroxypropionylnitrate (PPN), methylhydroperoxide, formic acid, acetone and butanone. Decay rates of these organic species were measured relative to one or more of the following reference compounds; ethene, ethane, chloroethane, chloromethane, and methane. Using rate constants of 9.29×10–11, 5.7×10–11, 8.04×10–12, 4.9×10–13, and 1.0×10–13 cm3 molecule–1 sec–1 for the reaction of Cl atoms with ethene, ethane, chloroethane, chloromethane, and methane respectively, the following rate constants were derived, in units of cm3 molecule–1 s–1: PAN, <7×10–15; PPN, (1.14±0.12)×10–12; HCOOH, (2.00±0.25)×10–13; CH3OOH, (5.70±0.23)×10–11; CH3COCH3, (2.37±0.12)×10–12; and CH3COC2H5, (4.13±0.57)×10–11. Quoted errors represent 2 and do not include possible systematic errors due to errors in the reference rate constants. Experiments were performed at 295±2 K and 700 torr total pressure of nitrogen or synthetic air. The results are discussed with respect to the previous literature data and to the modelling of nonmethane hydrocarbon oxidation in the atmosphere.In recent discussions with Dr. R. A. Cox of Harwell Laboratory, UKAEA, we learnt of a preliminary value for the rate constant of the reaction of Cl with acetone of (2.5±1.0)×10–12 cm3 molecule–1 sec–1 measured by R. A. Cox, M. E. Jenkin, and G. D. Hayman using molecular modulation techniques. This value is in good agreement with our results.  相似文献   

12.
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   

13.
Bulk transfer coefficient over a snow surface   总被引：1，自引：0，他引：1  
The drag coefficient C D and the bulk transfer coefficient for sensible heat C H over a flat snow surface were determined experimentally. Theoretical considerations reveal that C D depends on the friction velocity u * as well as on the geometrical roughness h of the snow surface. It is found that C D increases with increasing u * and/or h. The dependency of C H on u * and h is so small that it is possible to consider C H as a constant for practical purposes: C H, 1 = 2.0 × 10–3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at C E, 1 = 2.1 × 10–3 for a reference height of 1 m.  相似文献   

14.
A combined study of the OH gas phase reaction and uptake on aqueous surfacesof two carbonates, dimethyl and diethyl carbonate has been carried out todetermine the atmospheric lifetimes of these compounds. Rate coefficients havebeen measured for gas phase reactions of OH radicals with dimethyl and diethylcarbonate. The experiments were carried out using pulsed laser photolysis– laser induced fluorescence over the temperature range 263–372K and the kinetic data were used to derive the following Arrhenius expressions(in units of cm3 molecule–1 s–1):for dimethyl carbonate, k1 = (0.83±0.27)×10–12 exp [–(247± 98)/T] and fordiethyl carbonate, k2 = (0.46±0.15)×10–12 exp [(503± 203)/T]. At 298 K, therate coefficients obtained (in units of 10–12 cm3molecule–1 s–1) are: k1 =(0.35± 0.04) and k2 = (2.31± 0.29). The results arediscussed in terms of structure-activity relationships.The uptake coefficients of both carbonates on aqueous surfaces were measuredas a function of temperature and composition of the liquid phase, using thedroplet train technique coupled to a mass spectrometric detection. Dimethyland diethyl carbonate show very similar results. For both carbonates, themeasured uptake kinetics were found to be independent of the aqueous phasecomposition (pure water, NaOH solutions) but dependent on gas-liquid contacttime which characterises a surface saturation effect. The uptake coefficientvalues show a slight negative temperature dependence for both carbonates.These values vary from 1.4×10–2 to0.6×10–2 in the temperature range of 265–279 Kfor dimethyl carbonate, from 2.4×10–2 to0.9×10–2 in the temperature range of 270–279 Kfor diethyl carbonate. From the kinetic data, the following Henry's lawconstants were derived between 279 and 265 K: dimethyl carbonate,H1 = 20–106 M atm–1; and diethyl carbonate,H2 = 30–98 M atm–1. The reported data showthat the OH reaction is the major atmospheric loss process of these twocarbonates with lifetimes of 33 and 5 days, respectively, while the wetdeposition is a negligible process.  相似文献   

15.
The absolute rate constants for the gas-phasereactions of the NO3 radical with a series ofaldehydes such as acetaldehyde, propanal, butanal,pentanal, hexanal and, heptanal were measured overthe temperature range 298–433 K, using a dischargeflow system and monitoring the NO3 radical byLaser Induced Fluorescence (LIF).The measured rate constants at 298 K for thereaction of NO3, in units of 10–14 cm3molecule–1 s–1, were as follows:acetaldehyde 0.32 ± 0.04, propanal 0.60 ± 0.06, butanal 1.46± 0.16, pentanal 1.75 ±0.06, hexanal 1.83 ± 0.36, and heptanal 2.37 ±0.42. The proposed Arrhenius expressions arek1 = (6.2 ± 7.5) × 10–11 exp[–(2826 ± 866)/T] (cm3 molecule–1s–1),k2 = (1.7 ± 1.0) × 10–11 exp[–(2250 ± 192)/T] (cm3 molecule–1s1), k3 =(7.6 ± 9.8) × 1011 exp[–(2466 ± 505)/T] (cm3 molecule–1s–1),k4 = (2.8 ± 1.4) × 10–11 exp[–(2189 ± 156)/T] (cm3 molecule–1s–1), k5 = (7.0 ± 1.8) ×10–11 exp [–(2382 ± 998)/T](cm3 molecule–1 s–1), andk6 = (7.8 ± 1.0) × 10–11 exp[–(2406 ± 481)/T](cm3 molecule–1 s–1).Tropospheric lifetimes for these aldehydes werecalculated at night and during the day for typicalNO3 and OH average concentrations and showed thatboth radicals provide an effective tropospheric sinkfor these compounds and that the night-time reactionwith the NO3 radical can be an important, if notdominant, loss process for these emitted organics andfor NO3 radicals.  相似文献   

16.
The following temperature-dependent rate coefficients (k/cm3 molecule–1 s–1) of the reactions of hydroxyl radicals with aliphatic ethers have been determined over the temperature range 247–373 K by a competitive flow technique: diethyl ether,k OH=5.2×10–12 exp[(262±150)/T]; methyln-butyl ether,k OH=5.4×10–12 exp[(309±150)/T]; ethyln-butyl ether,k OH=7.3×10–12 exp[(335±150)/T]; di-n-butyl ether,k OH=5.5×10–12 exp[(502±150)/T] and di-n-pentyl ether,k OH=8.5×10–12 exp[(417±150)/T]. The data have been measured relative to the rate coefficientk(OH + 2,3-dimethylbutane)=6.2×10–12 cm3 molecule–1 s–1 independent of temperature.Previous discrepancies in the room-temperature rate coefficients for the OH reactions with ethyln-butyl ether and di-n-butyl ether, obtained in the flow and static experiments of Bennett and Kerr (J. Atmos. Chem. 8, 87–94, 1989;10, 29–38, 1990) compared with those of Wallingtonet al. (Int. J. Chem. Kinet. 20, 541–547, 1988;21, 993–1001, 1989) and of Nelsonet al. (Int. J. Chem. Kinet. 22, 1111–1126, 1990) have been resolved. The results are considered in relation to the available literature data and evaluated rate expressions are deduced where possible. The data are also discussed in terms of structure-activity relationships.  相似文献   

17.
We present here experimental determinations of mass accommodation coefficients using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase.We report in this article measurements of exp=(6.0±0.8)×10–2 at 298 K and with a total pressure of 38 Torr for SO2, (5.0±1.0)×10–2 at 297 K and total pressure of 52 Torr for HNO3, (1.5±0.6)×10–3 at 298 K and total pressure of 50 Torr for NO2, (2.4±1.0)×10–2 at 290 K and total pressure of 70 Torr for NH3.These values are corrected for mass transport limitations in the gas phase leading to =(1.3±0.1)×10–1 (298 K) for SO2, (1.1±0.1)×10–1 (298 K) for HNO3, (9.7±0.9)×10–2 (290 K) for NH3, (1.5±0.8)×10–3 (298 K) for NO2 but this last value should not be considered as the true value of for NO2 because of possible chemical interferences.Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.  相似文献   

18.
Rate coefficients have been measured for the gas phasereactions of hydroxyl (OH) radicals and ozone with twounsaturated esters, allyl acetate(CH3C(O)OCH2CH=CH2) and isopropenylacetate (CH3C(O)OC(CH3)=CH2). The OHexperiments were carried out using the pulsed laserphotolysis – laser induced fluorescence technique overthe temperature range 243–372 K and the kinetic dataused to derive the following Arrhenius expressions (inunits of cm3 molecule-1 s-1): allylacetate, k 1 = (2.33 ± 0.27) ×10-12 exp[(732 ± 34)/T]; and isopropenyl acetate,k 2 = (4.52 ± 0.62) × 10-12exp[(809 ± 39)/T]. At 298 K, the rate coefficients obtained (inunits of 10-12 cm3 molecule-1 s-1)are: k 1 = (27.1 ± 3.0) and k 2= (69.6± 9.4). The relative rate technique has been usedto determine rate coefficients for the reaction ofozone with the acetates. Using methyl vinyl ketone asthe reference compound and a value of4.8 × 10-18 cm3 molecule-1s-1 asthe rate coefficient for its reaction with O3,the following rate coefficients were derived at 298 ± 4 K (in units of10-18 cm3molecule-1 s-1): allyl acetate, (2.4 ± 0.7) andisopropenyl acetate (0.7 ± 0.2). Theresults are discussed in terms of structure-activityrelationships and used to derive atmospheric lifetimesfor the acetates.  相似文献   

19.
An analytical error analysis of profile-derived fluxes of heat, moisture, and momentum, along with stability and roughness length, is performed using the accuracies of the constituent temperature, humidity, and wind speed measurements. Five experiments, representing more than two thirds of the existing marine profile data presently contained in the literature, are compared. Much of the profile data examined was used to develop the transfer coefficients presently employed by a large number of competing bulk aerodynamic flux schemes. Depending upon the experiment, typical profile-method measurement errors were found to range from 15 to 35% for a sensible heat flux of ± 10 W m-2, from 15 to 105% for a latent heat flux of ± 100 W m-2, from 10 to 40% for a stress of 0.05 N m-2, from 15 to 60% for a Monin-Obukhov stability of ± 0.05, and from 25 to 100% for a roughness length of 2 × 10-4 m. Smaller magnitude flux values were found to contain typical errors as large as 100% for sensible heat flux, 300% for latent heat flux, and 60% for stress.  相似文献   

20.
A procedure for the formulation of bulk transfer coefficients over water   总被引：3，自引：0，他引：3  
A method suitable for predicting bulk transfer coefficients appropriate to any reasonable height of measurement in the atmospheric surface boundary layer and incorporating the effects of atmospheric stability is based on the assumption that eddy and molecular diffusivities are additive near a water surface. This assumption is supported in the case of sensible heat, by results obtained over Lake Michigan and over an industrial cooling pond at Dresden, Illinois, as well as by published measurements made over Lake Flevo, Holland. The verification appears to extend to wind speeds in the range 10–15 m s–1. The results permit evaluation of transfer coefficients applicable in the demanding situations of inland lakes and artificial cooling ponds.Work performed under the auspices of the U.S. Energy Research and Development Administration.  相似文献   

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