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1.
In order to study energy and water cycles in the Huaihe River Basin, micrometeorological measurements were carried out in Shouxian County, Anhui Province, during HUBEX/IOP (May to August 1998 and June to July 1999). The employed techniques included Bowen Ratio-Energy Balance (BREB) and Eddy Covariance (EC) methods. In this paper, the basic characteristics of the energy balance components in the district are analyzed. Furthermore, the results are compared with those from other regions of China.The main results are as follows: (1) There was a consistency between the available energy (Rn-G) and the sum of sensible (H) and latent (E) heat fluxes measured by the EC method (H+E)ec, but Ebr was slightly larger (about 10%) than Eec; (2) Most of the net radiation (Rn) was used to evaporate water from the surface. During HUBEX/IOP in 1998 and 1999, the mean daily amounts of Rn were 13.89 MJ m-2 d-1 and 11.83 MJ m-2 d-1, and the mean Bowen Ratios (β) were 0.14 (over ruderal) and 0.06 (over paddy) respectively; (3) The diurnal variation characteristic of β was larger and unsteady at sunrise and sunset, and smaller and steady during the rest of the daytime. Local advection appeared in the afternoon over paddy areas in 1999; (4) In comparison with the results from other regions of China, the nean β was the lowest (0.06) over paddy areas in the Huaihe River Basin and the highest (0.57) during June-August 1998 in Inner Mongolia grassland. The Bowen Ratio β is mainly related to the soil humidity.  相似文献   

2.
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   

3.
Previous measurements of urban energy balances generally have been limited to densely built, central city sites and older suburban locations with mature tree canopies that are higher than the height of the buildings. In contrast, few data are available for the extensive, open vegetated types typical of low-density residential areas that have been newly converted from rural land use. We made direct measurements of surface energy fluxes using the eddy-covariance technique at Greenwood, a recently developed exurban neighbourhood near Kansas City, Missouri, USA, during an intensive field campaign in August 2004. Energy partitioning was dominated by the latent heat flux under both cloudy and near clear-sky conditions. The mean daytime Bowen ratio (β) values were 0.46, 0.48, and 0.47 respectively for the cloudy, near clear-sky and all-sky conditions. Net radiation (R n ) increased rapidly from dawn (−34 and −58W m−2) during the night to reach a maximum (423 and 630W m−2) after midday for cloudy and near clear-sky conditions respectively. Mean daytime values were 253 and 370W m−2, respectively for the cloudy and near clear-sky conditions, while mean daily values were 114 for cloudy and 171W m−2 for near clear-sky conditions, respectively. Midday surface albedo values were 0.25 and 0.24 for the cloudy and near clear-sky conditions, respectively. The site exhibited an angular dependence on the solar elevation angle, in contrast to previous observations over urban and suburban areas, but similar to vegetated surfaces. The latent heat flux (Q E ), sensible heat flux (Q H ), and the residual heat storage ΔQ s terms accounted for between 46–58%, 21–23%, and 18–31% of R n , respectively, for all-sky conditions and time averages. The observed albedo, R n , and Q E values are higher than the values that have been reported for suburban areas with high summer evapotranspiration rates in North America. These results suggest that the rapidly growing residential areas at the exurban fringe of large metropolitan areas have a surface energy balance that is more similar to the rural areas from which they were developed than it is to the older suburbs and city centres that make up the urban fabric to which they are being joined.  相似文献   

4.
Sensible (H) and latent (LvE) heat fluxes are obtained by a combined energy budget – similarity model applied to observations from Melle in Belgium and Cabauw in The Netherlands. The sensitivity to both the stability functions and the accuracy of input data is investigated. In a first step, fluxes are calculated for a selection of stability functions and compared to values obtained with pre-defined (reference) functions. For the diurnal fluxes higher than 10 W m−2 in 1996 at Melle, the root-mean-square rmsreaches 9 W m−2 for H and 6 W m−2 for LvE, depending on the chosen functions. A lesser sensitivity is obtained at Cabauw and can be explained by lower absolute values of the stability parameter ζ (L involving the Obukhov length) mainly induced by higher mean wind speeds. Different stability bins are also considered. It is concluded that a more accurate assessment of the stability functions is already desirable for absolute values of L above a few metres. These values are not so scarce at Melle and should be captured in the future by an increasing number of new developing long-term measurement stations. In a second step, a statistical approach is proposed with errors depicted by both systematic biases and random fluctuations represented by means of Gaussian distributions. The results show that very accurate measurements are needed in order to maintain the mean annual value of the bias and rms below 5–10 W m−2, and thus to allow the discrimination between the sensitivity to errors on input data and to the stability functions selection.  相似文献   

5.
Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z i , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, φ m , has been found to be a function of both z/L and z i /L. At constant z/L, φ m decreases with decreasing boundary layer height. Since φ m is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m−2 or about 1% and the mean surface stress increase by 1.4 mN m−2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m−2 and an increase of the mean annual surface stress by 2.6 mN m−2.  相似文献   

6.
An extensive meteorological dataset obtained from the plumevalidation experiment conducted by the Electric Power Research Institute (EPRI) atKincaid during 1980–1981 is analysed for studying the characteristic differences in thesurface-layer parameters in strong and weak wind stable conditions. The surface-layerparameters are computed using the similarity functions m and h proposed byBeljaars and Holtslag. The weak winds are characterized using the geostrophic wind speedas well as the wind speed at the 10-m level. The surface fluxes are found to be finitein weak wind conditions.Empirical formulations for the eddy diffusivities of momentum(KM) and heat (KH), and drag (CD) and heat exchange (CH) coefficients, as powerlaw functions of the bulk Richardson number (RiB), are proposed under both strong andweak wind conditions. Results are close to those based on observations taken from the IndianInstitute of Technology low wind diffusion experiment, the Land surface processes experiment,the Hanford diffusion experiment, the Cabauw field experiment and the Cooperative Atmospheric SurfaceExchange Study 1999 (CASES-99) experiment. In addition, the fluxes obtained fromthe proposed empirical relations are in good agreement with those based on similarity theory as wellas the turbulence measurements taken from the CASES-99 experiment.  相似文献   

7.
In unstable conditions, the set of equations defining the aerodynamic resistance to sensible heat transfer, r a , cannot be solved analytically. An iterative technique must be used to obtain r a exactly, but this is cumbersome and time consuming. In this paper, a new, empirical equation is presented relating the ratio, Q, of the aerodynamic resistances in neutral and unstable conditions, to the bulk Richardson number, Ri B . The equation takes the form Q = a + b(–Ri) c , where a, b and c are empirical functions of (z – d)/z om . This model is shown to predict r awith a mean absolute error of 0.06 s m–1 over the ranges -15 < Ri B < 0 and 10 < (z – d)/z om < 2300. Statistical comparison with other equations that have been proposed for r a in unstable conditions indicates the superior precision of the model presented here.  相似文献   

8.
Local Scales of Turbulence in the Stable Boundary Layer   总被引：1，自引：1，他引：0  
Local, gradient-based scales, which contain the vertical velocity and temperature variances, as well as the potential temperature gradient, but do not include fluxes, are tested using data collected during the CASES-99 experiment. The observations show that the scaling based on the temperature variance produces relatively smaller scatter of empirical points. The resulting dimensionless statistical moments approach constant values for sufficiently large values of the Richardson number Ri. This allows one to derive predictions for the Monin–Obukhov similarity functions φ m and φ h , the Prandtl number Pr and the flux Richardson number Rf in weak turbulence regime.  相似文献   

9.
 The diurnal range of surface air temperature (rT a ) simulated for present and doubled CO2 climates by the CSIRO9 GCM is analysed. Based on mean diurnal cycles of temperature and surface heat fluxes, a theory for understanding the results is developed. The cycles are described as the response to a diurnal forcing which is represented well by the diurnal mean flux of net shortwave radiation at the surface (SW) minus the evaporative (E) and sensible (H) fluxes. The response is modified by heat absorbed by the ground, and by the cycle in downward longwave (LW) radiation, but these effects are nearly proportional to the range in surface temperature. Thus in seasonal means, rT a is approximately given by SW–E–H divided by 6 W m-2/°C. A multiple regression model for (rT a ) is developed, based on quantities known to influence SW, E and H, and applied to both spatial variation in seasonal means, and day-to-day variation at a range of locations. In both cases, rT a is shown to be influenced by cloud cover, snow extent and wind speed. It is influenced by soil moisture, although this effect is closely tied to that of cloud. In seasonal means rT a is also well correlated with precipitable water, apparently because of the latter’s influence on E+H. The regression model describes well the spatial variation in the doubled CO2 change in rT a . The annual mean change in rT a over land on doubling CO2 was −0.36 °C, partly because of a decrease in the mean diurnal forcing (as defined in the theory), but also apparently because of the effect of nonlinearity in T s of the upward longwave emission. A diagnostic radiation calculation indicates that the CO2 and water vapour provide a small increase in rT a through the downward LW response, which partially counters a decrease due to a reduction of SW by the gases. Received: 8 November 1995 / Accepted: 3 January 1997  相似文献   

10.
Increased precipitation during the vegetation periods was observed in and further predicted for Inner Mongolia. The changes in the associated soil moisture may affect the biosphere-atmosphere exchange of greenhouse gases. Therefore, we set up an irrigation experiment with one watered （W） and one unwatered plot （UW） at a winter-grazed Leymus chinensis-steppe site in the Xilin River catchment, Inner Mongolia. UW only received the natural precipitation of 2005 （129 mm）, whereas W was additionally watered after the precipitation data of 1998 （in total 427 mm）. In the 3-hour resolution, we determined nitrous oxide （N20）, methane （CH4） and carbon dioxide （CO2） fluxes at both plots between May and September 2005, using a fully automated, chamber-based measuring system. N20 fluxes in the steppe were very low, with mean emissions （±s.e.） of 0.9-4-0.5 and 0.7-4-0.5 μg N m^-2 h^-1 at W and UW, respectively. The steppe soil always served as a CH4 sink, with mean fluxes of -24.1-4-3.9 and -31.1-4- 5.3 μg C m^-2 h^-1 at W and UW. Nighttime mean CO2 emissions were 82.6±8.7 and 26.3±1.7 mg C m^-2 h^-1 at W and UW, respectively, coinciding with an almost doubled aboveground plant biomass at W. Our results indicate that the ecosystem CO2 respiration responded sensitively to increased water input during the vegetation period, whereas the effects on CH4 and N2O fluxes were weak, most likely due to the high evapotranspiration and the lack of substrate for N2O producing processes. Based on our results, we hypothesize that with the gradual increase of summertime precipitation in Inner Mongolia, ecosystem CO2 respiration will be enhanced and CH4 uptake by the steppe soils will be lightly inhibited.  相似文献   

11.
The empirical dependence of turbulence Prandtl number (Pr) on gradient Richardson number (Ri) is presented, derived so as to avoid the effects of self-correlation from common variables. Linear power relationships between the underlying variables that constitute both Pr and Ri are derived empirically from flux and profile observations. Pr and Ri are then reconstructed from these power laws, to indicate their interdependence whilst avoiding self-correlation. Data are selected according to the stability range prior to regression, and the process is iterated from neutral to higher stability until error analysis indicates the method is no longer valid. A Butterworth function is fitted to the resulting Pr −1(Ri) regression to give an empirical summary of the analysis. The form suggests that asymptotically Pr −1 decreases as Ri 3/2. Scatter in the data increases above Ri ~ 1, however, indicating additional constraints to Pr are not captured by Ri alone in this high stability regime. The Butterworth function is analytic for all Ri > 0, and may be included in suitable boundary-layer parameterisation schemes where the turbulent diffusivity for heat is derived from the turbulent diffusivity for momentum.  相似文献   

12.
Summary  Turbulent fluxes of CO2 were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period with snow cover, fluxes of CO2 to the atmosphere were small, typically 0.005 mg CO2 m−2 s−1 (0.41 g CO2 m−2 d−1), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the onset of vegetation growth, fluxes of CO2 to the atmosphere were typically 0.1 mg CO2 m−2 s−1 in the afternoon, and daily sums reached values up to almost 9 g CO2 m−2 d−1. After 4 July, downward fluxes of CO2 increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values of about −0.23 mg m−2 s−1 at midday and max values of daily sums of −12 g CO2 m−2 d−1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO2 m−2. Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements. On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO2 m−2 yr−1. Received October 6, 1999 Revised May 2, 2000  相似文献   

13.
The aerodynamic effects of various configurations of an urban array were investigated in a wind-tunnel experiment. Three aerodynamic parameters characterising arrays—the drag coefficient (C d ), roughness length (z o) and displacement height (d)—are used for analysis. C d is based on the direct measurement of the total surface shear using a floating element, and the other two parameters are estimated by logarithmic fitting of the measured wind profile and predetermined total drag force. The configurations of 63 arrays used for measurement were designed to estimate the effects of layout, wind direction and the height variability of the blocks on these parameters for various roughness packing densities. The results are summarised as follows: (1) The estimated C d and z o of the staggered arrays peak against the plan area index (λ p ) and frontal area index (λ f ), in contrast with values for the square arrays, which are less sensitive to λ p and λ f . In addition, the square arrays with a wind direction of 45° have a considerably larger C d , and the wind direction increases z o/H by up to a factor of 2. (2) The effect of the non-uniformity of roughness height on z o is more remarkable when λ f exceeds 20%, and the discrepancy in z o is particularly remarkable and exceeds 200%. (3) The effect of the layout of tall blocks on C d is stronger than that of short blocks. These results indicate that the effects of both wind direction and the non-uniformity of the heights of buildings on urban aerodynamic parameters vary greatly with λ p and λ f ; hence, these effects should be taken into account by considering the roughness packing density.  相似文献   

14.
Zusammenfassung Für eine Reihe von Atmosphärenmodellen, die untereinander gleiche Temperaturschichtung und konstante relative Feuchtigkeit, aber verschieden hohe Temperatur und Feuchtigkeit im festen Niveau haben, wird der NettostrahlungsstromE in 400 mb und am Boden durch Berechnungen in den bekannten Strahlungsdiagrammen bestimmt. Es läßt sich zeigen, daßE 400 (E B ) als Funktion der Temperaturt 400 (t B ) und der Feuchtigkeit bzw. des Taupunktes 400 ( B ) dargestellt werden kann [Abb. 1 und 3, Gl. (1) und (3)], ählich, wie das für die effektive Strahlung des BodensE B bereits früher durch die Formeln vonÅngström, Brunt u. a. geschehen ist. Wolkenschichten können durch Korrektionsfaktoren berücksichtigt werden. Die Abweichungen zwischen den so gewonnenen Strahlungssummen und den durch Planimetrieren im Strahlungsdiagramm berechneten sind im Durchschnitt kleiner als 3,5% (11%), für die Differenz E=E 400–E B sind sie kleiner als 6%.
Summary For a series of model atmospheres which have the same stratification of temperature and constant relative humidity but different temperatures and humidities at a fixed level, the net radiation fluxE at 400 mb and at the surface are determined by computations in the well known radiation charts. It is shown thatE 400 (E B ) can be represented as a function of the temperaturet 400 (t B ) and humidity resp. dew point 400 ( B ) [Fig. 1 and 3, formula (1) and (3)], in a similar manner as the outgoing radiationE B is already approximated by the formulae ofÅngström, Brunt, a. o. Cloud layers may be accounted for by correction factors. The differences between the radiation fluxes found in this way and those computed by integration in a radiation chart are in the average smaller than 3.5% (11%); with E=E 400–E B , the differences are smaller than 6%.
Résumé Pour plusieurs modèles de l'atmosphère qui ont la même structure par rapport à la température et une humidité relative constante, mais à un niveau fixe des températures et humidités différentes, on calcule à l'aide de diagrammes de radiation connus le flux net de radiationE pour 400 mb et au sol. On peut montrer queE 400 (E B ) peut être représenté comme fonction de la températuret 400 (t B ) et de l'humidité, resp. du point de rosée 400 ( B ) [Fig. 1 et 3, formules (1) et (3)], tout comme cela a déjà été fait pour la radiation effective au solE B grâce aux formules d'Ångström, deBrunt et d'autres. Par un facteur de correction on tient compte des couches de nuages. Les écarts entre les sommes de radiation ainsi obtenues et celles calculées à l'aide des diagrammes de radiation sont en moyenne plus petits que 3,5% (11%), la différence E=E 400–E B ne dépasse pas 6%.

Mit 4 Textabbildungen.  相似文献   

15.
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   

16.
A study of the oxidation mechanism of N-methyl pyrrolidinone (C5H9NO, NMP) initiated by hydroxyl radicals was made at EUPHORE at atmospheric pressure (1000 ± 10) mbar of air and ambient temperature (T = 300 ± 5 K). The main products were N-methyl succinimide (NMS) (52 ± 4)% and N-formyl pyrrolidinone (FP) (23 ± 9)%. The relative rate technique was used to determine the rate constants of OH with NMP, NMS and FP, the measured values were (in units of cm3 molecule − 1 s− 1): kNMP = (2.2 ± 0.4) × 10− 11, kNMS = (1.4 ± 0.3) × 10− 12 and kFP = (6 ± 1) × 10− 12. The results are presented and discussed in terms of the atmospheric impact.  相似文献   

17.
Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH4+ and ~32% NO3– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH4+ and NO3− in rainwater ranged from 1.7–55 μmol L−1 and 0.16–18 μmol L−1, respectively, accounting for ~87% by NH4+ and ~13% by NO3− of total inorganic N (median values for all data). A significant correlation (r = 0.74, p < 0.05, n = 10) between NH4+ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations were found between NH4+ and MSA in rainwater collected over the subarctic (r = 0.42, p > 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH3) by marine biological activity from the ocean could become a significant source of NH4+ over the South Pacific. While NO3− was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH4+ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m−2 d−1, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.  相似文献   

18.
Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (R n ) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λE) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H) and λE, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between T and the net ecosystem exchange (NEE) of carbon dioxide (CO2), forcing T to closure while retaining the Bowen ratio and applying the same factor to CO2 fluxes (F C ) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship between T, F C and C.  相似文献   

19.
Turbulent mixing across heat-stratified density interfaces was studied in the laboratory using oscillating-grid generated turbulence. The aim was to study the transition between the entrainment regimes dominated by interfacial wave-breaking and molecular diffusion, and to study the characteristics of the latter. It was observed that, above a critical Richardson number Ric, which depends on the Peclet number Pe, the mixing due to wave breaking disappears and that Ric Pe−n, where the mean value of the exponent n is approximately . Above Ric, the entrainment is molecular-diffusion dominated and takes place through a sequence of events: the buoyancy gradient of the initially sharp density interface is weakened by molecular diffusion until the mixed-layer eddies can engulf a portion of the interfacial layer wherefore the interface sharpens again. Thus, the entrainment events are recurrent with a rate-controlling diffusion stage between them. An entrainment law of the form E Ri−2Pe−2, where E is the entrainment coefficient and Ri is the Richardson number, is suggested for the diffusion-dominated entrainment regime.  相似文献   

20.
Air temperature T a , specific humidity q, CO2 mole fraction χ c , and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri b . For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri b < 0.5. For strongly stable conditions (Ri b > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χ c have with Ri b are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO2 near the ground being 5–7 μmol mol−1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri b -binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO2 advection over heterogenous, complex terrain are discussed.  相似文献   

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