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1.
Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective fluxes, primarily of CO2. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget, different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime energy budget closure and might change ecosystem respiration fluxes considerably.  相似文献   

2.
Towards Closing the Surface Energy Budget of a Mid-latitude Grassland   总被引：4，自引：1，他引：3  
Observations for May and August, 2005, from a long-term grassland meteorological station situated in central Netherlands were used to evaluate the closure of the surface energy budget. We compute all possible enthalpy changes, such as the grass cover heat storage, dew water heat storage, air mass heat storage and the photosynthesis energy flux, over an averaging time interval. In addition, the soil heat flux was estimated using a harmonic analysis technique to obtain a more accurate assessment of the surface soil heat flux. By doing so, a closure of 96% was obtained. The harmonic analysis technique appears to improve closure by 9%, the photosynthesis for 3% and the rest of the storage terms for a 3% improvement of the energy budget closure. For calm nights (friction velocity u * < 0.1 m s−1) when the eddy covariance technique is unreliable for measurement of the vertical turbulent fluxes, the inclusion of a scheme that calculates dew fluxes improves the energy budget closure significantly.  相似文献   

3.
In the context of CO2 surface exchange estimation, an analysis combining the basic principles of diffusion and scalar conservation shows that the mixing ratio is the appropriate variable both for defining the (eddy covariance) turbulent flux and also for expressing the relationship between the turbulent flux and surface exchange in boundary-layer budget equations. Other scalar intensity variables sometimes chosen, both the CO2 density and molar fraction, are susceptible to the influence of surface exchange of heat and water vapour. The application of a hypsometric analysis to the boundary-layer “control volume” below the tower measurement height reveals flaws in previously applied approaches: (a) incompressibility cannot be assumed to simplify mass conservation (the budget in terms of CO2 density); (b) compressibility alone makes the analysis of mass conservation vulnerable to uncertainties associated with resultant non-zero vertical velocities too small to measure or model over real terrain; and (c) the WPL (Webb et al. (1980) Quart J Roy Meteorol Soc 106:85–100) “zero dry air flux” assumption is invalidated except at the surface boundary. Nevertheless, the definition and removal of the WPL terms do not hinge upon this last assumption, and so the turbulent CO2 flux can be accurately determined by eddy covariance using gas analysers of either open- or closed-path design. An appendix discusses the necessary assumptions and appropriate interpretations for deriving the WPL terms.  相似文献   

4.
In order to provide high quality data for climate change studies, the data quality of turbulent flux measurements at the station of SACOL (Semi-Arid Climate & Environment Observatory of Lanzhou University), which is located on a semi-arid grassland over the Loess Plateau in China, has been analyzed in detail. The effects of different procedures of the flux corrections on CO2, momentum, and latent and sensible heat fluxes were assessed. The result showed that coordinate rotation has a great influence on the momentum flux but little on scalar fluxes. For coordinate rotation using the planar fit method, different regression planes should be determined for different wind direction sectors due to the heterogeneous nature of the ground surface. Sonic temperature correction decreased the sensible heat flux by about 9%, while WPL correction (correction for density fluctuations) increased the latent heat flux by about 10%. WPL correction is also particularly important for CO2 fluxes. Other procedures of flux corrections, such as the time delay correction and frequency response correction, do not significantly influence the turbulent fluxes. Furthermore, quality tests on stationarity and turbulence development conditions were discussed. Parameterizations of integral turbulent characteristics (ITC) were tested and a specific parameterization scheme was provided for SACOL. The ITC test on turbulence development conditions was suggested to be applied only for the vertical velocity. The combined results of the quality tests showed that about 62%–65% of the total data were of high quality for the latent heat flux and CO2 flux, and as much as about 76% for the sensible heat flux. For the momentum flux, however, only about 35% of the data were of high quality.  相似文献   

5.
Summary Based on measurements made in March 1982 in the Inn valley during the MERKUR experiment, an attempt was made to compute the mass budget of a large alpine valley during periods of mountain and valley wind.The computations come from measurements of the alongvalley mass flux and assumptions on the fluxes in the slope layer and tributaries.Vertical motions in the valley's atmosphere have been evaluated from the mass budget computations. These motions, including the subsidence which compensates for daytime upslope winds and the subsidence which compensates for the valley wind flowing into tributaries during the day, are of great importance for the understanding of the thermal circulation.The results allow better estimation of vertical advection, which contributes to the budgets of momentum and energy.With 8 Figures  相似文献   

6.
Carbon dioxide, latent and sensible heat fluxes were measured by means of the eddy covariance method above a mountain meadow situated on a steep slope in the Stubai Valley in Austria, based on the hypothesis that, due to the low canopy height, measurements can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were subjected to a rigorous testing procedure using a series of quality control measures established for surface-layer flows. The resulting high quality dataset comprised 36% of the original observations, the substantial reduction being mainly due to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition periods. The validity of the high quality dataset was further assessed by two independent tests: (i) a comparison with the net ecosystem carbon dioxide exchange measured by means of ecosystem chambers, and (ii) the ability of the eddy covariance measurements to close the energy balance. The net ecosystem CO2 exchange measured by the eddy covariance method agreed reasonably well with ecosystem chamber measurements. The assessment of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated by 28% and 29%, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain.  相似文献   

7.
Summary Eddy correlation measurements of surface energy fluxes above a meadow site are presented and discussed in terms of energy balance closure. The degree of closure changed with time and wind direction, which can be explained by temporal and spatial variations of the surrounding vegetation. More frequently than not, a tendency towards an energy “deficit” was observed, that means, the sum of turbulent fluxes was less than the available energy. As a possible reason, we cautiously suggest that large-scale flux components may have to be taken into account. If so, then a convenient way to obtain fairly accurate total surface energy fluxes would be to correct the turbulent fluxes with an appropriate factor that forces the budget to perfect closure. Received October 14, 1996 Revised October 1, 1998  相似文献   

8.
Based on the micrometeorological measurements at a heterogeneous farmland in South China, this work detects the effects of vegetative heterogeneity and patch-scale harvest on the energy balance closure and turbulent fluxes. As a quality control, the integral turbulent characteristics are analyzed in the framework of Monin-Obukhov similarity theory. Spatial representativeness of the measurements is studied in terms of footprint and source area. Firstly, in two wind sectors, the nondimensional standard deviations of turbulent quantities generally agree with some foregoing studies. Discrepancies exist in the other sectors due to the instrument-induced flow distortion. Secondly, energy balance closure is examined with two types of linear regression, which confirms that mismatching source areas between the available energy and turbulent fluxes have no preference to either energy “deficit” or “surplus”. Thirdly, turbulent fluxes exhibit greater variability when they represent smaller source areas. The patch-scale harvest adjacent to the flux mast causes notable increase and decrease in the sensible heat and latent heat fluxes, respectively, while the CO2 exchange almost vanishes after the harvest. Interestingly, energy balance closure is less influenced despite the notable effects on the turbulent fluxes and Bowen ratio, implying that the energy balance closure check may mask some variability in the turbulent fluxes. Thus, to adjust the heat fluxes with a single “closure factor” for a perfect closure is dangerous at a patchy site.  相似文献   

9.
In this work, three turbulence closure models, Mellor andYamada level 2.5, E - l and E - implemented in a circulation model, are compared in neutral condition over complex terrain. They are firstly applied to a one-dimensional case on flat terrain and then to a schematic two-dimensional valley. The simulation results, in terms of wind field and turbulent kinetic energy, are tested against measurements from a wind-tunnel experiment. The empirical constants defining the characteristic length scales of the closures are modified based on turbulence parameters estimated in the experiment. The formulation of the diffusion coefficients is analysed to explain the differences among the various closures in the simulation results. Regarding the mean flow, both on flat and complex terrain, all the closures yield satisfactory results. Concerning the turbulent kinetic energy, the best results are obtained by E - l and E - closures.  相似文献   

10.
Model Simulations of the Boundary-Layer Evolution over an Arid Andes Valley   总被引：1，自引：1，他引：0  
The boundary layer of the Elqui valley in the arid north of Chile exhibits several interesting phenomena, such as a very shallow convective boundary layer (CBL) during the day. In the morning, warming is observed in and above the CBL, while the humidity decreases in the CBL. At midday, in and above the CBL of the valley, the temperature stagnates. In the afternoon in the CBL the temperature decreases and humidity increases, although the latent heat flux is very low. Because the characteristic features of the valley atmosphere are hard to interpret from observations alone, model simulations were applied. The simulations indicate that all components of the budget equations, i.e. the turbulent flux divergences, advection via the sea breeze, the upvalley and upslope wind systems, as well as subsidence, contribute to the evolution of the valley atmosphere.  相似文献   

11.
We examine the space–time structure of the wind and temperature fields, as well as that of the resulting spatial temperature gradients and horizontal advection of sensible heat, in the sub-canopy of a forest with a dense overstorey in moderately complex terrain. Data were collected from a sensor network consisting of ten stations and subject to orthogonal decomposition using the multiresolution basis set and stochastic analyses including two-point correlations, dimensional structure functions, and various other bulk measures for space and time variability. Despite some similarities, fundamental differences were found in the space–time structure of the motions dominating the variability of the sub-canopy wind and temperature fields. The dominating motions occupy similar spatial, but different temporal, scales. A conceptual space–time diagram was constructed based on the stochastic analysis that includes the important end members of the spatial and temporal scales of the observed motions of both variables. Short-lived and small-scale motions govern the variability of the wind, while the diurnal temperature oscillation driven by the surface radiative transfer is the main determinant of the variability in the temperature signal, which occupies much larger time scales. This scale mismatch renders Taylor’s hypothesis for sub-canopy flow invalid and aggravates the computation of meaningful estimates of horizontal advective fluxes without dense spatial information. It may further explain the ambiguous and inconclusive results reported in numerous energy and mass balance and advection studies evaluating the hypothesis that accounting for budget components other than the change in storage term and the vertical turbulent flux improves the budget closure when turbulent diffusion is suppressed in plant canopies. Estimates of spatial temperature gradients and advective fluxes were sensitive to the network geometry and the spatial interpolation method. The assumption of linear spatial temperature gradients was not supported by the results, and leads to increased spatial and temporal variability of inferred spatial gradients and advection estimates. A method is proposed to estimate the appropriate minimum network size of wind and temperature sensors suitable for an evaluation of energy and mass balances by reducing spatial and temporal variability of the spatially sampled signals, which was estimated to be on the order of 200 m at the study site.  相似文献   

12.
Summary ?Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above ground level, νval, and the large-scale flow velocity, νlar, are between νval/νlar ≈ 0.1 and νval/νlar ≈ 1. The νval/νlar ratio exhibits a strong dependence on thermal stratification, δ, and decreases from νval/νlar ≈ 1 at δ = 0 K m−1 to νval/νlar ≈ 0.2 at δ = 0.0075 K m−1. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the νval/νlar ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The νval/νlar ratio is characterized by small values in the direct lee side (νval/νlar ≈ 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (νval/νlar ≈ 0.8). Received October 22, 2001; revised June 18, 2002; accepted June 23, 2002  相似文献   

13.
Numerical analysis of flux footprints for different landscapes   总被引：5，自引：0，他引：5  
Summary A model for the canopy – planetary boundary layer flow and scalar transport based on E- closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns – forest with clear-cuts as well as hypothetical heterogeneous relief – a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown – the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends on the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Solling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites.  相似文献   

14.
This contribution investigates the nature of turbulent kinetic energy (TKE) in a steep and narrow Alpine valley under fair-weather summertime conditions. The Riviera Valley in southern Switzerland was chosen for a detailed case study, in which the evaluation of aircraft data (obtained from the MAP-Riviera field campaign) is combined with the application of high-resolution (350-m horizontal grid spacing) large-eddy simulations using the numerical model ARPS. The simulations verify what has already been observed on the basis of measurements: TKE profiles scale surprisingly well if the convective velocity scale w * is obtained from the sun-exposed eastern slope rather than from the surface directly beneath the profiles considered. ARPS is then used to evaluate the TKE-budget equation, showing that, despite sunny conditions, wind shear is the dominant production mechanism. Therefore, the surface heat flux (and thus w *) on the eastern slope does not determine the TKE evolution directly but rather, as we believe, indirectly via the interaction of thermally-driven cross-valley and along-valley flows. Excellent correlation between w * and the up-valley wind speed solidifies this hypothesis.  相似文献   

15.
Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near the coast and in the valley. Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze winds of 3–4 m s−1 prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night. The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day, no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about 150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence is of minor relevance to the moisture budget of the boundary layer. Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany  相似文献   

16.
We investigate the evolution of the early-morning boundary layer in a low-mountain valley in south-western Germany during COPS (convective and orographically induced precipitation study) in summer 2007. The term low-mountain refers to a mountainous region with a relief of gentle slopes and with an absolute altitude that remains under a specified height (usually 1,500 m a.s.l.). A subset of 23 fair weather days from the campaign was selected to study the transition of the boundary-layer flow in the early morning. The typical valley atmosphere in the morning hours was characterized by a stable temperature stratification and a pronounced valley wind system. During the reversal period—called the low wind period—of the valley wind system (duration of 1–2 h), the horizontal flow was very weak and the conditions for free convection were fulfilled close to the ground. Ground-based sodar observations of the vertical wind show enhanced values of upward motion, and the corresponding statistical properties differ from those observed under windless convective conditions over flat terrain. Large-eddy simulations of the boundary-layer transition in the valley were conducted, and statistical properties of the simulated flow agree with the observed quantities. Spatially coherent turbulence structures are present in the temporal as well as in the ensemble mean analysis. Thus, the complex orography induces coherent convective structures at predictable, specific locations during the early-morning low wind situations. These coherent updrafts, found in both the sodar observations and the simulation, lead to a flux counter to the gradient of the stably stratified valley atmosphere and reach up to the heights of the surrounding ridges. Furthermore, the energy balance in the surface layer during the low wind periods is closed. However, it becomes unclosed after the onset of the valley wind. The partition into the sensible and the latent heat fluxes indicates that missing flux components of sensible heat are the main reason for the unclosed energy balance in the considered situations. This result supports previously published investigations on the energy balance closure.  相似文献   

17.
Summary Two- and three-dimensional fields of meteorological variables are constructed from measurements in a small Alpine end valley during daytime of two fine weather days. These are fields of the surface energy balance, of temperature and wind. With these data the energy budget of the valley atmosphere is calculated. In the morning, the sensible heat which is released on the valley surface is used to heat the valley atmosphere. The advection terms are rather small. In the afternoon, the energy gain is nearly completely exported out of the valley region. The transport is managed by advection with the valley wind and the related upward motion, which are branches of the valley wind system which overlap into the next larger valley. The role of a small valley in the interplay of the thermal circulation systems in the heating of the mountainous boundary layer is discussed.
Erwärmung eines kleinen Alpentals
Zusammenfassung Messungen in einem kleinen alpinen Endtal gestatten die Berechnung zwei- und dreidimensionaler Felder meteorologischer Größen tagsüber an zwei Schönwettertagen. Dies sind Felder der Terme der Energiebilanz am Untergrund, der Temperatur und des Windes. Daraus läßt sich der Energiehaushalt der Talatmosphäre berechnen. Am Vormittag wird die am Untergrund freigesetzte fühlbare Wärme zur Erwärmung des Tals genutzt, die Advektionsterme sind ziemlich klein. Am Nachmittag wird dieser Wärmegewinn nahezu vollständig aus dem Talbereich exportiert. Dieser Transport wird durch die Advektion mit dem Talwind und dem resultierenden Aufwind bewerkstelligt, die Zweige eines ins nächste Tal übergreifenden Talwindsystems sind. Die Rolle eines kleinen Alpentals im Zusammenspiel der verschiedenen thermischen Zirkulationssysteme bei der Erwärmung der Gebirgsgrenzschicht wird diskutiert.

With 6 Figures  相似文献   

18.
The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations.  相似文献   

19.
An investigation into high Reynolds number turbulent flow over a ridge top in New Zealand is described based on high-resolution in-situ measurements, using ultrasonic anemometers for two separate locations on the same ridge with differing upwind terrain complexity. Twelve 5-h periods during neutrally stratified and weakly stable atmospheric conditions with strong wind speeds were sampled at 20 Hz. Large (and small) turbulent length scales were recorded for both vertical and longitudinal velocity components in the range of 7–23 m (0.7–3.3 m) for the vertical direction and 628–1111 m (10.5–14.5 m) for the longitudinal direction. Large-scale eddy sizes scaled to the WRF (Weather Research and Forecasting) numerical model simulated boundary-layer thickness for both sites, while small-scale turbulent features were a function of the complexity of the upwind terrain. Evidence of a multi-scale turbulent structure was obtained at the more complex terrain site, while an assessment of the three-dimensional isotropy assumption in the inertial subrange of the spectrum showed anisotropic turbulence at the less complex site and evidence of isotropic turbulence at the more complex site, with a spectral ratio convergence deviating from the 4/3 or unity values suggested by previous theory and practice. Existing neutral spectral models can represent locations along the ridge top with simple upwind complexity, especially for the vertical wind spectra, but sites with more orographic complexity and strong vertical wind speeds are often poorly represented using these models. Measured spectra for the two sites exhibited no significant diurnal variation and very similar large-scale and small-scale turbulent length scales for each site, but the turbulence energy measured by the variances revealed a strong diurnal difference.  相似文献   

20.
中国北方疆域辽阔,多种植被类型交错分布,陆面过程较为复杂.研究该区域地表能量平衡及分配过程,有助于进一步了解其生态系统的生产力及气候变化.本文使用"中国北方协同观测实验"观测资料,选取中国北方四种植被类型——高寒草甸、荒漠草地、玉米农田及半干旱草地,在深入认识地表能量分配特征的基础上,讨论了环境因子对能量分配的影响.结...  相似文献   

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