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1.
An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length \(z_{0}\) varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant \(z_{0}\) of 1 mm leads to a mean friction velocity bias of \(24\,\%\) in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat \(z_{0t}\) to \(z_{0}\) on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached \(100\,\%\) when \(z/L > 1\); (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.  相似文献   

2.
We have proposed in a previous study a non-stationary wind model to represent the typhoon record as a summation of a time-varying mean wind speed (TVM) and a stationary turbulence. This note further suggests a quantitative scheme, rather than the previous qualitative method, to find the best TVM for any given wind record. Trial TVMs are first extracted from the wind record by a data-processing technique named empirical mode decomposition. For each TVM, its corresponding turbulent component is computed by removing the TVM from the original wind record, and the degree of stationarity of the turbulence component is checked. The best TVM is taken as the one that leads to the maximum degree of stationarity. The degree of stationarity of turbulence is quantified by two indicators: ?? the ratio of horizontal wind variability and wind speed; and ?? the ratio of friction velocity at different Reynolds averaging periods. The applicability of the suggested scheme is validated with 550 typhoon and 3300 monsoon records of 10 minute duration and at different measurement heights. Threshold values for the two stationary indicators ?? and ?? are determined using field measurements and their sensitivities to the Reynolds averaging periods are discussed. Observations in this study demonstrate that the suggested scheme is proper for finding the TVM of a wind record. For a stationarity quantification of 10 minute duration record, the ?? indicator with 30 second Reynolds averaging period is recommended.  相似文献   

3.
Field measurements were carried out to calculate the threshold friction velocity for snow saltation, and mass fluxes during snow drift. The wind was measured in three components by an ultrasonic anemometer, and the mass fluxes were determined using an optical sensor (snow particle counter), acoustic sensors (Flowcapt) and mechanical traps. The threshold friction velocity was found to be correlated to the grain size (R2=0.75). The mass flux measurements were compared with numerical simulations of snow drift, and it was demonstrated that the maximum snow transport takes place at shear stress values of roughly two times the average shear stress over 20 min. By implementing a probability distribution for the shear stress the mass flux was simulated with only the mean measured value of the shear stress as input. This procedure enables the future use of the numerical model for operational applications.  相似文献   

4.
A set of micro-meteorological data collected over a horizontal, uniform terrain (the plain of La Crau, France) in June 1987 is analysed. Conditions were predominantly sunny and arid, while due to the Mistral the wind speed could exceed 10 m/s. Verification of several methods to evaluate surface fluxes of heat, momentum and water vapour from the standard deviation of temperature, wind and specific humidity is presented. Also, a similar approach using the structure parameter of temperature is considered. These methods are all based on Monin-Obukhov (M-O) similarity theory. It is found that the standard deviation of temperature, vertical and horizontal wind speed as well as the structure parameter for temperature behave according to M-O similarity. It is shown that the sensible heat flux and friction velocity can be determined from a fast response thermometer and a cup anemometer. Also, it appears that the analytic solution of the set of governing equations as derived by the first author yields good results. M-O theory does not appear to work for the standard deviation of specific humidity. This may be due to the relative importance of large eddies.  相似文献   

5.
In this paper,the observational data from Marine and Meteorological Observation Platform(MMOP)at Bohe,Maoming and buoys located in Shanwei and Maoming are used to study the characteristics of air-sea temperature and specific humidity difference and the relationship between wind and wave with the tropical cyclones over the South China Sea(SCS).The heat and momentum fluxes from eddy covariance measurement(EC)are compared with these fluxes calculated by the COARE 3.0 algorithm for Typhoon Koppu.The results show that at the developing and weakening stages of Koppu,both these differences between the sea surface and the near-surface atmosphere from the MMOP are negative,and data from the buoys also indicate that the differences are negative between the sea surface and near-surface atmosphere on the right rear portion of tropical cyclones(TCs)Molave and Chanthu.However,the differences are positive on the left front portion of Molave and Chanthu.These positive differences suggest that the heat flux is transferred from the ocean to the atmosphere,thus intensifying and maintaining the two TCs.The negative differences indicate that the ocean removes heat fluxes from the atmosphere,thus weakening the TCs.The wind-wave curves of TCs Molave and Chanthu show that significant wave height increases linearly with 2-min wind speed at 10-m height when the wind speed is less than 25 m/s,but when the wind speed is greater than 25 m/s,the significant wave height increases slightly with the wind speed.By comparing the observed sensible heat,latent heat,and friction velocity from EC with these variables from COARE 3.0 algorithm,a great bias between the observed and calculated sensible heat and latent heat fluxes is revealed,and the observed friction velocity is found to be almost the same as the calculated friction velocity.  相似文献   

6.
The sensible heat flux (H) is determined using large-aperture scintillometer (LAS) measurements over a city centre for eight different computation scenarios. The scenarios are based on different approaches of the mean rooftop-level \((z_{H})\) estimation for the LAS path. Here, \(z_{H}\) is determined separately for wind directions perpendicular (two zones) and parallel (one zone) to the optical beam to reflect the variation in topography and building height on both sides of the LAS path. Two methods of \(z_{H}\) estimation are analyzed: (1) average building profiles; (2) weighted-average building height within a 250 m radius from points located every 50 m along the optical beam, or the centre of a certain zone (in the case of a wind direction perpendicular to the path). The sensible heat flux is computed separately using the friction velocity determined with the eddy-covariance method and the iterative procedure. The sensitivity of the sensible heat flux and the extent of the scintillometer source area to different computation scenarios are analyzed. Differences reaching up to 7% between heat fluxes computed with different scenarios were found. The mean rooftop-level estimation method has a smaller influence on the sensible heat flux (?4 to 5%) than the area used for the \(z_{H}\) computation (?5 to 7%). For the source-area extent, the discrepancies between respective scenarios reached a similar magnitude. The results demonstrate the value of the approach in which \(z_{H}\) is estimated separately for wind directions parallel and perpendicular to the LAS optical beam.  相似文献   

7.
A Simple Method of Estimating Scalar Fluxes Over Forests   总被引：1，自引：0，他引：1  
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions (friction velocity greater than 0.1 ms–1), and uses measurements of mean wind speed, and standard deviations of temperature and CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible heat flux and CO2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat and CO2 flux estimates that are well correlated with the measured values.  相似文献   

8.
The impact of sea waves on sensible heat and momentum fluxes is described. The approach is based on the conservation of heat and momentum in the marine atmospheric surface layer. The experimental fact that the drag coefficient above the sea increases considerably with increasing wind speed, while the exchange coefficient for sensible heat (Stanton number) remains virtually independent of wind speed, is explained by a different balance of the turbulent and the wave-induced parts in the total fluxes of momentum and sensible heat.Organised motions induced by waves support the wave-induced stress which dominates the surface momentum flux. These organised motions do not contribute to the vertical flux of heat. The heat flux above waves is determined, in part, by the influence of waves upon the turbulence diffusivity.The turbulence diffusivity is altered by waves in an indirect way. The wave-induced stress dominates the surface flux and decays rapidly with height. Therefore the turbulent stress above waves is no longer constant with height. That changes the balance of the turbulent kinetic energy and of the dissipation rate and, hence the diffusivity.The dependence of the exchange coefficient for heat on wind speed is usually parameterized in terms of a constant Stanton number. However, an increase of the exchange coefficient with wind speed is not ruled out by field measurements and could be parametrized in terms of a constant temperature roughness length. Because of the large scatter, field data do not allow us to establish the actual dependence. The exchange coefficient for sensible heat, calculated from the model, is virtually independent of wind speed in the range of 3–10 ms-1. For wind speeds above 10 ms-1 an increase of 10% is obtained, which is smaller than that following from the constant roughness length parameterization.The investigation was in part supported by the Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organization for Scientific Research (NWO).  相似文献   

9.
We investigate the momentum and energy exchange across the wave boundary layer (WBL). Directly at the air–sea interface, we test three wave-growth parametrizations by comparing estimates of the wave-induced momentum flux derived from wave spectra with direct covariance estimates of the momentum flux. An exponential decay is used to describe the vertical structure of the wave-induced momentum in the atmospheric WBL through use of a decay rate, a function of the dimensionless decay rate and wavenumber (A?=?α k). The decay rate is varied to minimize the difference between the energy extracted from the WBL and the energy flux computed from wave spectra using our preferred wave-growth parametrization. For wave ages (i.e. the peak phase speed to atmospheric friction velocity ratio) in the range \( 15 < c_{p}/u_{*} < 35 \) we are able to balance these two estimates to within 10%. The decay rate is used to approximate the WBL height as the height to which the wave-induced flux is 0.1 of its surface value and the WBL height determined this way is found to be between 1–3 m. Finally, we define an effective phase speed with which to parametrize the energy flux for comparison with earlier work, which we ultimately attempt to parametrize as a function of wind forcing.  相似文献   

10.
A Wind Tunnel Model for Quantifying Fluxes in the Urban Boundary Layer   总被引：9，自引：6，他引：3  
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes.  相似文献   

11.
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12.
Inverse methods are widely used in various fields of atmospheric science. However, such methods are not commonly used within the boundary-layer community, where robust observations of surface fluxes are a particular concern. We present a new technique for deriving surface sensible heat fluxes from boundary-layer turbulence observations using an inverse method. Doppler lidar observations of vertical velocity variance are combined with two well-known mixed-layer scaling forward models for a convective boundary layer (CBL). The inverse method is validated using large-eddy simulations of a CBL with increasing wind speed. The majority of the estimated heat fluxes agree within error with the proscribed heat flux, across all wind speeds tested. The method is then applied to Doppler lidar data from the Chilbolton Observatory, UK. Heat fluxes are compared with those from a mast-mounted sonic anemometer. Errors in estimated heat fluxes are on average 18 %, an improvement on previous techniques. However, a significant negative bias is observed (on average $-63\,\%$ ) that is more pronounced in the morning. Results are improved for the fully-developed CBL later in the day, which suggests that the bias is largely related to the choice of forward model, which is kept deliberately simple for this study. Overall, the inverse method provided reasonable flux estimates for the simple case of a CBL. Results shown here demonstrate that this method has promise in utilizing ground-based remote sensing to derive surface fluxes. Extension of the method is relatively straight-forward, and could include more complex forward models, or other measurements.  相似文献   

13.
Observations using a three-dimensional scanning coherent Doppler lidar in an urban area revealed the characteristics of streaky structures above a rough, inhomogeneous surface for a high-Reynolds-number flow. The study focused on two points: (1) the frequency of occurrence and conditions required for the presence of streaky structures, and (2) the universal scaling of the spacing of streaky structures (\(\lambda )\). The horizontal snapshots of the radial velocity were visually classified into six groups: Streak, Mixed, Fishnet, No streak, Front, and Others. The Streak category accounted for more than 50% of all possible flows and occurred when the horizontal wind speed was large and the atmospheric stratification was near-neutral. The spacing (\(\lambda )\) was estimated from the power spectral density of the streamwise velocity fluctuations along the spanwise direction. The spacing \(\lambda \) decreased with an increase in the local velocity gradient. Furthermore, it was revealed that the local velocity gradient normalized by the friction velocity and the boundary-layer height (\(z_i )\) comprehensively predicts \(\lambda /z_i \) under various experimental and environmental conditions, in terms of the scale of motion (i.e., indoor and outdoor scales), thermal stratification (i.e., from weakly unstable to stable stratification), and surface roughness (i.e., from flat to very rough surfaces).  相似文献   

14.
The influence of wave-associated parameters controlling turbulent \(\hbox {CO}_2\) fluxes through the air–sea interface is investigated in a coastal region. A full year of high-quality data of direct estimates of air–sea \(\hbox {CO}_2\) fluxes based on eddy-covariance measurements is presented. The study area located in Todos Santos Bay, Baja California, Mexico, is a net sink of \(\hbox {CO}_2\) with a mean flux of \(-1.3\, \upmu \hbox {mol m}^{-2}\hbox {s}^{-1}\) (\(-41.6\hbox { mol m}^{-2}\hbox {yr}^{-1}\)). The results of a quantile-regression analysis computed between the \(\hbox {CO}_2\) flux and, (1) wind speed, (2) significant wave height, (3) wave steepness, and (4) water temperature, suggest that the significant wave height is the most correlated parameter with the magnitude of the flux but the behaviour of the relation varies along the probability distribution function, with the slopes of the regression lines presenting both positive and negative values. These results imply that the presence of surface waves in coastal areas is the key factor that promotes the increase of the flux from and into the ocean. Further analysis suggests that the local characteristics of the aqueous and atmospheric layers might determine the direction of the flux.  相似文献   

15.
Wind speed profiles above a forest canopy relate to scalar exchange between the forest canopy and the atmosphere. Many studies have reported that vertical wind speed profiles above a relatively flat forest can be classified by a stability index developed assuming wind flow above a flat plane. However, can such a stability index be used to classify vertical wind speed profiles observed above a sloping forest at nighttime, where drainage flow often occurs? This paper examines the use of the bulk Richardson number to classify wind speed profiles observed above a sloping forest at nighttime. Wind speed profiles above a sloping forest were classified by the bulk Richardson number Ri B . Use of Ri B distinguished between drainage flow, shear flow, and transitional flow from drainage flow to shear flow. These results suggest that Ri B is useful to interpret nighttime CO2 and energy fluxes above a sloping forest. Through clear observational evidence, we also show that the reference height should be high enough to avoid drainage-flow effects when calculating Ri B .  相似文献   

16.
Summary Hourly lysimetric and micrometeorological data taken over a grass surface at the Meteorological Research Unit, Cardington U.K. have been analysed. A temperature difference and measurements of wind speed at only one height, combined with an independently estimated effective roughness length allowed sensible heat and momentum fluxes determination by the profile method on an hourly basis. The estimates are compared with direct measurements of sensible heat and friction velocity obtained by the eddy correlation method. The sensible and latent heat fluxes are also modelled by the resistance method. Equations based on the Monin—Obukhov similarity theory are used to account for stability effects through various forms of parameterization Aerodynamic and surface resistances, necessary for the Penman—Monteith equation are calculated from routinely measured meteorological data. The profile method for estimation of sensible heat flux and friction velocity is found to work excellently on the discussed daytime experimental data which correspond mainly to near neutral or slightly unstable conditions.Surface latent and sensible heat fluxes can also be described very well by the resistance method. A slightly better estimate of the sensible heat flux is achieved when stability corrections are taken into account. On the contrary Penman-Monteith equation for estimating latent heat flux is insensitive to adjustments for atmospheric stability.The comparison of the various methods leads to the establishment of empirical relationships which correlate various quantities such as soil heat flux, resistances, evapotranspiration etc. to routinely measured meteorological data.With 8 Figures  相似文献   

17.
This note is devoted to the problem of the appropriate scaling of parameters relevant for sea waves, such as wave height, peak frequency, duration, and fetch. In the past, the growth of sea waves has often been analysed in terms of the wind velocity at a fixed height, despite the fact that many authors have stressed the importance of scaling with the friction velocity. This problem would be immaterial if the ratio between the friction velocity and the wind speed at a fixed height were a constant. There is, however, ample evidence that this ratio increases with wind speed (Smith and Banke, 1975; Smith, 1980), in agreement with dimensional considerations by Charnock (1955) on the friction height. As a result, the scaling problem is an important one. In this note we conjecture that the correct procedure is to scale wave parameters with friction velocity, and we discuss experimental evidence for the correctness of this conjecture. Comparing two independent datasets (JONSWAP and KNMI), we find some evidence supporting our ideas. Further confirmation remains desirable, however, and suggestions are made as to how this might be obtained.  相似文献   

18.
The turbulence characteristics measured in the surface layer over a real underlying surface are presented. The measurements are carried out at the 3- and 16-m levels using acoustic anemometers; at the lower level, the measurements are doubled. The diurnal cycle is obtained of the temperature flux and friction velocity. To obtain the temperature flux, so-called “acoustic” temperature is used as calculated from the sound speed data with corrections for air humidity and pressure. The normalized characteristics of turbulence are presented as dependent on dimensionless height z/L. The values of temperature flux and friction velocity increase with height: their day-integral values at 16-m level exceed those at 3-m level for about 20 and 35%, respectively.  相似文献   

19.
The statistics of momentum exchange in the urban roughness sublayer are investigated. The analysis focuses on the characteristics of the dimensionless friction velocity, \({u_{*}}/U\) , which is defined as the square root of the drag coefficient. The turbulence observations were made at a height of 47 m above the ground on the 325-m meteorological tower, which is located in a very inhomogeneous urban area in Beijing. Under neutral conditions, the dependence of the drag coefficient on wind speed varies with wind direction. When the airflow is from the area of densely built-up buildings, the drag coefficient does not vary with wind speed, while when the airflow is from the area covered by vegetation, the drag coefficient appears to decrease with increasing wind speed. Also, the drag coefficient does not vary monotonically with the atmospheric stability. Both increasing stability and increasing instability lead to the decrease of the drag coefficient, implying that the roughness length and zero-plane displacement may vary in urban areas.  相似文献   

20.
Remarks on the Definition and Estimation of Friction Velocity   总被引：3，自引：1，他引：2  
One of the mainscaling parameters in similarity theory of the atmospheric boundary layer is friction velocity. Unfortunately, several definitions of friction velocity exist in the literature. Some authors use the component of the horizontal Reynolds stress vector in the direction of the mean wind vector to define friction velocity. Others define the friction velocity by means of the absolute value of the horizontal Reynolds stress vector. The two definitions coincide only if the direction of the mean wind vector is parallel to the horizontal Reynolds stress vector. In general, the second definition gives larger values for the friction velocity. Over complex terrain the situation is further complicated by the fact that the terrain following flow is not necessarily horizontal. Thus, several authors have proposed to use terrain following coordinate systems for the definition of friction velocity. By means of a large dataset of fast-response wind measurements with an ultrasonic anemometer the friction velocities resulting from the different definitions are compared. Furthermore, it is shown that friction velocity can be well estimated from horizontal wind speed, and even better from simple horizontal or vertical turbulence parameters.  相似文献   

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