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1.
A radar equation which can be applied to any bistatic acoustic sounder is derived. The equation reduces to the expression normally used for the special case of the monostatic sounder. Numerical results using this equation are given for specific acoustic sounders, including the relative contributions to the scattering from the temperature and velocity parameters (C T 2 and C V 2), the effect of wind speed on the scattered intensity and the measurement of horizontal wind velocity.Now with the Department of Electrical and Electronic Engineering, Portsmouth Polytechnic, Anglesea Road, Portsmouth PO1 3DJ.  相似文献   

2.
The friction velocity, the surface heat flux and the height of the Atmospheric Boundary Layer (ABL) are important parameters. In this work, vertical velocity variance ( w 2 ) and wind velocity structure parameter (C v 2 ) profiles estimated by acoustic sounder measurements are used, along with similarity relations, to estimate these parameters in the unstable Atmospheric Boundary Layer and the friction velocity in the stable one. The data were collected by two acoustic sounders with different height range and resolution under various atmospheric conditions (stability) and at two experimental sites in different terrain. The C v 2 profiles are estimated using gate difference of the vertical velocity measurements and the assumption of local isotropy. The vertical velocity data are corrected for the significant effects of noisy measurements and sampling volume averaging on the w 2 and C v 2 estimations using original techniques that are presented in this work. The results of the similarity method using acoustic sounder data are compared against estimates of the corresponding atmospheric parameters obtained from direct measurements. The comparison confirms the ability of the method to provide reasonably accurate estimates of these parameters especially in the middle of the day.  相似文献   

3.
Mean and fluctuating wind velocities were measured above a flexible stand (weeping-lovegrass). A waving phenomenon Honami appeared over the stand during the observation period. Some spectral parameters were derived from the vertical wind fluctuations. A dependency of frequency on mean horizontal wind velocity was found. The result, n m = 0.66u, was obtained under the range of wind speeds from 0.9 m s-1 to 3.1 m s-1 just above the canopy.  相似文献   

4.
Results are presented which show that reasonably accurate estimates of the atmospheric temperature structure paparmeter C T 2 can be derived from the returns of a monostatic acoustic sounder in both stable and convective atmospheric conditions.  相似文献   

5.
The spatial variability and temporal behavior of the vertical flux of ozone have been investigated from turbulence measurements collected on aircraft flight legs in the daytime period during two consecutive summer experimental field programs. The data were obtained during horizontal flight legs conducted over agricultural crops and forested land in three different regions of the eastern United States.Results from individual experimental cases and statistics derived from all cases in each region are presented. Ozone flux generally exhibited a significant height dependency. The strongest negative (downward) fluxes in the lowest-level flight legs were primarily attributed to the uptake of ozone by the surface and vegetative cover. Fluxes were near-zero in the middle of the convective boundary layer (CBL) in the afternoon period. As ozone flux was proportional to concentration, slightly stronger fluxes were found in low-level urban plume segments where ozone concentrations were 10–20 ppb higher than in the surrounding area. The derived deposition velocity showed no such bias as a function of position across the urban plume. Ozone flux differences were not apparent between the more heavily forested sections and the primarily agricultural cropland areas in these regions. During the afternoon period when no clear temporal trend was evident, means from values obtained below 0.15Z i (Z i being the CBL height) were -0.421 and -0.431 ppb m-2 s-1 for ozone flux and 0.81 and 0.82 cm s-1 for the derived mean deposition velocity in the southeastern Pennsylvania and central Ohio areas, respectively. These experimental results for ozone provide support to a dry deposition parameterization module which computes grid-area averaged deposition velocities for use in regional-scale models.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.  相似文献   

6.
A dataset from two campaigns conducted at the Vielsalm experimental site in Belgium was used as a basis for discussing some methodological problems and providing intermediate results on estimating CO2 advection. The analysis focused on the horizontal [CO2] gradient and on the vertical velocity w, the variables most affected by uncertainty. The sampling error for half-hourly horizontal [CO2] gradients was estimated to be 1.3 μmol mol−1. Despite this important random error for half-hour estimations of [CO2], the mean horizontal [CO2] gradients in advective conditions were shown to be representative at the ecosystem scale and to extend only to the lowest part of a drainage sub-layer, which developed in the trunk space. By contrast, under daytime conditions, this gradient was shown to be more sensitive to local source heterogeneities. The estimation of the short-term averaged vertical velocity ( was the greater source of error when computing advection terms. The traditional correction methods used to obtain are discussed and a (co)sine correction is tested to highlight the instrumental origin of the offset in w. A comparison of measurements by sonic anemometers placed close together above the canopy showed that the uncertainty on was 0.042 m s−1, which is of the same order of magnitude as the velocity itself. In addition, as the drainage sub-layer is limited to the lowest part of the canopy, the representativeness of is questionable. An alternative computation using the divergence of the horizontal wind speed in the trunk space produced a estimation that was four times lower than the single-point measurement. However, this value gives a more realistic estimate of the vertical advection term and improves the CO2 budget closure at the site.  相似文献   

7.
Acoustic sounder and tower data obtained at the Boulder Atmospheric Observatory (BAO) are used to examine several features of the wave and turbulence structure associated with a disturbed nocturnal inversion. General features, including mean fields and Richardson number, for the case selected for this study are presented. Spectral analysis of the tower data reveals a separation of energy into wavelike and turbulent fluctuations. Analysis of the heat flux, however, shows upward counter-gradient fluxes in the vicinity of a low-level jet and near the top of the inversion. Cospectral analysis shows that the major contribution to the upward heat flux occurs at frequencies that would normally be considered characteristic of waves. In some cases, the upward flux is associated with a phase shift between vertical velocity w and fluctuating temperature θ different from the quadrature relation that would be expected of internal waves. Time series analysis reveals that these unexpected positive fluxes occur in relatively short bursts. Analysis of time series of θ and w in other cases, as well as inspection of acoustic sounder records, shows that sometimes such upward fluxes can result from a combination of wave motion and horizontal temperature advection. In this case the advection is associated with a shallow cold front.  相似文献   

8.
Quasi-two-dimensional turbulence was generated by towing an array of vertical cylinders through a tank which was filled with a two-layer stratified fluid. Sugar and Epsom salts were used, to give matching refractive indices for the two layers. The interface between the two layers was seeded with approximately 1000 neutrally buoyant particles. The evolution of this quasi-two-dimensional turbulence was visualized by photographing the fluorescent particles illuminated by a horizontal laser sheet traversing in the vertical direction. The three-dimensional particle velocity was obtained by digitizing the streaks. The evolution of the velocity correlations, length scales, one-dimensional and two-dimensional velocity and vorticity spectra were obtained for N = 5.72 s−1, N = 4.43 s−1, and N = 2.55 s−1 (where N is the Brunt-Väisälä frequency). The results showed the physical process of inverse energy cascading and the formation of dominant vortical structures under the influence of density stratification. Compared with idealized two-dimensional turbulence, the flow is highly dissipative at high N, as a result of the frictional dissipation between the interface and the unstratified layers.  相似文献   

9.
The system transfer function ¦H(v)¦2 at frequencyv (units of Hz) for a vertical velocity propeller anemometer in a statistically stationary and horizontally homogeneous turbulent flow is determined from: (1) experimental estimates of propeller velocity spectra; and (2) estimates of Eulerian vertical velocity spectra based on the hypothesis that degradation of the input vertical velocity Fourier components occurs in the inertial subrange. The experimental estimates of ¦H(v)¦2 were adequately summarized with the mathematical expression for the system transfer function of a first-order system with parameterT which has units of time and is analogous to the time constant of a horizontal velocity propeller anemometer. Dimensional analysis techniques and the Monin-Obukhov similarity hypothesis were used to construct a model for the system parameterT which yielded the result that w /D 1 ( w /)1/3, where w , andD 1 denote the standard deviation of the input vertical velocity fluctuations, the horizontal mean wind speed, and the diameter of the propeller, respectively. The system parameterT is interpreted in terms of the time required for the propeller velocity statistics to become asymptotically independent of time upon being released from rest in a statistically stationary turbulent flow.Currently on leave of absence from the Indian Institute of Technology, New Delhi, India.  相似文献   

10.
Micro-scale turbulent transport processes over the marginal ice zone have been studied by use of a two-dimensional numerical model. It has been found that internal boundary layers (IBLs) of horizontal mean velocity, temperature, and specific humidity reveal a near field and a far field. In the near field, the change in surface roughness dominates the height and growth rate of a velocity IBL. The change in surface heat flux governs the near field of a temperature and humidity IBL. In the far field, approximately x/¦L *2 ¦ ~ 20, where L *2 is the downstream Obukhov length, the downstream stratification more and more influences the growth rate of IBLs basically by modifying the eddy viscosity.Above more complex terrain consisting of an ensemble of ice strips and leads, a merging height h M develops, below which the horizontal variability of the surface modification is clearly observed; h M varies with the length scale L of surface modification approximately in proportion to h M /L ~ 1/20 – 1/10, as a rule of thumb. Above the merging height, an enveloping IBL exists, whose growth depends on the ice cover, i.e., on the integral of surface modification, but changes very little with L.Local advection of momentum, heat, and moisture clearly affects the local surface heat fluxes. Sensible and latent heat fluxes are found to show also a near and far field. However, if areally averaged surface fluxes are to be deduced from grid-averaged flow variables, then details of local advection can be neglected to a reasonably good approximation.  相似文献   

11.
The morning development of the daytime convective boundary layer (CBL) during fine weather has been observed with an acoustic Doppler sodar of the C.R.P.E. In particular, the vertical profile of the vertical velocity third-order statistic W* 3 has been obtained. This quantity is a maximum near 0.3z I where z I, is the height of the CBL. The histogram of vertical velocity in the CBL shows a relationship between W 3 and the convective velocity W * and is useful for convective plume determination.  相似文献   

12.
A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols c entrainment parameter for sensible heat - c p specific heat of air at constant pressure, 1010 J kg–1 K–1 - d 1 the thickness of velocity shear at the mixed-layer top, m - Q H surface sensible heat flux, W m–2 - u m mean mixed-layer wind speed, m s–1 - u * friction velocity at the surface, m s–1 - w subsidence velocity, m s–1 - W subsidence warming,oC s–1 - w e entrainment velocity, m s–1 - w * convection velocity in the mixed layer, m s–1 - x downwind horizontal distance from the water-land interface, m - y dummy variable forx, m - Z height above the surface, m - Z i height of capping inversion, m - Z m mixed-layer depth, i.e.,Z i–Zs, m - Z s height of the surface layer, m - lapse rate of potential temperature aboveZ i, K m–1 - potential temperature step atZ i, K - u h velocity step change at the mixed-layer top - m mean mixed-layer potential temperature, K  相似文献   

13.
Turbulence measurements of the vertical velocity component were obtained by an instrumented aircraft under fair weather conditions in the St. Louis, Missouri, metropolitan area. Time series of vertical velocity fluctuations from horizontal flight segments made in the lower part of and near the middle of the convective boundary layer (CBL) over the urban area and surrounding region were subjected to various statistical and objective analyses. Higher order vertical velocity moments, and positive and negative velocity statistics, were computed. The horizontal dimensions of updrafts and downdrafts, and related properties of these turbulent eddies were derived by conditional sampling analysis. Emphasis is on a comparison of the results from urban and selected rural measurements from the lower part of the CBL.The vertical velocity probability density distribution for each segment was positively skewed and the mode was negative. The means and standard deviations of positive and negative velocity fluctuations were greater over the urban area. The urban vertical velocity variance was 50% greater than rural values, and power spectra revealed greater production of vertical turbulent energy in the urban area over a wide frequency range.The mean and maximum widths of downdrafts were generally larger than the corresponding values for updrafts. Differences between urban and rural eddy sizes were not statistically significant. The widths of the largest updraft and downdraft are comparable to the boundary-layer depth, Z i, and the mean value of the ratio of spectral peak wavelength to Z iwas about 1.3 and 1.1 for urban and rural areas, respectively. Convective similarity scaling parameters appeared to order both the urban and rural measurements.On assignment from the National Oceanic and Atmospheric Administration, U.S. Dept. of Commerce.  相似文献   

14.
Nocturnal convection, originating in a well-mixed marine cloud-topped boundary layer, advected onshore, was observed using a Doppler sodar on the Tyrrhenian coast in Italy. The horizontal and vertical dimensions of the downdrafts were evaluated. The oscillation frequency triggered by the downdrafts at the inversion layer, derived from the harmonic analysis of the sodar measured vertical velocity (w), is compared with the Brunt-Vaisala frequency, obtained from the rawinsonde temperature profile. A similarity function for the 2w vertical profile was used to fit the sodar experimental data and to retrieve the depth of the mixing layer and the sensible heat flux at the top of the cloud layer. The results are in agreement with the convection layer depth observed in the sodar echoes facsimile record, and with the energy budget evaluated at the top of the cloud layer using the rawinsonde profiles.  相似文献   

15.
The winter-time arctic atmospheric boundary layer was investigated with micrometeorological and SF6 tracer measurements collected in Prudhoe Bay, Alaska. The flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. The relatively warm maritime air mass originating over the nearby, partially frozen Beaufort Sea is cooled at the tundra surface resulting in strong (4 to 30 °C · (100 m)-1) temperature inversions with light winds and a persistent weak (1 to 2 °C · (100 m)-1) surface inversion with wind speeds up to 17 m s-1. The absence of any diurnal atmospheric stability pattern during the study was due to the very limited solar insolation. Vertical profiles were measured with a multi-level mast from 1 to 17 m and with a Doppler acoustic sounder from 60 to 450 m. With high wind speeds, stable layers below 17 m and above 300 m were typically separated by a layer of neutral stability. Turbulence statistics and spectra calculated at a height of 33 m are similar to measurements reported for non-arctic, open terrain sites and indicate that the production of turbulence is primarily due to wind shear. The distribution of wind direction recorded at 1 Hz was frequently non-Gaussian for 1-hr periods but was always Gaussian for 5-min periods. We also observed non-Gaussian hourly averaged crosswind concentration profiles and assume that they can be modeled by calculating sequential short-term concentrations, using the 5-min standard deviation of horizontal wind direction fluctuations () to estimate a horizontal dispersion coefficient ( y ), and constructing hourly concentrations by averaging the short-term results. Non-Gaussian hourly crosswind distributions are not unique to the arctic and can be observed at most field sites. A weak correlation between horizontal ( v ) and vertical ( w ) turbulence observed for both 1-hr and 5-min periods indicates that a single stability classification method is not sufficient to determine both vertical and horizontal dispersion at this site. An estimate of the vertical dispersion coefficient, z , could be based on or a stability classification parameter which includes vertical thermal and wind shear effects (e.g., Monin-Obukhov length, L).  相似文献   

16.
Poisson's equation is solved numerically by two direct methods, viz. Block Cyclic Reduction (BCR) method and Fourier Method. Qualitative and quantitative comparison between the numerical solutions obtained by two methods indicates that BCR method is superior to Fourier method in terms of speed and accuracy. Therefore. BCR method is applied to solve (?)2(?)= ζ and (?)2X= D from observed vorticity and divergent values. Thereafter the rotational and divergent components of the horizontal monsoon wind in the lower troposphere are reconstructed and are com pared with the results obtained by Successive Over-Relaxation (SOR) method as this indirect method is generally in more use for obtaining the streamfunction ((?)) and velocity potential (X) fields in NWP models. It is found that the results of BCR method are more reliable than SOR method.  相似文献   

17.
Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (e){(\varepsilon)} are reasonably well correlated with concurrent measurements of vertical velocity variance (sw2){(\sigma_{w}^{2})} obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100 m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e{\varepsilon} can exhibit values as low as 10−7 m2 s−3. We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e{\varepsilon} to provide a proxy measurement for sw2{\sigma_{w}^{2}} at altitudes higher than tower heights.  相似文献   

18.
An experimental study of sulfur and NOx fluxes over grassland   总被引：1，自引：0，他引：1  
Three independent sulfur sensors were used in a study of sulfur eddy fluxes to a field of wheat stubble and mixed grasses, conducted in Southern Ohio in September, 1979. Two of these sensors were modified commercial instruments; one operated with a prefilter to measure gaseous sulfur compounds and the other with a denuder system to provide submicron particulate sulfur data. The third sensor was a prototype system, used to measure total sulfur fluxes. The data obtained indicated that the deposition velocity for gaseous sulfur almost always exceeded that for particulate sulfur; average surface conductances were about 1.0 cm s–1 for gaseous sulfur in the daytime and about 0.4 cm s–1 for particulate sulfur. The data indicate that nighttime values were probably much lower. The total sulfur sensor provided support for these conclusions. The boundary-layer quantity ln(z 0 /z H )was found to be 2.75 ± 0.55, in close agreement with expectations and thus providing some assurance that the site was adequate for eddy flux studies. However, fluxes derived using a prototype NOx sensor were widely scattered, partially as a consequence of sensor noise but also possibly because of the effects of nearby sources of natural nitrogen compounds.Formerly with Argonne National Laboratory.  相似文献   

19.
R.E. Munn 《大气与海洋》2013,51(4):144-147
An acoustic echo sounder situated in downtown Toronto has been used to detect convective plumes in the planetary boundary layer and to measure, by means of the Doppler effect, the vertical air motions associated with them. The plumes observed were the order of 390 m in horizontal extent, were detectable to a height of about 400 m, and were characterized by peak upward velocities in excess of 1 m s?1. The sounder measurements are shown to be consistent with surface meteorological parameters, and suggest that free convection over an urban area of considerable surface roughness and non‐uniformity is not greatly different from that over uniform land surfaces or water.  相似文献   

20.
We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales > 10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr−1 navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales > 10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX. The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

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