首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The development of droplet size, supersaturation, and temperature in an ascending, unmixed parcel of cloudy air was investigated using a numerical model in which condensation and supersaturation are explicitly calculated. Of particular interest was the steady‐state value which the super‐saturation attains, and the time required to reach this value. The results were found to be in reasonable agreement with an approximate analytical solution of the equations that predicts a steady‐state supersaturation equal to G(p, T)U/vr, where G(p, T) is a known thermodynamic function of temperature and pressure, U is the updraft speed (assumed constant), v is the number of droplets per unit mass of air (assumed constant), and r is the droplet radius. The approach to this limiting value is very nearly exponential, with a time constant equal to G(p, T)/Q 1 vr, where Q 1 is a function of temperature. Without giving up the possibility of approximate analytical solutions, the equations can be elaborated to allow an accelerating updraft or to simulate the effects of rapid droplet coalescence.  相似文献   

2.
Under growing wind-wave conditions the shear velocity,u *, over the water surface equalsg 2 H s 2 B a 2 C p 3 , whereg is the gravitational acceleration,H s is the significant wave height,B a is a constant, andC p is the wave celerity. From an independent field experiment in a lake environment which provided all three parameters (u *,H s , andC p ), the value ofB a is found to be 0.89, which is slightly lower than but consistent (within 20%) with the literature value between 0.90 and 1.06 obtained from an oceanic environment. Since thisu * equation does not include the wind speed,U 10, anotheru * formulation withU 10 in addition to the wave information is also evaluated. It is shown that the latter equation which includesU 10 is superior to the former withoutU 10.  相似文献   

3.
Reliable predictions of the daytime dispersal of heavy particles in the unstable atmospheric boundary layer are important in a variety of disciplines. For many applications, particles disperse from area sources near the ground, and corresponding theoretical solutions are desired to reveal insight into the physical processes. Here, theoretical solutions recently developed for neutral conditions are modified to include the effects of atmospheric instability. The Obukhov length L O and convection velocity w ? are introduced to characterize the patterns of particle dispersion, in additional to friction velocity u ? and settling velocity w s used in the neutral case. The major effects of atmospheric instability are accounted for by modifying the vertical velocity variance profile and considering the ratio of velocity scales w ?/u ?. Theoretical predictions including the mean concentration profile, plume height, and horizontal transport above the source, and ground deposition flux downwind from the source agree well with large-eddy simulation results while the particle plume is within the atmospheric surface layer. The deposition curve is characterized by a power-law decay whose exponent depends on u ?, w s, and w ?. A second steeper power-law develops once the plume extends into the mixed layer. This effect is enhanced with increasing atmospheric instability, implying that particles disperse farther from the source.  相似文献   

4.
The deviation of the sea surface temperature from the water temperature below is calculated as a function of the heat flow through the air-sea interface, using wind tunnel determinations of the effective thermal diffusivity in a boundary layer. The influence ofQ, shortwave radiation, andH, latent and sensible heat transfer plus effective back radiation, and U, wind speed, can be described by:T 0 –T w =C 1 ·H/U +C 2 ·Q/U. The calculated coefficients vary slightly with reference depth, Tables II and III. They are in good agreement with independent observations.On leave at Department of Oceanography, Oregon State University, Corvallis, Oregon in 1969–70.  相似文献   

5.
The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u * /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ w /U) is the smallest over the downslope surface whereas that of the u component (σ u /U) has a minimum over the upslope surface. Consequently, σ w /u * has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ u /u * has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface.  相似文献   

6.
Dispersion estimates with a Gaussian plume model are often incorrect because of particle settling (β), deposition (γ) or the vertical gradient in diffusivity (K v (z) = K 0 + μz). These “non-Gaussian” effects, and the interaction between them, can be evaluated with a new Hankel/Fourier method. Due to the deepening of the plume downwind and reduced vertical concentration gradients, these effects become more important at greater distance from the source. They dominate when distance from the source exceeds L β = K 0 U/β 2, L γ  = K 0 U/γ 2 and L μ = K 0 U/μ 2 respectively. In this case, the ratio β/μ plays a central role and when β/μ = 1/2 the effects of settling and K gradient exactly cancel. A general computational method and several specific closed form solutions are given, including a new dispersion relation for the case when all three non-Gaussian effects are strong. A more general result is that surface concentration scales as C(x) ~ γ −2 whenever deposition is strong. Categorization of dispersion problems using β/μ, L γ and L μ is proposed.  相似文献   

7.
An easily-evaluated expression for the dimensionless concentration profile (z/z 0,/z 0, z 0/L) = = cu */kQ (or z 0cu*/kQ) downwind of a continuous ground-level area (or line) source in the stable surface layer is obtained by integrating the diffusion equation using the Shwetz approximation method (c = concentration, Q = source strength, k = von Kárman's constant). The analytical solution compares closely with concentration profiles obtained using a trajectory-simulation model over a useful range of heights, the important discrepancies occurring at the upper edge of the plume. The analytical solution is used to generate predictions of ground-level concentration for the Project Prairie Grass experiments; good agreement with the observations is obtained at all downwind distances (50 to 800 m).  相似文献   

8.
By integrating the Fokker-Planck equation corresponding to a Lagrangian stochastic trajectory model, which is consitent with the selection criterion of Thomson (1987), an analytical solution is given for the joint probability density functionp(xi, ui, t) for the position (x i) and velocity (u i) at timet of a neutral particle released into linearly-sheared, homogeneous turbulence. The solution is compared with dispersion experiments conforming to the restrictions of the model and with a shortrange experiment performed in highly inhomogeneous turbulence within and above a model crop canopy. When the turbulence intensity, wind shear and covariance are strong, the present solution is better than simpler solutions (Taylor, 1921; Durbin, 1983) and as good as any numerical Lagrangian stochastic model yet reported.  相似文献   

9.
Turbulence measurements above a pine forest   总被引：1，自引：0，他引：1  
Eddy fluxes of momentum, sensible and latent heat, and turbulence spectra measured over the Thetford Forest during 10 days in the Spring of 1973 are described. The measured total heat flux (H + E) for 122 20-min periods agreed closely on average with independent estimates from an energy balance method. There was evidence that the energy balance data gave small systematic overestimates of available energy during the hours before noon, compensated by slight underestimates for the remainder of the day. A comparison of measured wind speeds and friction velocities in neutral stability confirmed the validity of the aerodynamic method for estimating momentum fluxes at heights of a few roughness lengths above the canopy. In stable conditions the log-linear wind profileU = (u */k)(ln ((z -d)/z o) + (z -d -z o)/L) with = 3.4 ± 0.4 provided a good fit to the data. Spectra in unstable conditions were generally more sharply peaked than those measured by other workers over smoother terrain: differences were less marked in the case of vertical velocity in stable conditions. Temperature spectra in these stable conditions showed high energy at relatively low wavenumbers, andwT cospectra showed a cospectral gap; both of these results were associated with an intermittent sawtooth structure in the temperature fluctuations.Now at the Meteorological Office, Bracknell  相似文献   

10.
The fog meteorology, fog chemistry and fog deposition on epiphytic bryophytes were investigated from July 2000 to June 2001 in the Yuanyang Lake forest ecosystem. The elevation of the site ranges from 1650 to 2420 m, at which the high frequency of fog occurrence throughout the year has been thought to be of benefit to the establishment of the primary Taiwan yellow cypress forest [Chamaecyparis obtusa var. formosana (Hayata) Rehder] and to the extensive growth of the epiphytic bryophytes. A weather station including a visibility sensor and an active fog collector was installed for fog meteorological and chemical study. The fog deposition rate on epiphytic bryophytes was estimated by measuring the increase rate in plant weight when exposed to fog. Average fog duration of 4.7 and 11.0 h per day was measured in summer months (June to August) and the rest of the year, respectively. November 2000 was the foggiest month in which the average fog duration reached 14.9 h per day. The ionic composition of fog water revealed that the area was less polluted than expected from literature data. The in situ exposure experiments done with the dominant epiphytic bryophytes showed an average fog deposition rate of 0.63 g H2O g−1 d. w. h−1, which approximated 0.17 mm h−1 at the stand scale. The nutrient fluxes estimated for February 2001 showed that for all ions, more than 50% of the ecosystem input was through fog deposition. These results demonstrate the importance of epiphytic bryophytes and fog deposition in nutrient cycling of this subtropical montane forest ecosystem. The incorporation of fog study in the long-term ecosystem research projects is necessary in this area.  相似文献   

11.
An attempt is made to construct a model, coupling land surface and atmospheric processes in the planetary boundary layer (PBL). A grassland strip in a semi-desert (hereinafter called desert) is presupposed, so as to simulate the case of heterogeneous vegetation cover.Modeling results indicate that every term in the equation of the surface energy balance changes as the air flows over the grassland. The striking contrast of water and energy conditions between the grassland and the desert means that the air over the grassland is cooler and wetter than that over the desert. Consequently, in the heating and dynamic forcing of the air by the underlying surface, heterogeneities arise and are then transferred upward by the turbulent motions. Horizontal differences thus develop in the PBL, resulting in a local circulation. Meanwhile, the horizontal differences affect the free atmosphere through vertical motion at the top of the PBL.List of symbols d 1,d 2,d 3 depths of surface, middle and lower layers of soil - T c ,T 1,T 2,T 3 temperatures of canopy, surface, middle and lower layers of soil - R nc net radiation of canopy layer - c shielding factor of vegetation - Ew, Etc evaporation from wet fraction of foliage and transpiration from dry fraction of foliage - Et 1,Et 2 transpiration of foliage water absorbed by the root in the upper and lower soil, respectively - H c sensible heat of canopy - P c ,D c precipitation rate and drainage of canopy - C s ,C c ,C w heat capacity of soil, canopy and water - w , s density of water and air near the surface - D hydraulic permeability of soil - s saturated value of the ratio of volumetric soil moisture - S g , g solar radiation and surface reflection - H g ,R L g turbulent heat flux and long wave radiation of surface - P g ,E g precipitation rate and evaporation of soil surface - K s soil thermal diffusivity - K (m),K (H),K (q) eddy coefficients of momentum, heat and moisture - u, v, w components of wind speed in three directions - air potential temperature - e turbulent kinetic energy - p atmospheric pressure - C p specific heat of air under constant pressure - R d gas constant - u * friction velocity - * feature temperature - h height of the PBL - f Coriolis parameter - L 0 Monin-Obukhov length - latent heat of vaporization - q specific humidity - M c ,M cm interception water storage of canopy and its maximum - 0 Exner number of largescale background field - perturbation Exner number - u g ,v g components of the geostrophic wind speed Sponsored by the National Natural Science Foundation of China.  相似文献   

12.
Numerical simulation of turbulent convective flow over wavy terrain   总被引：1，自引：1，他引：0  
By means of a large-eddy simulation, the convective boundary layer is investigated for flows over wavy terrain. The lower surface varies sinusoidally in the downstream direction while remaining constant in the other. Several cases are considered with amplitude up to 0.15H and wavelength ofH to 8H, whereH is the mean fluid-layer height. At the lower surface, the vertical heat flux is prescribed to be constant and the momentum flux is determined locally from the Monin-Obukhov relationship with a roughness lengthz o=10–4 H. The mean wind is varied between zero and 5w *, wherew * is the convective velocity scale. After rather long times, the flow structure shows horizontal scales up to 4H, with a pattern similar to that over flat surfaces at corresponding shear friction. Weak mean wind destroys regular spatial structures induced by the surface undulation at zero mean wind. The surface heating suppresses mean-flow recirculation-regions even for steep surface waves. Short surface waves cause strong drag due to hydrostatic and dynamic pressure forces in addition to frictional drag. The pressure drag increases slowly with the mean velocity, and strongly with /H. The turbulence variances increase mainly in the lower half of the mixed layer forU/w *>2.  相似文献   

13.
Vapor phase concentrations of acetone, acetaldehyde and acetonitrile over their aqueous solutions were measured to determine Henry's law partition coefficients for these compounds in the temperature range 5–40 °C. The results are for acetone: ln(H 1/atm)=–(5286±100)T+(18.4±0.3); acetaldehyde: ln(H 1/atm)=–(5671±22)/T+(20.4±0.1); and acetonitrile: ln(H 1/atm)=–(4106±101)/T+(13.8±0.3). Artificial seawater of 3.5% salinity in place of deiionized water raisesH 1 by about 15%. A similar technique has been used to measure the equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite in aqueous solution. The results are ln(K 1/M –1)=(4972±318)/T–(11.2±1.1) and ln(K 1/M –1)=(6240±427)/T–(8.1±1.3), respectively. The results are compared and partly combined with other data in the literature to provide an average representation.  相似文献   

14.
Wind-wave relationship from SEASAT radar altimeter data   总被引：2，自引：0，他引：2  
We present a nonlinear relationship between ocean surface wind at 10 m height (U 10 ) and significant wave height of wind-generated gravity waves, (H 1/3)gw, over the open oceans using SEASAT radar altimeter data. The data represent a variety of fetches, durations and strength of winds. Concurrent measurement of significant wave height, (H 1/3 ), which may contain a measure of swell and U 10 obtained from the processed geophysical data record (GDR) of the SEASAT radar altimeter were used in the analysis. The total wave energy, E alt, characterised by altimeter H 1/3 measurements was compared with the energy of a fully developed sea, E fd derived from U 10 measurements using the Pierson-Moskowitz model. The criteria E alt E fd was used in data selection to minimise the influence of swell. (H 1/3)gw thus obtained was used in a regression in terms of U 10 in a second-degree polynomial. Verification with independent radar altimeter data confirmed the validity of the proposed wind-wave model, which could be used for operational wave forecasting.  相似文献   

15.
Recent papers by Wilson et al. (1981b) and Legg and Raupach (1982) give methods for the calculation of particle trajectories in turbulence with a gradient in vertical velocity variance 2 w. However the two methods seem contradictory.This paper demonstrates that in systems in which l(d w /dz) (where / is the length scale) varies only slowly with height z, the two methods give similar predictions, and indicates why this is the case. For a particular system in which the restriction on l(d w /dz) is not satisfied, it is shown that neither method is correct but that a simple modification of the method of Wilson et al. (1981b) gives reasonable predictions.  相似文献   

16.
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   

17.
A higher order closure model is applied to simulate the dynamics in an area with a deep valley characterized by complex terrain in the southwestern US. The simulation results show generally good agreement with measured profiles at two locations within the valley. Both the measurements and the simulations indicate that the flow dynamics in the area are highly influenced by the topography and meandering of the valley, and can be resolved only by the full three-dimensional model code. The wind veering simulated over the range of the topographic elevations is often larger than 100 deg and in some cases as large as 180 deg, as a consequence of topographic forcing. In the case of an infinitely long valley, as is assumed in two-dimensional test simulations, a strong low-level jet occurs within the valley during stable conditions. The jet is mainly a consequence of the Coriolis effect. However, the jet development is significantly reduced due to asymmetric effects of the actual topography treated in the three-dimensional simulations. Tests with the two-dimensional nonhydrostatic version of the model show significant wave responses for a stable stratified flow over the valley. The structure resembles nonlinear mesoscale lee waves, which are intrinsically nonhydrostatic. However, considering the three-dimensional nature of this valley system, a better understanding and verification of the nonhydrostatic effects requires both a three-dimensional nonhydrostatic numerical model and an observational data set which is fully representative in all three dimensions.List of symbols (unless otherwise defined in the text) B 1 closure constant - f Coriolis parameter - g acceleration of gravity - K M ,K H ,K R turbulent exchange coefficients for momentum, heat and moisture - k von Karman constant - L Monin-Obukhov length - q 2 twice the turbulent kinetic energy - R specific humidity - s height of the model top - T g ground surface temperature - t time - U, V horizontal components of wind - U g ,V g geostrophic wind components - u, w perturbation components ofU andW wind components - u * friction velocity - W vertical wind component in the terrain-following coordinates - x, y horizontal coordinates - Z actual height above sea level - z actual height above ground - z 0 roughness length - z g terrain height - z i depth of the convective boundary layer - 1 closure constant - coefficient of thermal expansion - height in the terrain-following coordinate - master length scale in the turbulent parameterization - scaled pressure (Exner function) - potential temperature - m normalized vertical wind shear  相似文献   

18.
Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February 1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations of the wind vector u′, v′, w′ and the sonic temperature θ s ′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h (the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ w /u * is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller compared to other urban turbulence studies. The σθ/θ* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The profile of friction velocity u * is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities S w (f)/S u (f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2. Received September 26, 1997 Revised February 15, 1998  相似文献   

19.
A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface, in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z 01)/ln(δ i /z 01), where z, δ i and z 01 are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different positions downwind of the transition.  相似文献   

20.
The Langevin equation is used to derive the Markov equation for the vertical velocity of a fluid particle moving in turbulent flow. It is shown that if the Eulerian velocity variance wE is not constant with height, there is an associated vertical pressure gradient which appears as a force-like term in the Markov equation. The correct form of the Markov equation is: w(t + t) = aw(t) + b wE + (1 – a)T L ( wE 2)/z, where w(t) is the vertical velocity at time t, a random number from a Gaussian distribution with zero mean and unit variance, T L the Lagrangian integral time scale for vertical velocity, a = exp(–t/T L), and b = (1 – a 2)1/2. This equation can be used for inhomogeneous turbulence in which the mean wind speed, wE and T L vary with height. A two-dimensional numerical simulation shows that when this equation is used, an initially uniform distribution of tracer remains uniform.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号