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1.
Summary During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths () and irradiances at the top of the atmosphere. In addition, water vapor optical depths (wv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (ray) and ozone (oz) optical depths were obtained from published tabulations. Subtracting ray, oz, and wv from yielded estimates of aerosol optical depth aer. The components ray and oz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; aer also decreases towards the higher elevations. Particularly pronounced is the decrease of aer and from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.With 4 Figures  相似文献   

2.
Standard deviations of concentration in horizontal andvertical directions i.e. y andz have been estimated by using fivedifferent schemes based on empirical(due to Pasquill and Briggs)schemes and sophisticated methods(due to Irwin, Draxler, Taylor, Hanna et al.). The fiveschemes are discussed atlength. The purpose of this study is to make use ofmeteorological observations whichare routinely available, to test all the above methods andintercompare the resultswith one another and observations so that the sensitivityof each schemeunder various atmospheric stability conditions could beassessed. It has beenfound that the existing schemes are good enough to providereasonable estimates ofdispersion coefficient (y) during highly unstableconditions (Pasquill stability classes A and B). However, thesame is not true for the case when the stability increasesfrom C to F and turbulencedecreases, specifically during stable atmospheric conditions,when the observedvalues were found to be much higher than all the existingschemes. This suggests thatwhile we continue to use the current methods of estimatingthe dispersion parameters,a rigorous search is required for methods which give predictionswhich are close-to-realityduring such conditions which are represented by lowlevels (in terms of magnitude)of atmospheric turbulence leading to higher levelsof pollution.As one of the sophisticated methods requiresthe use of v and w (standard deviationsof wind velocity fluctuation in y and z directions),these have been estimated andvalidated with observed data (field experiments conductedby EPRI at Kincaid).Statistical evaluation of v and wbased on performance measures indicate a goodperformance of the parameterisations adopted in thisstudy. In the case of w duringunstable conditions a comparison of three differentschemes with observations is made.  相似文献   

3.
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   

4.
A random-walk model for dispersion of heavy particles in turbulent air flow   总被引：1，自引：0，他引：1  
A random-walk model is presented for calculating the dispersion of heavy particles in a turbulent air flow when only air turbulence statistics and the drag characteristics of the particle are known. Algebraic expressions for the modification of air velocity variance 2 and Lagrangian autocorrelation tune-scale T L,due to particle inertia effects, are derived. These expressions introduce only a very small computational overhead on the random-walk models for inertia-less particles of Wilson et al. (1983). Measurements of T Land by Snyder and Lumley (1971) for four different particles are used to determine constants in the heavy-particle model. It is shown that the agreement between the model, for a single set of constants, and the dispersion measurements is good for the 47 m hollow glass, 87 m glass, and 47 m copper particles. The predictions for the 87 m corn pollen particles show less satisfactory agreement by underestimating dispersion measurements by 15% after 0.4s. Finally, some aspects of the model's application to spray dispersion in and above a crop canopy are considered.  相似文献   

5.
A Comparative Analysis of Transpiration and Bare Soil Evaporation   总被引：4，自引：0，他引：4  
Transpiration Ev and bare soil evaporation Eb processes are comparatively analysed assuming homogeneous and inhomogeneous areal distributions of volumetric soil moisture content . For a homogeneous areal distribution of we use a deterministic model, while for inhomogeneous distributions a statistical-deterministic diagnostic surface energy balance model is applied. The areal variations of are simulated by Monte-Carlo runs assuming normal distributions of .The numerical experiments are performed for loam. In the experiments we used different parameterizations for vegetation and bare soil surface resistances and strong atmospheric forcing. According to the results theEv()-Eb() differences are great, especially in dry conditions. In spite of this, the available energy flux curves of vegetation Av() and bare soil Ab() surfaces differ much less than the Ev() and Eb() curves. The results suggest that Ev is much more non-linearly related to environmental conditions than Eb. Both Ev and Eb depend on the distribution of , the wetness regime and the parameterization used. With the parameterizations, Eb showed greater variations than Ev. These results are valid when there are no advective effects or mesoscale circulation patterns and the stratification is unstable.  相似文献   

6.
Shear convection     
The paper discusses convection in the presence of wind shear, a condition analysed previously by Zilitinkevich (1971). This region (between the forced and fully developed convective layers) was also considered by Betchov and Yaglom (1971). In the present paper the author endeavors to develop a consistent analysis from the basic hypothesis of a very weak interaction between the vertical convective motions and mechanical turbulence, employing a new similarity model of the turbulent regime. Additional experimental data are introduced. Unlike the notation used in the references quoted above, this regime is termed shear convection rather than free convection. The latter is traditionally regarded as synonymous with the terms pure or fully developed convection.  相似文献   

7.
Turbulence mechanisms at an agricultural site   总被引：8，自引：0，他引：8  
An extensive set of turbulence data from the 3- and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from ideal sites.In unstable air, Monin-Obukhov similarity is found to be valid for the non-dimensional gradients of wind, m , temperature, h , and humidity, e , for (only a few data), for T /|T *|,/ E /|E *| and for the non-dimensionalized inertial subrange spectra of temperature and humidity. Where comparison is possible, the unstable data also agree with those found in the Kansas study, with one remarkable exception, the inertial subrange constant of the temperature spectrum, 1, being only 0.39, compared to the value 0.80 found at the Kansas site.On the stable side, most similarity predictions break down, with most of the data differing systematically from the corresponding Kansas results, the only exception being . The inertial subrange constants for temperature, 1, and for humidity, 1 are found to have the same values, 0.39 and 0.30, respectively, as they do on the unstable side. Remarkable similarity is found for the shape of the stable u- and - and e-spectra. In addition, this shape is found to be identical with that found in Kansas. The peak wavelength of the stable u-, and -spectra is found to be about four times larger than it is for the corresponding Kansas spectra. This is interpreted to be a result of the increased macro-roughness at the Marsta site as compared with that at the Kansas site. A possible explanation for the low 1-value is discussed, suggesting that 1 is not a universal constant, but instead dependent on the turbulent structure.  相似文献   

8.
A Field Study of the Mean Pressure About a Windbreak   总被引：3，自引：0，他引：3  
To provide additional field data for assessingwindbreak flow models, mean ground-level pressurehas been measured upstream and downstream from along porous fence (height H = 1.25 m, resistancecoefficient k r = 2.4). Measurements were madeduring periods of near-neutral stability and near-normallyincident flow, with the fence standing on bare soil(roughness length, z 0 0.8 cm;H/z 0 160), or within a plant canopy. The mean pressure field,measured far from the ends of the fence, was foundto be quite insensitive to mean wind direction( , zero for perpendicular flow), for| | less than about 25°.In the absence of a canopy, during each measurementperiod the minimum pressure occurred at the closestsampling location to leeward of the windbreak, thepressure-gradient in most cases beingmaximally-adverse in the immediate lee, and decayingwith increasing downwind distance (x). On one day ofmeasurements, however, the pressure gradient over2 x/H 6 (H = windbreak height) resembled theleeward plateau identified by Wang and Taklein their numerical studies. Perhaps thisoccasional feature was only due to instrumenterror. Nevertheless a plateau of sorts wasindicated in similar measurements by Judd andPrendergast (with H = 1.92 m, z 0 1.2 cm;H/z 0 160, k r 3). Therefore,existence of a leeward pressure plateau behind athin fence cannot be definitely ruled out.When the windbreak was placed in a canopy, minimumsurface pressure was displaced downwind. Thisagrees with the wind-tunnel study of Judd, Raupach and Finnigan,and is consistent with a simple simulation reported here.  相似文献   

9.
Summary An analysis of the report of the (U.S.) National Academy of Sciences (NAS) on atmospheric effects of a nuclear exchange leads to conclusions that differ from those of the NAS and of the earlier TTAPS and AMBIO studies. Any cooling of the earth's surface is likely to beshort-lived because of rapid removal of the smoke clouds originating from nuclear burst-initiated fires, andminor because of appreciable green-house effects due to several distinct physical causes. (One of these, neglected in prior analyses, is the infrared absorption from cirrus clouds produced directly by the nuclear bursts.) Taken together, these effects may even induce slight surface warming (nuclear summer) instead of cooling (nuclear winter). The consequences to atmospheric ozone are similarly ambiguous; depending on the detailed nuclear scenario, the net ozone content may increase-rather than decrease as argued by TTAPS. Experiments could settle some uncertainties.With 2 Figures  相似文献   

10.
Energy partitioning and evaporation were measured over three wetland surfaces in a subarctic coastal marsh during pre-growing and growing periods. These surfaces included an alder/willow woodland, a sedge marsh and a raised backshore sedge meadow. A combination model analysis was used to assess the relative importance of surface resistance and meteorological conditions on the magnitude of the Bowen ratio, , during the growing period.Overall, the three surfaces experienced important site-to-site and seasonal differences in and evaporation, Q E. During the non-foliated period, Q E was largest and was smallest for the open water marsh, while the dry backshore site experienced the smallest Q E and largest . The non-foliated woodland assumed intermediate values of and Q E. After the vegetation covers were established, the woodland assumed the smallest and largest Q E flux. It was also found that at the marsh site increased with the presence of a vegetation cover.Wind direction was always an important factor in determining Q E and at all sites. was substantially larger and Q E was smaller for onshore winds (i.e., originating from James Bay) than for offshore winds. The combination model analysis showed that canopy resistance at all sites was largest during warm offshore winds, which were associated with large saturation deficits. However, the effect of increased canopy resistance on during offshore winds was offset by a large climatological resistance, resulting in small values and large Q E. When winds originated from James Bay, canopy resistance was smaller than for offshore winds, but the climatological resistance also was much smaller, resulting in larger and small Q E. The results have important implications for changes in land cover and climate on the regional water balance.  相似文献   

11.
A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO3 (0.01), N2O5 (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.  相似文献   

12.
Selected field measurements of evening stable boundary layers are presented in detail comparable with published Large Eddy Simulation results. Such models appear to match idealized theories more closely than do some boundary-layer observations. Any attempt to compare detailed observations with idealized models therefore highlights the variability of the real boundary layer.Here direct turbulence measurements across the stable boundary layer from a heterogeneous and an ideal site are contrasted. Recommendations are made for the information needed to distinguish heterogeneous and ideal cases.The companion paper (Part II) discusses further the issues of data, analysis in the presence of variability, and the effects of averaging over heterogeneous terrain.Part of UK Meteorological Office Atmospheric Process Research Division.  相似文献   

13.
Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.  相似文献   

14.
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15.
Summary A radiative transfer model has been used to determine the large scale effective 6.6 GHz and 37 GHz optical depths of the vegetation cover. Knowledge of the vegetation optical depth is important for satellite-based large scale soil moisture monitoring using microwave radiometry. The study is based on actual observed large scale surface soil moisture data and observed dual polarization 6.6 and 37 GHz Nimbus/SMMR brightness temperatures over a 3-year period. The derived optical depths have been compared with microwave polarization differences and polarization ratios in both frequencies and with Normalized Difference Vegetation Index (NDVI) values from NOAA/AVHRR. A synergistic approach to derive surface soil emissivity from satellite observed brightness temperatures by inverse modelling is described. This approach improves the relationship between satellite derived surface emissivity and large scale top soil moisture fromR 2=0.45 (no correction for vegetation) toR 2=0.72 (after correction for vegetation). This study also confirms the relationship between the microwave-based MPDI and NDVI earlier described and explained in the literature.List of Symbols f frequency [Hz] - f i(p) fractional absorption at polarizationp - h surface roughness - h h cos2 - H horizontal polarization - n i complex index of refraction - p polarization (H orV) - R s microwave surface reflectivity - T B(p) brightness temperature at polarizationp - T * normalized brightness temperature - T polarization difference (T v-T H) - T s temperature of soil surface - T c temperature of canopy - T max daily maximum air temperature - T min daily minimum air temperature - V vertical polarization - soil moisture distribution factor; also used for the constant to partition the influence of bound and free water components to the dielectric constant of the mixture - empirical complex constant related to soil texture - microwave transmissivity of vegetation (=e –) - * effective transmissivity of vegetation (assuming =0) - microwave emissivity - s emissivity of smooth soil surface - rs emissivity of rough soil surface - vs emissivity of vegetated surface - soil moisture content (% vol.) - K dielectric constant [F·m–1] - K fw dielectric constant of free water [F·m–1] - K ss dielectric constant of soil solids [F·m–1] - K m dielectric constant of mixture [F·m–1] - K o permittivity of free space [8.854·10–12 F·m–1] - high frequency limit ofK wf [F·m–1] - wavelength [m] - incidence angle [degrees from nadir] - polarization ratio (T H/T V) - b soil bulk density [gr·cm–3] - s soil particle density [gr·cm–3] - R surface reflectivity in red portion of spectrum - NIR surface reflectivity in near infrared portion of spectrum - eff effective conductivity of soil extract [mS·cm–1] - vegetation optical depth - 6.6 vegetation optical depth at 6.6 GHz - 37 vegetation optical depth at 37 GHz - * effective vegetation optical depth (assuming =0) - single scattering albedo of vegetation With 12 Figures  相似文献   

16.
The relation between the turbulence Reynolds numberR and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale and the Kolmogorov length scale .  相似文献   

17.
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   

18.
The aerodynamic classification of the resistance laws above solid surfaces is based on the use of a so-called Reynolds roughness number Re s =h s u */, whereh s is the effective roughness height, -viscosity,u *-friction velocity. The recent experimental studies reported by Toba and Ebuchi (1991), demonstrated that the observed variability of the sea roughness cannot be explained only on the basis of the classification of aerodynamic conditions of the sea surface proposed by Kitaigorodskii and Volkov (1965) and Kitaigorodskii (1968) even though the latter approach gains some support from recent experimental studies (see for example Geernaertet al. 1986). In this paper, an attempt is made to explain some of the recently observed features of the variability of surface roughness (Toba and Ebuchi, 1991; Donelanet al., 1993). The fluctuating regime of the sea surface roughness is also described. It is shown that the contribution from the dissipation subrange to the variability of the sea surface can be very important and by itself can explain Charnock's (1955) regime.  相似文献   

19.
The validity of a common radiometeorological application of Monin-Obukhov (M-O) similarity theory to potential refractivity (), which is a nonlinear combination of and q, is determined by whether the properly nondimensionalized gradient is a universal function of z/L. We develop expressions for the flux of (and its scaling parameter, *) in terms of temperature and moisture fluxes, and an M-O similarity expression for the vertical gradient. Results show that even if and q are accepted as exactly following M-O similarity expressions, when the surface layer is stable, does not obey such an expression. That is, when properly nondimensionalized, the vertical gradient of does not collapse to a single universal function of z/L. The assumption that behaves as a similarity variable is approximately correct for well-mixed surface layers under unstable and near-neutral conditions.The gradient of is an important factor in determining microwave propagation conditions. We demonstrate the error induced in a simple algorithm when is assumed to obey M-O similarity theory. An alternative methodology, consistent with the application of similarity theory to and q, is then developed without requiring that itself satisfy similarity theory.  相似文献   

20.
Summary This study presents an analytical investigation of the local behaviour of the solution to a mesoscale model with Newtonian nudging when observed winds are time varying. The analysis examines each Fourier component of the time series of observed winds. Unlike the case with a constant observed wind, the nudged wind vector does not asymptotically approach the observed wind. In response to sinusoidal oscillation of the observed wind, the nudged wind vector is always on a half circle connecting the vector ends of the observed and un-nudged modelled winds. When nudging parameter 0, the nudged wind vector approaches the un-nudged wind; when , the nudged wind vector approaches the observed wind. For commonly used values of nudging parameter , the modelled wind field always carries errors.A target nudging scheme is devised in this study in order to ensure the model result is identical to observed winds with sinusoidal oscillation. Investigation shows that such a target wind exists for a finite value of , and the magnitude of the target-nudging term is about the same as that of a normal nudging term if f, wheref is the Coriolis parameter and is the frequency of the wind oscillation.With 7 Figures  相似文献   

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