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1.
Jackson and Hunt's (1975) equation for the depth of the inner layer of flows over low hills does not depend on any closure assumption as contrarily supposed in literature. This equation contains a constant which can arbitrarily be specified. It is suggested that this inner-layer constant should be determined from experimental data. A preliminary check with some data from the Askervein experiment suggests that Jackson and Hunt's equation fits these data almost as well as Jensen's equation provided that fitted inner-layer constants are used.  相似文献   

2.
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   

3.
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   

4.
Observations have been made of the structure of turbulence and turbulent exchange within plant canopy layers. A new three-dimensional anemometer was used to measure the eddy fluxes of heat and momentum, and the related cospectra, within and above a corn crop and above a red pine forest. Measured values of momentum and heat fluxes, at each height within the corn canopy, were relatively constant proportions of the flux above the canopy, for the period of a day's observation. Extensive regions obeying a –5/3 power relation were found. Isotropy was found above the forest at high frequencies while above and within the corn crop, the ratios of the lateral and vertical spectral densities to the longitudinal component were less than the expected value in the – 5/3 region. In all situations, the vertical velocity spectra were more peaked than a universal curve, particularly a vertical velocity spectrum from above the forest. It is suggested that the additional variance results from the mixing caused by the individual roughness elements. As expected, the spectra could not be normalized using the height above the soil surface to calculate a non-dimensional frequency, but scaling heights were estimated by matching the frequencies of the peak of each curve with that of the universal curve. Cospectra of uw and wT within the corn canopy were of similar shape and frequency regime, and were basically similar in shape to cospectra above the crop. All of the cospectra were more sharply peaked than universal cospectral curves.  相似文献   

5.
The yields of products have been calculated for the reactions of hydroxyl radicals and ozone with 19 of the two-through-six carbon anthropogenic alkenes. Based on their rate of reaction, mechanisms of reactions and the ambient air distribution for these alkenes their seasonal ambient air yields have been estimated.Aldehydes predominate as products irrespective of season, with smaller yields of several ketones. Other minor products include carboxylic acids, carbon monoxide, carbon dioxide, and alkenes. About a two-fold increase is estimated in the yields of hot biradicals and their products from summer to winter.One sensitivity analysis was made by recomputing yields at a different OH radical to O3 concentration than assumed most likely in the calculations discussed above. In addition, the sensitivity of product yields to an estimated range of seasonally averaged sunset-to-sunrise NO3 radical concentrations was calculated. The effects of free radical reactions are discussed, but these are believed to make a relatively minor contribution within the NO x -rich atmospheres that contain anthropogenic alkenes.The uncertainties in product yields associated with the range of NO3 radical concentrations assumed present is relatively small for aldehydes, as is the decrease in yield of the one carbon hot biradical. Larger uncertainties occur for ketones. Significant decreases in yields occur for larger hot biradicals, especially the branched-chain hot radicals in the presence of NO3 radicals.  相似文献   

6.
The influence of the main large-scale wind directions on thermally driven mesoscale circulations at the Baltic southwest coast, southeast of Sweden, is examined. The aim of the study is to highlight small-scale alterations in the coastal atmospheric boundary layer. A numerical three-dimensional mesoscale model is used in this study, which is focused on an overall behaviour of the coastal jets, drainage flows, sea breezes, and a low-level eddy-type flow in particular. It is shown that synoptic conditions, together with the moderate terrain of the southeast of Sweden (max. height h0 206 m), governs the coastal mesoscale dynamics triggered by the land-sea temperature difference T. The subtle nature of coastal low-level jets and sea breezes is revealed; their patterns are dictated by the interplay between synoptic airflow, coastline orientation, and T.The simulations show that coastal jets typically occur during nighttime and vary in height, intensity and position with respect to the coast; they interact with downslope flows and the background wind. For the assigned land surface temperature (varying ±8 K from the sea temperature) and the opposing constant geostrophic wind 8 m s-1, the drainage flow is more robust to the opposing ambient flow than the sea breeze later on. Depending on the part of the coast under consideration, and the prevailing ambient wind, the sea breeze can be suppressed or enhanced, stationary at the coast or rapidly penetrating inland, locked up in phase with another dynamic system or almost independently self-evolving. A low-level eddy structure is analyzed. It is governed by tilting, divergence and horizontal advection terms. The horizontal extent of the coastal effects agrees roughly with the Rossby radius of deformation.  相似文献   

7.
Summary The electromagnetic radiation of cloud discharge known as atmospheric radio noise field strength (ARNFS) shows a gradual fall from a frequency of 9 kHz to 81 kHz as studied over a period of two years at Calcutta, very close to Bay of Bengal. The main characteristic features of ARNFS at Calcutta are that-(i) ARNFS shows that midday median value is smaller than midnight median value in all months, (ii) level of daily minimum is higher in February and monsoon compared to other seasons, (iii) sunrise effect and sunset effect are well correlated with local sunrise and sunset times, (iv) the magnitude of sunrise fade and sunrise fade rate are maximum in April and lowest during winter period, (v) the magnitude of sunset fade is higher in premonsoon and postmonsoon while it is lowest in monsoon, (vi) number of occurrence of both sunrise effect and sunset effect is remark-ably smaller in monsoon. The positions of the sun and of atmospheric sources are jointly the causes of seasonal and diurnal variations. The missing of sunrise effect and sunset effect are due to local cloud activity and variation of electron density during geomagnetic storms.With 7 Figures  相似文献   

8.
Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and aerosol carbon to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (SO) ranged from 0.52 to 0.99 with an average of 0.80. Aerosol carbon was scavenged less efficiently with an average value (AC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both SO and AC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, SO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of 0.3 g/m3. In the case of aerosol carbon, we obtained a more gradual increase of AC up to an LWC of 0.5 g/m3. At higher LWCs, _ remained relatively constant at 0.60. As the differences between SO and A varied across the LWC range observed at SBO, we assume that part of the aerosol carbon was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.  相似文献   

9.
The impact of climate change on the river rhine: A scenario study   总被引：3，自引：0，他引：3  
This paper concerns the impact of human-induced global climate change on the River Rhine discharge. For this purpose a model for climate assessment, named ESCAPE, is coupled to a water balance model, named RHINEFLOW. From climate scenarios, changes in regional annual water availability and seasonal discharge in the River Rhine Basin are estimated. The climate scenarios are based on greenhouse gases emissions scenarios. An assessment is made for best guess seasonal discharge changes and for changes in frequencies of low and high discharges in the downstream reaches of the river. In addition, a quantitative estimation of the uncertainties associated with this guess is arrived at.The results show that the extent and range of uncertainty is large with respect to the best guess changes. The uncertainty range is 2–3 times larger for the Business-as-Usual than for the Accelerated Policies scenarios. This large range stems from the doubtful precipitation simulations from the present General Circulation Models. This scenario study showed the precipitation scenarios to be the key-elements within the present range of reliable climate change scenarios.For the River Rhine best guess changes for annual water availability are small according to both scenarios. The river changes from a present combined snow-melt-rain fed river to an almost entirely rain fed river. The difference between present-day large average discharge in winter and the small average discharge in autumn should increase for all scenarios. This trend is largest in the Alpine part of the basin. Here, winter discharges should increase even for scenarios forecasting annual precipitation decreases. Summer discharge should decrease. Best guess scenarios should lead to increased frequencies of both low and high flow events in the downstream (Dutch) part of the river. The results indicate changes could be larger than presently assumed in worst case scenarios used by the Dutch water management authorities.  相似文献   

10.
The probability of setting global temperature records is reconsidered in light of cooling due to the Mt. Pinatubo eruption. The cooling resets temperature by moving it away from the top of its 100-year range. Depending on the statistical model for temperature, this reset can lead to a much lower probability for a record in the next few years. The exercise illustrates how record setting depends on the underlying model, the current record value, and - if there is serial correlation- the current temperature.  相似文献   

11.
Two shape profiles are in common use in plume modelling to describe the variation of velocity and buoyancy across the cross-section of a stack plume. Conditions for equivalence of the Gaussian profile and the top hat profile are discussed in detail.  相似文献   

12.
A recent noticeable climatic anomaly in some meteomarine quantities is analysed based on observations during the period March 1, 1971-February 28, 1993 taken in Trieste (Italy). Marked deviations from the normal values of the coupled quantities atmospheric pressure-solar irradiance and atmospheric pressure-sea level elevation, during the last 22 years, are analysed. The procedures used bring out a clear and persistent configuration of high pressure, high irradiance and low sea level from 1989 on, previously unmatched.  相似文献   

13.
Increasing reliance on natural gas (methane) to meet global energy demands holds implications for atmospheric CO2 concentrations. Analysis of these implications is presented, based on a logistic substitution model viewing energy technologies like biological species invading an econiche and substituting in case of superiority for existing species. This model suggests gas will become the dominant energy source and remain so for 50 years, peaking near 70 percent of world supply. Two scenarios of energy demand are explored, one holding per capita consumption at current levels, the second raising the global average in the year 2100 to the current U.S. level. In the first (efficiency) scenario concentrations peak about 450 ppm, while in the second (long wave) they near 600 ppm. Although projected CO2 concentrations in a methane economy are low in relation to other scenarios, the projections confirm that global climate warming is likely to be a major planetary concern throughout the twenty-first century. A second finding is that data on past growth of world per capita energy consumption group neatly into two pulses consistent with longwave theories in economics.  相似文献   

14.
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   

15.
Ozone has been observed in elevated concentrations by satellites over areas previously believed to be background. There is meteorological evidence, that these ozone plumes found over the Atlantic Ocean originate from vegetation fires on the African continent.In a previous study (DECAFE-88), we have investigated ozone and assumed precursor compounds over African tropical forest regions. Our measurements revealed large photosmog layers at altitudes from 1.5 to 4 km. Both chemical and meteorological evidence point to savanna fires up to several thousand km upwind as sources.Here we describe ozone mixing ratios observed over western Africa and compare ozone production ratios from different field measurement campaigns related to vegetation burning. We find that air masses containing photosmog ingredients require several days to develop their oxidation potential, similar to what is known from air polluted by emissions from fossil fuel burning. Finally, we estimate the global ozone production due to vegetation fires and conclude that this source is comparable in strength to the stratospheric input.  相似文献   

16.
The evaluation of the impact of climatic change on deltaic coastal systems requires establishing a reference situation. As deltaic coastal zones are highly dynamic areas, it is necessary to define the reference situation as a function of their present evolutive stage. This implies characterizing coastal processes as a complex system defined by the links between driving or forcing terms and the coastal response at different temporal and spatial scales (long/large, medium and episodic scales). Starting from classical previous works on deltaic systems and including all available field and conceptual knowledge to properly consider present conditions (largely regulated rivers, subsidence, etc.) a detailed methodology to analyse forcing or driving agents at the three mentioned scales is presented. This methodology which developed the general approach presented in a companion paper, will be followed by a future paper focusing on the analysis of the coastal response.  相似文献   

17.
The use of analytical solutions of the diffusion equation for footprint prediction is explored. Quantitative information about the footprint, i.e., the upwind area most likely to affect a downwind flux measurement at a given height z, is essential when flux measurements from different platforms, particularly airborne ones, are compared. Analytical predictions are evaluated against numerical Lagrangian trajectory simulations which are detailed in a companion paper (Leclerc and Thurtell, 1990). For neutral stability, the structurally simple solutions proposed by Gash (1986) are shown to be capable of satisfactory approximation to numerical simulations over a wide range of heights, zero displacements and roughness lengths. Until more sophisticated practical solutions become available, it is suggested that apparent limitations in the validity of some assumptions underlying the Gash solutions for the case of very large surface roughness (forests) and tentative application of the solutions to cases of small thermal instability be dealt with by semi-empirical adjustment of the ratio of horizontal wind to friction velocity. An upper limit of validity of these solutions for z has yet to be established.  相似文献   

18.
The standard E – model generates aplanetary boundary layerthat appears to be much too deep. The cause of theproblem is traced to the equation for the dissipationrate () of turbulent kinetic energy (E), specifically theparameterization of dissipation production anddestruction. In the context of atmosphericboundary-layer modelling, we argue that a part of thedissipation production should be modelled as the inputto the spectral cascade from the energy-containingpart of the spectrum, with a characteristic length , while the equilibrium imbalancebetween local production and destruction ofdissipation is modelled as proportional toE2/E, as in the standard model. Wepropose an E – – turbulence closurescheme, in which both the mixing length, m, and are prescribed. The importance ofthe equation is diminished, though itstill determines the dissipation rate in the Eequation.  相似文献   

19.
The term variangular is introduced to emphasize a significant difference between the present and certain earlier solutions to the problem of organized airmotion within the planetary boundary layer. The latter belong to the family of equiangular wind spirals and have the characteristic that the angle () formed by the vectors of shearing stress and geostrophic departure is invariant with height; it is shown that in this spiral-family, parabolic height-dependency of the effective (eddy) diffusivity (K) alone is permitted, including the asymptotic case of constant K; the famous Ekman spiral as well as the Rossby spiral are two prominent members of the family of equiangular wind spirals. The new variangular theory, as the name implies, permits variation of with height (z) and produces more versatile profiles of wind and stress due to less restraint in K (z). As an example of comparison with observed data, monthly mean wind profiles obtained at Plateau Station, Antarctica, are selected since they exhibit a noteworthy degree of variangularity, in relatively satisfactory agreement with properties of the new theoretical model for wind spirals.National Research Council Visiting Scientist Research Associate, Regional Environments Division, Earth Sciences Laboratory.  相似文献   

20.
The gradient diffusion parameterization of the Reynolds stress is modified by adding a bulk momentum transport term. Comparisons to available data indicate that this term accounts for the gusts or downsweeps of higher momentum fluid into the canopy and ejections of lower momentum fluid out of the canopy. This term results in realistic mean wind speed profiles.  相似文献   

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