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1.
The paper describes various aspects of the wind regime in coastal areas as obtained from several experimental programs along the Baltic coast of Sweden. The studies include the change with distance inland of mean wind structure as well as the turbulence structure in various conditions. It is found that wind spectra are usually well described by local similarity in its high frequency part, even in complex terrain. The low frequency parts of the spectra show clear spectral lag effects. The effects of a small slope, ca 5 m height change over 300 m travel distance, is clearly seen in some spectra, and it is shown that current flow over a hill theory can be used to account for it. The change of mean wind speed throughout the entire boundary layer as the wind passes a low but wooded island (Gotland), ca 30 km wide, has been studied in a series of relatively strong wind, near neutral cases. Some unexpected features are found; in particular the wind speed decreases more rapidly with distance inland than predicted by current numerical models. Two cases with a low level jet are discussed in some detail. Arguments are presented for the phenomenon to be caused by frictional decoupling at the Latvian coast, about 200 km upwind — the jet being thus an analogy in space to the classical Blackadar nocturnal jet frequently observed in continental areas.  相似文献   

2.
Island tropical montane cloud forests may be among the most sensitive of the world's ecosystems to global climate change. Measurements in and above a montane cloud forest on East Maui, Hawaii, document steep microclimatic gradients. Relatively small climate-driven shifts in patterns of atmospheric circulation are likely to trigger major local changes in rainfall, cloud cover, and humidity. Increased interannual variability in precipitation and hurricane incidence would provide additional stresses on island biota that are highly vulnerable to disturbance-related invasion of non-native species. Because of the exceptional sensitivity of these microclimates and forests to change, they may provide valuable listening posts for detecting the onset of human-induced global climate change.  相似文献   

3.
The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (u, w) and temperature (T) are more planar homogeneous than their vertical flux of momentum (u* 2) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO2 and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u*, especially when u* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (Iw) and the arrival frequency of organized structures (/h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.  相似文献   

4.
Sensible heat (H) and latent heat (LE) fluxes and turbulence statistics in St. Louis, Missouri and the surrounding region are presented. The urban-scale analyses were derived from a series of aircraft transects at 150 m above ground across the metropolitan area during the afternoon convective period. The results revealed that H varied by a factor of two to four in the region; the largest values were associated with the urban heat island. LE varied across the urban area by about a factor of four, but low values of LE overlaid the urban heat island. Consequently, the Bowen ratio (H/LE) exhibited large spatial variability, with a maximum value greater than 1.5 over the city and values less than 0.2 in nonurban areas. The areas along the Mississippi River and adjacent low lying marshland northeast of the downtown area displayed significantly smaller H and Bowen ratio. The derived surface heat storage term (G) for this area as well as for the urban area exceeded either H or LE.The spatial patterns for the standard deviations of the three velocity components ( u,v,w ), temperature ( T ), and absolute humidity ( q ), are also presented. The patterns of u,v,w were similar to the pattern of H. the highest values associated with the urban heat island. The correlation coefficient between the vertical velocity and temperature fluctuations was highest over the city, and a noteworthy minimum was observed in the upwind area over the river and marshland in association with low H. The convective similarity relationships for u,v,w appeared to be approximately valid spatially, as variations were typically less than 10% from theory over the urban area and nonurban region, except for a 40% anomaly in the lowland around the river northeast of the city.Measurements of H from 30-m towers within various land-use areas were contrasted with the aircraft data. Land-use differences in H at the surface were at least as large as those observed at 150 m across the city. This was primarily because of the measurement requirement that the minimum resolvable fetch increases with measurement height.  相似文献   

5.
The reactions of alkoxy radicals determine to a large extent the products formed during the atmospheric degradations of emitted organic compounds. Experimental data concerning the decompositions, 1,5-H shift isomerizations and reactions with O2 of several classes of alkoxy radicals are inconsistent with literature estimations of their absolute or relative rate constants. An alternative, although empirical, method for assessing the relative importance under atmospheric conditions of the reactions of alkoxy radicals with O2 versus decomposition was derived. This estimation method utilizes the differences in the heats of reaction, (H)=(Hdecomposition–HO 2 reaction), between these two reactions pathways. For (H)[22–0.5(HO 2 reaction)], alkoxy radical decomposition dominates over the reaction with O2 at room temperature and atmospheric pressure of air, while for (H)[25-0.5(HO 2 reaction)], the O2 reaction dominates over decomposition (where the units of H are in kcal mol–1). The utility and shortcomings of this approach are discussed. It is concluded that further studies concerning the reactions of alkoxy radicals are needed.  相似文献   

6.
A vapour of radio-lead (212Pb) has been used to measure the Sherwood number, Sh, of model leaves at various angles of incidence,, to the airstream in a wind tunnel. The results for=0 are compared with Pohlhausen's formula and the results for 0, with Powell's experiments. The local values of Sh on the upwind and downwind sides of discs have been obtained. For leaves in the canopy, Sh was found to be about 25% greater than would be predicted by applying Pohlhausen's equation without correction for orientation.  相似文献   

7.
Summary This study describes a reconstruction of rainfall characteristics in Southern Spain from 1501 to 1700 AD, during the beginning of the period called the Little Ice Age. Weather information was taken from original documentary sources (urban annals, local and religious histories, municipal documents, relations, etc.) in the region. A numerical index was established to characterize the rainfall, its characteristics, evolution and geographical distribution. Results were characteristics compared with modern precipitation data and with the results of other studies of historical climate. The general conclusion is that rainfall in Western Andalusia increased from approximately 1550 to 1650 AD. Some perspectives for future research work are outlined.With 6 Figures  相似文献   

8.
Measurements of the snow surface emissivity for infrared radiation have been made with an infrared radiation thermometer. The mean value of the emissivity is estimated at = 0.970 ± 0.008.  相似文献   

9.
A two dimensional model has been set up to investigate the circulation induced by an urban heat island in the absence of synoptic winds. The boundary conditions need to be formulated carefully and due to difficulties arising here, we restrict our attention to cases of initially stable thermal stratification. Heat island circulations are allowed to develop from rest and prior to the appearance of the final symmetric double cell pattern, a transitional multi-cell pattern is observed in some cases. The influence on the steady state circulation of various parameters is studied, among which are eddy transfer coefficients, the heat island intensity, the initial temperature stratification and the heat island size. Some results are presented for a case in which differential surface cooling beneath an initially stable atmosphere produces a circulation and an unstable layer capped by an elevated inversion over the city. It is hoped that this case is vaguely representative of the night-time heat island with no geostrophic wind.Notation cp Specific heat at constant pressure - g Acceleration due to gravity - H Top of integration region - Kz Vertical eddy transfer coefficient - Kx, KxH, Kxm Horizontal eddy transfer coefficients for heat and momentum - l ixing length - p Pressure - p0 Reference surface pressure (1000 mb) - PH (x, t) Pressure at z = H - R Specific gas constant for dry air - t Time - u, w Horizontal and vertical velocities - x, z Horizontal and vertical coordinates - x1, x2 Positions of discontinuities in surface temperature field (see Figure 2) - xa Heat island half-width - xb Boundary of integration region - Parameter in formula for eddy coefficients (variable-K case) = 18.0 - s Intensity of heat island - Potential temperature field - Reference absolute temperature (variable-K case) - r Reference temperature (° C) - s Surface temperature - Q Air density  相似文献   

10.
We formulate a method for determining the smallest time interval Tover which a turbulence time series can be averaged to decompose it intoinstantaneous mean and random components. From the random part the method defines the optimal interval (or averaging window) AW over which this part should be averaged to obtain the instantaneous spectrum. Both T and AW vary randomly with time and depend on physical properties of the turbulence. T also depends on the accuracy of the measurements and is thus independent of AW. Interesting features of the method are its real-time capability and the non-equality between AW and T.  相似文献   

11.
An observation of waves and turbulence in the earth's boundary layer   总被引：1，自引：1，他引：1  
An account is given of an observation of a wave-like phenomenon obtained during a study of nocturnal inversions. Associated bursts of turbulent activity are also described.  相似文献   

12.
A study of the temperature field for the Baltimore-Washington region reveals that since 1950 there has been the development of an urban heat corridor. Trend surface analysis shows that there has been an inversion of the thermal topography in the region as a saddle of rising temperatures has emerged, replacing a trough of lowered temperatures through three decades. Steady population growth throughout the area is seen to be the most important contributor. The strengthening of the heat corridor is best expressed in the summer months and weakest during the winter months. As population continues to grow in this region, the thermal topography is sure to be modified even further.It is recommended that further study be devoted to temperature and precipitation changes in regions experiencing urban growth.  相似文献   

13.
A simple energy balance model which simulates the thermal regime of urban and rural surfaces under calm, cloudless conditions at night is used to assess the relative importance of the commonly stated causes of urban heat islands. Results show that the effects of street canyon geometry on radiation and of thermal properties on heat storage release, are the primary and almost equal causes on most occasions. In very cold conditions, space heating of buildings can become a dominant cause but this depends on wall insulation. The effects of the urban greenhouse and surface emissivity are relatively minor. The model confirms the importance of local control especially the relation between street geometry and the heat island and highlights the importance of rural thermal properties and their ability to produce seasonal variation in the heat island. A possible explanation for the small heat  相似文献   

14.
In-water and remote measurements of ocean color   总被引：2，自引：0，他引：2  
Spectral measurements of downwelling irradiance, E d(), above the surface, and of upwelling irradiance just below the surface, E u(), allow computation of spectral values of the diffuse reflectance R() = E u()/E d(); this yields full information about the true color and brightness of the ocean. Typical results are presented and interpreted for waters very different in turbidity and phytoplankton content. Conversely, the possibility of infering the water content from R() data at selected wavelengths is examined in terms of the respective number of equations and unknowns. The necessary use of assumptions and of empirical laws is emphasized.The magnitude of the useful signal emerging from the water, and the magnitude of the additional signals due to specular reflexion at the interface and to atmospheric scattering, are compared on the basis of spectroradiometric measurements performed within and above the sea, from different altitudes. These unwanted signals are dominant, causing a drastic change in the spectral composition of the light received by a remote sensor. The evaluation of the atmospheric effect must be very precise in order to recover the marine signal with a sufficient accuracy for a meaningful application of any kind of algorithm.This work was supported in part by the Centre National d'Exploitation des Océans (under contract CNEXO 77/1695) and in part by the Centre National de la Recherche Scientifique (RCP 247 & ERA 278).  相似文献   

15.
A model is presented which calculates the changes of the velocity variances and stress uw in flow over gentle isolated hills. At intermediate frequencies spectra of the velocity components are modified according to rapid distortion theory. At low frequencies spectral densities change in proportion to the square of the mean wind. The inner and outer layer of the flow are distinguished. Streamline curvature effects are accounted for in the vertical velocity variance and the covariance.The sensitivity of the model to several parameters is investigated. Then, its results are compared with measurements of turbulent flow over various hills and an escarpment. The model is able to simulate the structure of the modified variance and covariance fields although larger differences occur at individual positions. The calculated modified spectra compare well with observed spectra.  相似文献   

16.
Plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focusis on highly-buoyant plumes that loft near or become trapped in the CBL capping inversion and resistdownward mixing. Such plumes are defined by dimensionless buoyancy fluxes F* 0.1, where F* = Fb/(U w* 2 zi), Fb is the stack buoyancy flux,U is the mean wind speed, w* is the convective velocity scale, and zi is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, c) concentration fields as a function of F* and the dimensionless distance X = w*x/(U zi), where x is the distance downstream of the source.The experiments reveal the following mainresults: (1) For 3 X 4and F* 0.1, the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below zi with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff. (2) The lateral dispersion (y) variation with X is consistent with Taylor's theory for * 0.1 and a buoyancy-enhanced dispersion, y/zi F* 1/3X2/3, forF* = 0.2 and 0.4. (3) The entrapment, the plume fraction above zi, has a mean (E) that follows a systematic variationwith X and F*, and a variability (e/E) that is broad ( 0.3 to 2) near the source but subsides to 0.25 far downstream. (4) Vertical profiles of the concentration fluctuation intensity (c/C) are uniform for z < zi and X > 1.5, but exhibit significant increases: (a) at the surface and close to the source (X 1.5), and(b) in the entrainment zone. (5) The cumulative distribution functions (CDFs) of the scaled concentration fluctuations (c/c) separate into mixed-layer and entrainment-layer CDFs for X 2, with the mixed-layer group collapsing to a single distribution independent of z.These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous buoyant releases in a convection tank. They also are the first tank experiments to demonstrate agreement with field observations of: (1) the scaled ground-level concentration along the plume centreline, and (2) the dimensionless lateral dispersion _y/z_i of buoyant plumes.  相似文献   

17.
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   

18.
Data from National Centre for Atmospheric Research (NCAR) Queen Air boundary-layer flights over the Nebraska Sandhills are analyzed to investigate the effects of these low hills on boundary-layer turbulence. The Sandhills are an area of anisotropic rolling terrain with characteristic wavelengths of order 2km and rms height variations of order 25m. The biggest impact is found in early morning flight data where horizontal velocity perturbations appear at the same wavelengths as the terrain and variances (normalised by u 2 , where u is the local friction velocity) are significantly enhanced relative to standard flat terrain values. By contrast the vertical velocity variance seems less affected and terrain effects are much less evident in data from the afternoon convective boundary layer.  相似文献   

19.
A field experiment was carried out in which wind speed and direction were measured over flat terrain at a height of 10 m using 13 identical instruments spaced logarithmically along two perpendicular 10 km lines. Station separations ranged from 312 m to 10 km. One-minute data from 11 sampling periods of duration 6 to 10 h were studied. p ]The statistics showed little dependence on whether the line of instruments was oriented along the wind or across the wind. The correlation coefficients between wind fluctuations at two stations separated by distance x were found to vary exponentially with x, with an integral distance scale on the order of 1 km. The integral time scale derived from the variation of the single station variances with averaging time was found to equal several minutes. At a station separation of 10 km, the correlation coefficients between the wind components at the two sites were calculated to be 0.24, 0.37, and 0.47 for averaging times of 1, 10, and 60 min, respectively. These values for the correlation coefficients correspond to root-mean-square differences in wind speed at the two stations of about 1.3, 1.0, and 0.7 m/s, respectively.Exponential formulas based on dimensional analysis are suggested for fitting these observations. It is found that the observations of spatial correlations are best fit if two independent integral distance scales are used — a boundary-layer distance scale of about 300 m that best applies to small station separations and a mesoscale distance scale of about 10 km that applies to larger station separations.  相似文献   

20.
Analysis of recent rainfall normals reveals an unusual spatial pattern where the rainfall increases with the distance from the sea over the central coastal plain of Israel. Applying a multiple regression technique separately to rainfall amounts for the two randomized experiments of cloud seeding of seeded days and not seeded days, leads to the conclusion that the inadvertent influence of recent urbanization processes along the coastal plain are the cause of this phenomenon, rather than the advertent artificial cloud seeding. The seeding effect at the target area, on the other hand, seems to increase the inhomogeneity of the spatial structure of the rainfall distribution.  相似文献   

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