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1.
The influence of both spatial and temporal temperature inhomogeneities of the underlying surface on the temperature field in an unstably stratified atmospheric surface layer is considered. The methods of correlation and spectral analyses are proposed to estimate statistical characteristics of surface-air temperature fluctuations caused by both turbulent mixing and inhomogeneities in the temperature of the underlying surface. Analysis of experimental data obtained from measurements in the atmospheric surface layer yields estimates for the contribution made by the time-dependent thermal properties of the underlying surface to the total variance of air-temperature fluctuations. It is shown that the additional air-temperature fluctuations generated by surface-temperature inhomogeneities and unrelated to shear flow may reach 70% and 30% of the total variance of measured fluctuations under variable cloudiness and clear skies, respectively. For the height z = 2 m within the wave-number range 2 × 10?3 rad m?1 < k < 0.1 rad m?1, the contribution made by a spatial surface-temperature inhomogeneity to the variance of air-temperature fluctuations does not exceed 10% of the total variance. Correlation and spectral analyses of experimental data make it possible to isolate the spectra of properly turbulent fluctuations from the measured fluctuations and thus to obtain more accurate values of the universal function of similarity theory for temperature in the range of small wave numbers beyond the inertial range.  相似文献   

2.
Using a high-resolution LES numerical model, we calculated the turbulent thermal convection for high ratios of horizontal and vertical sizes of the computational domain (26: 26: 1). The natural analog of the simulated process is a planetary boundary layer (PBL) of the atmosphere growing with height in the background of stably stratified overlying air layers over a horizontally homogeneous heated surface under a weak average wind. We obtained the spectral distributions of variances of fluctuations in potential temperature and velocity components in ranges corresponding to scales from a few tens of meters to a few tens of kilometers. We found energetically significant segments of the spectrum of large-scale fluctuations in the potential temperature for which the power dependences S ～ k ?1/3 and S ～ k ?4/3 are satisfied with good accuracy. We calculated the characteristic spatial scales of horizontal fluctuations in velocity and temperature. We obtained a dependence of these scales on the height of the growing convective PBL. We discuss the characteristic features of large-scale distributions in terms of the self-similarity of the growing boundary layer behavior.  相似文献   

3.
Hourly fluctuations of vertical velocity in relation to components of flow and wind and temperature oscillations at a morring site in the shelf waters off the west coast of India are discussed. The vertical velocities were computed from a time series of vertical temperature profiles assuming that horizontal advection of temperature is negligible. The computed values at a depth of 40 m during the 72-h period of observation were of the order of 10−1 to 10−2cm s−1, with a mean value of −2·77 × 10−2 cm s−1 indicating a net upward movement of water. The computed vertical velocity showed fluctuations of about 2–3 h, in addition to weaker signals of about 12 h. Based on the spectral estimates, we speculate that these fluctuations of 2–3 h in the vertical velocity may be caused by the fluctuations in the along-shore wind. The oscillations of isotherms found in the temperaturedepth time series and the spectral estimates of temperature and cross-shore flow component showed a periodicity of about 12 h, which indicated the presence of semi-diurnal internal waves. The fact that these internal wave troughs were associated with the measured onshore flow suggested that the waves were propagating offshore. The computed stability parameters showed little evidence of instability or mixing. It was found that the isotherm troughs in the temperaturedepth time series at about 12-h period coincided with high vertical shear in the cross-shore direction and low values of Brunt Vaisälä frequency.  相似文献   

4.
A phenomenological model is proposed for the three-dimensional (3D) spectrum of temperature irregularities generated by internal waves in the atmosphere. This model develops a theory (Chunchuzov, 2002) based on the assumption that the field of the Lagrange displacements of the medium’s particles that are caused by a statistical ensemble of internal waves with randomly independent amplitudes and phases is stationary, homogeneous, axially symmetric in a horizontal plane, and Gaussian. To fit the model to measured spectra of fluctuations in the stratosphere and mesosphere, an additional assumption is introduced into the model that the degree of anisotropy of irregularities depends on their vertical size. An explicit expression is presented for the 3D spectrum. The model vertical spectrum follows a power law with an exponent of ?3. The horizontal spectrum has three asymptotically power portions. Two of these are characterized by an exponent of ?3, whereas an intermediate portion has an exponent of ?1 to ?3, depending on the rate at which the degree of anisotropy decreases as the vertical size of temperature irregularities increases. Simple asymptotic formulas are obtained for the horizontal spectrum. Within the range of a few decades, the model is in good agreement with the published results of measuring the spectra in the upper troposphere, stratosphere, and mesosphere.  相似文献   

5.
The nonlinear propagation of acoustic pulses from a point source of an explosive character (surface explosion or volcano) throughout the atmosphere with stratified wind-velocity and temperature inhomogeneities is studied. The nonlinear distortions of acoustic pulse and its transformation into an N-wave during its propagation to the upper atmosphere are analyzed in the context of a modified Burgers’ equation which takes into account a geometric ray-tube divergence simultaneously with an increase in both nonlinear and dissipative effects with height due to a decrease in atmospheric density. The problem of reflection of a spherical N-wave from an atmospheric inhomogeneous layer with model vertical wind-velocity and temperature fluctuations having a vertical spectrum that is close to that observed within the middle atmosphere is considered. The relation between the parameters (form, length, frequency spectrum, and intensity) of signals reflected from an atmospheric inhomogeneous layer and the parameters of the atmospheric fine layered structure at reflection heights is analyzed. The theoretically predicted forms of signals reflected from stratified inhomogeneities within the stratosphere and the lower thermosphere are compared to the observed typical forms of both stratospheric and thermospheric arrivals from surface explosions and volcanoes in the zones of an acoustic shadow.  相似文献   

6.
In experiments aimed at measuring the spectral volume scattering function of light, we obtain an additional confirmation of the existence of two-dimensional inhomogeneities of water on scales of 10-7–10-3 m. In the case of mixing of pure water with small amounts of filtered water with different temperature or salinity, we observe an 1.8–4.5-fold intensification of the scattering of light in the entire range of angles. It is shown that this effect can be explained neither by turbulence nor by the formation of dissipative structures. We estimate the sizes of inhomogeneities affecting the spectral volume scattering function of light.  相似文献   

7.
We describe a differential approach to the spectral composition of thin thermohaline inhomogeneities in the ocean depending on the mechanisms responsible for their generation and demonstrate the possibilities of this approach in getting realistic hydrophysical results. The statistical classification of fine-structure fluctuations according to their origin is performed on the basis of the joint analysis of the temperature, salinity, and density spectra computed in a section of the vertical profile where the variation of mean gradients of all three parameters is insignificant. On the basis of the constructed mean spectrum of wave elevations, we estimate the dissipative characteristics of the convection of salt fingers and small-scale internal waves. Translated by Peter V. Malyshev and Dmitry V. Malyshev  相似文献   

8.
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9.
Water samples from a Pacific Ocean basin have been analysed for a number of forms of chromium. Total Cr, almost entirely as dissolved Cr(VI), steadily increases with depth and correlates most closely with silica data. Mid-depth regeneration of Cr is suggested by a multiple regression equation which includes Cr(total), silica and phosphate data.The release of Cr and silica from bottom sediments is apparent as the continental margin is approached. Box model transport calculations suggest that a Cr flux from the sediments on the order of 100 nmol Cr m?2 day?1 could be balanced by horizontal and vertical eddy diffusion coefficients of 106 and 10 cm2 s?1, respectively, and by horizontal and vertical advection rates of 10?2 and 10?4 cm s?1, respectively.  相似文献   

10.
Vessel-based observations of water column structure and flow near a shallow-crested seamount are used to quantify the physical disturbance induced by that seamount. The implications of this disturbance on the ‘feeding hole’ hypothesis are then examined based on data from moored thermistors and acoustic current profilers, as well as vessel-based acoustic sounding-derived biomass estimates, currents, and conductivity-temperature-depth profiles from a 55 km square grid of stations around the crest. Mean currents in the region of 0.2 m s?1 are comparable to observations from surface drifters whereas the semidiurnal tidal flow amplitude was one third of this. Thorpe Scale-based estimates of energy dissipation rate were in the range 10?9 to 2.10?8 W kg?1 and vertical diffusivities Kz were in the range 10?4 to 10?3 m2 s?1. Turbulence levels were higher upstream of the seamount–likely due to the influence of nearby seamounts Rumble IV and V. There was no evidence of a Taylor Cap in the Rumble III velocity field. The sounder data provide some evidence of a feeding hole and analysis based on diffusivities suggests that this might persist downstream of the seamount for as much as 7 days.  相似文献   

11.
Application of a simple model describing regional variations in the contents of manganese and associated minor metals in deep-sea sediments suggests that solid manganese phases are being removed from the <0.5 μm fraction of seawater at ～1–7 · 1012g yr?1 in excess of the rate of stream-supplied manganese. This flux is consistent with: (1) the relative rates of sediment accumulation in the Atlantic and Pacific Oceans; (2) the contrast between the oceanic residence time of manganese calculated from stream-supply data (14 · 103 yr) and from the flux of manganese precipitating in marine sediments or as manganese nodules (0.38–2.4 · 103 yr); (3) the surplus mass of manganese revealed by geochemical balance calculations (22.9 · 102g). On this basis excess manganese is accumulating in deep-sea sediments at 0.2–2.0 · 10?6 g cm?2yr?1. Manganese supplied to the upper layers of marine sediments by diagenesis has been evaluated with the aid of vertical advection—diffusion—reaction models. The calculated diagenetic flux of manganese at the sediment surface in a near-shore environment is in agreement with the known accretion rate of manganese deposits (1.7 · 10?2 g cm?2 10?3 yr?1) and the regionally variable flux over the area assessed is consistent with the presence or absence of manganese nodules at or near the water-sediment interface. The diagenetic flux at the surface of deep-sea sediments has been calculated at 0.7 · 10?4 g cm?2 10?3 yr?1 when the upper, oxic, zone of the sediment is ～20 cm thick. A limiting factor on the in situ production flux of dissolved manganese in deep-sea sediments appears to be the availability of reducing agents for manganese dissolution rather than the rate of downward transport of manganese-rich sediment to a reaction boundary where dissolution takes place. Various estimates of the rate of upward-migrating manganese suggest that manganese precipitates in the oxic zone with a rate constant of ～10?7 sec?1 with the result that diagenetic processes cannot supply the flux of excess manganese through more than ～0.25 m of oxic sediment. However, estimates of the flux of manganese to the oceans by submarine volcanic processes (0.79–1.1 · 1012g yr?1) are similar to the surplus mass of manganese detected by geochemical balance calculations (0.7 · 1012g yr?1). If submarine hydrothermal solutions provide only 10% of this excess then their computed discharge rate (39 g cm?2 yr?1) and residence time in the upper layer of oceanic crust (130,000 yr) agree well with these parameters for continental thermal springs.  相似文献   

12.
Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF6 tracer. During the experiment the mixed layer depth zmld ranged between 50 and 80 m, with several re-stratifying events that brought zmld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating zmld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, Lc, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10−3 m2 s−1 in the pycnocline rising to over 10−2 m2 s−1 higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m2/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.  相似文献   

13.
The data of the CTD survey conducted in the Denmark Strait and Irminger Sea in May–June 2009 are used to calculate the vertical profiles of the turbulent overturning scale, which are then used to estimate the dissipation and entrainment rates in the overflow plume. The resulting estimates of the entrainment rate varied widely from 2 × 10–7 to 7 × 10–3 m/s. It is shown that such a wide range of entrainment rates is caused by the intermittency of turbulence. Large turbulent overturning at the interface of the Denmark Strait overflow plume is detected on the vertical temperature, salinity, and potential density profiles.  相似文献   

14.
A vertically integrated two‐dimensional (2‐D) and a five‐layer three‐dimensional (3‐D) numerical models were developed to compute the tides in the Gulf of Suez, Egypt. The computational grid used to schematize the Gulf has a horizontal resolution of 3 × 3 km and consists of a lattice of 23 × 100 points in the 2‐D model and five such lattices in the 3‐D model. Both 2‐D and 3‐D simulations clearly revealed the Kelvin wave nature of the tide with partial reflection. The M2 simulations also showed a strong tidal signature in the southern sector as compared to the northern part. For the 3‐D simulations, the horizontal and vertical eddy viscosity coefficients and the coefficient of bottom friction were respectively set to 3 × 106 cm2/s, 200 cm2/s, and 0.001. The tidal range decreases from the entrance of the Gulf of Suez toward the Bank of Tor where it reaches a small value and then increases again to about 1.5 m at Suez. A difference of six hours exists between the times of high water at the southern and northern ends of the Gulf. Quantitatively the agreement between the observed and computed tide is better in the 2‐D simulation than in the 3‐D simulation. However, the counterclockwise gyres in the top three layers of the 3‐D model (upper 30 m) during slack water following the flood tide do not appear in the 2‐D simulation.  相似文献   

15.
The strength of mixing due to turbulence in the Antarctic Slope Front (ASF) region was investigated using CTD (conductivity-temperature-depth profilers) observations and direct measurements of turbulence conducted off Adélie Land, East Antarctica along 140°E from the 12th–14th February, 2005. The strongest horizontal gradient of the ASF was located below 300 m depth near the 1000 m isobath. The turbulent measurements revealed that the energy dissipation rate frequently exceeded 10?8 Wkg?1 on the continental shelf and upper slope regions. Turbulent diffusivities near the shelf break were higher than 10?3 m2s?1. Near the ASF the average turbulent heat flux was 5.7 Wm?2 and 1.1 Wm?2 across the temperature minimum layer to 250 m and from 300 to 600 m, respectively. The distribution of the high dissipation rate was consistently explained by the characteristic curve of the M2 internal wave emanating from the shelf break and continental slope. The water mass observed in the ASF below 300 m in the continental slope comprised Modified Circumpolar Deep Water and low salinity Shelf Water originating from either the upper layer of the Adélie Depression or the Adélie Bank, and produced by boundary mixing near the shelf break.  相似文献   

16.
Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s?1 and the buoyancy frequency squared was in the range 10?5 to 10?3?s?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10?6?m2?s?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact.  相似文献   

17.
A long submerged horizontal circular cylinder of .90 m diameter was assembled off the beach at Reggio Calabria where the wind waves typically have significant height ranging within 0.20 and 0.40 m and dominant period within 1.8 and 2.6 s. Three ultrasonic probes recorded the waves, and two sets of pressure transducers, the first one at the cylinder and the second one in the undisturbed wave field, enabled to compare the force amplitude on the cylinder to the force amplitude on an equivalent mass of water in the undisturbed wave field (Froude-Krylov F-K force). After ten days of measurements, the experiment was repeated with a cylinder of .45 m diameter. The Keulegan-Carpenter number was within 2.5, and the wave forces proved to be inertial. The following general features emerged: (i) the force spectrum is usually very narrow even if the wave spectrum is broad; (ii) the vertical diffraction coefficient is somewhat smaller than the horizontal diffraction coefficient; (iii) the positive extremes of Fz (vertical force referred to the buoyancy force) markedly exceed the negative extremes; (iv) the pressure fluctuations induced by the highest waves at the cylinder are very similar to the measured pressure-surface displacement covariances. In each of the 580 records obtained in the course of the experiment it was found that the propagation speed reduces to about a half at the cylinder, and the amplitude of the pressure fluctuations increases of 10–15% at the upper half of the cylinder and decreases of about the same percentage at the lower half. These phenomena fully explain why the force amplitude on the cylinder is larger than the F-K force amplitude.  相似文献   

18.
The oceanographic conditions of the Mackenzie River plume in the Arctic Ocean were examined during a 12-day period in August 2007. Field observations in the river channel and the delta region (2–6 m depth), ship-based observations on the shelf and satellite observations of sea surface temperatures indicate that movements of plume density fronts cause changes in water temperatures of over 10  C over a few days. We used a 1D model to compare the strength of stratification versus surface wind stress, and a 3D numerical model to simulate the plume motions under forcing from the river flows, local wind and water level variations from tides and wind-driven surge. The results indicate that the coastal region is stratified with a ∼2 m thick surface plume even in water depths of 3–4 m, resulting in strong vertical variation of horizontal currents. Moderate easterly winds of 5–10 m/s are sufficient to induce offshore transport of the surface plume and onshore transport of the deeper shelf water, leading to large fluctuations in temperature and salinity in the coastal region. This study examined a period of offshore transport and mean water level set-down, and indicates the rapid response of the plume to wind over the shallow delta.  相似文献   

19.
The problem of the incompressible liquid convection between two horizontal planes heated on the underside is considered in the three-dimensional statement. The horizontal boundaries are assumed to be free of tangential stresses and isothermal. The calculated temporal spectrum of the temperature pulsations at the convection cell center at a supercriticality of 410 agrees well with the experimental measurement result. The Bolgiano-Obukhov (BO), k −11/5, k −3, and k −5, were obtain for the velocity pulsations. The Kolmogorov spectra k −5/3 and k −2.4 were obtained for the temperature pulsations. The presence of clearly identified spectra in the convective flow under study allows this process to be characterized as developed turbulence.  相似文献   

20.
To explain the conditions for dew and pollutant precipitation during still-weather radiative cooling of the Earth’s surface, attention is paid to the role of surface temperature inhomogeneities and convective air flows appearing near them. To understand how convection forms near cold spots, schematic cylinders with adiabatic walls and a cold bottom are considered. The results of this consideration agree with the results obtained in previous calculations and experimental studies, namely, the Nusselt number depends on the Rayleigh number to the power 1/5 with the proportionality coefficient γ. The quantitative characteristics were experimentally investigated by measuring the vertical temperature profiles and observing tracer motion near circular cold water surfaces about 30 cm and 10 m in size. As a result, it is found that, for isolated planar spots, the temperature difference between the spot surface and the environment decreases in accordance with the exponential law and air motions from the center of the spot toward its edges are of a laminar character. For the Rayleigh numbers that use the ratio of the area of the spot to the half of its perimeter as the characteristic size, the mean values of the coefficient γ are 1.1 for an isolated spot with Ra = 106 and 0.7–0.8 for spots with Ra = 106?5 × 1010 located at one level with the surrounding surface. For deepened spots, the processes of heat exchange significantly slow down and the character of temperature profiles changes depending on the material of the surrounding walls. The process of establishing the equilibrium temperature of an isolated spot that is caused by radiative cooling and the convective inflow of heat and moisture is considered in the Appendix. A method for preventing radiation fog by air drying during convection near the surrounding cold surfaces is proposed, and the principles of the intensification of the condensed-moisture collection are formulated.  相似文献   

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