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1.
We investigate the processes by which an accelerating stratified shear flow undergoes the transition to turbulence in a sequence of experiments in a tilted tank. We observe that the processes by which the flow undergoes breakdown are both complex and diverse, and suggest that the ratio, D, of the depth of the shear layer to the total tank depth and/or the (nondimensional) total density difference are important parameters in the determination of the dominant structures. In general, inherently three-dimensional, and relatively large-scale, flow structures strongly suppress simple subharmonic vortex pairing, and appear to dominate totally the transition to turbulence. In certain circumstances, the primary instabilities of the flow, namely Kelvin-Helmholtz billows, are able to develop in a quasi-two-dimensional manner before interaction between neighbouring billows becomes significant. In these circumstances, narrow secondary streamwise ‘tubes’ of vorticity are observed between neighbouring billows. Alternatively subharmonic, quasi-two-dimensional vortex mergings may be observed; these are not just simple pairings, but also three vortices are observed to merge into a single secondary billow, or two merge and the other persists, as predicted theoretically by Klaassen and Peltier (J. Fluid Mech., 202: 367–402, 1989). Three-dimensional vortex merging (knotting) of initially quasi-two-dimensional billows is also observed. Such knots are observed not only as pairwise transitions, as discussed by Thorpe (Geophys. Astrophys. Fluid Dyn., 34: 175–199, 1985), but also single billows are observed to knot with both adjacent neighbours simultaneously. Also, billows are observed to bow during merging events. However, particularly at larger density differences, higher Reynolds number and when the depth ratio, D, is sufficiently small, billow-billow interactions are apparent essentially immediately upon instability onset. Although the structures which develop resemble secondary tubes, these structures appear to be a primary instability of the flow, analogous to an instability observed by other researchers in both forced and unforced homogeneous shear layers.  相似文献   

2.
Nocturnal Low-Level Jet Characteristics Over Kansas During Cases-99   总被引：5，自引：1，他引：4  
Characteristics and evolution of the low-level jet (LLJ)over southeastern Kansas were investigated during the 1999 Cooperative Surface-AtmosphereExchange Study (CASES–99) field campaign with an instrument complement consisting of ahigh-resolution Doppler lidar (HRDL), a 60 m instrumented tower, and a triangle of Dopplermini-sodar/profiler combinations. Using this collection of instrumentation we determined thespeed UX, height ZX and direction DX of the LLJ. We investigate here the frequencyof occurrence, the spatial distribution, and the evolution through the night, of these LLJcharacteristics. The jet of interest in this study was that which generates the shear and turbulencebelow the jet and near the surface. This was represented by the lowest wind maximum.We found that this wind maximum, which was most often between 7 and 10 m s1,was often at or just below 100 m above ground level as measured by HRDL at the CASEScentral site. Over the 60 km profiler–sodararray, the topography varied by 100 m. The wind speed anddirection were relatively constant over this distance (with some tendency for strongerwinds at the highest site), but ZX was more variable. ZX was occasionally about equal at allthree sites, indicating that the jet was following the terrain, but more often it seemed to berelatively level, i.e., at about the same height above sea level. ZX was also more variable thanUX in the behaviour of the LLJ with time through the night, and on some nights $UX wasremarkably steady. Examples of two nights with strong turbulence below jet level were furtherinvestigated using the 60 m tower at the main CASES–99 site. Evidence of TKE increasing withheight and downward turbulent transport of TKE indicates that turbulence was primarilygenerated aloft and mixed downward, supporting the upside–down boundary layer notion in thestable boundary layer.  相似文献   

3.
In this paper, measurements of the first 150 m of the atmospheric boundary layer obtained by a high-frequency acoustic mini-sounder are compared with measurements obtained by a full complement of instruments including sonic anemometers mounted on the Boulder Atmospheric Observatory tower. The acoustic mini-sounder, starting as low as 6 m from the ground, measures in the monostatic mode the profiles of the vertical wind speed, w, and of the temperature structure parameter, C T 2 with enhanced height resolution of the order of 1 m and time resolution of the order of 30 s. The results of the comparison show that the high-frequency mini-sounder is an effective atmospheric boundary-layer profiler that is also portable and relatively inexpensive. Measurements of the spectrum of C T 2 are presented that provide information on the local isotropy of the temperature field. Statistics of the variability of C T 2 in both stable and unstable conditions are also given. The sounder's capabilities are further demonstrated by some detailed observations of the structure and time evolution of a thermal plume root at noon and of a nocturnal, stably stratified layer in which a dynamic instability develops. The plume starts at a height of less than 5 m, possesses substantial internal structure, and includes vertical velocities in excess of 2 m s-1.  相似文献   

4.
Centred at the interface between the sea-breeze and the return flow aloft, Kelvin-Helmholtz billows (KHB) are an important feature of the turbulent structure of some sea-breeze circulations (SBCs). In other SBCs, there are no prominent KHBs observed. Factors governing the appearance of billows are determined from a database of 139 sea breezes, constructed from two years of summertime surface observations at a site on the south coast of England. Post-frontal oscillations occur in the surface data for some SBCs and are interpreted as indicating possible KHBs aloft. The SBCs are formed under a wide range of synoptic conditions, enabling various measures of possible billow occurrence to be related to properties of the large-scale, ambient flow. Consistent with laboratory experiments of density currents, KHBs are suppressed for propagation into a head wind and enhanced with a tail wind. They are also found to be enhanced for stronger ambient wind speeds, while large-scale coast-parallel flow is effective in suppressing the billows.  相似文献   

5.
Two different Doppler acoustic sounders have been operated at the Kernforschungszentrum Karlsruhe (KfK) since 1982. It has been investigated whether meteorological data from these sounders can be used for dispersion modeling and monitoring in the environment of pollutant-emitting plants. Data from the sounders and from a 200 m high meteorological tower have been sampled continuously for intercomparison.Two schemes of stability classification are presented. They are based on 30-min mean values of the following meteorological data measured by the acoustic sounders: (a) standard deviation σw of the vertical wind speed and horizontal wind speed u, at a height of 100 m; and (b) standard deviation σφ of the vertical wind direction at a height of 100 m and vertical profile of the backscattered amplitude Aw.The class limits applied in these schemes are determined by “statistical equivalence” with a standard classification scheme. This standard scheme is based on σφ, measured by a vector vane at the 100 m level of the tower. Statistical equivalence in this context means that the frequency distributions of the classes are approximately equal at the same site and during the same period.The reliability of these schemes is investigated and compared to the standard scheme by correlation analysis. Finally, the schemes are compared with other commonly applied classification methods.  相似文献   

6.
The spatial variability of both turbulent flow statistics in the roughness sublayer (RSL) and temperature profiles within and above the canopy layer (CL) were investigated experimentally in a densely built-up residential area in Tokyo, Japan. Using five towers with measuring devices, each tower isolated from the others by at least 200 m, we collected high-frequency measurements of velocity and temperature at a height z=1.8 z H, where z H, the mean building height in the area, is 7.3 m. Also, temperature profiles were measured from z=0.4 to 1.8 z H. The ‘areal mean’ geometric parameters that were obtained for the areas within 200 m of each tower were fairly homogeneous among the tower sites. The main results are as follows: (1) The spatial variability of all RSL turbulent statistics, except the sensible heat flux, was comparable to that reported in a pine forest. Also, the variability decreased with increasing friction velocity. (2) The spatial variability of the RSL sensible heat flux was larger than that reported in a pine forest. Also, the variability depended on the time of the day and became larger in the morning. The difference among the sites was well related to the areal fraction of vegetation. (3) The spatial variability of the CL temperature profile depended on the time of the day and became larger in the morning. Nevertheless, the spatial standard deviation of CL temperature was always below 0.7 K. (4) It is suggested that the “warming-up” process in the morning when heat storage is dominant increases the spatial variation of RSL sensible heat flux and CL temperature according to the local properties around each tower and the variation decreases once there is further convective mixing in the midday  相似文献   

7.
Observations near the BAO tower site have suggested a roughness length of about 0.01 m. Wind profiles from the tower yield values between 0.04 and 0.30 m. We now analyze fluctuations of vertical velocity at 10 m, and get intermediate values, differing with wind direction. Thus, inferred z 0 values increase systematically with height.  相似文献   

8.
We have analyzed measurements of vertical velocity w statistics with the NOAA high resolution Doppler lidar (HRDL) from about 390 m above the surface to the top of the convective boundary layer (CBL) over a relatively flat and uniform agricultural surface during the Lidars-in-Flat-Terrain (LIFT) experiment in 1996. The temporal resolution of the zenith-pointing lidar was about 1 s, and the range-gate resolution about 30 m. Vertical cross-sections of w were used to calculate second- to fourth-moment statistics of w as a function of height throughout most of the CBL. We compare the results with large-eddy simulations (LES) of the CBL and with in situ aircraft measurements. A major cause of the observed case-to-case variability in the vertical profiles of the higher moments is differences in stability. For example, for the most convective cases, the skewness from both LES and observations changes more with height than for cases with more shear, with the observations changing more with stability than the LES. We also found a decrease in skewness, particularly in the upper part of the CBL, with an increase in LES grid resolution.  相似文献   

9.
This paper describes some of the results that were obtained in an experimental study of an inversion capping a convective boundary layer, during which a balloon-borne probe was flown through the actual interface at the same time as it was scanned by a high power 10-cm Doppler radar. The initial analysis has revealed several interesting features including the possible presence of small-scale Kelvin-Helmholtz billows at the crest of the hummocks of the inversion and the absence of an inertial subrange. A model is proposed for the transfer of sensible heatdownwards across the inversion. This is hypothesised as a two-stage process — sporadic thickening by the entrainment of air from above and below due to the action of the breaking billows, and a continual erosion of the lower surface by convective activity.  相似文献   

10.
Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a significant portion of the imbalance was caused by: (i) the overestimation of net radiation (R n ) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation of the latent heat flux (λE) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H) and λE, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between T and the net ecosystem exchange (NEE) of carbon dioxide (CO2), forcing T to closure while retaining the Bowen ratio and applying the same factor to CO2 fluxes (F C ) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship between T, F C and C.  相似文献   

11.
Mixing depth structure and its evolution have been diagnosed from radar wind profiler data in the Chamonix and the Maurienne valleys (France) during summer 2003. The behaviour of refractive index structure parameter C n 2 peaks coupled with the vertical velocity variance σ w 2 was used to estimate the height of the mixed layer. Tethersonde vertical profiles were carried out to investigate the lower layers of the atmosphere in the range of approximately 400–500 m above ground level. The tethersonde device was especially useful to study the reversal of the valley wind system during the morning transition period. Specific features such as wind reversal and the convective mixed layer up to approximately the altitude of the surrounding mountains were documented. The wind reversal was observed to be much more sudden in the Maurienne valley than in the Chamonix valley  相似文献   

12.
The evolution of the lower portion of the planetary boundary layer is investigated using the Small Multifunction Research and Teaching Sonde (SMARTSonde), an unmanned aerial vehicle developed at the University of Oklahoma. The study focuses on the lowest 200 m of the atmosphere, where the most noticeable thermodynamic changes occur during the day. Between October 2010 and February 2011, a series of flights was conducted during the evening hours on several days to examine the vertical structure of the lower boundary layer. Data from a nearby Oklahoma Mesonet tower was used to supplement the vertical profiles of temperature, humidity, and pressure, which were collected approximately every 30?min, starting 2 h before sunset and continuing until dusk. From the profiles, sensible and latent heat fluxes were estimated. These fluxes were used to diagnose the portion of the boundary layer that was most affected by the early evening transition. During the transition period, a shallow cool and moist layer near the ground was formed, and as the evening progressed the cooling affected an increasingly shallower layer just above the surface.  相似文献   

13.
A laboratory study in a rotating stratified basin examines the instability and long time evolution of the geostrophic double gyre introduced by the baroclinic adjustment to an initial basin-scale step height discontinuity in the density interface of a two-layer fluid. The dimensionless parameters that are important in determining the observed response are the Burger number S=R/R0 (where R is the baroclinic Rossby radius of deformation and R0 is the basin radius) and the initial forcing amplitude (H1 is the upper layer depth). Experimental observations and a numerical approach, using contour dynamics, are used to identify the mechanisms that result in the dominance of nonlinear behaviour in the long time evolution, τ>2−1 (where τ is time scaled by the inertial period TI=2π/f). When the influence of rotation is moderate (0.25≤S≤1), the instability mechanism is associated with the finite amplitude potential vorticity (PV) perturbation introduced when the double gyre is established. On the other hand, when the influence of rotation is strong (S≤0.1), baroclinic instability contributes to the nonlinear behaviour. Regardless of the mechanism, nonlinearity acts to transfer energy from the geostrophic double gyre to smaller scales associated with an eddy field. In the lower layer, Ekman damping is pronounced, resulting in the dissipation of the eddy field after only 40TI. In the upper layer, where dissipative effects are weak, the eddy field evolves until it reaches a symmetric distribution of potential vorticity within the domain consisting of cyclonic and anticyclonic eddy pairs, after approximately 100TI. The functional dependence of the characteristic eddy lengthscale LE on S is consistent with previous laboratory studies on continuously forced geostrophic turbulence. The cyclonic and anticyclonic eddy pairs are maintained until viscous effects eventually dissipate all motion in the upper layer after approximately 800TI. The outcomes of this study are considered in terms of their contribution to the understanding of the energy pathways and transport processes associated with basin-scale motions in large stratified lakes.  相似文献   

14.
A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW).  相似文献   

15.
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels.  相似文献   

16.
Springtime fetch in the Cape Bathurst Polynya System may present opportunities for winds to generate waves capable of propagating into the thick pack ice formed over the winter. A waves-in-ice event at a study site located on the Canadian Shelf in the southern Beaufort Sea that occurred 22–23 May 2011 is presented and analyzed for wave energy attenuation and dissipation characteristics. The event was monitored near the ice edge and, therefore, presents information on attenuation of waves from the ice edge into the pack. Waves of T?=?5?s, λ?=?37.5?m were observed up to approximately 143?m and approximately 77?m away from the ice edge during two separate observation periods of ice edge wave propagation. We estimated reflection coefficients of 53% and 52% and wave attenuation coefficients of α?=?2.4?×?10?2?m?1 and α?=?5.4?×?10?2?m?1, respectively, for the two periods. Estimated attenuation rates are an order of magnitude greater than in comparable studies and are inconsistent with previous findings of a “rollover” effect in attenuation rates for short-period waves.  相似文献   

17.
We investigate numerically and theoretically the nonlinear evolution of a parallel shear flow at moderate Reynolds number which has embedded within it a mixed layer of intermediate fluid. The two relatively thin strongly stratified density interfaces are centered on the edges of the shear layer. We are particularly interested in the development of primary and secondary instabilities. We present the results of a stability analysis which predicts that such flows may be unstable to stationary vortical disturbances which are a generalization of an inviscid instability first considered by G.I. Taylor. We investigate the behavior of these “Taylor billows” at finite amplitude through two-dimensional numerical simulations. We observe that the braid regions connecting adjacent primary Taylor billows are susceptible to secondary, inherently two-dimensional instabilities. We verify that these secondary instabilities, which take the form of small elliptical vortices, arise due to a local intensification of the spanwise vorticity in the braid region.  相似文献   

18.
A statistical model, based on a method of Vulf'son, is used to examine some of the plume-like temperature structures formed in the unstable boundary layer. The model assumes that the plume diameter changes slowly with height so that a cylindrical approximation may be made. Measurements of the vertical velocity and temperature were used to determine the temperature dependent portion of the vertical velocity field. Temperature data were collected from sensors on a tower and from aircraft; velocity data were collected only from the tower.Using this model for analysis of the data indicates that: (1) the average isotherm diameter and the population of isotherms are a function ofz/L; (2) the distribution of core temperatures is approximately a uniform distribution.Independent of the model, a convective velocity was determined and found to have approximately the same profile as the temperature; from this the average velocity of the plumes was found to be a linear function ofz/L, fromz/L - 0.1 toz/L - 1.0. As a consequence of this functional dependence, the entrainment into the plumes is approximately constant over this range. The cumulative temperature distribution function was found to be an asymmetric function ofz/L. A simple relation which is independent ofu * is given to determine the heat flux.Contribution No. 269 Dept. of Atmospheric Sciences, University of Washington.  相似文献   

19.
Simultaneous observations were made of the Marine Boundary Layer at Tarapur, a site near Bombay on the sea coast, by acoustic sounder and instrumented tower. The meteorological tower was used to sense wind and temperature at various levels up to a height of 120 m while the acoustic sounder was used to examine the thermal structure of the boundary layer up to a height of 700 m. Data recorded for the year 1982 have been analysed.Analysis of the data shows that while the normal structures of thermal echoes and shear echoes represent the mixing depth of the atmospheric boundary layer, the often observed elevated layers are due to sea breeze reversals with their base giving a measure of the depth of the sea-breeze circulation during the day. A sea breeze has been detected during both spring (March to May) and autumn (October to December) months. The onset times are around 1000 hr during spring months and around noon during the autumn period, the height of development being respectively up to 500 and 350 m. The capability of the sodar to detect the base and thickness of the sea breeze, is clearly revealed.  相似文献   

20.
Simultaneous observations were made by an acoustic sounder and on a meteorological tower during the month of May 1978 at the Atomic Power Station Tarapur. The probing range of the acoustic sounder was 700 m. The meteorological tower could sense wind and temperature at various levels up to a height of 120 m.The site being close to the sea shore, the thermal environment of the lower atmosphere is controlled mostly by land and sea breeze circulations. Thermal convective structures were seen during the daytime and also at night. The frequency of plume formation and the height of the plumes were, however, low during the night. The convective boundary layer in the daytime ranged from 400–500 m while at night it was mostly under 200 m. The observation of thermals at night is explained by the presence of a naturally stable marine layer above 30 m at this site. In the morning hours, winds suddenly change their direction allowing advection of a land breeze which is responsible for the formation of surface-based shear echoes to a height of 200 m during the transition period and for the subsequent development of an elevated layer due to mixing of two different air masses. A marine layer was also seen over Tarapur for a few days during the early evening and night hours. Its height was mostly around 400 m. It may indicate the presence of a subsidence inversion at Tarapur. The need for collection of supporting meteorological data to a height of 500 m by tethered balloon or some other suitable in-situ technique is stressed.  相似文献   

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