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1.
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2.
Measurements of the temperature and zonal velocity fields which develop in a rotating annulus of fluid with an upper surface, differentially heated from the inner to outer cylinder, are described for the lower symmetric regime (small radial temperature differences). The temperature field is essentially conductive for moderate to large rotation rates, Ω (>1.0 sec−1). The zonal velocity field is poorly approximated by the thermal wind equation.Measurements of the transition to waves from the lower symmetric regime at very large rotation rates are presented for positive and negative radial temperature differences. They suggest that the centrifugal buoyancy force and the free surface curvature may be important factors for the lower symmetric-wave transition at large Ω. By varying the stratification of the fluid over a range of 103 independently of the radial temperature difference, ΔrwT, it is conclusively shown that several theories are correct in predicting that the lower symmetric transition is independent of the stratification at small ΔrwT > 0 for large enough Ω.  相似文献   

3.
Formation of horizontal convective rolls in urban areas   总被引：6，自引：0，他引：6  
The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with − zi / L < 25 (zi: the boundary-layer depth, L: the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes.  相似文献   

4.
The response of a two-dimensional thermohaline ocean circulation model to a random freshwater flux superimposed on the usual mixed boundary conditions for temperature and salinity is considered. It is shown that for a wide range of vertical and horizontal diffusivities and a box geometry that approximates the Atlantic Ocean, 200–300 yr period oscillations exist in the basic-state, interhemispheric meridional overturning circulation with deep convection in the north. These fluctuations can also be described in terms of propagating salinity anomalies which travel in the direction of the thermohaline flow. For large horizontal (K h = 15 × 103 m2/s) and small vertical (K v = 0.5 × 10–4 m2/s) diffusivities, the random forcing also excites deca-millennial oscillations in the basic structure of the thermohaline circulation. In this case, the meridional circulation pattern slowly oscillates between three different stages: a large positive cell, with deep convection in the North Atlantic and upwelling in the south; a symmetric two-cell circulation, with deep convection in both polar regions and upwelling near the equator; and a large negative cell, with deep convection in the South Atlantic and upwelling in the north. Each state can persist for 0 (10 kyr).  相似文献   

5.
In order to investigate flows over topography in an atmospheric context, we have studied experimentally the wake structure of axi-symmetric Gaussian obstacles towed through a linearly stratified fluid. Three dimensionless parameters govern the flow dynamics: F, the Froude number based on the topography height h; Re, the Reynolds number and the aspect ratio r = h/L, where L is the topography horizontal scale. Two-dimensional (2-D), saturated lee wave (SLW) and three-dimensional (3-D) regimes, as defined in Chomaz et al. (1993), are found to be functions of F and r only (Fig. 1) as soon as Re is larger than Rec ≈ 2000. For F < 0.7 the flow goes around the obstacle and the motion in the wake is quasi-two-dimensional. This 2-D layer is topped by a region affected by lee wave motions with amplitude increasing with r and F. For 0.7 < F < 1/r, the flow is entirely dominated by a lee wave of saturated amplitude which suppresses the separation of the boundary layer from the obstacle. Above the critical value 1/r, the lee wave amplitude decreases with F and a recirculating zone appears behind the obstacle. Simultaneously, coherent large-scale vortices start to be shed periodically from the wake at a Strouhal number which decreases as 1/F until it reaches its neutral asymptotic value.  相似文献   

6.
To investigate the stability of the bottom boundary layer induced by tidal flow (oscillating flow) in a rotating frame, numerical experiments have been carried out with a two-dimensional non-hydrostatic model. Under homogeneous conditions three types of instability are found depending on the temporal Rossby number Rot, the ratio of the inertial and tidal periods. When Rot < 0.9 (subinertial range), the Ekman type I instability occurs because the effect of rotation is dominant though the flow becomes more stable than the steady Ekman flow with increasing Rot. When Rot > 1.1 (superinertial range), the Stokes layer instability is excited as in the absence of rotation. When 0.9 < Rot < 1.1 (near-inertial range), the Ekman type I or type II instability appears as in the steady Ekman layer. Being much thickened (100 m), the boundary layer becomes unstable even if tidal flow is weak (5 cm/s). The large vertical scale enhances the contribution of the Coriolis effect to destabilization, so that the type II instability tends to appear when Rot > 1.0. However, when Rot < 1.0, the type I instability rather than the type II instability appears because the downward phase change of tidal flow acts to suppress the latter. To evaluate the mixing effect of these instabilities, some experiments have been executed under a weak stratification peculiar to polar oceans (the buoyancy frequency N2  10−6 s−2). Strong mixing occurs in the subinertial and near-inertial ranges such that tracer is well mixed in the boundary layer and an apparent diffusivity there is evaluated at 150–300 cm2/s. This suggests that effective mixing due to these instabilities may play an important role in determining the properties of dense shelf water in the polar regions.  相似文献   

7.
Local free convection scaling is one of the obvious triumphs of boundary-layer similarity theory. In free convection, there is no dynamic velocity scale; the sensible and latent heat fluxes, therefore, scale directly with the temperature and humidity structure parameters C t 2 and C q 2. By using scintillation to measure the refractive index structure parameter C n 2 at two electromagnetic (EM) wavelengths, we can obtain C t 2 and C q 2 and, thus, in effect, measure path-averaged values of the sensible and latent heat fluxes. Here I describe this so-called two-wavelength method for free convection, derive quantitative guidelines for optimizing the method, and evaluate its potential accuracy. I show that the two-wavelength method works best when one EM wavelength is in the visible or infrared region and the other is in the millimeter or radio region. When the Bowen ratio is between -5 and -0.1 or between 0.1 and 5, the expected accuracy of the measured fluxes is ±10–20% — typical of what is possible with eddy-correlation measurements. With the two-wavelength method, however, the fluxes represent spatial averages.  相似文献   

8.
The stability of large-scale horizontal motion in the atmosphere is discussed in this paper by using qualitative analysis theory of non-linear ordinary differential equations. Both the non-linear distribution of basic Zephyr flow and the variation of geostrophic vorticity along the latitude (f=f0 + βδy ) are all included in this paper's mathematical model so as to analogue the background field of large-scale horizontal air motion more really in the rotating reference frame of the earth. Some significant results are drawn out from this paper and the conclusions of Li(1986)'s and Wan et al.(1990)'s are extended widely.  相似文献   

9.
In a series of laboratory experiments, a partially mixed patch was produced in thick linear concentration gradients favorable to salt-finger convection. Salt-fingers, which give rise to an up-gradient flux of buoyancy, can reduce and invert the density gradient in the initial imposed patch. This leads to overturning convection within the patch if (a) the ratio of ambient T and S gradients, Rρ=αTz/βSz, is near one; (b) the initial imposed turbulence results in a nearly well-mixed patch; and (c) the patch thickness is large enough that convective eddies are able to transport T and S faster than salt-fingers. Once overturning occurs, subsequent turbulent entrainment can lead to growth of the patch thickness. Experimental results for one-dimensional patches (layers) agree well with the theoretical prediction. This thickening is in contrast to the collapse that a partially mixed three-dimensional patch would experience due to lateral intrusion in a wide tank.  相似文献   

10.
In this study, we evaluate four different parameterizations of the turbulent Prandtl (Schmidt) number Prt = νt/Γt where νt is the eddy viscosity and Γt is the scalar eddy diffusivity, for stably stratified flows. All four formulations of Prt are strictly functions of the gradient Richardson number Ri, which provides a measure of the strength of the stratification. A zero-equation (i.e. no extra transport equations are required) turbulence model for νt in a one-dimensional, turbulent channel flow is considered to evaluate the behavior of the different formulations of Prt. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems such as atmospheric boundary layer flows and tidally driven estuarine flows, to quantify the behavior of each of the four formulations of Prt. We perform model-to-model comparisons to highlight which of the models of Prt allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of buoyancy forces resulting from linear (continuous) stratification as well as two-layer stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to emphasize the considerable variability in the different formulations and the importance of choosing an appropriate parameterization of Prt given a model for νt in stably stratified flows.  相似文献   

11.
Laboratory experiments are conducted on a physical system in which an oscillatory, along-shore, free stream flow of a homogeneous fluid occurs in the vicinity of a long coastline with vertical slope; the model sea-floor is horizontal. Particular attention is given to the resulting rectified (mean) current which is along the coastline with the shore on the right, facing downstream. In the lateral far field region defined by (1), where y is the offshore coordinate and H is the depth of the fluid, the motion field is approximately independent of the lateral distance from the coast. The vertical structure of the cross-stream motion in this region consists of Ekman layers near the sea-floor and interior adjustment flows, both periodic in time. In the near field, defined by (1), the motion is strongly dependent on the cross-stream coordinate as well as time, and rectified currents are observed. The mechanism responsible for the rectification is a complex nonlinear coupling between laterally directed adjustment flows driven by the transport in the bottom Ekman layers, and the free stream motion field. The rectified current is found to be substantially wider than the Stewartson layer thickness but much narrower than the Rossby deformation radius. The characteristic width, δy, of the rectified current is shown to scale as , where Ro is the Rossby number Rot is the temporal Rossby number and E is the Ekman number. Experiments are presented which support this scaling.  相似文献   

12.
We have analyzed the fine-structure of 131 electric field (E) waveforms that were radiated during the onset of first return strokes in cloud-to-ocean lightning. The dE/dt waveforms were recorded using an 8-bit waveform digitizer sampling at 100 MHz, and the E waveforms were sampled at 10 MHz using a 10-bit digitizer. 49 (or 37%) of the dE/dt waveforms contain one or more large pulses within ± 1 μs of the largest (or dominant) peak in dE/dt, i.e. within an interval from − 1 μs to + 1 μs, where t = 0 μs is the time of the dominant peak, and 37 (or 28%) have one or more large pulses in the interval from 4 μs before to 1 μs before the dominant peak, i.e. − 4 μs to − 1 μs, and only the dominant peak within ± 1 μs . We give statistics on the amplitude and timing of dE/dt pulses that are near the dominant peak, and we show how the presence of these pulses adds considerable fine-structure to the shape of Eint, the integrated dE/dt waveform, on a time-scale of tens to hundreds of nanoseconds. This fine-structure includes fast pulses near the beginning of the slow front, large pulses and shoulders within the slow front and during the fast-transition, and very narrow peaks in Eint. Our overall conclusion is that the electromagnetic environment near the point(s) where lightning leaders attach to the surface is often more complicated than what would be produced by a single current pulse propagating up a single channel at the time of onset.  相似文献   

13.
Historical dates of ice-affected flows for 16 rural, unregulated rivers in northern New England, USA were analyzed. The total annual days of ice-affected flow decreased significantly (p < 0.1) over the 20th century at 12 of the 16 rivers. On average, for the nine longest-record rivers, the total annual days of ice-affected flow decreased by 20 days from 1936 to 2000, with most of the decrease occurring from the 1960s to 2000. Four of the 16 rivers had significantly later first dates of ice-affected flow in the fall. Twelve of the 16 rivers had significantly earlier last dates of ice-affected flow in the spring. On average, the last dates became earlier by 11 days from 1936 to 2000 with most of the change occurring from the 1960s to 2000. The total annual days of ice-affected flow were significantly correlated with November through April air temperatures (r = −0.70) and with November through April precipitation (r = −0.52). The last spring dates were significantly correlated with March through April air temperatures (r = −0.73) and with January through April precipitation (r = −0.37). March mean river flows increased significantly at 13 of the 16 rivers in this study.The U.S. Government right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.  相似文献   

14.
A new statistical two-point closure EDQNM (Eddy Damped Quasi Normal Markovian) model for axisymmetric stratified turbulence is presented. This model takes into account the detailed anisotropic structure of the flow, including angular dependence, and its closure assumption may involve explicit effects of the stratification. In the present paper, a simplified closure assumption is used, in which these explicit effects are not taken into account. The model is tested against three-dimensional direct numerical simulations (DNS) of decaying homogeneous stratified turbulence. A very good agreement is found between DNS results and EDQNM predictions, when energy transfers between the different modes of motion are considered. However, transfers among wavenumbers are insufficiently damped by the EDQNM model, because of the simplified closure assumption, thus yielding a somewhat less satisfactory agreement. An interesting prediction of the EDQNM model is the existence of a ‘directional’ anisotropy at all scales of motion, to the smallest: the level of the total (kinetic + potential) energy density spectrum E(k,θk) varies with the angle θk that the wavenumber vector k makes with the vertical.  相似文献   

15.
The analytical model of finite-amplitude, quasi-geostrophic ‘free mode’ baroclinic eddies and mean zonal flows in a Cartesian channel, presented recently by Read, is extended to take account of vertical variations in the buoyancy frequency N. A series of exact solutions is presented to illustrate the effect of monotonically varying static stability on the structure and properties of the flow. The analytical solutions are then compared with a corresponding series of numerical simulations of steady wave flows in a rotating fluid annulus subject to internal heating and sidewall cooling. By suitable choices of internal heating distributions and boundary conditions, several different forms of N2 profile could be obtained in the simulated flows, in which N2 was concentrated to a greater or lesser extent towards the upper boundary. The resulting steady flows exhibited strong qualitative similarities in their structure and dependence upon the form of N2(z) to that of the analytical solutions when realistic profiles of N2 were included in the latter, especially when an equivalent-barotropic component was included, although the latter component is unable to satisfy the simplest (internal jet) form of horizontal boundary condition as usually applied to Rossby waves.The relatively weak, though crucially important, forcing and dissipation processes in the annulus are examined using approximate quasi-geostrophic diagnostics of the major terms in the budget of potential enstrophy for the numerical simulations. Internal heating is found to be the major source of potential enstrophy for the mean zonal flow, solely by virtue of the variation of N2 with height, but has only a minor direct effect upon the eddy flow component. Because of the presence of critical layers in the flow, all non-linear terms (including the third-order potential enstrophy flux divergence) are found to be significant in certain regions. Some implications for the value and applicability of EP flux diagnostics are discussed. Potential enstrophy budgets for horizontal regions enclosed by geostrophic streamlines are used to shed further insight into the maintenance of the flow against ‘friction’, and on the form of the potential vorticity-streamfunction relationship. Some implications of the results for other systems of geophysical interest are also discussed.  相似文献   

16.
Microphysical theory has proven essential for explaining sea spray's role in transferring heat and moisture across the air–sea interface. But large-scale models of air–sea interaction, among other applications, cannot afford full microphysical modules for computing spray droplet evolution and, thus, how rapidly these droplets exchange heat and moisture with their environment. Fortunately, because the temperature and radius of saline droplets evolve almost exponentially when properly scaled, it is possible to approximate a droplet's evolution with just four microphysical endpoints: its equilibrium temperature, Teq; the e-folding time to reach that temperature, τT; its equilibrium radius, req; and the e-folding time to reach that radius, τr.Starting with microphysical theory, this paper derives quick approximation formulas for these microphysical quantities. These approximations are capable of treating saline droplets with initial radii between 0.5 and 500 μm that evolve under the following ambient conditions: initial droplet temperatures and air temperatures between 0 and 40 °C, ambient relative humidities between 75% and 99.5%, and initial droplet salinities between 1 and 40 psu.Estimating Teq, τT, and τr requires only one-step calculations; finding req is done recursively using Newton's method. The approximations for Teq and τT are quite good when compared to similar quantities derived from a full microphysical model; Teq is accurate to within 0.02 °C, and τT is typically accurate to within 5%. The estimate for equilibrium radius req is also usually within 5% of the radius simulated with the full microphysical model. Finally, the estimate of radius e-folding time τr is accurate to within about 10% for typical oceanic conditions.  相似文献   

17.
A laboratory study of scalar diffusion in the convective boundary layer has found results that are consistent with a 1999 large-eddy simulation (LES) study by Jonker, Duynkerke and Cuijpers. For bottom-up and top-down scalars (introduced as ‘infinite’ area sources of passive tracer at the surface and inversion, respectively) the dominant length scale was found to be much larger than the length scale for density fluctuations, the latter being equal to the boundary-layer depth h. The variance of the normalized passive scalar grew continuously with time and its magnitude was about 3–5 times larger for the top-down case than for the bottom-up case. The vertical profiles of the normalized passive scalar variance were found to be approximately constant through the convective boundary layer (CBL) with a value of about 3–8c*2 for bottom-up and 10–50c*2 for top-down diffusion. Finally, there was some evidence of a minimum in the variance and dominant length scale for scalar flux ratios (top-down to bottom-up flux) close to −0.5. All these convection tank results confirm the LES results and support the hypothesis that there is a distinct difference in behaviour between the dynamic and passive variables in the CBL.  相似文献   

18.
Summary  The fourth generation of numerical weather prediction (NWP) models is currently under development at the Deutscher Wetterdienst (DWD) consisting of a global grid point model (GME) and limited-area Lokal-Modell (LM). The nonhydrostatic fully compressible LM has been designed to meet high-resol ution regional fore-cast requirements at meso-β and meso-γ scales. The initial LM implementation is based on the NCAR/Penn State MM5 with the addition of a novel generalized terrain-following coordinate and rotated lat-lon grid. A fully 3D semi-implicit time-stepping scheme has been implemented by retaining the full buoyancy term instead of the approximate form found in MM5. In contrast with earlier schemes, mass-lumping is not applied to simplify the elliptic operator on an Arakawa-C/Lorenz grid. The resulting variable-coefficient elliptic problem is solved using a minimal residual Krylov iterative method with line relaxation preconditione rs. The new semi-implicit scheme is compared with a variant of the Klemp–Wilhelmson split-explicit scheme (horizontal explicit, vertical implicit) on the basis of computational efficiency and accuracy at resolutions ranging from 7 km to 400 m. Both idealized 3D mountain wave flows and naturally occuring flows are analyzed. Below the tropopause, the 3D semi-implicit scheme can be more efficient for low Mach number M ≪ 1 flows when the number of small time steps Δt s of the split-explicit approach increases with the sound-speed Courant number. Revised October 7, 1999  相似文献   

19.
Laboratory experiments are described which provide insight into the interaction of intermediate depth boundary currents (IDBCs) with interrupted sloping topography. Specifically, they contribute to the debate over meddy formation on the Iberian continental slope. The experiments were performed in a rectilinear rotating tank filled initially with a linearly-stratified fluid. A false bottom sloped away from the side-wall along which the current flowed, and was interrupted by a gap of variable length. The effects of varying gap length and rotation rate on the boundary current were observed.In the first of two sets of experiments, the current flowed above the slope, along the vertical sidewall. In the second, the current flowed along the sloping bottom. In the former, current nose speed was consistent with geostrophic predictions, but decreased in the presence of a gap in the topography. Kelvin wave radiation is postulated as a reason for this. The IDBCs exhibited vortical lateral intrusions at values of the Burger number Bu=(N0/Ω)2 at which counterpart flat-bottom studies had been stable, implying that the sloping topography had a de-stabilising effect. Energy measurements and qualitative observations suggest the intrusions were due to mixed barotropic/baroclinic instabilities, the latter dominating at higher rotation rates.In the second configuration, four distinct flows were observed, distinguished by the deformation radius:gap width ratio RD/G*. For a range of values of RD/G*, attached eddies formed at the upstream end of the gap. They remained at this position, unlike those in similar studies of surface boundary currents (Klinger, 1993). Their persistence and ability to move downstream – salient factors for meddy – formation were greater for a finite gap size than a permanent change from sloping to flat bottom.  相似文献   

20.
In this paper warm cloud microphysical parameters including cloud droplet number concentration (Nc), liquid water content (ql) and effective radius (re) from 75 flights around the Beijing area during 2005 and 2006 are summarized. Average Nc (cm− 3) for Cu, Sc, Ac, As and Ns are 376 ± 290, 257 ± 226, 147 ± 112, 60 ± 35 and 60 ± 84, respectively. Many records of high Nc above 1000 cm− 3 are observed. The large standard deviations indicate a large variation of Nc and ql in this region. The maxima of ql reach 1.4 g m− 3 in Cu and 1.0 g m− 3 in Sc, respectively. Different parameterizations of effective radius are examined with the in-situ data in this area. There are different ways to obtain the prefactor representing the relationship between effective radius and mean volume radius. Significant systematic errors are found to be at the large sizes when the prefactor is expressed with relative dispersion under the Gamma Distribution. Fixed prefactor of 1, which was widely used, even produces much larger error. A prefactor of 1.22 is found to be better than the former two methods by fitting with the observed data. The effective radius is further parameterized as functions of mean volume radius, liquid water content and cloud droplet number concentration. We suggest that the effective radius can be parameterized as re,p ≈ 1.20rv + 0.22–1.28/rv2, which is a practical and more accurate scheme without too much computation complexity.  相似文献   

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