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1.
The sea surface is aerodynamically rough even under light winds 总被引：3，自引：0，他引：3
Jin Wu 《Boundary-Layer Meteorology》1994,69(1-2):149-158
The sea surface is generally considered to be aerodynamically rough at high winds (U>7 m/s), where the roughness length increases with wind velocity; below this velocity, the atmospheric surface layer enters a transition region and then becomes aerodynamically smooth as the wind velocity further decreases. The sea surface is shown, however, to reach its smoothest condition at a wind velocity of about 5 m/s, and then become rough again at lower velocities. In the latter case, the roughness length increases as the wind velocity decreases in accordance with the surface-tension relation governing wind-wave interactions. 相似文献
2.
The air flow above breaking monochromatic Stokes waves is studied using a numerical nonlinear model of the turbulent air flow above waves of finite amplitude. The breaking event (spilling breaker) is parameterized by increasing the local roughness at the downwind slope of the wave, just beyond the crest. Both moderate slope waves and steep waves are considered. Above steep breaking waves, a large increase (typically 100%) in the total wind stress — averaged over the wave profile — is found compared to nonbreaking moderate slope waves. This is due to the drastic increase of the form drag, which arises from the asymmetrical surface pressure pattern above breaking waves. Both increase of wave slope (sharpening of the crest) and increase of local roughness in the spilling breaker area cause this asymmetrical surface pressure pattern. A comparison of the numerical results with the recent experimental measurements of Banner (1990) is carried out and a good agreement is found for the structure of the pressure pattern above breaking waves and for the magnitude of enhanced momentum transfer.
Also: Dept. of Applied Physics, Techn. Univ. Delft, Netherlands. 相似文献
3.
S. Sethuraman 《Boundary-Layer Meteorology》1979,16(4):279-291
Momentum flux measurements by eddy correlation method and wave height measurements with a capacitance-type wave staff were carried out from a stable air-sea interaction buoy anchored 5 km off Tiana Beach, Long Island, New York State. A characteristic height of sea surfaceh
s was estimated from wave height spectra. A roughly linear variation of surface shear stress withh
s
was found for an aerodynamically rough sea surface. 相似文献
4.
M. Segal 《Boundary-Layer Meteorology》1990,52(1-2):193-198
Calculations are made of the effects of thermal stability under a range of conditions, over the sea and land, on the physical factors (including the critical wind speed) affecting dust-storm generation, snow drift, and rough sea conditions. The computational procedure involves the surface friction velocity, u
*, and its relation with the aerodynamic roughness over aerodynamically rough, mobile surfaces. The results indicated that even at relatively high wind speeds, thermal effects under extreme advection situations may be significant, particularly for those properties of the agitated surface dependent on u
*
3 and u
*
4. 相似文献
5.
A theory for the scalar roughness and the scalar transfer coefficients over snow and sea ice 总被引：1，自引：0，他引：1
Edgar L. Andreas 《Boundary-Layer Meteorology》1987,38(1-2):159-184
Although the bulk aerodynamic transfer coefficients for sensible (C
H
) and latent (C
E
) heat over snow and sea ice surfaces are necessary for accurately modeling the surface energy budget, they have been measured rarely. This paper, therefore, presents a theoretical model that predicts neutral-stability values of C
H
and C
E
as functions of the wind speed and a surface roughness parameter. The crux of the model is establishing the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces on the basis of a surface-renewal model in which turbulent eddies continually scour the surface, transferring scalar contaminants across the interface by molecular diffusion. Matching these interfacial sublayer profiles with the semi-logarithmic inertial sublayer profiles yields the roughness lengths for temperature and water vapor. When coupled with a model for the drag coefficient over snow and sea ice based on actual measurements, these roughness lengths lead to the transfer coefficients. C
E
is always a few percent larger than CH. Both decrease monotonically with increasing wind speed for speeds above 1 m s–1, and both increase at all wind speeds as the surface gets rougher. Both, nevertheless, are almost always between 1.0 × 10–3 and 1.5 × 10–3. 相似文献
6.
The atmospheric surface layer over sea has a density stratification which varies with moisture content and air/sea temperature difference. This influences the growth of water waves. To study the effect quantitatively, the Reynolds equations are solved numerically. For given wind speed and surface roughness, wave growth is found to be more rapid in unstably stratified conditions than in stable conditions. This is due to an increase in turbulence, primarily caused by an increase of mixing length.Under the assumption of a Charnock relation between surface roughness and friction velocity, it is found that for large inverse wave age (u
*/c>0.07), the effect of stratification on wave growth is weell described by Monin-Obukhov scaling of the friction velocity. For smaller values ofu
*/c, Monin-Obukhov scaling overpredicts.The effect on duration-limited wave growth is studied with the third-generation WAM surface wave model driven by 10 m winds. Effects of stratification on the significant wave height are found to be of the order of 10%. The results are comparable to those of a recent reanalysis of field measurements, although the measured stratification effect is somewhat stronger. Implementation of a stratification-dependent growth in wave models is recommended, as it can lead to small but significant improvements in wave forecasts when accurate air and sea temperatures are available. 相似文献
7.
A nonlinear numerical model is developed for turbulent boundary-layer flowover a train of water waves of finite amplitude or slope. The airflow isassumed to be steady, two-dimensional, and neutrally-stratified. The wavesurface is assumed to be aerodynamically rough and flow conditions at thewave surface are prescribed. The numerical model used in this study adoptsthree turbulence closure schemes with different degrees of physicalcompleteness. Two of these are second-order schemes, whichare believed to describe turbulent flow more completely than thesimpler closures used in most previous studies. Although models with all turbulence closures agree qualitatively in the prediction of the amplitude of the surface normal stress perturbation, the lower- and higher-order closures differ significantly in predictions of phase, and hence the form drag and energy transfer rate between wind and waves. Our model results are in reasonable agreement with field and laboratory measurements, although predicted energy transfer rates are generally at the low end of the range of experimental values. Cases with airflow at various angles to the wave direction are also considered. 相似文献
8.
《Dynamics of Atmospheres and Oceans》2006,41(1):1-27
An analytical model is developed for the initial stage of surface wave generation at an air–water interface by a turbulent shear flow in either the air or in the water. The model treats the problem of wave growth departing from a flat interface and is relevant for small waves whose forcing is dominated by turbulent pressure fluctuations. The wave growth is predicted using the linearised and inviscid equations of motion, essentially following Phillips [Phillips, O.M., 1957. On the generation of waves by turbulent wind. J. Fluid Mech. 2, 417–445], but the pressure fluctuations that generate the waves are treated as unsteady and related to the turbulent velocity field using the rapid-distortion treatment of Durbin [Durbin, P.A., 1978. Rapid distortion theory of turbulent flows. PhD thesis, University of Cambridge]. This model, which assumes a constant mean shear rate Γ, can be viewed as the simplest representation of an oceanic or atmospheric boundary layer.For turbulent flows in the air and in the water producing pressure fluctuations of similar magnitude, the waves generated by turbulence in the water are found to be considerably steeper than those generated by turbulence in the air. For resonant waves, this is shown to be due to the shorter decorrelation time of turbulent pressure in the air (estimated as ∝ 1/Γ), because of the higher shear rate existing in the air flow, and due to the smaller length scale of the turbulence in the water. Non-resonant waves generated by turbulence in the water, although being somewhat gentler, are still steeper than resonant waves generated by turbulence in the air. Hence, it is suggested that turbulence in the water may have a more important role than previously thought in the initiation of the surface waves that are subsequently amplified by feedback instability mechanisms. 相似文献
9.
10.
The dependence of the turbulent airflow over water waves on the angle,, between mean wind and wavedirections is investigated. To this end,an existing semi-analytical model is extended. In this model, the main simplification of the problem is obtained by using the well-established divisionof the wave boundary layer into inner and outer regions for modelling turbulence. The effect of waves on turbulence is restricted to the thin inner region. Simulations show that the influence of the wind speed component transverse to the wave direction on the air flow, and hence on the growth rate of the waves, is small. This is confirmed by calculations with a numerical model that solves the full Reynolds equations using a second-order turbulence closure scheme. The growth rate of slowly moving waves (as compared to the wind speed) is then proportional to cos2, whereas, for faster waves, it has a narrower angular distribution. 相似文献
11.
A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 × 40 km2, the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15ms-1 for the vertical wind component, 0.3ms-1 for the cross-wave wind component and 0.15ms-1 for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient.Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones. 相似文献
12.
13.
A Study of the Internal Boundary Layer due to a Roughness Change in Neutral Conditions Observed During the LINEX Field Campaigns 总被引：1，自引：0，他引：1
Summary As an aspect of the LINEX field studies (1996–1997; Lindenberg near Beeskow, Germany), the characteristics of the internal
boundary layer (IBL) that is associated with a step change of the surface roughnesses in neutral constant stress layers was
investigated and is reported in this paper. Both smooth to rough (in 1996) and rough to smooth (in 1997) types of flow, have
been studied based upon the profiles of mean wind and temperature realised from a 10-m mast and eddy correlation measurements
taken at two levels (2 m and 5 m). Depending upon wind direction, the fetch at the site varied between 140 m and 315 m within
the wind sector (200° to 340°) used for the field investigations. The height of the IBL, δ, had been determined from the intersect
of the logarithmic wind-profiles below (< 2 m) and above (> 6 ) the interface. Values of δ obtained at the experimental site
compared fairly well to the existing theoretical/empirical fetch-height relationships of the form: δ=aċx
b
, where a, b, are empirical constants. The ratio for the friction velocities below and above the IBL as measured directly by the eddy
correlation techniques showed that for fetches less than 250 m there was an increase (decrease) of about 20% of the momentum
flux arising from the smooth to rough (rough to smooth) transitions. Influences of distant obstructions (e.g., bushes, pockets
of trees) on the surface flow were markedly important on the examined wind profiles and such can be indicative as multiple
IBLs.
Received September 1, 1997 Revised August 5, 1998 相似文献
14.
By means of a large-eddy simulation, the convective boundary layer is investigated for flows over wavy terrain. The lower surface varies sinusoidally in the downstream direction while remaining constant in the other. Several cases are considered with amplitude up to 0.15H and wavelength ofH to 8H, whereH is the mean fluid-layer height. At the lower surface, the vertical heat flux is prescribed to be constant and the momentum flux is determined locally from the Monin-Obukhov relationship with a roughness lengthz
o=10–4
H. The mean wind is varied between zero and 5w
*, wherew
* is the convective velocity scale. After rather long times, the flow structure shows horizontal scales up to 4H, with a pattern similar to that over flat surfaces at corresponding shear friction. Weak mean wind destroys regular spatial structures induced by the surface undulation at zero mean wind. The surface heating suppresses mean-flow recirculation-regions even for steep surface waves. Short surface waves cause strong drag due to hydrostatic and dynamic pressure forces in addition to frictional drag. The pressure drag increases slowly with the mean velocity, and strongly with /H. The turbulence variances increase mainly in the lower half of the mixed layer forU/w
*>2. 相似文献
15.
Peter C. Smith 《大气与海洋》2013,51(1):181-209
Abstract Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ2 ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10?12 ms?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath. A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional. Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements. 相似文献
16.
The global energy cycle of stationary and transient atmospheric waves: Results from ECMWF analyses 总被引：3，自引：0，他引：3
Summary The role of stationary (monthly mean) and transient (departure from monthly mean) waves within the atmospheric energy cycle is examined using global analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for the period 1980–1987. Only January and July averages are considered.It is confirmed that planetary stationary waves are basically baroclinic. Their contribution to the globally averaged energy cycle of the atmosphere is comparable to that of the transient waves. In January they contribute about 40% to the baroclinic conversion (CA) from zonal mean to eddy available potential energy. Local values for the northern hemisphere even show a predominant role of the stationary wave conversions over those originating from transient waves. Part of the available potential energy of stationary waves (A
SE) is converted to kinetic energy by warm air rising and cold air sinking. Nonlinear energy conversion, which can be interpreted as destruction of stationary temperature waves by transients, is the second sink forA
SE. The order of magnitude of these two processes is similar.Barotropic nonlinear conversions, though negligible in the global average, reveal large conversion rates between the mean positions of the polar and the subtropical jets. Their orientation is suggestive of a tendency to increase stationary wave kinetic energyK
SE at its local minimum between the jets at the expense of the synoptic scale transients.While all terms of the energy cycle related to stationary waves reveal a predominance of the planetary scale (zonal wave numbers 1–3) transient waves are governed by synoptic scale waves (zonal wave numbers 4–9) only with respect to the baroclinic and barotropic conversions: a significant amount of transient wave energy (50% for the global average ofA
TE) is due to planetary scale waves.With 15 Figures 相似文献
17.
E. L. Deacon 《Boundary-Layer Meteorology》1981,21(1):31-37
The rates of transfer of radon from sea to air estimated by Peng et al. (1979) from an extensive series of observations of radon profiles made during the Geosecs oceanographical cruises, are reexamined in relation to wind speed dependence. It is concluded that there is a significant increase with wind speed, but the extent of the increase is uncertain. At 7 m s-1, the transfer velocity is indicated to be some 33%; greater than the BOMEX value of Broecker and Peng (1974) — a reasonably close agreement These rates exceed the theoretical smooth-surface value by a factor of two or three. It is shown that little of this excess can be attributed to the surface dilation effect of capillary waves. 相似文献
18.
Structure of air flow separation over wind wave crests 总被引：1，自引：0，他引：1
Sanshiro Kawai 《Boundary-Layer Meteorology》1982,23(4):503-521
Air flow over wind waves generated in a wind-wave tunnel was visualized by numerous tiny suspended particles (zinc stearate), and instantaneous air flow fields over about one wavelength of wind waves were obtained. Air flow separation was detected over the wave crest in about a half of the samples. In such cases, the separation started near the crest about half of the time, with a vortex trapped over the convergence point of the surface flow which appeared at the leeward face of the crest. This structure was much different from a previously imagined picture in which the separation started at the convergence point. The high frequency of its occurrence suggested the stability of this structure. However, even when this structure was clearly seen, the structure behind the vortex to the next wave crest had various patterns. This variety seems to be related to an instability of the high-shear layer accompanied by separation. Other varieties were also seen, such as the occurrence of separation without the above mentioned structure, as well as the existence of non-separated air flow structures. These varieties seem to be related to the variability of individual wind wave crests. An analysis of correlation between the wave form and the air flow structure over it shows that there is a critical value of local gradient of wave form, above which the air flow always separates. This fact suggests a strong coupling between the air and the water, i.e., the local stress exerted on the water surface changes the nature of a wave crest, especially its form, and as a result, the air flow structure over it changes drastically.Decreased 21 November, 1981. Final draft of the paper prepared by Professor Yoshiaki Toba, Geophysical Institute, Tohoku University. 相似文献
19.
一次暴风雪过程中的中尺度重力波特征及其影响 总被引：2，自引：0，他引：2
应用地面自动气象站观测资料、数字化多普勒天气雷达探测资料和WRFV2.2.1中尺度数值模拟资料,分析了中尺度重力波与基本气流的相互作用,以及重力波活动对暴雪和大风天气的重要影响。结果表明,在波导中传播的中尺度重力波能够与基本气流进行动量交换,使得对流层中上层4.5—8 km气层内的水平平均风速趋于均匀,形成斜穿整个对流层的饱和湿空气急流,即"湿急流"。在高空急流出口区激发的垂直向下传播的重力波,使基本气流的水平风速在垂直方向上出现了加速和减速的交替变化,水平风加速的气层,反射率增大;水平风减速的气层,反射率减小。随着波动下传及其随基本气流的移动,反射率回波强度沿高空风的方向(由西南向东北)出现周期性变化,回波带呈西北—东南走向,强回波中心之间为宽约40 km的弱回波区。重力波下传期间,当地面气压迅速下降时,东北风快速增长,风向有明显的改变,反射率强度开始减弱;气压脊线过后,反射率降低到最低点。地面大风中心出现在反射率回波强度周期性变化的地带,沿西南—东北方向间隔着分布。雷达探测表明,对流层低层风速在风向切变层上下边界对称相等,因此推测在重力波与切变层汇合的高度层存在垂直环流,由风切变层上下边界附近的西南气流和东北气流与受重力波影响形成的垂直方向上的上升和下沉气流共同组成。切变层上方的动量通过垂直环流的下沉支到达地面,强风中心对应着下沉气流,出现在降水回波开始减弱之际。 相似文献
20.
S. E. Larsen H. E. Jørgensen L. Landberg J. E. Tillman 《Boundary-Layer Meteorology》2002,105(3):451-470
The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surface-layer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T*, the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features.The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere. 相似文献