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1.
In this paper, a possible increase in wind wave heights in the south-eastern south American continental shelf between 32°S and 40°S is investigated. Both time series of in situ (1996–2006) and topex (1993–2001) annual mean significant wave heights gathered at the continental shelf and adjacent ocean present apparent positive trends. Even though these trends are not statistically different from zero, it must be taken into account that the available in situ and satellite data have a short span and, moreover, in situ data present several gaps. Several papers presented evidence about a possible change on the low atmospheric circulation in this region of the southern hemisphere. Consequently, a weak increase in wave height might be occurring, which would be hard to quantify due to the shortness and the insufficiency of the available observations. In order to study a possible trend in mean annual wind wave heights simulating waves nearshore (swan) model forced with ncep/ncar surface wind was implemented in a regional domain for the period 1971–2005. The annual root-mean-square heights of the simulated wave show significant trends at several locations of the inner continental shelf and the adjacent ocean. The most significant increase is observed between 1991–2000 and 1981–1990 decades. The largest difference (0.20 m, 9%) occurs around 34°S–48°W. The wave height increase is somewhat lower, 7%, in the continental shelf and in the río de la plata estuary. The annual mean energy density (spatially averaged) also presents a significant positive trend (0.036 m2/yr) and relatively high inter-annual variability. The possible link between this inter-annual variability and el niño–southern oscillation (enso) was investigated but no apparent relationship was found. A possible increase in the annual mean energy density of waves would be able to produce changes in the littoral processes and, consequently, in the erosion of the coast.  相似文献   

2.
Zusammenfassung Aus Windprofilmessungen in feststehenden H?hen über den Wellenk?mmen lassen sich keine zuverl?ssigen Schlüsse über die effektive Schubkraft des Windes an der Meeresoberfl?che ziehen. Der Grund liegt im wesentlichen in der verschiedenartigen vertikalen Windgeschwindigkeitsverteilung über Wellenbergen und Wellent?lern und in der Tatsache, da? die Windprofilmessungen im wesentlichen nur das Windprofil über den Bergen widerspiegeln. Der Versuch, die Windprofilmessungen zu „reduzieren“, ist mit zu gro?en Fehlern behaftet, zeigt aber qualitativ eine systematische Erh?hung der aus Windmessungen berechneten Reibungsfaktoren an. Damit kann der Anschlu? an die aus Windstaubeobachtungen ermittelten Reibungsfaktoren gewonnen werden, so da? der vermeintliche „Sprung“ von einer „glatten“ zu einer „rauhen“ Grenzfl?che verschwindet, und eine „kritische Windgeschwindigkeit“, die diesen Sprung angeben soll, gar nicht existiert.
On the problem of the “critical wind velocity” at the air — sea boundary surface
Summary From the measurements of wind profiles at fixed heights above the wave crests no reliable conclusions can be drawn with regard to the wind's effective shearing force at the sea surface. This is essentially due to the diversified vertical distribution of the wind speed above the waves' crests and troughs as well as to the fact that when measuring wind profiles above the waves it is mainly the profiles above the wave crests that are obtained. Attempts to “reduce” wind profiles have proved to be subject to considerable errors; however, they show qualitatively a systematic increase of the frictional factors as computed from wind measurements. This links up with the frictional factors ascertained from observations of the wind effect so that the supposed “leap” from a “smooth” to a “rough” boundary surface and the “critical wind speed” corresponding to this leap do not exist at all.
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3.
In the English Channel, extreme surge heights did not occur at the time of extreme high tides during the last decades and maximum recorded heights usually do not exceed the maximum astronomical tide by more than a few decimetres. To understand whether this lack of coincidence may be due to specific phenomena or only to chance, we have studied hourly tide records lasting a few decades from nine English and nine French stations as well as air pressure and wind data from nearby meteorological observatories. Among the case studies of moderate flooding at several coastal stations occurring during spring tide, we have selected those of 24–25/10/1980 and of 30/01/1983 to 02/02/1983 as representative of a normal situation without any special chance. The third case study 26–28/02/1990 was potentially more dangerous because of the storm intensity and duration; however, by chance, surge peaks occurred near the low tide. Finally, the propagation of the surge peak of 15–16/10/1987, which reached the maximum height recorded during all the instrumental period at several stations, has been followed all along the English Channel, using the hourly records of 12 tide-gauge stations and of 16 meteorological stations. The surge peak of this great storm, probably the strongest in the last two centuries, occurred everywhere at high tide and spread with the same velocity of the tidal wave. Fortunately, no major flooding occurred because it was the day after a neap tide. In conclusion, some good fortune has saved the low coastal areas of the English Channel from major floods during the last decades. However, the occurrence of the peak of a strong storm surge arriving near the western entrance of the Channel at the time of a great astronomical high tide is a possible event that could be devastating along both sides of the Channel coasts. Main parts of this paper have been presented orally in June 2005 at the joint INQUA–IGCP 495 Meeting “Dunkerque 2005” and in February 2006 at the ASLO-TOS-AGU “Ocean Sciences Meeting” (Honolulu, HI).  相似文献   

4.
Quantifying the long-term variability in wave conditions incident on a coastline is critical for predicting its resilience to future changes in the wave climate. In this study, a 40-year wave hindcast of the southern Indian Ocean has been created to assess the inter-annual variability and longer-term changes in the wave climate around Western Australia (WA) between 1970 and 2009. The model was validated against measurements from five wave buoys located along the WA coast. Changes in the mean annual significant wave height, 90th percentile wave height, peak period and mean wave direction were assessed, and the tracks of all wave events generating wave heights above 7 m were digitised and analysed for significant changes. Results show strong annual and inter-annual variability in the mean significant wave height, the 90th percentile wave height and the number of large events (wave height > 7 m) that impact the WA coastline. A significant positive trend in annual mean wave height was found in the southwest region of WA over the 40-year simulation. This appears to be due to an increase in intensity of the storm belt in the Southern Ocean which is associated with an increasing positive polarity in the Southern Annular Mode. However, no significant trends were found in the 90th percentile wave height or the number of large wave events impacting Western Australia. Although the number of large wave events in the southern Indian Ocean have increased, their potential to impact the coastal regions of Western Australia are reduced due to storm tracks being located further south, therefore balancing the number of large wave events reaching the WA coast.  相似文献   

5.
The wave velocity for two types of granitoids was measured using the analytic method of full-wave vibration at high pressure and high temperature. The laws of velocity changes for them differ with the pressure boost and temperature rise, and the velocity change of S-type is more violent than that of I-type. The “softening point” of compressional wave velocity (V μ) is also revealed during the measurement for two types of granitoids imitating the pressure and temperature at a certain depth. But the depth of “softening”, Vp after “softening” and the percentage of Vp’s drop around the “sofrening point” for two types of granitoids are obviously different. The depth of “softening” is 15 km approximately and Vp after “softening” is 5.62 km/s for S-type granitoid. But for I-type granitoid the depth of “softening” is 26 km approximately and Vp after “softening” is 6. 08 km/s. Through careful analysis of rock slices after the experiment, it was found that the “softening” of elastic-wave velocity is caused by the partial melting of granite. Combined with the results of geophysical prospecting, these results suggest that the low-velocity layers developing in the interior of Earth crust are related to thc partial melting of different types of granitoids. The formation of the low-velocity layer in the upper-middle Earth crust is closely related to the development of S-type granitoid, but that in the lower Earth crust is closely related to the development of I-type granitoid.  相似文献   

6.
张怡辉  胡维平  彭兆亮 《湖泊科学》2020,32(4):1177-1188
基于实测数据,利用验证良好的SWAN风浪模型开展了2018年巢湖风浪变化及分布特征研究.巢湖2018年平均有效波高和波周期分别为0.16 m和1.22 s,整体春季风浪大,秋季风浪小.月均最大值出现在4月,分别为0.22 m和1.36 s,月均最小值出现在11月,分别为0.11 m和1.06 s,变化幅度分别为最大值的52%和22%.月均值整体中巢湖最大,东巢湖次之,西巢湖最小.巢湖月最大有效波高和波周期主要出现在东巢湖或中巢湖,各值月间差异显著,最大变化幅度分别为最大值的61%和27%.不同湖区计算的月均有效波高和波周期较大值分布范围所占湖区的比例不同,中巢湖与东巢湖较大,西巢湖最小.不同月份及湖区较大有效波高出现的时间占比是不一致的,9-11月份时间占比较小,将有利于蓝藻水华的出现.  相似文献   

7.
太湖不同湖区风浪的季节变化特征   总被引:5,自引:3,他引:2  
为明晰太湖风浪的空间分布及季节变化,在湖心区设立波浪观测站,利用其记录的波浪数据证明SWAN模型能够较好地模拟太湖风浪.基于所建模型,对2013年自然风场条件下太湖不同湖区风浪季节动态进行模拟分析,结果表明:受岸线、地形和岛屿等地理因素影响,大太湖的风浪总是最强,其有效波高均值为0.523 m;而东太湖风浪最小,有效波高均值为0.305 m.受盛行风场季节变化影响,太湖春、夏季有效波高均值明显大于秋、冬季.太湖波浪的能量主要来源于风场,其有效波高随风速增大而增大,两者呈极显著正相关.而风向则可以通过改变风区长度来影响风浪生消.在偏东风作用下,太湖湖西区的风浪大于东部湖区;而受盛行于冬季的偏北风影响,太湖南部水域风浪要大于北部.同时,太湖风浪的时空分布特征是造成太湖水质参数、沉积物和水生植物空间分布差异的重要原因之一.  相似文献   

8.
The German Wadden Sea (southern North Sea) sediments are composed of both cohesive and non-cohesive deposits. The spatial distribution patterns are mainly driven by wind-induced waves and tidal currents. Transport intensity and duration depend on the hydrodynamic conditions, which vary over time. In this paper, the transport of suspended sediment was investigated on seasonal, tidal and hourly time scales in the back-barrier system of Spiekeroog Island. Long- and short-term data of fair weather periods and two storm events were investigated based on stationary and mobile measurements of currents and waves by Acoustic Doppler Current Profiler (ADCP), in situ particle size and suspended sediment concentration (SSC) measurements by laser in situ scattering and transmissometry (LISST) as well as wind records. The ADCP backscatter intensities were calibrated by means of LISST volume concentration data in order to quantify longer term SSCs and fluxes in the back-barrier system. Values up to 120 mg l−1 were recorded, but concentrations more commonly were below 60 mg l−1. The long-term results confirm former observations of a balanced budget during low-energy (fair weather) conditions in the study area. In general, SSCs were higher during spring tides than during neap tides. The data also clearly show the remobilisation of sediment by tidal current entrainment. The records include two severe storm events, “Britta” (1st November 2006) and “Kyrill” (18th January 2007). The data reveal very complex temporal flow and transport patterns. During both storm events, the export of material was mainly controlled by the interaction of wind, waves and tidal phase. The typical ebb-dominance occurring during fair-weather conditions was temporarily neutralised and even reversed to a flood-dominated situation. During “Kyrill”, the wind and high-waves setup in conjunction with the tidal phase was even able to compress the duration of two successive ebb cycles by over 70%. Although SSCs increased during both storms and higher turbulence lifted particle clouds upwards, an export of suspended matter towards the North Sea was only observed under the conditions taking place during “Britta”. Such fluxes, however, are currently still difficult to quantify because the backscatter intensity during high energy events includes a substantial amount of noise produced by the high turbulence, especially near the water surface.  相似文献   

9.
In this paper, mean sea level changes in the German Bight, the south-eastern part of the North Sea, are analysed. Records from 13 tide gauges covering the entire German North Sea coastline and the period from 1843 to 2008 have been used to derive high quality relative mean sea level time series. Changes in mean sea level are assessed using non-linear smoothing techniques and linear trend estimations for different time spans. Time series from individual tide gauges are analysed and then ‘virtual station’ time series are constructed (by combining the individual records) which are representative of the German Bight and the southern and eastern regions of the Bight. An accelerated sea level rise is detected for a period at the end of the nineteenth century and for another one covering the last decades. The results show that there are regional differences in sea level changes along the coastline. Higher rates of relative sea level rise are detected for the eastern part of the German Bight in comparison to the southern part. This is most likely due to different rates of vertical land movement. In addition, different temporal behaviour of sea level change is found in the German Bight compared to wider regional and global changes, highlighting the urgent need to derive reliable regional sea level projections for coastal planning strategies.  相似文献   

10.
We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields at wind speeds of 3.7 and 4.4 m s−1 and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along the windward side, whereas, the momentum transfer is from wind to waves along the leeward side.  相似文献   

11.
Sandy beaches are often characterized by the presence of sand bars, whose characteristics (growth, migration speed, etc.) strongly depend on offshore wave conditions, such as wave height and angle of wave incidence. This study addresses the impact of a sinusoidally time-varying wave angle of incidence with different time-means on the saturation height, migration speed and longshore spacing of sand bars. Model results show that shore-transverse sand bars (so-called TBR bars) eventually develop under a time-varying wave angle. Depending on the time-mean, amplitude and period of the varying angle of wave incidence, the mean heights and mean migration speeds of the bars can be larger or smaller than their corresponding values in the case of time-invariant angles. Bars might not even form when the wave angle varies around a too large oblique mean value, whereas bars exist in the case of a time-invariant wave angle. The oscillations in both bar height and migration speed are large if the period of the time-varying wave angle is close to the adjustment timescale of the system and if large differences in the local growth and migration rates of the bars occur during one oscillation period. The oscillations in bar height are a combination of harmonics with the principal period and half the period of the time-varying wave angle, whereas those of migration speed contain only the principal period. Bars that are subject to time-varying wave angles have larger longshore crest-to-crest spacings than those which form under fixed wave angles. Physical explanations for these findings are given. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

12.
张怡辉  胡维平 《湖泊科学》2020,32(1):236-245
基于实测和数值模拟方法分析了台风“摩羯”和“温比亚”过境巢湖流域时的影响.台风“摩羯”和“温比亚”过境时,在巢湖产生较大的风速和风浪过程,其中对东巢湖的影响最大,中巢湖次之,西巢湖最小.东巢湖、中巢湖、西巢湖在台风“摩羯”过境时出现了0.68、0.67和0.48 m的最大有效波高和2.25、2.33和1.95 s的最大平均波周期;而台风“温比亚”过境时则最大有效波高可达1.50、1.47和1.18 m,最大平均波周期可达2.99、3.04和2.74 s.影响较大的区域位于东巢湖与中巢湖连接的湖心水域,总体对东巢湖的影响最为显著,中巢湖次之,西巢湖最小.不同湖区湖心水域由于水深、风区长度大,往往是出现最大风浪强度的区域.  相似文献   

13.
浅水湖泊风浪过程对于湖泊生态系统具有重要的意义.基于巢湖风场、风浪和水环境参数同步高频观测结果,详细分析了快速变化风场下的风浪快速变化特征及其对湖泊水环境的影响特征.浅水湖泊风浪的有效波高和平均波周期均随风速的快速变化有较好的同步响应规律.在风速快速衰减阶段,相较有效波高,波周期有更好的稳定性.湖泊水体pH、水温、溶解氧会快速响应风浪的变化,随着风浪强度增强,对水体浊度、总磷浓度以及藻密度和生物量的扰动影响逐渐呈现.强烈的风浪扰动引起水体浊度变化的滞后时间可达3 d.快速变化的风浪场下,风浪的强烈扰动会改变水体固有的理化参数分布特征,扰动藻类常规的水体分布规律,风浪强度是造成差异的主要因子.  相似文献   

14.
The impact of the gustiness on surface waves under storm conditions is investigated with focus on the appearance of wave groups with extreme high amplitude and wavelength in the North Sea. During many storms characterized by extremely high individual waves measured near the German coast, especially in cold air outbreaks, the moving atmospheric open cells are observed by optical and radar satellites. According to measurements, the footprint of the cell produces a local increase in the wind field at sea surface, moving as a consistent system with a propagation speed near to swell wave-traveling speed. The optical and microwave satellite data are used to connect mesoscale atmospheric turbulences and the extreme waves measured. The parameters of open cells observed are used for numerical spectral wave modeling. The North Sea with horizontal resolution of 2.5?km and with focus on the German Bight was simulated. The wind field “storm in storm,” including moving organized mesoscale eddies with increased wind speed, was generated. To take into account the rapid moving gust structure, the input wind field was updated each 5?min. The test cases idealized with one, two, and four open individual cells and, respectively, with groups of open cells, with and without preexisting sea state, as well the real storm conditions, are simulated. The model results confirm that an individual-moving open cell can cause the local significant wave height increase in order of meters within the cell area and especially in a narrow area of 1–2?km at the footprint center of a cell (the cell's diameter is 40–90?km). In a case of a traveling individual open cell with 15?m·s?1 over a sea surface with a preexisting wind sea of and swell, a local significant wave height increase of 3.5?m is produced. A group of cells for a real storm condition produces a local increase of significant wave height of more than 6?m during a short time window of 10–20?min (cell passing). The sea surface simulation from modeled wave spectra points out the appearance of wave groups including extreme individual waves with a period of about 25?s and a wavelength of more than 350?m under the cell's footprint. This corresponds well with measurement of a rogue wave group with length of about 400?m and a period of near 25?s. This has been registered at FiNO-1 research platform in the North Sea during Britta storm on November 1, 2006 at 04:00 UTC. The results can explain the appearance of rogue waves in the German Bight and can be used for ship safety and coastal protection. Presently, the considered mesoscale gustiness cannot be incorporated in present operational wave forecasting systems, since it needs an update of the wind field at spatial and temporal scales, which is still not available for such applications. However, the scenario simulations for cell structures with appropriate travel speed, observed by optical and radar satellites, can be done and applied for warning messages.  相似文献   

15.
An experimental investigation of the airflow structure in the near surface region over the wind-sheared air–water interface is reported. The two-dimensional velocity fields in a plane perpendicular to the water surface were measured using particle image velocimetry (PIV) technique over a wind speed range from 1.5 to 4.4 m s−1. The results show a reduction in the mean velocity magnitudes and the tangential stresses when gravity waves appear on the surface. An enhanced vorticity layer was observed immediately above the water surface that extended to a height of approximately 2 cm. The vorticity was enhanced by an order of magnitude, and the energy dissipation rate was enhanced by a factor of 7 in this layer at all wind speeds. The vertical profiles of Reynolds stress, energy production, and dissipation indicate the contribution of surface waves in the enhanced transfer of momentum and energy between the two fluids. The results in this study show that the flow dynamics in a layer immediately adjacent to the water surface whose thickness is of the order of the significant wave height is significantly different from that at greater heights. Thus, it is concluded that the quantitative investigation of the flow in the immediate vicinity of the interface is vital for an improved understanding of the heat, mass, and momentum exchange between air and water. The present study demonstrates that PIV is an effective technique to accurately measure the velocity fields in this region.  相似文献   

16.
A model for the air–sea interface, based on the coupled pair of similarity relations for “aerodynamically” rough flow in both fluids, is presented, which is applied to fetch-limited and high wind speed conditions which occur, for example, in hurricanes. It is shown that the specification of the maximum 10-m drag coefficient and the 10-m wind speed and the peak wave speed at which it occurs are sufficient to uniquely determine the drag law, which asymptotes at low wind speeds to a Charnock constant similar to that for the fully developed wind wave sea and is almost independent of the peak wave speed at the maximum in drag coefficient. A feature of the drag law is that it is of Charnock form, almost independent of the wave age, consistent with the transfer of momentum to the wave spectrum being due to the smaller rather than the dominant wavelengths. The analysis is also applied to a variable sea state in which either the surface wind or the surface Stokes drift vary, but the peak wave speed is kept constant. The corresponding variability in the Charnock constant is in general accord with observations.  相似文献   

17.
Radar measurements at Aberystwyth (52.4°N, 4.1°W) of winds at tropospheric and lower stratospheric heights are shown for 12–13 March 1994 in a region of highly curved flow, downstream of the jet maximum. The perturbations of horizontal velocity have comparable amplitudes in the troposphere and lower stratosphere with downward and upward phase propagation, respectively, in these two height regions. The sense of rotation with increasing height in hodographs of horizontal perturbation velocity derived for hourly intervals show downwards propagation of energy in the troposphere and upward propagation in the lower stratosphere with vertical wavelengths of 1.7 to 2.3 km. The results indicate inertia-gravity waves propagating in a direction similar to that of the jet stream but at smaller velocities. Some of the features observed contrast with those of previous observations of inertia-gravity waves propagating transverse to the jet stream. The interpretation of the hodographs to derive wave parameters has taken account of the vertical shear of the background wind transverse to the direction of wave propagation.  相似文献   

18.
This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and “wave drag events” is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flux.  相似文献   

19.
An analysis of the extreme wave conditions in 1958–2002 in the North Sea as obtained from a regional model hindcast is presented. The model was driven by hourly wind fields obtained from a regional atmosphere model forced with reanalysis data from the National Center for Environmental Prediction (NCEP/NCAR). Furthermore, observed sea ice conditions from the Norwegian Meteorological Institute have been accounted for in the simulation. It is shown that the model is capable of reproducing extreme wave height statistics at a reasonable degree of approximation. The analysis of severe wave height events reveals that for much of the Southern North Sea, their number has increased since the beginning of the simulation period (1958), although the increase has attenuated later and leveled off around about 1985. On the other hand, the intensity and duration of severe wave height events decreased within the last few years of the simulation so that annual 99%-ile wave heights have also reduced since about 1990–1995. For the UK North Sea coast, a different behavior was found characterized by a reduction in severe wave conditions over much of the hindcast period.  相似文献   

20.
太湖北部风浪波高计算模式观测分析   总被引:10,自引:7,他引:3  
通过用无量纲分析和线性回归方法对2002-2003年太湖4测点1000多组波浪资料的分析,给出了太湖不同时段风浪平均波高(H)与风区长度(F)、水深(d)、风速(v)6个关系模式.在此基础上,开展了模式的误差分析及与前人模式对比.结果表明,太湖北部不同区域风浪平均波高的计算应选用不同计算模式,但是可用如下形式表示:其中,a1-a8为和地形及水生植物覆盖度等相关的参数.误差分析结果显示:离岸距离大于1km区域的参数a1-a8的取值分别为0.217456、1、0.15、0.6、0.09、0.6、1.0、0.0052,模式估算平均波高的误差小于24%;近岸区参数a7大于1,a8取0,a4取值介于0.6-0.72,a5介于0.00131-0.00168,模式估算平均波高的误差较大,表明近岸区波浪还需进行进一步的观测研究.  相似文献   

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