Sea surface wind stress and drag coefficients: The hexos results

Turbulent fluxes have been measured in the atmospheric surface layer from a boom extending upwind from the Dutch offshore research platform Meetpost Noordwijk (MPN) during HEXMAX (Humidity Exchange over the Sea Main Experiment) in October–November, 1986. We started out to study eddy flux of water vapour, but discrepancies among simultaneous measurements made with three different anemometers led us to develop methods to correct eddy correlation measurements of wind stress for flow distortion by nearby objects. We then found excellent agreement among the corrected wind stress data sets from the three anemometers on the MPN boom and with eddy correlation measurements from a mast on a tripod. Inertial-dissipation techniques gave reliable estimates of wind stress from turbulence spectra, both at MPN and at a nearby ship. The data cover a range of wave ages and the results yield new insights into the variation of sea surface wind stress with sea state; two alternative formulas are given for the nondimensional surface roughness as a function of wave age.     

Propagation and Effects of a Mesoscale Gravity Wave Over a Weakly-Stratified Nocturnal Boundary Layer During the SABLES2006 Field Campaign

During the SABLES2006 (Stable Atmospheric Boundary Layer Experiment in Spain 2006) field campaign, a gravity-wave episode was observed on the night of July 11 by the microbarometers deployed at the surface and on the 100-m tower. The high-amplitude, low-frequency periodic pressure fluctuations were very well correlated with the wind speed and direction. Data from neighbouring automatic stations showed that the gravity wave was not local, but long-lived and mesoscale. The propagation of the wave over the experimental site had significant effects on the structure of the weakly-stratified nocturnal boundary layer that developed that night: the stability increased, turbulent vertical motions were suppressed, the nocturnal low-level jet was disrupted, and periodic temperature fluctuations of amplitude up to 3–4 K were observed. In this work we analyse the different available data sources (tower data, RASS-SODAR, microbarometric, satellite imagery, automatic stations) to describe the phenomena in depth and to find a suitable explanation for the generation and propagation of the wave. The linear wave theory explains remarkably well most of the observations, and the wave parameters could be estimated by applying a wavelet-based technique to surface microbarometric measurements. We also analyse the vertical structure of the wave and find wave ducting conditions above the surface. Finally, by means of the multi-resolution flux decomposition, we analyse in detail the changes in vertical turbulent fluxes and the spectra of turbulent motions produced by the interaction between the gravity wave and the local flow.     

Determination of the momentum flux at the air-sea interface under variable meteorological and oceanographic conditions: Further application of the wind-wave interaction method

Simultaneous observations of wind, wave, and stability parameters made recently by several authors provide an evaluation of the contribution of these factors to the determination of wind stress on the sea surface. It is shown that under diabatic conditions the wind-wave interaction method of determining wind stress is superior to the method utilizing correction for stability. The implication is that the contribution from waves is more important to the stress than that from stability. Thus, the wind-wave interaction method may be applicable under a variety of conditions. For general meteorological-oceanographic applications, a nomograph is also provided for estimating the wind stress from commonly available wind and wave parameters.     

The KNMI HEXMAX stress data – a reanalysis

An earlier discussion of the wind stress measurements of the 1986 HEXMAX experiment off the Dutch coast left some uncertainty about the actual relationship between the stress and the wave field. In this paper we try to find a more definitive answer by looking at the consequences for gradients of wind speed, stress and phase speed of the waves if we assume the Charnock coefficient to be either a constant or to have an inverse wave age dependence. It turns out that both assumptions have a wind speed range where they produce coherent results, but that they break down when the wave length exceeds a value that is related to the water depth.     

Theoretical meteorology and the operational meteorologist

Abstract

Values of incoming solar and long‐wave radiation measured at the vessel Quadra during the three phases of GATE are used to assess the daily performance of three models, one for solar and two for long‐wave radiation. The solar radiation model, which uses data on precipitable water and cloud amount at three levels in the atmosphere performed satisfactorily during the first phase but gave poor results in the other two phases when cumulonimbus became more dominant. Both the flux‐emissivity approach using measured and interpolated Upper air data and Paltridge's empirical procedure produced estimates of long‐wave radiation which compared very closely with the measurements.     

Wind wave climate change: Impacts on the littoral processes at the Northern Buenos Aires Province Coast, Argentina

Several works reported wind-wave climate changes at Buenos Aires Province continental shelf. The aim of this work is to investigate the impact of these changes in the coastal processes of the region. This study is carried out by means of visual wave parameters gathered at the surf zone of Pinamar and by a conveniently implemented and validated numerical wave model (SWAN). Numerical results corresponding to a grid point located 30 km off Pinamar show a significant increase of wave heights from the S and SSE directions and in the frequency of occurrence of waves coming from the S, SSE and E. It is shown that these slight offshore appreciated trends would not have significant effects on the breaker heights observed at the surf zone at Pinamar. On the contrary, the slight positive trend observed offshore in the frequency of occurrence could be affecting the incidence of waves onshore, producing an increase in the number of cases of normal incidence at the surf zone and, consequently, a significant decrease in the alongshore wave energy flux assessed at Pinamar. This reduction in the alongshore wave energy flux could be responsible for some coastal changes detected in the region as, for instance, the remarkable shortening of Punta Rasa spit located 70 km northward Pinamar.     

Trend analysis of wave direction and associated impacts on the Catalan coast

The effects of climate change are currently a red-hot issue in the scientific community. However, little attention has been paid to the effects of climate change on the wave climate and, in particular, on wave directionality. In this study, we developed a methodology of trend analysis and extrapolation of mean wave climate. We used the parameters of a typical wave rose: frequencies of wave height intervals and directional sectors. The trend over time was estimated by means of linear regression analysis after applying a transformation according to the nature of the (compositional) data. Then, the wave climate was extrapolated up to 2050 assuming the same previously estimated trend and part of the uncertainty was assessed with the bootstrapping technique. Additionally, to get an idea of the magnitude of the consequences of the identified trends, we carried out an impact assessment, in terms of coastal morphodynamics and harbour operability, comparing the present situation with the extrapolation. To assess the impact on coastal morphodynamics, we compared the gradients of long-shore sediment transport to evaluate possible changes in beach retreat/accretion tendency, and to assess harbour operability, we studied the possible impact on harbour agitation in three port case studies. This study was carried out on the Catalan coast (NW Mediterranean Sea) and was based on 40 nodes of 44?years of wave hindcast data. The main impacts on the wave climate were found to be a reduction in the waves coming from the north and northeast and an increase in events coming from the south. This apparently significant change in directionality could result in changes in the prevailing dynamic pattern along more than 70?% of the Catalan coast (e.g. some apparently stable beaches could become erosive) although these results have associated a large uncertainty and are not statistically significant. On the other hand, harbour agitation is expected to increase with statistical significance (by up to 18?% on average) because the studied ports, like most of Catalonia’s ports, are oriented towards the south-southwest.     

On the Dependence of the Equilibrium Range of Sea Wave Spectra with Wave Age

Donelan and Pierson have proposed a semiphysical model of the equilibrium sea wave spectrum, based upon a parameterization of wave growth and dissipation terms. Their model is applicable for fully developed seas only. In the framework of Donelan and Pierson's approach, this paper explores the dependence of the equilibrium spectrum upon wave age. To this end, we examine how the dissipation through wave breaking is expected to vary with wave age, according to the approach proposed by Longuet-Higgins in 1969. The constraint imposed by Longuet-Higgins' theory requires an increase of the equilibrium spectrum F(k,0) in the wind direction with increasing inverse wave age U/C_p. This is in accordance with Banner's empirically deduced statement that F(k,0) is proportional to (U/C_p)^0.5 in the equilibrium range. Our inferred F(k,0) tends to increase more or less linearly with U/C_p (we find F(k,0) proportional to 1 + 0.25(U/C_p - 0.83), rather than through a power law. If a power law is fitted we obtain F(k,0) approximately proportional to (U/C_p)^0.35 for the range 0.83 < U/C_p > 5. Finally, the roughness length of the air-sea interface is inferred from our modelled spectrum through integration of the form drag over wave number under rough conditions. This shows a wave age dependence that is compatible with measurements of wind stress performed in the field at various wave ages.     

基于新型往返式探空观测的下平流层重力波特征分析

利用2018年6月9日—7月10日安庆、长沙、赣州、南昌、宜昌、武汉6个探空站的往返式探空观测试验数据,研究分析了重力波参数(能量密度、固有频率、波长及传播方向),统计了试验地区6—7月的重力波特征,比较了各站之间以及上升与下降段之间重力波的差异.主要结果如下:(1)该区域重力波动能分布在0.2—1.2 J/m3,平均...     

DOES WIND STRESS DEPEND ON SEA-STATE OR NOT? – A STATISTICAL ERROR ANALYSIS OF HEXMAX DATA

There have been several claims, either explicit or by implication, either based on experimental evidence or on theoretical reasoning, that the wind stress is modified by the stage of development of the wind sea. However, the overall evidence is weak, because theories are still incomplete and because it is questionable whether the sea-state effect, which is of the order of 10%, can be separated from experimental noise, which is of the order of 20%. In this paper a rigorous statistical analysis of HEXMAX data is pursued in order to establish the significance of sea-state effects. It appears that the enhanced drag, especially at high winds, which has already been established by previous analyses, cannot be attributed to the effect of young waves. The analysis provides no clues for the actual mechanism, which could be related to breaking or shoaling waves. As the effect of sea-state on wind stress is much smaller than the experimental noise level, it is hard to detect. Nevertheless, HEXMAX seems to contain a wave effect that is at the edge of statistical significance. It is, however, not the wave age itself that influences the drag, but a parameter involving wave height.Because the HEXMAX evidence is only indicative, we conclude that the issue set out in this paper cannot be answered on the basis of the HEXMAX data alone. It is recommended that error analyses are also carried out for other relevant observational data sets and that new measurements with suppressed noise will be taken up.     

New evidence for a relation between wind stress and wave age from measurements during ASGAMAGE

Data from the 1996 ASGAMAGE experiment, performed in the southern North Sea at research platform Meetpost Noordwijk (MPN), are analysed for the parameters affecting the momentum flux. The stress turns out to be quadratically related to the 10-m wind speed and linearly to the wind speed at a wavelength related level. The Charnock parameter (dimensionless roughness length) shows a pronounced correlation with wave age. This implies, due to a coupling between wave age and the steepness of the waves, a connection between the stress and the steepness. We find that our North Sea results are consistent withopen ocean observations. For a given wind speed the mean stress at MPN turns out to be higher because the wave age there is in general lower. We define and give an expression for a drag coefficient at a wavelength related level that can be calculated straightforwardly from the wave age and then reduced to a standard level.     

An Approach to Quantify the Heat Wave Strength and Price a Heat Derivative for Risk Hedging

Mitigating the heat stress via a derivative policy is a vital financial option for agricultural producers and other business sectors to strategically adapt to the climate change scenario. This study has provided an approach to identifying heat stress events and pricing the heat stress weather derivative due to persistent days of high surface air temperature (SAT). Cooling degree days (CDD) are used as the weather index for trade. In this study, a call-option model was used as an example for calculating the price of the index. Two heat stress indices were developed to describe the severity and physical impact of heat waves. The daily Global Historical Climatology Network (GHCN-D) SAT data from 1901 to 2007 from the southern California, USA, were used. A major California heat wave that occurred 20-25 October 1965 was studied. The derivative price was calculated based on the call-option model for both long-term station data and the interpolated grid point data at a regular 0.1 ×0.1 latitude-longitude grid. The resulting comparison indicates that (a) the interpolated data can be used as reliable proxy to price the CDD and (b) a normal distribution model cannot always be used to reliably calculate the CDD price. In conclusion, the data, models, and procedures described in this study have potential application in hedging agricultural and other risks.     

Stability Effects on Heat and Moisture Fluxes at Sea

During the 1996 ASGAMAGE experiment we measured windspeed, air temperature T_a, watertemperature T_s, humidity and the momentum,heat and moisture fluxes at a research platform offthe Dutch coast. For each quantity we used several(sets of) instruments simultaneously. This allowed usto make an extensive assessment of the quality of themeasurements and to find optimal values for thevarious quantities for each run. From these values wecalculated C_H and C_E, theStanton and Dalton numbers, and reduced them to 10-mheight and neutral conditions. For this reductionwe made a separate analysis for the effect ofinclusion or non-inclusion of the assumption that theroughness length for heat or moisture is the same forthe neutral and non-neutral cases. Differences inthe reduced data due to this assumption turned out tobe well within the measurement error.For C_H we distinguished three separategroups of data: stable (A), unstable with_{a s} (B) and unstablewith thetas;_a > _s (C), with indicating the potential temperature.The stable data separate into two groups, depending onwater temperature and/or the wave field. The data ofgroup B showed a relation with wave age. The data ofgroup C consistently gave negative values forC_H, a result that might indicate conversion oflatent heat into sensible heat through condensation ofwater vapour just above the water surface. An attemptto re-analyse the data in terms of density fluxes,combining the effects of heat and moisture, still gavenegative transfer coefficients for group C.For the moisture flux we found the more conventionalresult of a separation in stable and unstable values;these categories showed a clear difference, but notrends with, for example, wind speed.We conclude that standard Monin-Obukhov similaritytheory cannot explain our data.     

TropFlux wind stresses over the tropical oceans: evaluation and comparison with other products

In this paper, we present TropFlux wind stresses and evaluate them against observations along with other widely used daily air-sea momentum flux products (NCEP, NCEP2, ERA-I and QuikSCAT). TropFlux wind stresses are computed from the COARE v3.0 algorithm, using bias and amplitude corrected ERA-I input data and an additional climatological gustiness correction. The wind stress products are evaluated against dependent data from the TAO/TRITON, PIRATA and RAMA arrays and independent data from the OceanSITES mooring networks. Wind stress products are more consistent amongst each other than surface heat fluxes, suggesting that 10 m-winds are better constrained than near-surface thermodynamical parameters (2 m-humidity and temperature) and surface downward radiative fluxes. QuikSCAT overestimates wind stresses away from the equator, while NCEP and NCEP2 underestimate wind stresses, especially in the equatorial Pacific. QuikSCAT wind stress quality is strongly affected by rain under the Inter Tropical Convergence Zones. ERA-I and TropFlux display the best agreement with in situ data, with correlations >0.93 and rms-differences <0.012 Nm^?2. TropFlux wind stresses exhibit a small, but consistent improvement (at all timescales and most locations) over ERA-I, with an overall 17 % reduction in root mean square error. ERA-I and TropFlux agree best with long-term mean zonal wind stress observations at equatorial latitudes. All products tend to underestimate the zonal wind stress seasonal cycle by ~20 % in the western and central equatorial Pacific. TropFlux and ERA-I equatorial zonal wind stresses have clearly the best phase agreement with mooring data at intraseasonal and interannual timescales (correlation of ~0.9 versus ~0.8 at best for any other product), with TropFlux correcting the ~13 % underestimation of ERA-I variance at both timescales. For example, TropFlux was the best at reproducing westerly wind bursts that played a key role in the 1997–1998 El Niño onset. Hence, we recommend the use of TropFlux for studies of equatorial ocean dynamics.     

On the deep‐water fetch laws for wind‐generated surface gravity waves

Abstract

With the object of providing an accurate set of open‐sea wave spectra in a variety of conditions, we deployed, in conjunction with CASP, an array of 9 wave buoys (3 directional, 6 non‐directional) along a 30‐km line offshore from Martinique Beach, N.S. A large set of high‐quality wave spectra was collected in conjunction with extensive meteorological information. The data set is unique in the sense that a large onshore swell component was normally present.

Offshore‐wind cases for three windows: ±5°, ±15° and ±30° with respect to the shore normal, have been considered. Wind speed was found to be a strong function of fetch, and attempts were made to allow for this in the analysis. Power‐law regressions have been produced of dimensionless sea energy, peak frequency and high‐frequency spectral level (the Kitaigorodskii “alpha” parameter) vs dimensionless fetch and wind speed (inverse wave age). The regressions are compared with earlier work: the Joint North Sea Waves Project (Jonswap) and the Canada Centre for Inland Waters (CCIW) Lake Ontario study.

The comparisons indicate that dimensionless wave energies, peak frequencies and alpha values in this experiment are comparable with those from earlier experiments; in spite of different wind analysis methods, the CASP and CCIW fetch‐limited growth laws are consistent within the contexts of the two experiments. Differences among the estimated parameters are as large within the analyses of the three windows as they are among the three experiments we compare.     

不同阶曲线拟合扰动场对下平流层重力波气候特征影响研究

重力波参数气候特征是确定大气模式中重力波参数化方案的重要条件之一,高垂直分辨率探空资料扰动场是获取重力波参数气候特征的基础数据;目前,获取扰动场的方法较多,但基于不同方法计算的扰动场对重力波参数气候特征影响的研究较少.基于2014—2017年山西太原气象台高垂直分辨率探空资料,利用2—4阶曲线拟合方法获取下平流层(17...     

Laboratory Studies Of Wind Stress Over Surface Waves

Simultaneous laboratory observations of wind speed, wind stress, and surfacewind-wave spectra are made under a variety of wind forcing patterns using cleanwater as well as water containing an artificial surfactant. Under typical experimentalconditions, more than half of the total stress is supported by the wave-induced stressrather than by the surface viscous stress. When the surfactant reduces the shortwind-wave spectra, the wind stress also decreases by as much as 20–30% at agiven wind speed. When the wind forcing is modulated in time, the wind stresstends to be higher under decreasing wind than under increasing wind at a givenwind speed, mainly because the response of short wind-wave spectra to varyingwind forcing is delayed in time. These examples clearly demonstrate that therelationship between the wind speed and the wind stress can be significantlymodified if the surface wave field is not in equilibrium with the wind forcing.Next, we examine whether the wind stress is estimated accurately if the wave-inducedstress by all surface wave components is explicitly evaluated by linear superpositionand is added to the surface viscous stress. It is assumed that the surface viscous stressis uniquely related to the wind speed, and that the wind input rate is determined by thelocal, reduced turbulent stress rather than the total stress. Our wind stress estimatesincluding the wave contributions agree well with observed wind stress values, evenif the surface wave field is away from its equilibrium with the wind in the presenceof surface films and/or under time-transient wind forcing. These observations stronglysuggest that the wind stress is accurately evaluated as a sum of the wave-induced stressand the surface viscous stress. At very high winds, our stress estimates tend to be lowerthan the observations. We suspect that this is because of the enhancement of wind stressover very steep (or breaking) short wind-waves.     

Nonlinear Formulation of the Bulk Surface Stress over Breaking Waves: Feedback Mechanisms from Air-flow Separation

Historically, our understanding of the air-sea surface stress has been derived from engineering studies of turbulent flows over flat solid surfaces, and more recently, over rigid complex geometries. Over the ocean however, the presence of a free, deformable, moving surface gives rise to a more complicated drag formulation. In fact, within the constant-stress turbulent atmospheric boundary layer over the ocean, the total air-sea stress not only includes the traditional turbulent and viscous components but also incorporates surface-wave effects such as wave growth or decay, air-flow separation, and surface separation in the form of sea-spray droplets. Because each individual stress component depends on and alters the sea state, a simple linear addition of all stress components is too simplistic. In this paper we present a model of the air-sea surface stress that incorporates air-flow separation and its effects on the other stress components, such as a reduction of the surface viscous stress in the separated region as suggested by recent measurements. Naturally, the inclusion of these effects leads to a non-linear stress formulation. This model, which uses a variable normalized dissipation rate of breaking waves and normalized length of the separation bubble, reproduces the observed features of the drag coefficient from low to high wind speeds despite extrapolating empirical wave spectra and breaking wave statistics beyond known limits. The model shows the saturation of the drag coefficient at high wind speeds for both field and laboratory fetches, suggesting that air-flow separation over ocean waves and its accompanying effects may play a significant role in the physics of the air-sea stress, at least at high wind speeds.     

淮河流域夏季降水与低纬地区波列结构的关系

利用NCEP/NCAR再分析资料,通过合成分析、诊断分析等方法,研究了淮河流域夏季降水异常年份的大尺度环流背景,并且分析了造成这种环流背景的可能原因。得到以下主要结论:在副热带地区,对流层低层存在一个自低纬向高纬传播的波列结构。在多雨年从低纬到高纬呈现"+-+-"相间的波列结构,而少雨年则相反,但是多雨年的波列强度要比少雨年强。利用波活动通量进行诊断发现:在多雨年,菲律宾到南海是明显的波源区,我国淮河流域和印度尼西亚是波汇区;在少雨年,印度尼西亚和我国淮河流域是明显的波源区,南海地区是明显的波汇区。这种波列结构的差异可能与淮河流域降水异常存在密切的联系。大气加热的不同可能是产生波列结构的一个重要原因。研究表明,赤道西太平洋大气热源的变化与我国淮河流域降水有密切的联系。印度尼西亚附近的赤道地区加热强,则淮河多雨;加热较弱,则淮河少雨。诊断分析表明,赤道地区加热强度的变化所产生的Rossby波的不同位相,与我国淮河流域降水的异常有密切的联系。