强风条件下海面拖曳系数和粗糙长度研究

进一步研究强风条件下海-气湍流动量交换以及海浪特征,有助于提高数值天气模式对台风强度演变、移动路径以及恶劣海况的预报能力。依照前人的方法将台风分为风向与浪向(1)相同,(2)相反,和(3)交叉3个扇形区,并结合台风路径数据,得到了浮标数据相对于台风的方位。分别对3种类型的浮标数据进行分析,进而发现了波浪高度和相速度随风速增加而变化的规律。并利用GWW参数化方案计算出摩擦速度(u_*)、拖曳系数(C_DN)和粗糙长度(z₀)。将这些结果与前人代表性的研究论文中所用观测数据和所得研究结论进行比较,结果表明二者有较强的一致性。该研究证明GWW参数化方案在强风条件下依然有很好的适用性。     

台湾海域海面粗糙度及曳力系数的研究

利用福建沿海海域南日岛、金门岛与广东南澳岛的近海面气象、水文观测资料及台湾海峡西南部与南部的浮标观测资料,通过COARE(Coupled-Ocean Atmosphere Response Experiment)算法(V3.0),计算了台湾海域近海面空气动力粗糙度、曳力系数及海气表面动量通量,并研究它们关于水平风速的分布规律。结果得出:摩擦速度与10m风速基本上呈线性分布,线性拟合系数在0.042～0.045之间。曳力系数对10m风速的线性分布与过去研究结果相近,线性拟合系数在0.064～0.067之间。海面空气动力粗糙度对10m风速的线性拟合系数在0.053～0.058之间。海面动量通量与10m风速呈明显的二次关系,其线性与非线性拟合系数对于进一步了解台海地区大气湍流结构,建立和完善台海地区数值预报模式可提供重要的参数依据。     

海气热通量算法的改进及应用

COARE模型是国际上常用的计算海气热通量的算法,其风速适用范围可达20m/s,但未包含飞沫等高风速下的影响因子,将其直接扩展到20m/s以上风速的海况存在不合理性。本文提出了适合各种风速条件下的包含飞沫影响的海面动力粗糙度长度参数化方案,并利用该方案改进了COARE 3.0模型。利用南海浮标的观测数据,根据改进的COARE 3.0模型计算了海气热通量,分析了飞沫对海气热通量的影响。结果表明,在0～20m/s风速范围内,感热通量与潜热通量主要由海气温差和海气湿差决定,与波龄的相关性很小,飞沫对热通量无显著影响。当风速大于20m/s,感热通量和潜热通量与海气温差和海气湿差的相关性减小,与波龄的相关性增加,潜热通量与波龄呈现负相关。考虑飞沫的效应后,总热通量明显增加,飞沫所增加的感热通量平均可占界面感热通量的38.89%,飞沫所增加的潜热通量平均占界面潜热通量的39.19%。     

基于“黑格比”和“灿都”台风期间湍流数据的曳力系数和动力粗糙度长度参数化研究*

海面的曳力系数和空气动力学粗糙度长度是计算海气动量、感热和水汽通量交换必需的参数。基于在“黑格比”和“灿都”台风期间收集的涡动相关系统观测数据, 文章研究了10m风速和摩擦速度之间、10m风速和曳力系数之间、以及10m风速和动力粗糙度长度之间的参数化关系。结果表明: 曳力系数和摩擦速度及10m风速之间存在抛物线关系, 动力粗糙度长度与摩擦速度及10m风速之间存在自然指数关系; 临界摩擦速度为0.83m·s^-1, 临界10m级风速为23.69m·s^-1。     

基于遥感和实测数据对台风LEO和SOULIK期间的飞沫热通量研究

本文应用高风速条件下海面动力粗糙度长度,拓展了COARE3.0块体通量算法,考虑高风速下,海洋飞沫对热通量的贡献。利用GSSTF3(Goddard Satellite-based Surface Turbulent Fluxes Version 3)遥感产品、GSSTF_NCEP(National Centers Environmental Prediction)再分析资料和浮标KEO实测数据,探讨了中国南海台风LEO和西北太平洋台风SOULIK期间湍流热通量的变化。研究结果表明:感热通量与潜热通量相比很小;台风的轨迹与潜热通量的分布密切相关且在台风轨迹的东偏北区域潜热通量数值大;在热带低压之前,原潜热通量与改进后潜热通量的差值即飞沫热通量很小,随着台风等级的增加,飞沫热通量也增加。当台风LEO达到最高即台风级别时原潜热通量达到300W/m2,飞沫热通量与原通量的比值高达12%,而台风SOULIK达到强台风级别时原潜热通量达到1000W/m2,飞沫热通量与原通量的比值达到20%,显著高于台风LEO,飞沫效应更明显。     

定量评价中国北方粉尘源区地表覆盖类型对表土风蚀强度的影响

根据中国北方不同地表覆盖类型下表土的微团粒粒度分布和粗糙度长度参数,应用DPM模型分别计算了高草覆盖下、低草覆盖下、开垦耕地、砂砾质戈壁、砾质戈壁和裸露沙地在不同摩擦速度时的跃移通量。结果表明,裸露的粉细砂和细砂的跃移通量分别是砂砾质戈壁的几十倍;只有在极高风速条件下低草和高草覆盖下的砾质砂地才出现风蚀,其跃移通量与前面几种覆盖类型存在着1—2个数量级的差异。不同覆盖条件下地表可蚀性组分粒度分布和粗糙度长度的差异是造成风蚀强度差异的主要原因。在此基础上。从风蚀机制讨论了中国北方第四纪粉尘和现代粉尘的主要源区。     

Influence of the type of sea waves on the backscattered radar cross section at medium incidence angles

We consider the influence of the sea surface state on the backscattered radar cross section and the accuracy of the wind speed retrieval from the scatterometer data. We used a joint set of radars and buoys to determine the type of sea waves. Three types of sea waves were distinguished: developing wind waves, fully developed wind waves, and mixed sea. It is shown that the retrieval error of the near surface wind speed using a one-parameter algorithm is minimal in the case of fully developed wind waves. We compared these data with the results of radio-altimeter data analysis and showed that in both cases underestimation of the retrieval wind speed exists for developing wind waves and overestimation occurs for mixed sea. A variety of swell parameters (length of the dominating wave, swell height, swell age) significantly influence the backscattered radar cross section, leading to a growth in the mean square error of the retrieved wind speed during vertical sounding (radio-altimeter data), and only slightly influence the mean square error of the scatterometer data (medium incidence angles). It is necessary to include the information about the parameters of sea waves in the algorithms and take into account the regional wave properties to increase the accuracy of wind speed retrieval.     

Sea-surface roughness length fluctuating in concert with wind and waves

When the nondimensional aerodynamic roughness parameter for the sea surface (gz ₀/u _* ²,g being the acceleration of gravity,u _* the air friction velocity) is plotted as a function of the wave age, the data points in the diagram are distributed mostly in a triangle area between the Charnock formula and the Toba-Koga formula; the nondimensional roughness perameter is not expressed as a unique function of the wave age, but rather there seem to be multiple regimes. In order to investigate the cause of the data point scattering, a reanalysis was made of the 4.5-hour time series of the wind profile and wind-wave statistics which were obtained at an oceanographic tower station under the conditions of a winter monsoon wind having slightly fluctuating speed and steadily growing wind waves.It is concluded that the averaged variation ofz ₀ is given by the Toba-Koga formula with a constant of value 0.015. However, as a result of the wind fluctuation on the time scales ranging from several minutes to an hour, data points show a conspicuous fluctuation on the nondimensional roughness parameter-wave age diagram in the direction transverse to the averaged variation. The variation inz ₀ directly reflects the degree of over- or under-saturation in the high-frequency range of the wind-wave spectra. Physical interpretation of these variations is also presented.     

海上不同高度风速换算关系的研究

本文用海上石油平台上观测的风梯度资料,计算了在不同风况下海面摩擦速度和粗糙高度。结果表明,海面摩擦速度u_*与海上10m高度处风速u_(10)的关系为u_*=0.055u_(10)-0.058。在4—5级风时,海面平均摩擦速度为50cm/s,粗糙度为0.022m,它相当于陆上的平均粗糙高度。在风速为2—3级时,二者与海上通常采用的数据接近。文中最后给出了上述两种风况下海上不同高度的换算系数。     

COARE算法估算海气界面热通量的个例对比分析

本文先对NCEP分析风、QSCAT/NCEP混合风、MM5中尺度模式分析风场进行了比对分析,发现具有高分辨率的QSCAT/NCEP混合风资料给出的高风速数值较好,但给出的高风速开始时间相对较早;NCEP分析风资料给出的高风速数值明显偏小;MM5分析风场较为可信,只不过模拟的高风速数值还是相对偏小.使用COARE算法（版本3.0）计算了四种资料情况下的渤、黄海海域一次冷空气大风过程的海表面湍流热通量,并与MM5诊断分析结果进行了对比分析.结果发现相同资料情况下,MM5结果跟COARE算法所算海气热通量（包括感热和潜热）在区域分布和时间变化规律上均较为一致,中、低风速情况下,结果比较接近;但是高风速情况下两者差异显著.     

Impact of swell on simulations using a regional atmospheric climate model

When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg⁻¹). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.     

Peculiarities of backscattering in multifrequency nadir probing of the sea surface

Backscattering of centimeter electromagnetic waves during nadir probing of the sea surface is considered in an approximation of the Kirchoff method and a two-scale model of the scattering surface. A model of the effective reflection coefficient is developed under the assumption of the specified roughness spectrum model, and the coefficients for Ku (0.021 m), C (0.055 m), and S (0.09 m) bands are calculated. The dependence of the backscattering cross section on the wind speed, wind fetch, and the height of the swell are numerically investigated. The proximity of the numerical estimates and the experimental data is demonstrated. The dependence of the difference of two cross sections on the wind speed or on one of the backscattering cross sections is analyzed in dual-frequency measurements. It is shown that the numerical model made it possible to describe the nonmonotonic behavior of the difference cross section for the Ku-C and Ku-S pairs for the first time. The developed effective reflection coefficient model made it possible to predict in the numerical experiment the behavior of the difference cross section of a new frequency pair (C and S) for which in situ measurements have not yet been performed.     

海面粗糙度方案的适用性研究

由量纲分析导出海面粗糙度与波龄及海面涌浪的状况有关,并对与波龄和波陡有关的两种海面粗糙度模型方案进行了敏感性测试,得出利用波龄模型方案计算粗糙度的误差比利用波陡模型方案小。基于国际上6个不同水深、风区和海况条件的海气交换试验资料,将其分为两组,一组是纯风海或以风海为主海况条件,另一组是以涌浪为主海况条件,测试波龄、波陡模型海面粗糙度方案在不同试验条件下的适用性。结果得出:在各种海况条件下,波龄模型粗糙度方案比波陡模型具有更好的普适性。在纯风海或以风海为主海况条件下,PS07,DGHQ03方案计算的粗糙度长度与实测资料计算结果最接近,GW06方案次之,O02方案计算结果明显偏高;涌浪为主海况条件下两种模型方案计算的粗糙度长度均偏低,O02方案计算的粗糙度长度误差相对较小,TY01方案误差最大。     

一种新型海-气湍流通量参数化方案的模式测试

将一种新型湍流通量参数化方案代入到大气环流模式CAM3中,对比分析了模式改进前后模拟的风应力、感热通量、潜热通量和降水的全球分布,以及风应力、潜热通量和降水的纬向平均。结果表明:改进后模式的模拟结果更接近ERS、ERA40湍流通量数据和CMAP综合降水数据,其中对西风带上洋面风应力、亚洲季风区的潜热通量和降水改进明显。由于该新型湍流通量参数化方案直接由整体理查逊数、空气动力学粗糙度和热力学粗糙度参数化稳定度参数,避免了通过循环迭代计算Obukhov长度,计算效率得到提高。     

OBS海洋环境信号分析与应用

为研究海洋环境信号在OBS(Ocean Bottom Seismograph)原始数据中的规律及应用,根据OBS原始数据的波形及频谱特征,将研究区划分为5个时间段,依次为旧涌浪阶段、风浪渐强阶段、风浪全盛阶段、风浪消退阶段和新涌浪阶段。结合海洋天气预报,认为上述现象是由偏南风风浪对海流的影响造成的。参考野外地震数据采集记录班报,得到各阶段的时长和距离,计算风浪渐强、全盛和消退阶段OBS附近海流的平均速度。结果表明:OBS原始资料中浅海海洋环境噪音增强的主要因素是风浪,且风浪引起的噪音信号的波形变化特征是渐进式的;OBS可用于接收某种特殊阶段(如台风、海啸等)的噪音信号,并根据噪音信号的波形特征、频谱变化规律和持续时间估算该阶段的海流速度变化。     

南沙群岛海域近海面粗糙度、中性曳力系数及总体交换系数研究

根据 1 994年 9月 1 8— 30日南沙群岛海域渚碧礁的近海面大气湍流观测实验资料 ,分别计算了该海域光滑海面和粗糙海面上的空气动力粗糙度 (z0 )、中性曳力系数 (CDN)。利用Brutsaert的假定 ,推导了一组求取标量粗糙度 (zT,zQ)、整体输送系数 (感热交换系数CHN、水汽交换系数CEN)的公式。在此基础上分别计算和分析了该海域近海面光滑海面和粗糙海面上z0 ,zT,zQ,CDN,CHN,CEN 及它们关于水平风速u分量的分布 ,并得到了一些有意义的结论。     

A New Empirical Formula for the Aerodynamic Roughness of Water Surface Waves

A new empirical formula for the aerodynamic roughness of water surface waves has been derived from laboratory experimental results using dimensional analysis. The formula has different forms according to wind speed: at moderate wind speeds the formula is a function of the friction velocity of wind, the surface tension, the water density, the kinematic viscosity of water and the acceleration of gravity; at strong winds the formula is expressed by the Charnock relation. The aerodynamic roughness does not depend on such wave state parameters as the spectral peak frequency or the steepness of waves, unlike almost all parameterizations that have been proposed to date. The drag coefficient at moderate winds depends on the surface tension of water and the water temperature through the temperature dependence of the kinematic viscosity of water.     

A physical radar cross-section model for a wind-driven sea with swell

A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the0deg-70degrange. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies (Lband) and near normal incidence but can be nearly eliminated by using higher frequencies (K_{u}band) and large angles of incidence (approx 50deg).     

考虑气动阻尼影响的海上风电筒型基础整机运行状态振动模拟研究

运行状态下由叶轮旋转效应产生的气动阻尼对海上风电结构振动影响显著,然而当前的数值模拟方法受到计算能力与施加气动阻尼方式的限制,无法真实高效地表征气动力对振动的影响,使得模拟结果与实际工程存在偏差。针对上述问题,以某海上风电场内1台筒型基础整机为研究对象,考虑气动阻尼效应搭建了整机顺风向振动的简化理论模型,研究气动阻尼对运行状态下整机振动的折减效果,并以折减系数形式引入到数值模型计算中再与原型观测结果进行对比。研究表明：所建理论模型与数值模型的一阶自振频率与实测数据反馈值均相差较小,两类模型对实际工程的适用性良好;理论模型与数值模型同风速下的振动差值稳定,两者动力学上具有很好的同步性;停机工况与运行工况下,模拟塔筒顶部振动位移均方根值与原型观测均方根值平均误差分别为 12.19%与7.99%,说明基于考虑气动阻尼效应的振动理论模型对数值模拟结果进行折减是可行的,模拟精度可以满足工程需求。     

Coupling winds to ocean surface currents over the global ocean

A Wind stress–Current Coupled System (WCCS) consisting of the HYbrid Coordinate Ocean Model (HYCOM) and an improved wind stress algorithm based on Donelan et al. [Donelan, W.M., Drennan, Katsaros, K.B., 1997. The air–sea momentum flux in mixed wind sea and swell conditions. J. Phys. Oceanogr. 27, 2087–2099] is developed by using the Earth System Modeling Framework (ESMF). The WCCS is applied to the global ocean to study the interactions between the wind stress and the ocean surface currents. In this study, the ocean surface current velocity is taken into consideration in the wind stress calculation and air–sea heat flux calculation. The wind stress that contains the effect of ocean surface current velocity will be used to force the HYCOM. The results indicate that the ocean surface velocity exerts an important influence on the wind stress, which, in turn, significantly affects the global ocean surface currents, air–sea heat fluxes, and the thickness of ocean surface boundary layer. Comparison with the TOGA TAO buoy data, the sea surface temperature from the wind–current coupled simulation showed noticeable improvement over the stand-alone HYCOM simulation.