Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.     

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.

     

Note on the hypergeometric distribution as an invalidation test for binary forecasts

One mechanism giving rise to the hypergeometric distribution is the number of matches in a random reordering of a sequence of forecasts of binary events. This provides a simple means of invalidating a time series of binary forecasts if in fact the forecasting method has no skill. The hypergeometric distribution has a long history of application in this context but appears not to have a high profile in the environmental sciences. Attention is drawn to the utility of the distribution as a simple nonparametric test of the null hypothesis of no skill when forecasting binary environmental outcomes.     

Revision of the basic equations of wave distribution function analysis

The basic equations of wave distribution function analysis are rewritten in forms that treat the electric and magnetic fields of the waves in a more symmetrical way than the original equations do, and are slightly better for computing.     

Ionospheric conductance distribution and MHD wave structure: observation and model

The ionosphere influences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are different eigenvalues and eigensolutions of the three dimensional toroidal wave equation if the height integrated Pedersen conductivity exceeds a critical value, namely the wave conductance of the magnetosphere. As a result a jump in frequency can be observed in ULF pulsation records. This effect mainly occurs in regions with gradients in the Pedersen conductances, as in the auroral oval or the dawn and dusk areas. A pulsation event recorded by the geostationary GOES-6 satellite is presented. We explain the observed change in frequency as a change in the wave structure while crossing the terminator. Furthermore, selected results of numerical simulations in a dipole magnetosphere with realistic ionospheric conditions are discussed. These are in good agreement with the observational data.     

The global distribution of gravity wave energy in the lower stratosphere derived from GPS data and gravity wave modelling: Attempt and challenges

Five years of global temperatures retrieved from radio occultations measured by Champ (Challenging Minisatellite Payload) and SAC-C (Satelite de Aplicaciones Cientificas-C) are analyzed for gravity waves (GWs). In order to separate GWs from other atmospheric variations, a high-pass filter was applied on the vertical profile. Resulting temperature fluctuations correspond to vertical wavelengths between 400 m (instrumental resolution) and 10 km (limit of the high-pass filter). The temperature fluctuations can be converted into GW potential energy, but for comparison with parameterization schemes GW momentum flux is required. We therefore used representative values for the vertical and horizontal wavelength to infer GW momentum flux from the GPS measurements. The vertical wavelength value is determined by high-pass filtering, the horizontal wavelength is adopted from a latitude-dependent climatology. The obtained momentum flux distributions agree well, both in global distribution and in absolute values, with simulations using the Warner and McIntyre parameterization (WM) scheme. However, discrepancies are found in the annual cycle. Online simulations, implementing the WM scheme in the mechanistic COMMA-LIM (Cologne Model of the Middle Atmosphere—Leipzig Institute for Meteorology) general circulation model (GCM), do not converge, demonstrating that a good representation of GWs in a GCM requires both a realistic launch distribution and an adequate representation of GW breaking and momentum transfer.     

Variations in the depth distribution of phosphorus in soil profiles and implications for model-based catchment-scale predictions of phosphorus delivery to surface waters

The PSYCHIC process-based model for predicting sediment and phosphorus (P) transfer within catchments uses spatial data on soil-P derived from the National Soil Inventory (NSI) data set. These soil-P values are based on bulked 0–15 cm depth and do not account for variations in soil-P with depth. We describe the depth distribution of soil-P (total and Olsen) in grassland and arable soils for the dominant soil types in the two PSYCHIC study catchments: the Avon and the Wye, UK. There were clear variations in soil-P (particularly Olsen-P) concentrations with depth in untilled grassland soils while concentrations of total-P were broadly constant within the plough layer of arable soils. Concentrations of Olsen-P in arable soils, however, exhibited maximum values near the soil surface reflecting surface applications of fertilisers and manures between consecutive ploughing events. When the soil-P concentrations for the surface soil (0–5 cm average) were compared to both the profile-averaged (0–15 cm) and the NSI (0–15 cm) values, those for the surface soil were considerably greater than those for the average 0–15 cm depth. Modelled estimates of P loss using the depth-weighted average soil-P concentrations for the 0–5 cm depth layer were up to 14% greater than those based on the NSI data set due to the preferential accumulation of P at the soil surface. These findings have important implications for the use of soil-P data (and other data) in models to predict P losses from land to water and the interpretation of these predictions for river basin management.     

Quantifying and predicting the accuracy of radar-based quantitative precipitation forecasts

On the basis that hydrological users need to know the forecast uncertainty at the time that the forecast is issued, we computed distributions of radar rainfall forecast uncertainty as a function of forecast lead time, basin size, and forecasted rainfall intensity using data from the US 3-D National Mosaic of radar data. We document how exceptional forecasts such as those of heavy rainfall are generally biased. Since forecast uncertainty is also weather dependent, we tried to find good predictors to help either reduce the forecast uncertainty or better define it. These predictors were based either on characteristics of the current precipitation field or on the performance of the nowcast in the immediate past. The value of some predictors, especially those based on the properties of large-scale rainfall patterns, was significant though modest, the predictors being generally more skillful at characterizing forecast uncertainty than at improving forecast accuracy.     

A new approximation of the velocity-depth distribution and its application to the computation of seismic wave fields

Summary A new approximation of the velocity-depth distribution in a vertically inhomogeneous medium is suggested. This approximation guarantees the continuity of velocity and of its first and second derivatives and does not generate false low-velocity zones. It is very suitable for the computations of seismic wave fields in vertically inhomogeneous media by ray methods and its modifications, as it removes many false anomalies from the travel-time and amplitude-distance curves of seismic body waves. The ray integrals can be evaluated in a closed form; the resulting formulae for rays, travel times and geometrical spreading are very simple. They do not contain any transcendental functions (such asln (x) orsin ^–1, (x)) like other approximations; only the evaluation of one square root and of certain simple arithmetic expressions for each layer is required. From a computational point of view, the evaluation of ray integrals and of geometrical spreading is only slightly slower than for a system of homogeneous parallel layers and even faster than for a piece-wise linear approximation.     

唐山地区尾波Q值时空分布特征

利用首都圈地震数字台网2009年1月至2016年3月记录的地震波形资料,采用Sato尾波单次散射模型,固定尾波窗长,计算唐山地区尾波Q值,时间和空间上分析Q值分布特征,结果显示,Q值在空间分布上呈现明显的横向不均匀性,在较长孕震时间上呈现降低—升高—降低—发震的过程,且中强震易发生在Q值高低值交界区偏高值一侧,震群易发生在Q值相对较低区域。     

玉树地震震源区速度结构与余震分布的关系

利用玉树震区21个应急流动地震台站和青海省地震台网固定地震台站的观测数据,采用双差层析成像方法,对2010年4月14日至6月15期间发生的地震进行了重定位,并反演得到了玉树地震震源区的三维速度结构.重定位结果揭示余震主要沿NW向成窄带状分布在断层的两侧,表明脆性破裂应力释放主要集中于一个狭窄的区域内.在西北端,余震偏离玉树—甘孜断裂分布,在SW向也有分布,推测可能与南西向次级断裂有关.双差层析成像得到的速度结构在浅部与地表地质构造相一致,中上地壳的速度结构显示巴颜喀拉地块为高速异常,羌塘地块为低速异常.玉树地震余震分布与特定的速度结构存在相关性：主震发生在高低速过渡带偏高速体的一侧,余震主要分布在高速体外围,高速体内部几乎没有余震分布.一般说来,中上地壳的高速体通常具有较高的强度,可以积累较强的孕震能量.主震发生后,高速体内积累的弹性能量向周边释放,可能是导致高速体周边余震发生的主要原因.     

Accuracy of the kinematic wave and diffusion wave approximations for time-independent flows with infiltration included

Errors in the kinematic wave and diffusion wave approximation for time-independent (or steady-state) cases of channel flow with infiltration were derived for three types of boundary conditions: zero flow at the upstream end, and critical flow depth and zero depth gradient at the downstream end. The diffusion wave approximation was found to be in excellent agreement with the dynamic wave approximation, with errors of less than 1·4% for KF²₀≥7·5, and up to 14% for KF²₀≤0·75 for the upstream boundary condition of zero discharge and finite depth, where K is the kinematic wave number and F₀ is the Froude number. The kinematic wave approximation was reasonably accurate except at the channel boundaries and for small values of KF²₀ (≤1). The accuracy of these approximations was significantly influenced by the downstream boundary, both in terms of the magnitude of the error and the segment of the channel reach for which these approximations would be applicable. © 1998 John Wiley & Sons, Ltd.     

具有离散裂缝空间分布的二维固体中地震波传播的有限差分模拟

结合有限差分方法和等效介质理论,模拟了离散分布裂缝介质中地震波的传播. 基于等效介质理论,利用二维有限差分实现封闭裂缝的离散分布;裂缝可以处理成固体岩石中的高度柔性界面,并可以用线性滑动或者位移间断模型进行裂缝的物理描述. 对于含有多组裂隙的破裂固体,其有效柔度可以认为是固体骨架背景柔度和裂缝附加柔度之和. 在一阶近似条件下,固体骨架和裂缝参数可以通过有效各向异性系数联系起来,有效各向异性系数决定了各向异性（裂缝效应）对于地震波传播的影响. 通过与射线理论方法的对比检验,说明本文提出的模拟方法的有效性,并通过几个数值算例说明本方法可有效模拟不同的裂缝分布效应. 结果表明,即使在裂缝密度很小的情况下,具有相同裂缝密度的不同的空间分布可以产生不同的波场特征. 同时,也验证了不同裂缝尺度对波长的不同影响,以及裂缝尺度具有幂率分布（分形）时,尺度对波场的影响. 最后得出结论：在运用建立在等效介质理论基础上的地震各向异性概念来描述裂缝固体的特征时,要倍加小心,等效介质理论中尚未合理处理的裂缝尺度和空间分布对波的传播特征具有重要的影响.     

华南块体中部上地壳剪切波分裂特征

本文采用剪切波分裂分析方法,使用华南流动地震台阵（HN Array）和国家地震台网记录的近场波形资料,分析华南块体上地壳介质的地震各向异性特征.结果显示,华南块体快剪切波优势偏振方向整体表现为近东西向,与区域最大主压应力方向基本一致.结合华北块体地震各向异性特征,推测太平洋板块西向俯冲和菲律宾板块北西向俯冲作用,是中国大陆东部中上地壳各向异性的重要构造背景,导致中国大陆东部表现出近东西向的构造应力特征.苏鲁—大别造山和江南造山带的快剪切波偏振方向发生明显区域性变化,揭示了块体之间相互作用产生局部复杂构造特征和应力分布特点.研究显示,各向异性观测结果会因地壳结构的多样性变得复杂,通过江汉盆地的地震各向异性特征揭示出盆地内存在不同尺度的应力场.研究认为,岩石组分对中上地壳地震各向异性有重要贡献,华夏块体东部广泛存在的变质岩导致东西华夏块体各向异性偏振方向存在明显差异.

     

Performance of operational systems with respect to water level forecasts in the Gulf of Finland

This paper is devoted to the validation of water level forecasts in the Gulf of Finland. Daily forecasts produced by four setups of operational, three-dimensional Baltic Sea oceanographic models are analyzed using statistical means and are compared with water level observations at three Finnish stations located on the northern coast of the Gulf of Finland. The overall conclusion is that the operational systems were skillful in forecasting water level variations during the study period from November 1, 2003, to January 31, 2005. The factors causing differences between the water level forecasts of different models are discussed as well. An important task of operational sea level forecasting services is to provide accurate and early information about extreme water levels, both positive and negative surges. During the study period, two major winter storms occurred which caused coastal flooding in the region. According to our analysis, the operational models forecast the rise of water levels during these events rather successfully. Nowadays, operational forecasts can provide early warnings of extreme water levels at least 1 day in advance, which may be regarded as a minimum requirement for an operational forecasting system. The paper concludes that the models generally performed very well, with over 93% of the hourly water level forecasts found to be within the range of ±15 cm of the observed water levels, and with the timing of the water level peaks accurately predicted. Further discussion and studies dealing with the assessment of the skills of both operational meteorological and oceanographic forecasts, especially in connection with rare surge events, will be necessary. Skill assessment of operational oceanographic models would be relatively easy if acceptable error limits or a quality system was developed for the Baltic Sea operational models.     

STFT与WVD在地震波时频分析中的应用

地震波是一种非平稳信号,传统的傅里叶变换已不适用,时频分析方法能在时间—频率域给出非平稳信号不同时刻的频率分布特征。Wigner-Ville分布(WVD)对非平稳信号具有较好的时频聚集性,但因其为双线性变换,会产生严重的交叉干扰项。短时傅里叶变换(STFT)是一种线性变换,不存在交叉项干扰,但时频聚集性较差。考虑到二者优点,在对地震波进行时频分析时,采用短时傅里叶变换和Wigner-Ville分布的联合算法,既有效抑制了Wigner-Ville分布交叉项干扰,又保持了较高的时频聚集性。通过3种方法,对模拟信号和实际数据处理结果进行对比分析,发现联合算法对获取地震波信号的时频特征和纵波到时检测,具有更好效果。     

Effects of estimating the ionospheric and thermospheric parameters on electron density forecasts

Based on the thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM), a thermospheric-ionospheric data assimilation and forecast system is developed. Using this system, we estimated the oxygen ions, neutral temperature, wind, and composition by assimilating the simulated data from Formosa Satellite 3/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) occultation electron density profiles to evaluate their effects on the ionospheric forecast. An ensemble Kalman filter data assimilation scheme and combined state and parameter estimation methods are used to estimate the unobserved parameters in the model. The statistical results show that the neutral and ion compositions are more effective than the neutral temperature and wind for improving the forecast of the ionospheric electron density, whose root mean square errors in the assimilation period decreased by approximately 40%, 30%, and 10% due to the estimations of the neutral composition, oxygen ions, and neutral temperature, respectively. Due to the different physical and chemical processes that these parameters primarily affect, their e-folding times differ greatly from longer than 12 h for neutral composition to approximately 6 h for oxygen ions and 3 h for neutral temperature. The effect of estimating the neutral composition on improving the ionospheric forecast is greater than that of estimating the oxygen ions, which can be also be seen in an actual data assimilation experiment. This indicates that the neutral composition is the most important thermospheric parameter in ionospheric data assimilations and forecasts.     

基于张衡一号卫星波动观测的人工甚低频台站信号特征统计分析

地基人工甚低频(10~30 kHz)台站信号可以穿透电离层, 甚至泄露进地球磁层, 从而导致内辐射带高能电子的沉降.研究人工甚低频台站信号的特征对于研究辐射带电子的损失具有重要的科学意义.基于张衡一号卫星2019—2022年的电场观测数据, 本文对全球10个人工甚低频台站在电离层中的信号特征进行了研究, 统计分析了电离层中人工甚低频台站信号的昼夜差异、季变规律、对地磁活动水平的依赖性, 及其在地磁共轭区的分布特征.结果表明, 人工甚低频台站信号会传播至台站上空电离层及其共轭区, 信号辐射范围及强度与台站的发射功率呈正相关.电离层电子密度对甚低频信号的传播具有重要影响, 在夜侧和当地冬季, 电离层电子密度相对较小时, 泄露进入内磁层的人工甚低频台站信号较强, 而信号强度受地磁活动影响很弱.在台站上空, 信号增强区域以穿刺点为中心呈现近圆形分布, 并存在明显的波模干涉效应, 在共轭半球, 信号增强区域中心相对共轭穿刺点会发生极向漂移(L＜2的台站)或赤道向漂移(L＞2的台站), 但所有台站的电波能量都被限制在1/2赤道电子回旋频率磁壳以内的区域.这些统计观测特征表明, 发射台站位于较低L值(L＜1.4)的人工甚低频台站信号在内磁层中主要以非导管模式传播到共轭半球, 而发射台站位于较高L值(L＞2.6)的人工甚低频台站信号主要以导管模式传播.

     

Normally incident wave propagation across a joint set with the virtual wave source method

This paper presents an analytical study on normally incident wave propagation across a joint set. Combined with the displacement discontinuity method (DDM), the virtual wave source method (VWSM) is adopted to take account of multiple wave reflections among joints. After theoretical formulation, a comparison between results from VWSM and those from the method of characteristics (MC) is carried out. These results agree well with each other, and the applicability of VWSM into wave propagation study is verified. Subsequently, an interesting phenomenon, i.e., the magnitude of transmission coefficient increases with joint number on some circumstances, is pointed out and studied. By analyzing differently arriving transmitted waves and their superposition, such phenomenon is explained.     

Internal structure of Piton de la Fournaise volcano from seismic wave propagation and earthquake distribution

Arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3-D velocity heterogeneity of Piton de la Fournaise.The lateral heterogeneity of the 2632 m high edifice is resolved as a high-velocity plug, 1.5 km in diameter, surrounded by a low-velocity ring, which may be interpreted as due to the construction of Fournaise on the flank of the older volcano Piton des Neiges. Wave mode conversion detected on three-component seismograms provides evidence for boundaries of contrasted velocities.Pre-eruptive swarm earthquakes cluster in the high-velocity zone, under the Dolomieu summit crater. Low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1–3 km radial flow of magma to the vents in Enclos Fouqué.Beneath the high-velocity plug the existence of a body with low velocity for P, and even for S, waves is well constrained. However, the walls and base are poorly defined because of the lack of deep earthquakes for sampling. The few earthquakes that are located in this depth region usually occur at a depth of around 1.5 km below sea level in the region of the cone. This can be considered providing the upper constraint on the lower limit of the aseismic part of the low-velocity body. The coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low-velocity aseismic volume 1.5 km below sea level under the high-velocity plug of the cone. Further down, the concentration of seismicity in two swarms, between 2 and 4 km, under the eastern flank does not allow the structure to be sampled effectively.