Wave Spectral Characteristics Research Articles

Spectral Wave Characteristics over the Head Bay of Bengal: A Modeling Study

Information on spectral wave characteristics is an essential prerequisite for ocean engineering-related activities and also to understand the complex wave environment at any given location. To the best of our knowledge, there are no comprehensive studies attempted so far to study the wave spectral characteristics over the head Bay of Bengal, a low-lying deltaic environment. The present study is an attempt to describe the spectral characteristics of wave evolution across different locations over this deltaic region based on numerical simulations. Therefore, it implements a multi-scale nested modeling approach using two state-of-art wave models, WAM and SWAN, and forced with ERA-Interim winds spanning the year of 2016. Model-computed integrated wave parameters are validated against wave rider buoy data as well as remotely sensed SARAL/AltiKa and merged altimeter data. Analysis of the monthly averaged one-dimensional spectrum reveals a single peak during the southwest monsoon and existence of double peaks from November to January, and occasionally up to March. Variance energy density undergoes inter-seasonal variation and attains its maxima during the month of July. Transformation of swell wave energy as a function of depth is found to be mostly associated with physical processes such as wave-bottom interaction and attenuation by opposing winds during the northeast (NE) monsoon. Fetch restriction for the evolution of wind seas (from the NE), modification in wind shear stress by opposing swells, and bottom effects remarkably contribute to the reduction in wind sea energy at shallow water depths. This study indicates that the influence of swells is higher along the eastern side of the basin as compared to the western side, and marginally higher variance is also observed over the east except during February–April. The two-dimensional wave spectra exhibit differential wave systems approaching from various directions attributed to a reflected swell system from the south-southeast throughout the year, southwest swells, reversing wind seas following local winds, and reflected wind seas from the land boundary.

OBSERVATION OF NEARSHORE WAVE-WAVE INTERACTION USING UAV

Infragravity waves, generated by nearshore wave-wave interaction, potentially increase the coastal hazard. Lack of detailed observation of nearshore wave fields however makes it difficult to fully understand the behavior of infragravity waves under wave-wave interactions. These days, UAVs (Unmanned Aerial Vehicles) have enabled us to easily capture the top-view images of the dynamic nearshore behavior with sufficiently high spatial and temporal resolutions. In this study, we conducted UAV-based observations of cross-shore variations of wave spectral characteristics to clarify the nearshore wave-wave interactions.

The influence of wave parameter definition over floating wind platform mooring systems under severe sea states

This paper explores the role of wave spectral characteristics and wave time history on the estimation of extreme mooring loads on floating offshore wind turbines. This research is applied to the DeepCwind semi-submersible platform located at the BiMEP test site in the North of Spain. Extreme sea states are selected using the inverse first-order reliability method (I-FORM). Mooring loads are modelled by quasi-static and dynamic numerical approaches. Different wave time series are generated numerically for each sea state to investigate the variability in predicted peak loads. All cases simulated incorporate the combined effect of wind and waves. Differences of approximately 30% in peak loads are found for the mooring system, reaching 40–79% for the most extreme sea states. Safety factors are proposed to account for sensitivity to wave groupiness in modelling loads under extreme work conditions of the DeepCwind platform (e.g., pitch and velocity control). A comparison between theoretical and real-sea wave spectra is also modelled to investigate possible differences due to the presence of multiple spectral peaks associated with swell and wind seas. In general, results show differences below 12% in the prediction of loads between both assumptions.

Spectral Characteristics of Gravity‐Capillary Waves, With Connections to Wave Growth and Microbreaking

AbstractIn order to improve our understanding of physical air‐sea interaction, it is essential to better describe the short‐scale ocean wave response to wind forcing. This is particularly true for waves which are small enough to evade observation by traditional buoy and point‐based gauge measurements but large enough to appreciably alter the transfer of momentum between atmosphere and ocean. Such waves are restored to equilibrium both by the Earth's gravity and air‐sea surface tension, hence the classification as “gravity‐capillary.” Radar remote sensing techniques depend greatly upon these waves in order to extract useful physical parameters from afar. Despite this importance, field observations of gravity‐capillary wave characteristics are uncommon and results vary from study to study. Furthermore, leading‐edge model wave number spectra generally do not match each other in shape or important spectral parameters. Here we present an extended analysis of short wave data collected via a polarimetric camera aboard Research Platform Floating Instrument Platform in the Santa Barbara Channel. Our wave number saturation spectra show the emergence of a peak in the gravity‐capillary subrange at low wind forcing magnitude (u*~0.045 m/s), consistent with critical wave growth in air side stability theory and previously only observed in the laboratory. Finally, the effects of microbreaking on wave spectral characteristics are discussed.

Spectral Wave Characteristics in the Nearshore Waters of Northwestern Bay of Bengal

The spectral wave characteristics in the nearshore waters of northwestern Bay of Bengal are presented based on the buoy-measured data from February 2013 to December 2015 off Gopalpur at 15-m water depth. The mean seasonal significant wave height and mean wave period indicate that the occurrence of higher wave heights and wave periods is during the southwest monsoon period (June–September). 74% of the sea surface height variance in a year is a result of waves from 138 to 228° and 16% are from 48 to 138°. Strong inter-annual variability is observed in the monthly average wave parameters due to the occurrence of tropical cyclones. Due to the influence of the tropical cyclone Phailin, maximum significant wave height of 6.7 m is observed on 12 October 2013 and that due to tropical cyclone Hudhud whose track is 250 southwest of the study location is 5.84 m on 12 October 2014. Analysis revealed that a single tropical cyclone influenced the annual maximum significant wave height and not the annual average value which is almost same (~ 1 m) in 2014 and 2015. The waves in the northwestern Bay of Bengal are influenced by the southwest and northeast monsoons, southern ocean swells and cyclones.

Spectral wave characteristics along the central coast of eastern Red Sea

The Jeddah coast lies in the central eastern Red Sea, which is characterized by the predominant northwest winds and the associated wind waves throughout the year. A detailed investigation on the spectral wave characteristics in the nearshore regions of the Jeddah coast has not been carried out yet, primarily due to the lack of data. In the present study, we have analyzed the available wave spectra measured at two nearshore locations along the Jeddah coast using wave gauges. The wave spectra were separated into wind sea and swell components using a frequency-based algorithm, and the integral wave parameters corresponding to each component were derived. Although the measurements were limited to the summer season, notable features such as the diurnal variability and the superimposition of wind seas and swells have been identified from the spectra. The superimposition is mainly due to the interaction of the young swells propagated from the northern Red Sea and the local breezes prevailing along the coast at certain periods in a diurnal cycle. Based on the wave speed calculations and the estimated time shifts between the wind and waves, the potential swell regions have been backtracked.

Wave Spectral Patterns during a Historical Cyclone: A Numerical Model for Cyclone Gonu in the Northern Oman Sea

The third generation wind-wave model Mike21-SW was used to study spectral characteristics of waves generated by the historical Cyclone Gonu in June 2007 along and off the Iranian coasts on the northern Oman Sea. The model was forced with the cyclone wind field generated using a Holland (1980) model based on cyclone data obtained from the Joint Typhon Warning Center (JTWC). The wave model was calibrated for the northern Oman Sea using bulk and spectral wave data at a station out of the Chabahar Bay. Evolution of directional-frequency spectra during the cyclone was investigated for two locations near the entrance and off the Chabahar Bay. At the offshore station, energy was contributed to the spectrum over an approximately 180 degree directional span that included different local and remotely generated waves. As the cyclone proceeded northwestward, all spectral directions continuously rotated in the clockwise direction at both locations. Frequency spectra at these locations were investigated for four different times corresponding to different locations of Cyclone’s eye and were justified using the sea growth parameter of the Joint North Sea Wave Project (JONSWAP) experiment. Using the modified JONSWAP parameters for hurricane conditions resulted in a frequency spectrum consistent with simulation results.

On the development of controlled stationary and travelling disturbances in the supersonic boundary layer of a swept wing

Experimental data on the evolution of controlled stationary and travelling disturbances in a 3D supersonic boundary layer, over a 45° swept-wing at Mach number 2.0, is presented. Travelling artificial disturbances were introduced in the boundary layer by periodical glow discharge, at a frequency of 20 kHz. Stationary disturbances were acquired by setting the roughness elements on the surface of the model. Spatial-temporal and spectral-wave characteristics of the wave train at the frequency 20 kHz, in the linear region of development, were obtained. It was found that the periodic modulation of mean flow can lead to the stabilization of unstable travelling disturbances in the supersonic boundary layer of a swept wing. These experiments have investigated the viability of using roughness elements to control laminar-turbulent transition.

An Analysis of Gravity Wave Spectral Characteristics in Moist Baroclinic Jet–Front Systems

Abstract This study investigates gravity wave spectral characteristics based on high-resolution mesoscale simulations of idealized moist baroclinic jet–front systems with varying degrees of convective instability, with the intent of improving nonorographic gravity wave parameterizations. In all experiments, there is a clear dominance of negative vertical flux of zonal momentum. The westward momentum flux is distributed around the estimated ground-based baroclinic wave phase velocity along the zonal direction, while strong moist runs indicate a dipole structure pattern with stronger westward momentum flux centers at slower phase velocity and weaker eastward momentum flux centers at faster phase velocity. The spectral properties of short-scale wave components (50–200 km) generally differ from those of medium-scale ones (200–600 km). Compared to the medium-scale wave components, the momentum flux phase speed spectra for the short-scale ones appear to be more sensitive to the increasing initial moisture content. The spectral behavior in horizontal wavenumber space or phase velocity space is highly anisotropic, with a noticeable preference along the jet direction, except for the short-scale components in strong moist runs. It is confirmed that the dry gravity wave source (i.e., upper jet and/or surface front) generates a relatively narrow and less symmetrical power spectrum (dominated by negative momentum flux) centered around lower phase velocity and horizontal wavenumber, whereas the moist gravity wave source (i.e., moist convection) generates a broader and more symmetrical power spectrum, with a broader range of phase speeds and horizontal wavenumbers. This study also shows that the properties of gravity wave momentum flux depend on the location relative to the baroclinic jet.

Spectral wave climatology off Ratnagiri, northeast Arabian Sea

Wave spectra of the measured data at 13 m water depth off Ratnagiri using the Datawell Directional waverider buoy from March 2010 to November 2014 are used to study the daily, monthly, seasonal and annual variations in the wave spectral characteristics. Wave data are also analyzed to identify them as single-peaked or multi-peaked spectra. Single-peaked spectra are then identified as belonging to wind-seas or swells. The monthly averaged wave spectrum indicates that the wave spectra are mainly single-peaked during June to August with peak frequency distributed around 0.08 Hz (peak wave period ~12.5 s) and double-peaked during the remaining period, except in September. During January to May and from October to December, the swell peak is around 0.07 Hz. The wind-sea peak during the non-monsoon period is mainly at 0.2 Hz except during March and April. During March and April, due to the strong local winds and weak swells, the wind-sea peak shifted to higher frequencies (~0.3 Hz). The monthly averaged spectrum is wind-sea dominated during January to March and swell dominated during rest of the year. The inter-annual variations in the monthly average wave spectrum are high during February, May and September to November. The inter-annual variations in the direction of swell are marginal (<10°), whereas significant variations in wind-sea are observed during June to November. Parameters of the JONSWAP and Donelan model spectra are estimated for the corresponding sea-state by using a least square error method.

Surfzone Wave Characteristics During Flood Tide on the Central West Coast of India

Surfzone wave characteristics, measured using a wave and tide gauge (WTG) during a flood tide, were studied at three different beaches having different nearshore slopes. The spectral wave characteristics viz., wave-height and mean wave period were estimated considering different sample sizes. Inter-comparisons of wave climate between each of three beaches for a similar tide level are presented. The wave-height and mean wave period values obtained from the wave analysis for each of the record using waves by wave method show that surfeone wave-height increased with time during a flood tide and the mean wave period decreased with time up to mid tide and then increased during the rest of the measurement period at Keri and Miramar beaches. At Candolim, the reversing trend of mean wave period increase is observed to occur much before reaching the mid tide level. The surfzone waves during a flood tide indicated that the wave-heights increased with time while the mean wave period showed a decreasing trend in general. The variation of breaker index showed a decreasing trend from low tide to high tide ranging between 0.45 and 0.23. The surf similarity parameter estimated corroborates with the visual observations.

Spectral analysis of irregular waves in wave–mud and wave–current–mud interactions

The dissipation of irregular waves passing over muddy beds is investigated through two series of wave flume laboratory experiments with and without currents, where commercial kaolinite is used as the muddy sediment bed material. The changes in spectral characteristics of waves along the muddy bed and the effects of following and opposing currents are investigated. A numerical multi-layered model was also employed to simulate the attenuation of regular/irregular waves assuming viscoelastic rheological behavior for fluid mud, and the outputs were compared with the laboratory data. The first series of the experiments show that propagation of a wave spectrum over a short fluid mud section does not result in a shift in peak frequencies of wave spectra. The comparisons of spectral width parameters of various wave spectra also reveal that higher values of spectral peakedness parameters generally result in higher rates of wave energy dissipation. This can be related to the frequency dependency of wave energy dissipation on the mud layer. The results of the second series of experiments show higher dissipation rates in the opposing current and lower rates in the following current, which can be attributed to the changes in incident wave heights due to existence of currents. The study confirms that the dynamic pressure of wave propagation on the mud surface is the governing factor in regular/irregular wave–current–mud interaction and the current itself has little direct effect on the mud layer.

Control of rapid phase oscillations in the modelling of large wave energy arrays

Several recently developed concepts for economically viable conversion of ocean wave energy are based on large arrays of point absorbers. Simulations of the hydrodynamic interactions between devices in wave energy parks provide guidelines for optimal configurations with regard to maximizing produced electricity while minimizing fluctuations and costs. Parameters that influence the performance include the geometrical lay-out of the park, the number of wave energy converters and their dimensions and separating distance, as well as the wave climate and the incoming wave spectral characteristics. However, the complexity of the simulations increases rapidly with growing number of interacting units, and simulations become a severe challenge that calls for new methods. Here we address the problem of rapid phase oscillations appearing in the simulation of large arrays of point absorbers using potential theory for the structure–fluid interaction. We do this by analytically integrating out the factors that are causing the oscillations. Our group has successfully utilized this method to model parks with up to 1000 point absorbers.

Inter-annual variations in wave spectral characteristics at a location off the central west coast of India

Abstract. The inter-annual variations in wave spectrum are examined based on the wave data measured at 9 m water depth off the central west coast of India from 2009 to 2012 using a wave rider buoy. The temporal variation of the spectral energy density over a calendar year indicates similar variation in all the four years studied. The inter-annual variations in wave spectrum are observed in all months with larger variations during January to February, May and October to November due to the changes in wind-sea. The seasonal average wave spectrum during the monsoon (June–September) is single-peaked and the swell component is high in 2011 compared to other years. The annual averaged wave spectrum had higher peak energy during 2011 due to the higher spectral energy present during the monsoon period. During the non-monsoon period, two peaks are predominantly observed in the wave spectra; with the average peak at 0.07 Hz corresponding to the swells from the Indian Ocean and another at 0.17 Hz due to the local wind field.

Source characteristics of ocean infragravity waves in the Philippine Sea: analysis of 3-year continuous network records of seafloor motion and pressure

Continuous 3-year records of broadband ocean-bottom seismometers and pressure gauges of the seafloor network (Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET)) in the Nankai Trough region made it possible to monitor incoming ocean infragravity (IG) waves. Application of a slant-stacking technique revealed that the most energetic IG waves are incoming across the Nankai Trough from the Philippine Sea with limited energy of reflected waves back from the nearest coast. The sources of the most energetic waves are narrowly and stably localized into two closely adjacent azimuthal windows with mutually different wave spectral characteristics. Both sources show a seasonal variation, weak in summer and strong in winter. Although less energetic, IG waves propagating parallel to the trough and coast are observed. Such waves are greatly amplified when IG waves from a distant typhoon are incoming to the trough, suggesting the secondary origin of IG waves that can emit even more energetic waves than the originally incoming waves.

Global Pc5 pulsations during strong magnetic storms: excitation mechanisms and equatorward expansion

Abstract. The dynamics of global Pc5 waves during the magnetic storms on 29–31 October 2003 are considered using data from the trans-American and trans-Scandinavian networks of magnetometers in the morning and post-noon magnetic local time (MLT) sectors. We study the latitudinal distribution of Pc5 wave spectral characteristics to determine how deep into the magnetosphere these Pc5 waves can extend at different flanks of the magnetosphere. The wave energy transmission mechanisms are different during 29–30 October and 31 October wave events. Further, we examine whether the self-excited Kelvin–Helmholtz instability is sufficient as an excitation mechanism for the global Pc5 waves. We suggest that on 31 October a magnetospheric magnetohydrodynamic (MHD) waveguide was excited, and the rigid regime of its excitation was triggered by enhancements of the solar wind density. The described features of Pc5 wave activity during recovery phase of strong magnetic storm are to be taken into account during the modeling of the relativistic electron energization by ultra-low-frequency (ULF) waves.

Spectral wave characteristics off Gangavaram, Bay of Bengal

Spectral wave characteristics were studied based on waves measured for 1 year during 2010 off Gangavaram, Bay of Bengal. Maximum wave height of 5.2 m was observed on 19 May 2010 due to the influence of cyclonic storm LAILA. The wave spectrum was single-peaked during 57 % of the time and the double-peaked spectrum observed was mainly swell-dominated. Low-frequency waves (0.05–0.15 Hz) were predominantly from 150° to 180°, whereas high-frequency waves (>0.15 Hz) during November–January were mainly from 90° to 120°, and during July and August from 180° to 210°. Annual average significant wave height was similar to the value (1 m) observed in the eastern Arabian Sea.

A convection‐based gravity wave parameterization in a general circulation model: Implementation and improvements on the QBO

AbstractIn order to simulate stratospheric phenomena, such as the Quasi‐Biennial Oscillation (QBO), atmospheric general circulation models (GCM) require parameterizations of small‐scale gravity waves (GW). In the tropics, the main source of GWs is convection, showing high spatial and temporal variability in occurrence and strength. In this study, we implement in the GCM ECHAM6 a source parameterization for GWs forced by convection. The GW source parameterization is based on the convective heating depth, convective heating rate, and the background wind. First, we show that the heating depth distribution of convective properties strongly influences the waves' source spectra. The strong sensitivity of spectral wave characteristics on heating property distributions highlights the importance of a realistic parameterization of convective processes in a GCM. Second, with the convection‐based GW scheme as the unique source of GWs, the GCM simulates a QBO with realistic features. While the vertical extent of the easterly jet shows deficiencies, the wind speeds of the jet maxima and the variance of wind alteration show a clear improvement, compared to the standard model which employs a parameterization with constant, prescribed GW sources. Furthermore, the seasonality of the QBO jets downward progression is modeled more realistically due to the seasonality of physically based gravity wave sources.

Linear evolution of controlled disturbances in the supersonic boundary layer on a swept wing

The linear development of controlled disturbances in the three-dimensional supersonic boundary layer on a swept model wing with a sharp leading edge is experimentally investigated at the Mach number 2. The spatial-temporal and spectral-wave characteristics of the wave train of unstable disturbances are obtained. The asymmetry of these characteristics, due to the secondary flow in the three-dimensional boundary layer, is confirmed.

Shallow water wave spectral characteristics along the eastern Arabian Sea

The spectral characteristics of shallow water waves were studied at two locations along the eastern Arabian Sea during 2011. Wave spectra were single-peaked from June to October and predominantly double-peaked during the rest of the year. Even though both locations were subjected to open sea conditions, the percentage of single-peaked spectra was large (63 %) in the southern location compared to a location 350 km north (46 %), because of variation in local winds. Throughout the year, the double-peaked spectra were mostly swell dominated in the southern location. In the northern location, the double-peaked spectra during January to May and December were sea dominated due to the strong local winds blowing from north-west. For the double-peaked wave spectra, the average difference between the spectral peaks was 0.11 Hz, and the average ratio of the spectral energy density at the two peaks was 0.5. Significant wave heights up to 4.2 m and a maximum wave height of 7 m were observed during the south-west monsoon period. Fifty per cent of the waves recorded had spectral peak wave periods between 6 and 12 s. The narrowest directional spectra were found for waves with 10–12-s peak wave periods. Inverse wave age values were biased towards lower values with peaks in the range of 0.2–0.6, indicating a swell-driven wave regime along the eastern Arabian Sea.