A new method for analysing nonlinear and non-stationary data has been developed. The key part of the method is the empirical mode decomposition method with which any complicated data set can be dec...

/pdf/the-empirical-mode-decomposition-and-the-hilbert-spectrum-qkmom366dy.pdf

The empirical mode decomposition and the Hilbert spectrum for nonlinear and non-stationary time series analysis

A new Ensemble Empirical Mode Decomposition (EEMD) is presented. This new approach consists of sifting an ensemble of white noise-added signal (data) and treats the mean as the final true result. Finite, not infinitesimal, amplitude white noise is necessary to force the ensemble to exhaust all possible solutions in the sifting process, thus making the different scale signals to collate in the proper intrinsic mode functions (IMF) dictated by the dyadic filter banks. As EEMD is a time–space analysis method, the added white noise is averaged out with sufficient number of trials; the only persistent part that survives the averaging process is the component of the signal (original data), which is then treated as the true and more physical meaningful answer. The effect of the added white noise is to provide a uniform reference frame in the time–frequency space; therefore, the added noise collates the portion of the signal of comparable scale in one IMF. With this ensemble mean, one can separate scales naturall...

/pdf/ensemble-empirical-mode-decomposition-a-noise-assisted-data-4htvj7tcuy.pdf

Ensemble empirical mode decomposition: a noise-assisted data analysis method

During the late 1990s, Huang introduced the algorithm called Empirical Mode Decomposition, which is widely used today to recursively decompose a signal into different modes of unknown but separate spectral bands. EMD is known for limitations like sensitivity to noise and sampling. These limitations could only partially be addressed by more mathematical attempts to this decomposition problem, like synchrosqueezing, empirical wavelets or recursive variational decomposition. Here, we propose an entirely non-recursive variational mode decomposition model, where the modes are extracted concurrently. The model looks for an ensemble of modes and their respective center frequencies, such that the modes collectively reproduce the input signal, while each being smooth after demodulation into baseband. In Fourier domain, this corresponds to a narrow-band prior. We show important relations to Wiener filter denoising. Indeed, the proposed method is a generalization of the classic Wiener filter into multiple, adaptive bands. Our model provides a solution to the decomposition problem that is theoretically well founded and still easy to understand. The variational model is efficiently optimized using an alternating direction method of multipliers approach. Preliminary results show attractive performance with respect to existing mode decomposition models. In particular, our proposed model is much more robust to sampling and noise. Finally, we show promising practical decomposition results on a series of artificial and real data.

Variational Mode Decomposition

One of the most ubiquitous and long-lasting recent changes to the surface of our planet is the accumulation and fragmentation of plastics. Within just a few decades since mass production of plastic...

/pdf/accumulation-and-fragmentation-of-plastic-debris-in-global-2ayi1pcd8z.pdf

Accumulation and fragmentation of plastic debris in global environments.

Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model [presentation]

Internal waves propagate along wave beams that are inclined with respect to the horizontal plane. It is conjectured that the internal waves generated in the Luzon Strait may be confined between the double ridges in the strait and concentrate to a closed trajectory, the so-called internal wave attractor, due to the reflection of wave beams from the lateral boundaries, sea surface and bottom. This work carried out two experiments using a three dimensional non-hydrostatic general circulation model, MITgcm, to investigate the possibility that the ridges in the Luzon Strait allows for internal wave attractors. Baroclinic current in both of the experiments demonstrate the forming of ring-like patterns in some section around 20° and 21°N, indicating that the development of the internal wave attractors are allowed in the Luzon Strait. The different resolutions and initial conditions in the two experiments also reveal that the internal-wave-attractor phenomenon is robust in this region.

Three dimensional simulation of internal wave attractors in the Luzon Strait

The northern South China Sea (SCS) including the Luzon Strait, from around 19° to 22°N latitude and from 114° to 122°E longitude, is an ocean area where energetic internal waves occur frequently. In this study, satellite ocean color products from SeaWiFS (Sea-viewing Wide Field-of-view Sensor) and MODIS (Moderate Resolution Imaging Spectroradiometer) including true color and chlorophyll images are used for the statistical analysis of internal wave occurrence. For spatial distribution, fewer internal waves distributed in the east of 118°E obviously originate from the Luzon Strait, and more internal waves in the west of 118°E may propagate from the east or evolve into solitons originating from the eastern boundary owing to the fission effect of the shoaling thermocline. No internal wave is found east of the Luzon Strait. The lunar daily observed internal wave occurrence frequencies show that more internal waves are found after full moon and new moon, that is the spring tide. This indicates that the generation of internal waves in the northern SCS may relate to the internal tide. The monthly distribution of internal wave occurrence frequencies reveals that the high frequencies are distributed from May to August and reach a peak in July with a maximum frequency of 21.5%. The low occurrence frequencies are found in winter from November to February with a minimum frequency of 0.5% in January.

Internal wave observations in the northern South China Sea from satellite ocean color imagery

The interaction of coastal water in I-Lan Bay, a bay near northeast Taiwan, and the Kuroshio Current is studied using the data from hydrologic survey and satellite remote sensing. An index for water mass distinguished is used for clarifying the origin of water mass in I-Lan Bay. The velocity profile from acoustic Doppler current profile data indicates a countercurrent that flows southward along the northeast coast of Taiwan toward I-Lan Bay with a speed around 0.1–0.2 m/s. The index of water mass indicates that the water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water, thus forming a clear ocean front between these two areas. The near surface water temperature in the I-Lan Bay is about 2 °C lower than that in the Kuroshio region. The seasonal average near surface salinity of I-Lan Bay can exist 0.4 psu fresher than the Kuroshio area. From the analysis of dynamic process, the coastal countercurrent in I-Lan Bay is primarily affected by 1) the occurrence of cold dome in northeast of Taiwan mainly occurs in summer, and 2) the wind-driven current from the Taiwan Strait along the north coast and the northeast coast of Taiwan caused by the southwesterly monsoon makes the countercurrents strongly in summer. The dynamics of the countercurrent occurrence can be explained by the Ekman transport theory.

Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan

This study aims to gain insight into the sea level response to typhoon in the northern South China Sea using typhoon records from 1978 to 1997, simultaneous tidal gauge records and satellite altimeter along-track sea level data. We find that the ocean sea level response signatures in the deep water agree well with a single sea level soliton, of which the average soliton amplitude is (34 ± 6) cm and the average characteristic half width is (115 ± 12) km. Evolution of the single soliton as propagating across a shoaling bottom topography obeys solutions of the perturbed Korteweg and de Vries equation. The solutions reveal that forced by the hyperbolic-tangent-shaped topography, the single soliton evolves to a solitary wave packet consisting of a large leading soliton and smaller following waves in an oscillatory tail as approaching the shallow water. This phenomenon is verified by observed sea level signatures of 19 storm surges. The correlation coefficient between theoretical solution and observations is as high as 0.91. Meanwhile, the soliton amplitude grows with a soliton amplitude growth ratio (SAGR), which depends on the deep water and shallow water depths. However, without consideration of viscous effects, the SAGR model overestimates the soliton amplitude growth. Thus the vorticity equation for viscous fluid is adopted to solve this issue. The results indicate that turbulent viscosities are responsible to soliton amplitude decay concurring with the growth and causes dependence of the storm surge amplitude on the typhoon incident angle.

Growth and dissipation of typhoon-forced solitary continental shelf waves in the northern South China Sea

A spatial integration filtering method is applied to process 4.5 years of Geosat altimeter sea level anomalies(SLA) recorded from 1 April 1985 to 17 September 1989. The integral limits are chosen as ±1.5° latitude centered at the equator and ±4° longitude centered at the each central longitude. These limits are chosen in order to eliminate the random noise and to enhance the equatorially trapped wave signals. The spatial-integration-filtered SLA time series show periodical signals with different time scales. Processing the SLA time series with a Short Time Fourier Transform (STFT) yields the time-frequency spectra of the SLA which show a frequency spectral splitting-combining phenomenon in the periods during and after the 1986–1987 El Ninio-Southern Oscillation (ENSO) event. Analysis of non-linear wave-wave interaction indicates that this phenomenon results from amplitude modulation of the short period oscillations in the 60–100 day band by a long period oscillation with a period of about 500 days. The wave components of the short period oscillations are further analysed using the equatorial wave modes to fit the meridional distribution of the SLA. The results indicate that the downwelling Kelvin wave mode before and during the peak phase of 1986–1987 ENSO and the upwelling Kelvin wave mode during the 1988 La Nina are the dominant components. The first and second Rossby wave modes play important roles for entire time series and occasionally become the dominant components. The mean phase speed of Kelvin wave mode during the 1986–1987 ENSO is 3.0 m s−1 which is 25% higher than those in non-ENSO periods.

Quanan Zheng

Papers

Three dimensional simulation of internal wave attractors in the Luzon Strait

Internal wave observations in the northern South China Sea from satellite ocean color imagery

Interaction of coastal countercurrent in I-Lan Bay with the Kuroshio northeast of Taiwan

Growth and dissipation of typhoon-forced solitary continental shelf waves in the northern South China Sea

Observation of equatorially trapped waves in the Pacific using Geosat altimeter data