Removing Spurious Low-Frequency Variability in Drifter Velocities

2013 ◽  
Vol 30 (2) ◽  
pp. 353-360 ◽  
Author(s):  
Rick Lumpkin ◽  
Semyon A. Grodsky ◽  
Luca Centurioni ◽  
Marie-Helene Rio ◽  
James A. Carton ◽  
...  
Keyword(s):  
Low Frequency ◽  
Original Data ◽  
Strain Sensors ◽  
Wind Forcing ◽  
Stokes Drift ◽  
Interannual Variations ◽  
Transmission Frequency ◽  
Low Frequency Variability ◽  
Frequency Variations ◽  
Drifting Buoys

Abstract Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from submergence to tether strain sensors. In this study, a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset.

scholarly journals Rotational and pulsational variability in the TESS light curve of HD 27463

2019 ◽  
Vol 490 (2) ◽  
pp. 2102-2111 ◽  
Author(s):  
V Khalack ◽  
C Lovekin ◽  
D M Bowman ◽  
O Kobzar ◽  
A David-Uraz ◽  
...  
Keyword(s):  
Rotation Period ◽  
Low Frequency ◽  
Balmer Line ◽  
Rotational Axis ◽  
Line Profiles ◽  
Stellar Rotation ◽  
Photometric Variability ◽  
Low Frequency Variability ◽  
Best Fitting ◽  
Frequency Variations

ABSTRACT The new photometric data on pulsating Ap star HD 27463 obtained recently with the Transiting Exoplanet Survey Satellite (TESS) are analysed to search for variability. Our analysis shows that HD 27463 exhibits two types of photometric variability. The low-frequency variability with the period P  = 2.834 274 ± 0.000 008 d can be explained in terms of axial stellar rotation assuming the oblique magnetic rotator model and presence of surface abundance/brightness spots, while the detected high-frequency variations are characteristics of δ Scuti pulsations. From the analysis of Balmer line profiles visible in two FEROS spectra of HD 27463 we have derived its effective temperature and surface gravity, finding values that are close to those published for this star in the TESS Input Catalogue (TIC). Knowing the rotation period and the v sin i value estimated from the fitting of Balmer line profiles we found that the rotational axis is inclined to the line of sight with an angle of $i=33\pm 8\deg$. Our best-fitting model of the observed pulsation modes results in an overshoot parameter value fov = 0.014 and values of global stellar parameters that are in good agreement with the data reported in the TIC and with the data derived from fitting Balmer line profiles. This model indicates an age of 5.0 ± 0.4 × 108 yr, which corresponds to a core hydrogen fraction of 0.33.

scholarly journals The Low Frequency Variability of Extragalactic Radio Sources: A Relativistic Effect or Galactic Scintillation?

1988 ◽  
Vol 129 ◽  
pp. 297-298 ◽  
Author(s):  
L. Padrielli ◽  
R. Fanti ◽  
A. Ficarra ◽  
L. Gregorini ◽  
F. Mantovani ◽  
...  
Keyword(s):  
Structural Changes ◽  
Low Frequency ◽  
Relativistic Electrons ◽  
List Type ◽  
Low Frequencies ◽  
Vlbi Observations ◽  
Low Frequency Variability ◽  
5 Ghz ◽  
Frequency Variations ◽  
3C 120

Results obtained on the Low Frequency Variability (LFV) phenomenon, by means of combined multifrequency observations of 50 sources, on a period of more than ten years on a frequency grid of 0.4, 2.3, 4.8, 8.0, and 14.4 GHz and two epoch VLBI observations at 18 cm can be summarized as follows: 1.15–20% of variables appear to have variations consisting either of quasi-simultaneous outbursts at all frequencies or of bursts which drift to lower frequencies with time and decreasing amplitude. In our sample, we find five good cases: 3C 120, 0605-085, 1510-089, 3C 345, BL Lac. Three of these are famous superluminals; the other two show significant structural changes between our 18 cm VLBI measurements. The corresponding expansion rate for these five sources is in agreement with the γ's derived from LFV with the usual causality arguments. For the sources of this class, the observations are therefore in agreement with models that explain the phenomenon of the variability as synchrotron emission of relativistic electrons beamed in a direction close to the line of sight.2.35% of variables show only low frequency (<1 GHz) variability and little or no intermediate high frequency variations. In DA 406, prototype of the category, no superluminal motions have been observed, even if the resolution of our VLBI observations should allow the detection of the structural change expected on the basis of intrinsic LFV. In this case we do not find direct evidence of relativistic motions associated with the LFV and the process is most easily explained if the variations are extrinsic (propagation effects through the interstellar medium as the slow refractive scintillation).3.The remaining 40–45% of variables show uncorrelated high (<5 GHz) and low frequency variability with a minimum of activity at the intermediate frequencies. The explanation of the phenomenon is less clear. It could be attributed to intrinsic (superluminal) variations at high frequencies, coexisting with unrelated processes at low frequencies.

scholarly journals Long-term Variability of Extragalactic Sources at 843 MHz

1996 ◽  
Vol 175 ◽  
pp. 103-104
Author(s):  
R. W. Hunstead ◽  
B. M. Gaensler
Keyword(s):  
Electron Density ◽  
High Frequency ◽  
Low Frequency ◽  
Time Variability ◽  
Radio Sources ◽  
Interstellar Scintillation ◽  
Low Frequency Variability ◽  
Signal Path ◽  
Frequency Variations

Time variability is commonly observed in the most compact extragalactic radio sources. Low-frequency variability (LFV)—at frequencies <1 GHz—is thought to arise through two different mechanisms, intrinsic and extrinsic. The former is just an extension of the often rapid high-frequency variations, delayed and reduced in amplitude. The latter is usually attributed to refractive interstellar scintillation (RISS; Rickett et al. 1984), whereby the variations in intensity are the result of wavefront distortions caused by transverse gradients in electron density. If RISS arises predominantly along the signal path through our Galaxy, we might expect to find evidence for a dependence on Galactic coordinates.

scholarly journals Low-Frequency Expansion Approach for Seismic Data Based on Compressed Sensing in Low SNR

2021 ◽  
Vol 11 (11) ◽  
pp. 5028
Author(s):  
Miaomiao Sun ◽  
Zhenchun Li ◽  
Yanli Liu ◽  
Jiao Wang ◽  
Yufei Su
Keyword(s):  
Reflection Coefficient ◽  
Compressed Sensing ◽  
Objective Function ◽  
Low Frequency ◽  
Original Data ◽  
Seismic Exploration ◽  
High Signal ◽  
Inversion Process ◽  
Frequency Information ◽  
Low Snr

Low-frequency information can reflect the basic trend of a formation, enhance the accuracy of velocity analysis and improve the imaging accuracy of deep structures in seismic exploration. However, the low-frequency information obtained by the conventional seismic acquisition method is seriously polluted by noise, which will be further lost in processing. Compressed sensing (CS) theory is used to exploit the sparsity of the reflection coefficient in the frequency domain to expand the low-frequency components reasonably, thus improving the data quality. However, the conventional CS method is greatly affected by noise, and the effective expansion of low-frequency information can only be realized in the case of a high signal-to-noise ratio (SNR). In this paper, well information is introduced into the objective function to constrain the inversion process of the estimated reflection coefficient, and then, the low-frequency component of the original data is expanded by extracting the low-frequency information of the reflection coefficient. It has been proved by model tests and actual data processing results that the objective function of estimating the reflection coefficient constrained by well logging data based on CS theory can improve the anti-noise interference ability of the inversion process and expand the low-frequency information well in the case of a low SNR.

scholarly journals Shift Detection in Hydrological Regimes and Pluriannual Low-Frequency Streamflow Forecasting Using the Hidden Markov Model

Water ◽  
2020 ◽  
Vol 12 (7) ◽  
pp. 2058 ◽  
Author(s):  
Larissa Rolim ◽  
Francisco de Souza Filho
Keyword(s):  
Time Series ◽  
Markov Model ◽  
Hidden Markov Model ◽  
Extreme Events ◽  
Hidden Markov ◽  
Low Frequency ◽  
Streamflow Forecasting ◽  
Hydrologic Time Series ◽  
Low Frequency Variability ◽  
The Impact

Improved water resource management relies on accurate analyses of the past dynamics of hydrological variables. The presence of low-frequency structures in hydrologic time series is an important feature. It can modify the probability of extreme events occurring in different time scales, which makes the risk associated with extreme events dynamic, changing from one decade to another. This article proposes a methodology capable of dynamically detecting and predicting low-frequency streamflow (16–32 years), which presented significance in the wavelet power spectrum. The Standardized Runoff Index (SRI), the Pruned Exact Linear Time (PELT) algorithm, the breaks for additive seasonal and trend (BFAST) method, and the hidden Markov model (HMM) were used to identify the shifts in low frequency. The HMM was also used to forecast the low frequency. As part of the results, the regime shifts detected by the BFAST approach are not entirely consistent with results from the other methods. A common shift occurs in the mid-1980s and can be attributed to the construction of the reservoir. Climate variability modulates the streamflow low-frequency variability, and anthropogenic activities and climate change can modify this modulation. The identification of shifts reveals the impact of low frequency in the streamflow time series, showing that the low-frequency variability conditions the flows of a given year.

scholarly journals A Chronology of El Niño Events from Primary Documentary Sources in Northern Peru*

2008 ◽  
Vol 21 (9) ◽  
pp. 1948-1962 ◽  
Author(s):  
R. Garcia-Herrera ◽  
D. Barriopedro ◽  
E. Hernández ◽  
H. F. Diaz ◽  
R. R. Garcia ◽  
...  
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Low Frequency ◽  
Primary Sources ◽  
Proxy Data ◽  
Documentary Sources ◽  
Low Frequency Variability ◽  
Northern Peru

Abstract The authors present a chronology of El Niño (EN) events based on documentary records from northern Peru. The chronology, which covers the period 1550–1900, is constructed mainly from primary sources from the city of Trujillo (Peru), the Archivo General de Indias in Seville (Spain), and the Archivo General de la Nación in Lima (Peru), supplemented by a reassessment of documentary evidence included in previously published literature. The archive in Trujillo has never been systematically evaluated for information related to the occurrence of El Niño–Southern Oscillation (ENSO). Abundant rainfall and river discharge correlate well with EN events in the area around Trujillo, which is very dry during most other years. Thus, rain and flooding descriptors, together with reports of failure of the local fishery, are the main indicators of EN occurrence that the authors have searched for in the documents. A total of 59 EN years are identified in this work. This chronology is compared with the two main previous documentary EN chronologies and with ENSO indicators derived from proxy data other than documentary sources. Overall, the seventeenth century appears to be the least active EN period, while the 1620s, 1720s, 1810s, and 1870s are the most active decades. The results herein reveal long-term fluctuations in warm ENSO activity that compare reasonably well with low-frequency variability deduced from other proxy data.

Dynamic Stability of a Water Brake Dynamometer

1998 ◽  
Vol 120 (1) ◽  
pp. 89-96 ◽  
Author(s):  
R. A. Van den Braembussche ◽  
H. Malys
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Lumped Parameter Model ◽  
Stable Operation ◽  
Pressure Oscillations ◽  
Lumped Parameter ◽  
Flow Oscillations ◽  
Brake Dynamometer ◽  
Low Torque ◽  
Frequency Variations

A lumped parameter model to predict the high frequency pressure oscillations observed in a water brake dynamometer is presented. It explains how the measured low frequency variations of the torque are a consequence of the variation in amplitude of the high frequency flow oscillations. Based on this model, geometrical modifications were defined, aiming to suppress the oscillations while maintaining mechanical integrity of the device. An experimental verification demonstrated the validity of the model and showed a very stable operation of the modified dynamometer even at very low torque.

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