On the reoccurrence of site specific response

The period and amplitude variations of local peaks in the Fourier amplitude spectra of free-field strong ground motion recorded at five stations in San Fernando Valley of metropolitan Los Angeles, California, are described, searching for peaks that reoccur during different earthquakes. The data suggest that some local peaks reoccur (about 50% of the time), during shaking by small local earthquakes (peak ground velocities, v_max<10–20 cm/s). During large strong motion amplitudes (v_max>20 cm/s), these peaks are shifted towards longer periods (by nonlinear response of soils) or disappear. The data also suggest that densification and settlement of soil, minutes and hours following the strong shaking may contribute towards fluctuations in the effective stiffness of the shallow surface layers.     

Morphology of NmF2 nighttime increases in the Eurasian sector

A statistical analysis of two peaks (pre-midnight and post-midnight) occurrence in NmF2 daily variations was made on a latitudinal chain of four ionosonde stations in the Eurasian longitudinal sector. Overall 6182 cases of the first and 5600 cases of the second peak occurrence were analyzed using all available foF2 observations for the years of solar maximum and minimum. Well-pronounced and systematic variations with season and solar activity were revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and equinoxes for midlatitude stations implying one and the same mechanism of their formation. The pre-midnight summer peak shows specific variations in particular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.     

中国东部夏季持续性降水日变化在淮河南北的差异分析

本文利用1961-2006年我国285站观测的逐时降水数据,分析了中国东部不同地区夏季平均降水日变化随降水持续时间的变化特征. 虽然整个东部地区都表现为短时降水峰值较一致地出现在下午17时左右,持续较长时间降水在清晨前后发生峰值降水,但持续性降水日变化的平均峰值时间以淮河为界存在显著南北差异. 北部地区的持续性降水峰值主要出现在02-06时前后;南部地区的持续性降水峰值时间出现在06-10时. 无论是降水强度或频次的日变化峰值的南北差异均较明显,但降水强度差异更为突出,且主要表现在持续性强降水中. 进一步分析发现：一方面北部地区持续性强降水开始时间较南部地区更早;另一方面,北部地区降水从开始到峰值经历的时间更短. 最后,对持续性强降水峰值时间南北差异的可能原因进行了初步讨论.     

A comparison of field-line resonances observed at the Goose Bay and Wick radars

Previous observations with the Goose Bay HF coherent-scatter radar have revealed structured spectral peaks at ultra-low frequencies. The frequencies of these spectral peaks have been demonstrated to be extremely consistent from day to day. The stability of these spectral peaks can be seen as evidence for the existence of global magneto spheric cavity modes whose resonant frequencies are independent of latitude. Fieldline resonances occur when successive harmonics of the eigenfrequency of the magnetospheric cavity or waveguide match either the first harmonic eigenfrequency of the geomagnetic field lines or higher harmonics of this frequency. Power spectra observed at the SABRE VHF coherent-scatter radar at Wick, Scotland, during night and early morning are revealed to show similarly clearly structured spectral peaks. These spectral peaks are the result of local field-line resonances due to Alfvén waves standing on magnetospheric field lines. A comparison of the spectra observed by the Goose Bay and Wick radars demonstrate that the frequencies of the field-line resonances are, on average, almost identical, despite the different latitudinal ranges covered by the two radars. Possible explanations for the similarity of the signatures on the two radar systems are discussed.     

Maximum Entropy Spectral Analysis of the geological time series of the oxygen isotope record from deep-sea cores

A Maximum Entropy Spectral Analysis of the geological time series of the oxygen isotope 18 for the last 782,000 years showed 98,000 years as the most prominent periodicity, followed by 40,000 years of less than half the amplitude and still smaller peaks at 24,000, 67,000, 84,000, 107,000 and 786,000 years. Among the astronomical parameters, only eccentricity showed a similar periodicity viz. 96,000 years. Obliquity had a prominent periodicity at 41,000 years which was reflected in the oxygen isotope series also. Corresponding to the bimodal precession peaks (23,000 and 19,000 years), the oxygen isotope series showed a peak at 24,000 years and smaller peaks at 22,000 and 19,000 years. Our results compare well with those obtained byThomson (1990) with a more sophisticated method.     

A note on the statistics of ordered peaks in stationary stochastic processes

Numerical simulation experiments have been carried out to evaluate the relative performance of three approximate formulations owing to Amini and Trifunac,[1] Gupta and Trifunac[2] and Basu et al.[3] for probability distributions of ordered peaks in stationary stochastic processes. The first two formulations are based on the assumption that the unordered peaks are statistically independent; whereas the formulation of Basu et al.[3] considers the dependence via Markov transition probability. In the formulation of Gupta and Trifunac,[2] the probability distribution of nth order peak is inherently conditioned by the fact that at most (n−1) peaks can occur with higher amplitudes and, thus, the ordered peaks are necessarily dependent. In this paper, ensembles of stationary time-histories are generated for three PSDFs representing a narrow-band, a broad-band and a band-limited white noise type of processes. Comparison of the results for the expected value and standard deviation for various orders of peaks, obtained by averaging over these ensembles, with the corresponding results obtained from the above mentioned three approximations, defined in terms of the moments of the PSDFs, has shown that the formulation of Gupta and Trifunac[2] describes the data well.     

Magnetic cloud in the solar wind: A comparison with the classical model

The solar wind magnetic field distribution near the Earth has been studied and compared with the distribution anticipated according to the classical model. It has been indicated that a two-hump distribution of the IMF values discovered previously is not an artifact of averaging but reflects the actual structure of the magnetic field within the sector. In this case the magnetic field of polarity corresponding to the leading spot in the Northern Hemisphere is encountered more frequently. Not only the magnetic field magnitude but also the fields of either polarity increase with increasing activity. The distance between the peaks on the histogram of the magnetic field near the Earth increases from 6 to 10 nT. The quasi-22-year, 11-year, and quasibiennial (2.6 ± 0.3 years) cycles are observed in an alternate increase in the peaks, in the strength of the fields of either polarity, and in the ratio of the peaks to the occurrence frequency of zero values, respectively. The classical model is violated in approximately 25% of cases.     

Changes in seasonality and timing of peak streamflow in snow and semi‐arid climates of the north‐central United States, 1910–2012

Changes in the seasonality and timing of annual peak streamflow in the north‐central USA are likely because of changes in precipitation and temperature regimes. A source of long‐term information about flood events across the study area is the U.S. Geological Survey peak streamflow database. However, one challenge of answering climate‐related questions with this dataset is that even in snowmelt‐dominated areas, it is a mixed population of snowmelt/spring rain generated peaks and summer/fall rain generated peaks. Therefore, a process was developed to divide the annual peaks into two populations, or seasons, snowmelt/spring, and summer/fall. The two series were then tested for the hypotheses that because of changes in precipitation regimes, the odds of summer/fall peaks have increased and, because of temperature changes, snowmelt/spring peaks happen earlier. Over climatologically and geographically similar regions in the north‐central USA, logistic regression was used to model the odds of getting a summer/fall peak. When controlling for antecedent wet and dry conditions and geographical differences, the odds of summer/fall peaks occurring have increased across the study area. With respect to timing within the seasons, trend analysis showed that in northern portions of the study region, snowmelt/spring peaks are occurring earlier. The timing of snowmelt/spring peaks in three regions in the northern part of the study area is earlier by 8.7– 14.3 days. These changes have implications for water interests, such as potential changes in lead‐time for flood forecasting or changes in the operation of flood‐control dams. Copyright © 2015 John Wiley & Sons, Ltd.     

The role of wind in generating turbidity maxima in the Tay Estuary

Variations of wind direction and strength in the Tay Estuary control wave generation and the resultant patterns of suspended sediment concentration in the waters over the extensive tidal flats. Bodies of water with high water concentration peaks advect to occupy sites at which turbidity maxima are normally present at low water. Other low water peaks are generated by ebb tidal current resuspension.     

Modelling impacts of agricultural practice on flood peaks in upland catchments: An application of the distributed TOPMODEL

Upland agricultural land management activities such as grazing, vegetation burning, and bare ground restoration impact hydrological elements of headwater catchments, many of which may be important for downstream flood peaks (e.g., overland flow and soil water storage). However, there is poor understanding of how these management practices affect river flow peaks during high magnitude rainfall events. Using the distributed TOPMODEL, spatial configurations of land management were modelled to predict flood response in an upland catchment, which contains different regions operating subsidized agricultural stewardship schemes. Heavy grazing leading to soil compaction and loss of vegetation cover in stewardship regions covering 79.8% of the catchment gave a 42‐min earlier flow peak, which was 82.2% higher (under a 1‐hr 15‐mm storm) than the current simulated hydrograph. Light grazing over the same regions of the catchment had much less influence on river flow peaks (18 min earlier and 32.9% increase). Rotational burning (covering 8.8% of the catchment), most of which is located in the headwater areas, increased the peak by 3.2% in the same rainfall event. Vegetation restoration with either Eriophorum or Sphagnum (higher density) in bare areas (5.8%) of the catchment provided a reduction of flood peak (3.9% and 5.2% in the 15‐mm storm event), whereas the same total area revegetated with Sphagnum in riparian regions delivered a much larger decrease (15.0%) in river flow peaks. We show that changes of vegetation cover in highly sensitive areas (e.g., near‐stream zones) generate large impacts on flood peaks. Thus, it is possible to design spatially distributed management systems for upland catchments, which reduce flood peaks while at the same time ensuring economic viability for upland farmers.     

Increase in the critical frequency of the ionospheric <Emphasis Type="Italic">F</Emphasis> region prior to the substorm expansion phase

Specific variations in the critical frequency of the ionospheric F ₂ layer during magnetospheric substorms have been found based on the data of vertical sounding stations in Europe and North America. Maximal attention has been paid to the positive peaks of ΔfoF2 with a duration of 6–8 h before the beginning of the substorm expansion phase (T ₀). The possible physical mechanisms by which these peaks are formed (related to the impact of fast particles in the foreshock region of the solar wind on the Earth’s magnetosphere and different for middle and high latitudes) have been considered. The positive peaks of ΔfoF2 can be used in a short-term prediction of the ionospheric disturbance onset and space weather on the whole.     

Least squares self-coherence for sub-nGal signal detection in the superconducting gravimeter records

In this paper, we use superconducting gravimeter (SG) data recorded at three stations of the global geodynamics project (GGP) network, with good geographical distribution, to search for possible significant peaks in the gravity spectrum that are in the assumed period range of the Slichter triplet. Seven-year long series from Cantley (Canada), and four-year long series from both Canberra (Australia) and Moxa (Germany) stations are used. First, a solid Earth and ocean tide model is subtracted from the data, followed by a local atmospheric pressure correction based on a frequency-, and location-dependent admittance estimated by the least squares response method. Subsequently, the residual series are filtered with a Parzen-based bandpass filter with a passband (12 h–78 s). A sub-nGal detection level is confirmed by injecting an artificial sine wave of different amplitudes. The Least Squares Self-Coherency spectrum shows the existence of many apparently statistically significant peaks at the 95% confidence level in the band (3–8 h). Although a few peaks are close to the claimed Slichter periods in previous research, the large number of candidate peaks may be related to other mechanisms such as global pressure variations, or hydrology.     

Spectral Reflectance Analysis of the Caribbean Sea

Reflectance spectral curves were analysed by a derivative method. Derivative reflectance spectra revealed concealed peaks of both reflectance and absorption curves of Caribbean seawater and elements contained within it. Reflectance curves showed a predominant blue colour (400–500 nm) characteristic of Caribbean oligotrophic waters, conspicuous peaks result from the optical properties of chlorophyll a and seawater. Reflectance curves had a similar spectral response. This paper analyses reflectance spectra of surface seawater at 31 stations in the Caribbean Sea during the summer of 2001.     

A Review of the Symposium on Distribution of Precipitation in Mountainous Areas

Abstract

A technique for determining baseflow adjustments to synthesized peaks produced from the US Geological Survey's rainfall-runoff model is presented. The antecedent precipitation index (API) is determined for each storm used in the calibration of a rainfall-runoff station. The antecedent precipitation index is related to the baseflow by a best fit straight line which can be represented by an equation. This equation is used to determine baseflow adjustments when using longterm rainfall data to synthesize peaks.     

On the shot-noise streamflow model and its applications

In this paper we consider the shot-noise model of streamflow series. We show how design discharge can be obtained by the stochastic intensity of thinned Poisson processes describing the peaks over a threshold. The main result concerns the stationary distribution of peaks. We derive an explicit expression for this limit distribution in terms of its Laplace transform. Approximation formulas are developed making use of the saddle point method for the asymptotic evaluation of contour integrals and the Post-Widder formula for inversion of Laplace transforms. We illustrate this methods on the case of Gamma-distributed shots. The stationary peak distribution is used to approximate the maximum value distribution for larger time intervals.     

Prediction of PM10 and SO2 exceedances to control air pollution in the Bay of Algeciras,Spain

In this paper, the authors apply different classification techniques in order to provide 24 h advance forecasts of the daily peaks of SO₂ and PM10 concentrations in the Bay of Algeciras. K-nearest-neighbours, multilayer neural network with backpropagation and support vector machines (SVMs) are the classification methods used. The aim of this research is to obtain a suitable prediction model that would enable us to predict the peaks of pollutant concentrations in critical meteorological situations caused by the widespread existing industry and population in the area. A resampling strategy with twofold crossvalidation has been applied, using different quality indexes to evaluate the performance of the prediction models. SVM models achieved better true positive rate and accuracy (ACC) quality indexes. Results of ACC index value of 0.795 for PM10 and 0.755 for SO₂ showed the ability of the model to predict peaks and non-peaks correctly.     

Using observations from the GPS and TOPEX satellites to investigate night-time TEC enhancements at mid-latitudes in the southern hemisphere during a low sunspot number period

The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating the total electron content (TEC) obtained by a dual-frequency group path and phase path GPS technique. For the low sunspot number time period of March 1995–February 1996, one week of data centred on the Priority Regular World Day for each month have been used to investigate night-time mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the night-time peaks. Although night-time TEC enhancements have been observed previously, there is no general agreement on their origin. From the results of the present study, the development of midnight TEC enhancements coincided with the low latitude processes occurring at around the time of vertical E×B drift velocity reversal. The TOPEX results confirmed that the upward E×B drift velocity reversal and the downward plasma flow from greater heights producing the night-time peaks at mid latitudes are triggered from a common source: the westward electric field.     

The response in the Pacific to the sun's decadal peaks and contrasts to cold events in the Southern Oscillation

van Loon et al. [2007. Coupled air–sea response to solar forcing in the Pacific region during northern winter. Journal of Geophysical Research 112, D02108, doi:10.1029/2006JD007378] showed that the Pacific Ocean in northern winter is sensitive to the influence of the sun in its decadal peaks. We extend this study by three solar peaks to a total of 14, examine the response in the stratosphere, and contrast the response to solar forcing to that of cold events (CEs) in the Southern Oscillation. The addition of three solar peak years confirms the earlier results. That is, in solar peak years the sea level pressure (SLP) is, on average, above normal in the Gulf of Alaska and south of the equator, stronger southeast trades blow across the Pacific equator and cause increased upwelling and thus anomalously lower sea surface temperatures (SSTs). Since the effect on the Pacific climate system of solar forcing resembles CEs in the Southern Oscillation, we compare the two and note that, even though their patterns appear similar in some ways, they are particularly different in the stratosphere and are thus due to separate processes. That is, in July–August (JA) of the year leading into January–February (JF) of the solar peak years, the Walker cell expands in the Pacific troposphere, and the stratospheric wind anomalies are westerly below 25 hPa and easterly above, whereas this signal in the stratosphere is absent in CEs. Thus the large-scale east–west tropical atmospheric (Walker) circulation is enhanced, though not to the extent that it is in CEs in the Southern Oscillation, and the solar influence thus appears as a strengthening of the climatological mean regional precipitation maxima in the tropical Pacific. Additionally, CEs have a 1-year evolution, while the response to solar peaks extends across 3 years such that the signal in the Pacific SLP of the solar peaks is similar but weaker in the year leading into the peak and in the year after the peak. The concurrent negative SST anomalies develop during the year before the solar peak, and after the peak the anomalies are still present but are waning. In the stratosphere in solar peaks, the equatorial quasi-biennial oscillation (QBO) is amplified when it is in its westerly phase in the lower stratosphere and easterly phase above; and the QBO is suppressed when in its easterly phase below–westerly phase above. Such an association is not evident in CEs.     

耀斑的短期生物效应

为研究太阳耀斑的短期生物效应,统计分析了198-199年中16万婴儿出生日与光学耀斑的关系. 结果表明,婴儿出生数的变化与耀斑辐射的能量,以及能量到达地球的时间有关. 太阳活动较平稳年白天爆发的各级耀斑的当天都对应着一个平均出生数高峰,而且耀斑的亮度和面积越大,对应的出生峰值越大. 2B耀斑当天平均出生数超过前后21d总均值的13.6髎,具有p＜0.001的统计高度显著性水平. 这些出生高峰与耀斑的电磁和高能辐射有关. 2B、2N和1B三类耀斑后的第2-4d还有一个出生峰,它们与耀斑引发的地球物理变化的时间符合.     

Relationship between the aerosol concentration in the stratosphere and ionization according to the data on conductivity and nitrate content of Greenland ice

This paper studies the data on the conductivity and concentration of nitrates (ions) in an ice core from central Greenland obtained with high time resolution. The performed analysis indicates that the abrupt increase in the concentration of sulfate aerosols in the stratosphere due to additional ionization resulted from precipitation of solar cosmic ray energetic particles is one of the main factors that cause simultaneous origination of powerful peaks in the nitrate conductivity and concentration. Thus, coincidences of peaks in both studied paleoseries are manifestations of the effect that has been experimentally registered with lidar and satellite equipment for the last 25 years. This demonstrates that the relationship between the aerosol concentration and the ionization rate in the stratosphere is real and makes it possible to expand the interval where this relationship exists to more than 200 years. The possible physical mechanisms of the observed phenomenon are discussed.