首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Summary Atmospheric total ozone contents over three stations in north India have been studied. A power spectrum analysis has been made of daily values recoreded at these stations during the winter season. Three types of periodicities have been observed in the available records, namely, oscillations with a period of (a) 2.5–3.5 days, (b) 4.0–5.3 days and (c) 8.0–9.6 days. The first and third type of oscillations were also observed when the data were extended to cover an entire year, instead of the winter season alone. A possible mechanism for the occurrence of these periodicities is discussed.  相似文献   

2.
The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion field dominated by quasi-monochromatic gravity waves with representative apparent periods of 30–40 min, amplitudes of up to 2.5 m s–1 and large vertical wavelength. In some instances waves appear to be ducted. Vertical profiles of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance profiles evaluated for consecutive days of recording show that the general shape of the variance profiles persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 m2s–2 to 6.5 m2s–2 and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of -1.36 ± 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.  相似文献   

3.
Summary Ozone observations made during 1964 and 1965 at nine Mediterranean, central and southeast European stations (latitudes 38–52°N, longitudes 9–23°E) reveal patterns of seasonal and shorter time-variations in total ozone as well as in vertical ozone distribution. During the winter-spring season, a significant increase (20%) of ozone occurs essentially simultaneously with the spring stratospheric warming, and is noticed at all stations.—Autocorrelation coefficients show that the total ozone on any day is strongly related to the total ozone of the preceding four days in summer or one or two days in winter-spring or autumn. Changes of total ozone in southeast Europe correlate closely with those in Mediterranean Europe, and less closely with those from north central Europe.—Power spectrum analysis detects the dependence of ozone changes on processes with periods longer than 6–8 days, and indicates a significant oscillation with a period of 14–15 days, perhaps a result of the direct influence of lower stratospheric circumhemispheric circulation. — Reliable vertical ozone soundings were not available from all stations. The mean vertical profiles at Arosa, Switzerland (47°N) and Belsk, Poland (51°) are very similar. More than 60% of the variability of the total ozone is contributed by changes in ozone concentration between 10 and 24 km; less than 10% is due to variations above 33 km. Changes in ozone partial pressure at different altitudes, and relationships of those changes to total ozone, indicates that a mean vertical ozone distribution may be described adequately by considering the ozone changes in four layers: a) the troposphere, b) the lower stratosphere up to 24 km, c) a transition layer from 24 km to a variable upper border at 33–37 km, and d) the layer above 33–37 km.Part of this paper was presented at the Ozone Seminar in Potsdam, Germany, 27 September 1966.  相似文献   

4.
To realistically assess the seismic risk relating to built infrastructures in Hong Kong and in the neighbouring coastal cities of southern Guangdong province, it is necessary to predict ground shaking induced by different earthquake scenarios with good accuracy. A companion paper has described the modelling of the spatial and temporal distribution of the diffused seismic activities in the region, based on the newly-developed ‘Expanding Circular Disc’ (ECD) method. Representative Magnitude–Distance (M–R) combinations for both near-field and far-field earthquakes (in relation to Hong Kong) have been derived using the ECD method. The present paper describes the modelling of the response spectrum on rock sites associated with the predicted M–R combinations, using the Component Attenuation Model (CAM) that was also developed recently by the authors, based on stochastic simulations of the seismological model. The significant effects of soil resonance on the response spectrum are described in a separate publication.The accuracy of CAM in modelling ground motion properties on rock sites has been tested here by comparisons with (i) strong motions recorded in Taiwan and South China from the 1999 ‘Chi-Chi’ earthquake in Taiwan (M=7.6), (ii) motions recorded in South China from another earthquake occurring in the southern Taiwan Strait in the same year (M=5.1), and (iii) historical seismic intensity data obtained within South China. The overall capability of CAM in modelling both near-field and far-field attenuation has been shown to be unmatched by existing empirical models. Results of the comparison studies confirm the accuracy of CAM, particularly within an epicentral distance of 300–400 km.This study shows that the developed serviceability response spectra (i.e. at short return periods) are controlled mainly by the earthquake recurrence behaviour of major distant seismic sources. In contrast, the ultimate response spectra (i.e. at long return periods) relate to events with magnitudes close to the maximum credible earthquake (MCE) limit, the effect of which may also be represented by the Characteristic Response Spectrum (CRS). Both types of earthquake scenario can be significantly affected by the regional crustal properties. The proposed response spectrum envelopes have been compared with previously developed recommendations, and a critical review has been conducted. The intrinsic advantages of the ECD–CAM modelling approach have been highlighted, emphasising its directness and transparency when compared with the more complex process required to implement traditional Probabilistic Seismic Hazard Assessment (PSHA).  相似文献   

5.
Summary Characteristic features of long-periodic oscillations in the general atmospheric circulation may be deduced from the correlation existing between the monthly, the three-monthly, the six-monthly and the twelve-monthly figures of the barometric pressure at two distant stations and from the autocorrelation of the same figures at each station.It is shown that the monthly figures of the barometric pressure at Easter Island and those at Djakarta during the six years 1950–1955 have, when plotted as a function of time, one oscillation in common, whereas further these figures have both their own rhythms and their own noise. The period of the common oscillation is 28.6 months, while the phase difference between both stations amounts to 180°. The r.m.s. value of the noise is 0.76 times the value of the amplitude of the oscillation. The rhythms differ in wave length, amplitude and phase.It is also shown that the monthly figures of the barometric pressure at Ponta Delgada and those at Stykkisholm have, when plotted as a function of time, a whole spectrum of oscillations jointly, while all oscillations have the same phase difference (238°) between both stations. These features are deduced from observations during 40 years (1896–1915 and 1921–1940). If yearly figures are used instead of monthly ones, then the greater part of the spectrum of oscillations is smoothed away and only a few common oscillations are left. The first common oscillation has a wave length of 26.5 months and the second one has a wave length of 38.7 months.  相似文献   

6.
Summary The technique for evaluating the natural illumination of direct solar radiation introduced byÅngström andDrummond [1]2) has been applied by the authors to the pyrheliometric observations in India and the values of natural illumination derived at eight representative stations are presented in this paper. The dirunal and seasonal variations of the illumination fluxes have been discussed. —In general, illumination fluxes in India show a maximum during summer season except at Madras, where the maximum occurs during winter. The illumination fluxes at various stations vary within the range 80–110 kilolux throughout the year. However, at Calcutta, it varies within a rather narrow range of 65–85 kilolux. Both in summer and winter, the illumination flux during forenoon hours is usually more than that during afternoon hours. An increase in daylight illumination is observed with height of the station above sea level.  相似文献   

7.
We use the dense Israel Seismic Network (ISN) to discriminate between low magnitude earthquakes and explosions in the Middle East region. This issue is important for CTBT monitoring, especially when considering small nuclear tests which may be conducted under evasive conditions. We explore the performance of efficient discriminants based on spectral features of seismograms using waveforms of 50 earthquakes and 114 quarry and underwater blasts with magnitudes 1.0–2.8, recorded by ISN short-period stations at distances up to 200 km. The single-station spectral ratio of the low and high-frequency seismic energy shows an overlap between explosions and earthquakes. After averaging over a subnet of stations, the resolving power is enhanced and the two classes of events are separated. Different frequency bands were tested; the (1–3 Hz)/(6–8 Hz) ratio provided the best discriminant performance. We also estimated normalized r.m.s. spectral amplitudes in several sequential equal frequency windows within the 1–12 Hz band and applied multiparametric automatic classification procedures (Linear Discrimination Function and Artificial Neural Network) to the amplitudes averaged over a subnetwork. A leave-one-out test showed a low rate of error for the multiparametric procedures. An innovative multi-station discriminant is proposed, based on spectral modulation associated with ripple-firing in quarry blasts and with the bubbling effect in underwater explosions. It utilizes a distinct azimuth-invariant coherency of spectral shapes for different stations in the frequency range (1–12 Hz). The coherency is measured by semblance statistics commonly used in seismic prospecting for phase correlation in the time domain. After modification, the statistics applied to the network spectra provided event separation. A new feature of all the above mentioned procedures is that they are based on smoothed (0.5 Hz window), instrument-corrected FFT spectra of the whole signal; they are robust to the accuracy of onset time estimation and, thus well suited to automatic event identification.  相似文献   

8.
Summary Measurements of the total incoming solar radiation on a horizontal surface have been recorded since 1967 at a network of actinometric stations in Iraq. The instrument used at each of these stations is a bimetallic actinograph of the Robitzsch type. The stations at Mosul, Baghdad and Nasiriya have been selected, to represent respectively the northern, central and southern climatic zones of Iraq.This paper examines from the available records at these stations, until August 1973, the differnt aspects of radiation climatology which are important in solar energy utilizations. The mean annual, monthly and daily radiation; the number of days in which the radiation has exceeded certain given limits and the periods of successive days having radiation less than 100 and 250 cal cm–2 day–1 have been studied and presented in tabular and graphical forms suitable for direct application.To architects and engineers interested in the availability of solar radiation on vertical and inclined surfaces, the hourly radiation values for Baghdad have been studied for the different months of the year and the corresponding isopleths on the horizontal have been drawn.  相似文献   

9.
Ground temperature, pressure and wind speed monthly averages in the area of the Italian Station at Terra Nova Bay, Antarctica, were analyzed for the period 1987–1991 by means of a network of nine AWS (automatic weather stations). Spatial configurations of temperature show a well-defined, relatively warm island in the area of Terra Nova Bay, between Drygalsky and Campbell ice tongues, throughout the year. A second warm island is present to the north along the coast, between Aviator and Mariner ice tongues, for most of the year. From February to March a rapid drop in temperature is observed at all stations. A strong thermal gradient develops during February, March, April and October, November, December, between the coastal region and inner highlands. The baric configuration follows the elevation of the area. Annual average pressure and temperature as functions of stations altitude show linear trends. Severe katabatic wind episodes are recorded at all stations, with wind speed exceeding 25 ms–1 and direction following the orographic features of the inner areas. Co-occurrences of these episodes were observed for stations located along stream lines of cold air drainage. The autocorrelation function of maximum wind speed time series shows wind persistence of 2–3 days and wind periodicity of about one week.  相似文献   

10.
N-type earthquakes sometimes occur before the eruption of andesitic volcanoes, but their source mechanism has not been understood well. Their waveforms have stationary periods and decay slowly resembling to damped oscillation. They have common characteristics of spectra among the different stations; these spectral peaks appear at almost equal intervals of 1.0 Hz with common sharpness. We made detailed analysis on the N-type earthquakes observed at Asama volcano in July 1995. During 10 days activity, the duration of each earthquake increased gradually from 40 s to 3 min, while the peak frequency decreased from 2.7 to 0.8 Hz. Hypocenters are distributed slightly west under the summit crater with 0.4–1.7 km above sea-level. Source mechanism of the N-type earthquake is determined using a waveform inversion technique. Synthetic waveforms are calculated using 2D finite difference method. Surface topography is included in the calculation to incorporate the case that the source region is higher than the stations. Since the optimum solution has a large volumetric component, we approximated the volumetric part as a volume change of fluid filled sphere or cylinder or plane crack, and decomposed the solution into volumetric and non-volumetric part. Consequently, it is revealed that the optimum solution can be expressed as “crack expansion (contraction)+small fault slip”, because the spectral peak distribution advocates the crack expansion model among these three candidates. The frequency change during this activity may be explained by the time variation of the sound speed of inner fluid from 300 to 100 m/s. The change of the void fraction of water–steam phase might have caused the decrease of the sound speed.  相似文献   

11.
The series of daily Ap-indices has been subdivided into pentades (1932–1936 etc.) and spectra with fine-frequency resolution have been calculated for the indices in each of these intervals. Daily sunspot numbers have been processed in the same way. The average spectrum from all spectra in the pentades, as well as the spectrum from the whole interval have been calculated, and significant peaks have been determined. There is a significant difference between the spectra in the pentades containing the solar activity minimum (1932–1936, 1942–1946 etc.) and those containing the solar activity maximum (1937–1941, 1947–1951 etc.). Most peaks can be interpreted as a response to solar rotation and to the structure of solar wind speed (two high-speed streams per solar rotation), both modulated by the 11-year, annual and semi-annual waves. No significant peak corresponding to the period of the synodic month, or its half has been found. This result suggests that the influence of lunar cycles on some natural phenomena (if any) is not mediated by geomagnetic activity.  相似文献   

12.
The systematic time differences observed in the onset of postsunset VHF scintillations recorded simultaneously at Ujjain (Geogr. lat. 23.2○N, Geogr. long. 75.6○E) and Bhopal (Geogr. lat. 23.2○N, Geogr. long. 77.6○E), situated at the peak of the anomaly crest in the Indian region, have been analysed to determine the zonal drifts of scintillation-producing irregularities. The method is based on the assumption that the horizontal movement of irregularities does not change while crossing the F-region cross-over points of these stations. The calculated velocities of irregularities indicate an eastward drift decreasing from about 180 m s−1 to 55 m s−1 during the course of night. In the premidnight period, the drifts are reduced under the magnetically disturbed conditions. The average east-west extension of irregularites is found to be in the range of 200–500 km.  相似文献   

13.
In this site response study we examined local earthquakes recorded at surface stations of a local seismic network and at a temporary underground seismic array installed in a tunnel underneath the Gran Sasso Massif in Abruzzo (central Italy). This allowed us to compare the seismic site response beneath the mountain and on the surface in similar geological environment (soft rock sites). We applied spectral ratios method on different segments of the seismograms and used different reference spectra in the 1–20 Hz frequency band. We found little or no amplification effects at most of the surface stations whereas site transfer functions evaluated with respect to underground sites show an amplification factor up to 6 in the 1–8 Hz frequency range. Coda spectral ratios estimated at soft rock sites are confirmed as good estimates of shear wave transfer function.  相似文献   

14.
Wavelet and cross-wavelet analysis are used to identify and describe spatial and temporal variability in Canadian seasonal precipitation, and to gain further insights into the dynamical relationship between the seasonal precipitation and the dominant modes of climate variability in the Northern Hemisphere. Results from applying continuous wavelet transform to seasonal precipitation series from 201 stations selected from Environment Canada Meteorological Network reveal striking climate-related features before and after the 1940s. The span of available observations, 1900–2000, allows for depicting variance and covariance for periods up to 12 years. Scale-averaged wavelet power spectra are used to simultaneously assess the temporal and spatial variability in each set of 201 seasonal precipitation time series. The most striking feature, in the 2–3-year period and in the 3–6-year period—the 6–12-year period is dominated by white noise and is not considered further—is a net distinction between the timing and intensity of the temporal variability in autumn, winter and spring–summer precipitation. It is found that the autumn season exhibits the most intense activity (or variance) in both the 2–3 year and the 3–6 year periods. The winter season corresponds to the least intense activity for the 2–3 year period, but it exhibits more activity than the spring–summer for the 3–6 year period.Cross-wavelet analysis is provided between the seasonal precipitation and four selected climatic indices: the Pacific North America (PNA), the North Atlantic Oscillation (NAO), the Northern Hemisphere Annular Mode (NAM) originally called the Arctic Oscillation, and the sea surface temperature series over the Niño-3 region (ENSO). The wavelet cross-spectra revealed coherent space–time variability of the climate–precipitation relationship throughout Canada. It is shown that strong climate/precipitation activity (or covariance) in the 2–6 year period starts after 1940 whatever the climatic index and the season. Prior to year 1940, only local and weaker 2–6 year activity is revealed in western Canada essentially in winter and autumn, but overall a non-significant precipitation/climate relationship is observed prior to 1940. Correlation analysis in the 2–6 year band between the seasonal precipitation and the selected climatic indices revealed strong positive correlations with the ENSO, the NAO, and the NAM in eastern and western Canada for the post-1940 period. For the period prior to 1940, the correlation tend be negative for all the indices whatever the region. A particular feature in the correlation analysis results is the consistently stronger and positive NAM–precipitation correlations in all the regions since 1940. The cross-wavelet spectra and the correlation analysis in the 2–6 year band suggest the presence of a change point around 1940 in Canadian seasonal precipitation—that is found to be more likely related to NAM dynamics.  相似文献   

15.
Summary Eleven STD stations by lowering and raising the sensor were occupied about 170 n. miles northeast of Cape Hatteras in June, 1968. The stations were located in the slope water region covered by the upper warm water from the Gulf Stream. Power spectra of temperature and salinity fluctuations at 1-meter depth intervals were computed versus vertical wave numbers for the upper layer (5–320 m) and lower layer (320–1000 m) at each station. The power law coefficients of the spectra about the vertical wave number are between –5/3 and –3. These coefficients indicate that the temperature and salinity fluctuations are influenced by stratification as well as by turbulence.  相似文献   

16.
This paper presents a comparison between different techniques for evaluation of predominant periods in soft soil, for the urban area of Pereira city, Western Colombia. In this study we used microtremor and strong ground motion records obtained by a local array of seven accelerographs stations deployed in the city. Response spectra and spectral ratios have been calculated and compared with strong seismic events recorded in solid rock and soft soil stations. These observations allowed the determination of dominant response spectra for several sectors in the urban area. For the microtremor measurements and earthquake data, dominant periods were determined using interpretation of Fourier amplitude spectra and Nakamura's technique. A comparison between dominant periods obtained from strong ground motion records and those obtained from microtremor measurements show similarities, which is in the range 0.2–0.5 s. A preliminary version of a site response map for Pereira city was obtained from this analysis.  相似文献   

17.
On the basis of MEM spectrum analysis, the main planetary scale fluctuations formed in the lower ionosphere are studied over a period of 3–25 days during the CRISTA campaign (October-November 1994). Three dominant period bands are found: 3–5, 6–8 and 15–23 (mainly 16–18) days. For 7–8 and 16–18 day fluctuations, propagation was eastward with wave numbers K = 3 and K = 1, respectively. The magnitude of planetary wave activity in the mid-latitudes of the Northern Hemisphere during the CRISTA campaign seems to be fairly consistent with the expected undisturbed normal/climatological state of the atmosphere at altitudes of 80–100 km.  相似文献   

18.
The first phase (1997–2003) of the Global Geodynamics Project (GGP) has now been completed. Data from superconducting gravimeters (SGs) within GGP have shown great capabilities in a wide spectrum of geophysical applications from the tidal studies to the long-period seismology. Here, we compare the noise levels of the different contributing stations over the whole spectrum. We use three different processing procedures to evaluate the combined instrument-plus-site noise in the long-period seismic band (200–600 s), in the sub-seismic band (1–6 h) and in the tidal bands (12–24 h). The analysis in the seismic band has demonstrated that SGs are particularly well suited for the studies of the long-period normal modes and thus are complementary to long-period seismometers. In the sub-seismic band, the power spectral densities, computed over a period of 15 continuous days for every GGP station, cross the New Low Noise Model of Peterson from T = 16 min to T = 4.6 h. SG data are therefore appropriate for studying long-period seismic and sub-seismic modes. In the tidal bands, the noise comparison is realised by a least-squares fit to tides, local air pressure and instrumental drift, leading to gravity residuals where we estimate a standard deviation and average noise levels in different tidal frequency bands. Tidal gravity observations using SGs have also shown to be an independent validation tool of ocean tidal models, and they are therefore complementary to tide gauge and altimetric data sets. Knowledge of the noise levels at each station is important in a number of studies that combine the data to determine global Earth parameters. We illustrate it with the stacking of the data in the search for the gravity variations associated with the sub-seismic translational motions of the inner core, the so-called Slichter triplet.  相似文献   

19.
Study of sporadic-E clouds by backscatter radar   总被引：1，自引：0，他引：1  
It is shown that swept-frequency backscatter ionograms covering a range of azimuths can be used to study the dynamics of sporadic-E clouds. A simple technique based on analytic ray tracing can be used to simulate the observed narrow traces associated with Es patches. This enables the location and extent of the sporadic-E clouds to be determined. The motion of clouds can then be determined from a time sequence of records. In order to demonstrate the method, results are presented from an initial study of 5 days of backscatter ionograms from the Jindalee Stage B data base obtained during March-April 1990. Usually 2–3 clouds were observed each day, mainly during the evening and up to midnight. The clouds lasted from 1–4 h and extended between 30°–80° in azimuth and 150–800 km in range. The clouds were mostly stationary or drifted generally westward with velocities of up to 80 ms–1. Only one cloud was observed moving eastward.  相似文献   

20.
We document the detailed dynamics of the dayside aurora in the ≈1200–1600 MLT sector in response to a sharp southward turning of the interplanetary magnetic field (IMF) under negative IMF By conditions. Features not documented in previous work are elucidated by using two meridan scanning photometers (separated by 2 h) and an all-sky auroral imager in Ny Ålesund, Svalbard (75.5^MLAT) in combination with magnetograms from stations on Svalbard, covering the latitude range 71^–75^MLAT. The initial auroral response may be divided into three phases consisting of: (1) intensification of both the red (630.0 nm) and green (557.7 nm) line emissions in the cusp aurora near 1200 MLT and ≈100 km equatorward shift of its equatorward boundary, at ≈75^MLAT, (2) eastward and poleward expansions of the cusp aurora, reaching the 1430 MLT meridian after 5–6 min, and (3) east-west expansion of the higher-latitude aurora (at ≈77^–78^MLAT) in the postnoon sector. The associated magnetic disturbance is characterized by an initial positive deflection of the X-component at stations located 100–400 km south of the aurora, corresponding to enhanced Sunward return flow associated with the merging convection cell in the post-noon sector. The sequence of partly overlapping poleward moving auroral forms (PMAFs) during the first 15 min, accompanied by corresponding pulsations in the convection current, was followed by a strong westward contraction of the cusp aurora when the ground magnetograms indicated a temporary return to the pre-onset level. These observations are discussed in relation to the Cowley-Lockwood model of ionospheric response to pulsed magnetopause reconnection.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号