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1.
Horizontal winds in the mesosphere and lower thermosphere over the Antarctic have been measured by a meteor radar at Rothera (67.5°S, 68.0°W) and MF radar at Davis (68.6°S, 78.0°E). Data from Rothera recorded over a 20-month interval in 2005–2006 and data from Davis recorded over the 13-year interval 1994–2006 are examined to investigate the monthly mean behaviour of the lunar semidiurnal tide. Both data sets show a clear signal of the 12.42-h lunar semidiurnal (M2) tide. The amplitude reaches values as large as 8 m s−1. The vertical wavelengths of the tide vary seasonally from 10 to 65 km. Comparisons of the phase of the tide measured over the two sites reveals that it does not purely consist of a migrating wavenumber 2 mode. This suggests that other, non-migrating, modes are likely to be present.  相似文献   

2.
《Journal of Atmospheric and Solar》2002,64(12-14):1455-1463
In this paper we examine the relationship between the tidal characteristics as observed by the MF radar at Tirunelveli (8.7°N, 77.8°E, geographic; 0.3°N magnetic dip) and the occurrence of afternoon counter electrojet (ACEJ) events. A reduction in the diurnal tide activity and/or an enhancement of semi-diurnal tide amplitude is observed on many of the ACEJ days. A clear anticorrelation is seen between the afternoon electrojet strength and the amplitude of the semi-diurnal tide in the solstitial months of June and July, 1995. The results presented herein provide observational support to the earlier numerical models that consider the interplay of various tidal modes in the evolution of equatorial ionospheric current system whose extreme manifestation in ground-geomagnetic field variations being the reversal of the electrojet in the afternoon hours.  相似文献   

3.
Twelve years of horizontal wind data from the Scott Base MF radar and the Halley SuperDARN radar recorded between January 1996 and December 2007 are analysed to study the interannual variability of the migrating (S=2) and non-migrating (S=1) components of the semidiurnal tide around 78°S in the Antarctic upper mesosphere. Significant quasi-biennial modulation of the summer time S=1 component is observed. During early summer the amplitude of the component is up to 4 ms?1 stronger during the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) measured at 30 hPa. No statistically significant effect is seen in amplitude of the migrating component of the tide, or in the phase (time of maximum) of either component. These results are discussed in the light of previous observations of the interannual variability of the semidiurnal tide.  相似文献   

4.
Seasonal variations in the auroral E-region neutral wind for different solar activity periods are studied. This work is based on neutral wind data obtained over 56 days between 95–119 km altitude under geomagnetic quiet conditions (Ap<16) during one solar cycle by the European Incoherent Scatter radar located in northern Scandinavia. In general, the meridional mean wind shifts northward, and the zonal mean wind increases in eastward amplitude from winter to summer. The zonal mean wind blows eastward in the middle and lower E-region for each season and for each solar condition except for the equinox, where the zonal mean wind blows westward at and below 104 km. Solar activity dependence of the mean wind exists during the winter and equinox seasons, while in summer it is less prominent. Under high solar activity conditions, the altitude profiles of the horizontal mean winds in winter and the equinoxes tend to resemble those in summer. The horizontal diurnal tide is less sensitive to solar activity except during summer when the meridional amplitude increases by ∼10 m s−1 and the corresponding phase shifts to a later time period (1–2 h) during high solar activity. Seasonal dependence of the semidiurnal tide is complex, but is found to vary with solar activity. Under low solar activity conditions the horizontal semidiurnal amplitude shows seasonal dependence except at upper E-region heights, while under high solar activity conditions it becomes less sensitive to seasonal effects (except for the meridional component above 107 km). Comparisons of mean winds with LF and UARS observations are made, and the driving forces for the horizontal mean winds are discussed for various conditions.  相似文献   

5.
Planetary waves with periods between two and four days in the middle atmosphere over Antarctica are characterized using one year of data from the medium-frequency spaced antenna (MFSA) radars at Scott Base, Rothera, and Davis. In order to investigate the origin of the observed waves, the ground-based data are complemented by temperature measurements from the Earth Observing System Microwave Limb Sounder (EOS MLS) instrument on the Aura satellite as well as wind velocity data from the United Kingdom Met. Office (UKMO) stratospheric assimilation. Observed characteristics of waves with a period of approximately two days in summer are consistent with the quasi-two-day wave (QTDW) generally found after the summer solstice at low- and mid-latitudes. The Scott Base observations of the QTDW presented here are the highest-latitude ground-based observations of this wave to date. Waves with preferred periods of two and four days occur in bursts throughout the winter with maximum activity in June, July, and August. The mean of the two- and four-day wave amplitudes is relatively constant, suggesting constant wave forcing. When several waves with different periods occur at the same time, they often have similar phase velocities, supporting suggestions that they are quasi-non-dispersive. In 2005, a “warmpool” lasts from late July to late August. An alternative interpretation of this phenomenon is the presence of a structure propagating with the background wind. Consideration of the role of vertical shear (baroclinic instabilities) and horizontal shear (barotropic instabilities) of the zonal wind suggests that instabilities are likely to play a role in the forcing of the two- and four-day waves, which are near-resonant modes and thus supported by the atmosphere.  相似文献   

6.
Abstract

The normal mode instability of steady Wu-Verkley (1993) wave and modons by Verkley (1984, 1987, 1990) and Neven (1992) is considered. All these flows are solutions to the vorticity equation governing the motion of an ideal incompressible fluid on a rotating sphere. A conservation law for infinitesimal perturbations to each solution is derived and used to obtain a necessary condition for its exponential instability. By these conditions, Fjörtoft's (1953) average spectral number of the amplitude of an unstable mode must be equal to a specific number that depends on the degree of the solution in its inner and outer regions as well as on spectral distribution of the mode energy in these regions. Some properties of the conditions for different types of modons are discussed. The maximum growth (and decay) rate of the modes is estimated, and the orthogonality of the amplitude of each unstable, decaying, or non-stationary mode to the basic solution is shown in the energy inner product.

The new instability conditions confine the unstable disturbances of the WV wave and modon to a hypersurface in the perturbation space and allow interpretation of their energy structure. They are also useful both in estimating the maximum growth rate of unstable modes and in testing the numerical algorithms designed for the linear stability study.  相似文献   

7.
本文利用GCITEM-IGGCAS模式,从电动力学耦合作用和直接上传两种作用方式,详细模拟研究了DE2潮汐4种Hough波模分量对电离层的影响.我们将不同种类的Hough波模分别输入到模式当中作为底层边界条件,驱动模式模拟得到电离层的电子密度变化,从中分离两种作用机制的响应进行分析.模拟结果发现电离层对DE2的4种Hough波模的响应都表现为半年变化,波峰出现在春季和秋季,波谷则出现在冬季和夏季.一天的变化特性上,赤道对称波模的响应出现明显的4个峰值和谷值,其他3种波模响应主要表现为一个峰值和谷值.4种波模当中赤道对称波模对电离层的作用最为明显,占据主导地位,对电离层的影响表现为波动效应,其中3波分量的响应最强,主要由电动力学作用控制.其他3种波模对电离层则是削弱作用.本研究可以帮助我们更深刻的理解非迁移潮汐对电离层的作用方式和效果.  相似文献   

8.
Introduction Fourier transform that summed up a series discomposed sin functions of signal has been one of the widely used methods in the field of signal process all along. While for time information be-ing thrown away by this transform, it is difficult for us to judge when a special signal occurs on the way. Though short time Fourier transform (STFT) was developed later, which can probe local features of signals, it can not reveal what is really there because uniform window functions de-fin…  相似文献   

9.
The nodal modulation of the diurnal (K1 and O1) and semi-diurnal (M2 and K2) tidal constituents at the coasts of the Mediterranean Sea and the eastern Atlantic is estimated and its spatial variability mapped. Fourteen hourly tide gauge records each spanning more than 18 years are considered in this analysis. Ten tide gauges are located in the Mediterranean Sea and four in the Bay of Biscay. The nodal modulation of the most energetic tidal constituent (M2) reaches up to 5 cm at the eastern Atlantic coasts, while within the Mediterranean Sea its modulation is in general less than 1.1 cm. The largest K2 nodal modulation found is 3.7 cm in the eastern Atlantic coasts. In the Mediterranean Sea, smaller modulation amplitudes, ranging between 0.4 and 1.4 cm are found. The K1 tide constituent has the largest amplitude nodal modulation within the Mediterranean Sea of 1.9 cm in the north Adriatic Sea, which is also larger than the modulation of this constituent at the eastern Atlantic coasts. The O1 tide constituent has the highest amplitude nodal modulation (1.4 cm) at the eastern Atlantic coasts. In the Mediterranean Sea the maximum value is 1 cm in the north Adriatic Sea.  相似文献   

10.
Polar Mesospheric Cloud (PMC) observations from the Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) spacecraft are used to investigate the role of planetary wave activity on global PMC variability in the summer polar mesosphere during the 2007 Northern hemisphere season. This is coupled with an analysis of contemporaneous measurements of atmospheric temperature by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere–Energetics and Dynamics (TIMED) spacecraft to characterize the importance of temperature as a dominant forcing mechanism of the dynamical state of the summer polar mesosphere. The study confirms results from a recent study using PMC data from the Student Nitric Oxide Explorer (SNOE) and temperature data from SABER, such that planetary wave activity is present in both PMCs and mesospheric temperature and that are strongly coherent and anti-correlated. The dominant wave present in the polar summer mesosphere in both PMCs and temperature is the 5-day wavenumber 1 Rossby normal mode. The maximum amplitude of the variation of the 5-day wave in temperature is small at 3 K but has a significant effect on PMC albedo. The phase relationship between PMC and temperature is variable between 150° and 180° out of phase, with PMC albedo reaching a maximum ～10 h before the minimum in temperature. We have identified two additional waves, the westward propagating 2-day wavenumber 2 (2DW2) and the eastward propagating 2-day wavenumber 1 (2DE1) are both present in PMC and temperature variability in the 2007 NH season. The 2DW2 wave is consistent with a Rossby normal mode excited by the instability in the zonal mean zonal wind. However, the source of the 2DE1 wave could be a nonlinear interaction of the 2DW2 with the migrating diurnal tide. This is the first time these two wave features have been detected in coincident PMC and temperature measurements. Analysis of the zonal variation of PMC occurrence and temperature shows they are also anti-correlated and supporting the conclusion that temperature is an important forcing mechanism in zonal variability.  相似文献   

11.
The study is based on measuring fluctuations of the intensity and rotational temperatures of the molecular emissions of hydroxyl OH(6,2) and the first atmospheric band of oxygen O2(0–1), excited at approximately 87 and 95 km, respectively. The measurements are conducted at Maimaga station (63°N, 129.5°E), located 150 km north of Yakutsk. The semidiurnal tide parameters were obtained using the database compiled from 1999 to 2005. The data obtained from October to March were analyzed. The measurements conducted during 214 nights were used to determine the semidiurnal tide parameters. The wave amplitude at the height of the molecular oxygen emission (～95 km) is 8 K, which is larger than the amplitude at the height of the hydroxyl emission (～87 km) by approximately 2 K. Except November, the 12-h oscillation at the height of molecular oxygen excitation leads the oscillation at the height of hydroxyl excitation. On average, the phase is ～5.7 h at the OH emission height and ～6.4 h at the O2 emission height. We note that an abrupt increase in the tide amplitude in March at the molecular oxygen height can be related to a seasonal decrease in the so-called “wave” turbopause height.  相似文献   

12.
Observations of semidiurnal internal tidal currents from three moorings deployed on the continental shelf off central Chile during summer and winter of 2005 are reported. The spectra of the baroclinic currents showed large peaks at the semidiurnal band with a dominant counterclockwise rotation, which was consistent with internal wave activity. The amplitude of the barotropic tidal currents varied according to the spring–neap cycle following the sea level fluctuations. In contrast, the amplitudes of the internal tide showed high spatial-temporal variability not directly related to the spring–neap modulation. Near the middle of the continental shelf and near the coast (San Vicente Bay) the variance of the semidiurnal baroclinic current is larger than the variance of its barotropic counterpart. The vertical structure of the baroclinic tidal current fluctuations was similar to the structure of the first baroclinic internal wave mode. In general, in the three study sites the variance of the baroclinic current was larger near the surface and bottom and tended to show a minimum value at mid depths. Kinetic energy related to semidiurnal internal waves was larger in winter when stratification of the water column was stronger. During summer, upwelling and the decrease of freshwater input from nearby rivers reduced the vertical density stratification. The amplitude of the semidiurnal internal tide showed a tendency to be enhanced with increasing stratification as observed in other upwelling areas. The continental shelf break and submarine canyons, which limit the continental shelf in the alongshore direction, represent near-critical slopes for the semidiurnal period and are suggested to be the main internal tide generation sites in the study region.  相似文献   

13.
Bad cement bond with water channeling or low strength cement is usually generated on the first and/or secondary interfaces which lie between the cement and casing, and the cement and formation, respectively. It is an inherently complicated problem to evaluate cement bond on the secondary interface (SICB) in a cased borehole. So we need to find a useful wave for evaluating SICB and provide its propagation features. To achieve this, we simulated the acoustic full waveform, 2-D spectrum and dispersion curves in cased boreholes based on the real axis integration method, and a propagator matrix. Simulation results indicated that “casing-cement mode” wave can be utilized to evaluate SICB, as it is most sensitive to water channeling on the secondary interface. Velocity of this mode wave is in between that of the casing wave and the formation wave, so that it is easy to extract it in time. The dispersion curves showed that the casing-cement wave is a dispersion wave, but not a pseudo-Rayleigh wave. Its velocity is obviously larger than vs of cement. Simulation results further indicated that the amplitude of casing-cement wave is decreasing with increasing cement sheath thickness, but not water channeling thickness, and its travel time is influenced by casing and cement together. Moreover, we investigated factors influencing the casing–cement wave and found that casing diameter and peak frequency are major factors influencing the amplitude and the travel time, while other factors, such as P-wave velocities of cement and wall thickness of casing, are minor factors. But none of these factors greatly influence bond index which is computed from the amplitude. So the bond index has an advantage over the amplitude during evaluating SICB. Formation wave is a helpful signal to evaluate low strength cement without water channeling, and its amplitude is mainly related to the velocities (or impedances) of both cement and formation except for formation attenuation and disturbance wave. Experimental data in calibration pits and field data detecting channeling demonstrated the simulation results.  相似文献   

14.
Meteor radar measurements of winds near 95 km in four azimuth directions from the geographic South Pole are analyzed to reveal characteristics of the 12-h oscillation with zonal wavenumber one (s = 1). The wind measurements are confined to the periods from 19 January 1995 through 26 January 1996 and from 21 November 1996 through 27 January 1997. The 12-h s = 1 oscillation is found to be a predominantly summertime phenomenon, and is replaced in winter by a spectrum of oscillations with periods between 6 and 11.5 h. Both summers are characterized by minimum amplitudes (5–10 ms–1) during early January and maxima (15–20 ms–1) in November and late January. For 10-day means of the 12-h oscillation, smooth evolutions of phase of order 4–6 h occur during the course of the summer. In addition, there is considerable day-to-day variability (±5–10 ms–1 in amplitude) with distinct periods (i.e., 5 days and 8 days) which suggests modulation by planetary-scale disturbances. A comparison of climatological data from Scott Base, Molodezhnaya, and Mawson stations suggests that the 12-h oscillation near 78°S is s = 1, but that at 68°S there is probably a mixture between s = 1 and other zonal wavenumber oscillations (most probably s = 2). The mechanism responsible for the existence of the 12-h s = 1 oscillation has not yet been identified. Possible origins discussed herein include in situ excitation, nonlinear interaction between the migrating semidiurnal tide and a stationary s = 1 feature, and thermal excitation in the troposphere.  相似文献   

15.
The destructive Pacific Ocean tsunami generated off the east coast of Honshu, Japan, on 11 March 2011 prompted the West Coast and Alaska Tsunami Warning Center (WCATWC) to issue a tsunami warning and advisory for the coastal regions of Alaska, British Columbia, Washington, Oregon, and California. Estimating the length of time the warning or advisory would remain in effect proved difficult. To address this problem, the WCATWC developed a technique to estimate the amplitude decay of a tsunami recorded at tide stations within the Warning Center’s Area of Responsibly (AOR). At many sites along the West Coast of North America, the tsunami wave amplitudes will decay exponentially following the arrival of the maximum wave (Mofjeld et al., Nat Hazards 22:71–89, 2000). To estimate the time it will take before wave amplitudes drop to safe levels, the real-time tide gauge data are filtered to remove the effects of tidal variations. The analytic envelope is computed and a 2 h sequence of amplitude values following the tsunami peak is used to obtain a least squares fit to an exponential function. This yields a decay curve which is then combined with an average West Coast decay function to provide an initial tsunami amplitude-duration forecast. This information may then be provided to emergency managers to assist with response planning.  相似文献   

16.
Over the past 30 years, reclamation projects and related changes have impacted the hydrodynamics and sediment transport in the Bohai Sea. Three-dimensional tidal current models of the Bohai Sea and the Yellow Sea were constructed using the MIKE 3 model. We used a refined grid to simulate and analyze the effects of changes in coastline, depth, topography, reclamation, the Yellow River estuary, and coastal erosion on tidal systems, tide levels, tidal currents, residual currents, and tidal fluxes. The simulation results show that the relative change in the amplitude of the half-day tide is greater than that of the full-day tide. The changes in the tidal amplitudes of M2, S2, K1, and O1 caused by coastline changes accounted for 27.76–99.07% of the overall change in amplitude from 1987 to 2016, and water depth changes accounted for 0.93–72.24% of the overall change. The dominant factor driving coastline changes is reclamation, accounting for 99.55–99.91% of the amplitude changes in tidal waves, followed by coastal erosion, accounting for 0.05–0.40% of the tidal wave amplitude changes. The contribution of changes in the Yellow River estuary to tidal wave amplitude changes is small, accounting for 0.01–0.12% of the amplitude change factor. The change in the highest tide level (HTL) is mainly related to the amplitude change, and the correlation with the phase change is small. The dominant factor responsible for the change in the HTL is the tide amplitude change in M2, followed by S2, whereas the influence of the K1 and O1 tides on the change in the HTL is small. Reclamation resulted in a decrease in the vertical average maximum flow velocity (VVAM) in the Bohai Sea. Shallower water depths have led to an increase in the VVAM; deeper water depths have led to a decrease in the maximum flow velocity. The absolute value of the maximum flow velocity gradually decreases from the surface to the bottom, but the relative change value is basically constant. The changes in the tidal dynamics of the Bohai Sea are proportional to the degree of change in the coastline. The maximum and minimum changes in the tidal flux appear in Laizhou Bay (P-LZB) and Liaodong Bay (P-LDB), respectively. The changes in the tidal flux are related to the change in the area of the bay. Due to the reduced tidal flux, the water exchange capacity of the Bohai Sea has decreased, impacting the ecological environment of the Bohai Sea. Strictly controlling the scale of reclamation are important measures for reducing the decline in the water exchange capacity of the Bohai Sea and the deterioration of its ecological environment.  相似文献   

17.
The climatology of mean wind, diurnal and semidiurnal tide during the first year (1996–1997) of simultaneous wind observations at Wakkanai (45.4°N, 141.7°E) and Yamagawa (31.2°N, 130.6°E) is presented. The locations of the radars allow us to describe the latitudinal dependence of the tides. Tidal amplitude and phase profiles are compared with those of the global scale wave model (GSWM). While the observed amplitude profiles of the diurnal tide agree well with the GSWM values, the observed phase profiles often indicate longer vertical wavelengths than the GSWM phase profiles. In contrast to the GSWM simulation, the observations show a strong bimodal structure of the diurnal tide, with the phase advancing about 6 hours from summer to winter.  相似文献   

18.
The lunar semidiurnal tide in winds measured at around 90 km altitude has been isolated with amplitudes observed up to 4 m s–1. There is a marked amplitude maximum in October and also a considerable phase variation with season. The average variation of phase with height indicated a vertical wavelength of more than 80 km but this, and other results, needs to be viewed in the light of the considerable averaging required to obtain statistical significance. Large year-to-year variations in both amplitude and phase were also found. Some phase comparisons with the GSWM model gave reasonable agreement but the model amplitudes above a height of 100 km were much larger than those measured. An attempt to make a comparison with the lunar geomagnetic tide did not yield a statistically significant result.  相似文献   

19.
We studied the 11 March 2011 Tohoku tsunami through analysis of the sea level records from 21 tide gauge and 16 DART (Deep-ocean Assessment and Reporting of Tsunamis) stations from across the Pacific Ocean. The extreme power of this trans-oceanic tsunami was indicated by the trough-to-crest heights of 3.03 m at Arena Cove on the western coast of the USA and 3.94 m at Coquimbo on the southern coast of Chile. The average value of the maximum amplitude was 163.9 cm for the examined tide gauge records. At many coastal tide gauge stations the largest wave arrived several hours after the first arrival of the tsunami wave, and the tsunami lasted for a long time with an average duration of 4 days. On the contrary, at most of the DART stations in the deep ocean, the first wave was the largest, the tsunami amplitudes were smaller with an average maximum of 51.2 cm, and the durations were shorter with an average of 2 days. The two dominant tsunami periods on the DART records were 37 and 67.4 min, which are possibly attributed to the width and length of the tsunami source fault, respectively. The dimensions of the tsunami source was estimated as 233 km × 424 km. Wavelet analyses of tide gauge and DART records showed that most of the tsunami energy was distributed at the wide period band of around 10–80 min during the first hour after the tsunami arrival, then it was concentrated in a relatively narrower band. The frequency-time plots showed the switches and lapses of tsunami energy at the 35- and 65-min period bands.  相似文献   

20.
Summary The joint interpretation of Lg and Sa waves has been carried out on the basis of the first higher mode of Love waves. A characteristic feature of the spectral amplitude curve for the first higher mode is the existence of two pronounced maxima separated by a gap. The short-period maximum may be related to the Lg wave, the long-period to the Sa wave.  相似文献   

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