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1.
On the basis of bispectral analysis applied to the hourly data set of neutral wind measured by meteor radar in the MLT region above Bulgaria it was demonstrated that nonlinear processes are frequently and regularly acting in the mesopause region. They contribute significantly to the short-term tidal variability and are apparently responsible for the observed complicated behavior of the tidal characteristics. A Morlet wavelet transform is proposed as a technique for studying nonstationary signals. By simulated data it was revealed that the Morlet wavelet transform is especially convenient for analyzing signals with: (1) a wide range of dominant frequencies which are localized in different time intervals; (2) amplitude and frequency modulated spectral components, and (3) singular, wave-like events, observed in the neutral wind of the MLT region and connected mainly with large-scale disturbances propagated from below. By applying a Morlet wavelet transform to the hourly values of the amplitudes of diurnal and semidiurnal tides the basic oscillations with periods of planetary waves (1.5/20 days), as well as their development in time, are obtained. A cross-wavelet analysis is used to clarify the relation between the tidal and mean neutral wind variability. The results of bispectral analysis indicate which planetary waves participated in the nonlinear coupling with the atmospheric tides, while the results of cross-wavelet analysis outline their time intervals if these interactions are local.  相似文献   

2.
Based on the horizontal winds measured using SKiYMET meteor wind radar during the period of June 2004–May 2007, the seasonal and interannual variability of the diurnal and semidiurnal amplitudes and phases in the mesospheric and lower thermospheric (MLT) region over a low-latitude station Trivandrum (8.5°N) are investigated. The monthly values of amplitudes and phases are calculated using a composite day analysis. The zonal and meridional diurnal tidal amplitudes exhibit both annual and semiannual oscillations. The zonal and meridional components of semidiurnal tide show a significant annual oscillation. The phase values of both diurnal and semidiurnal tides exhibit annual oscillation above 90 km. The effect of background wind in the lower atmosphere on the strength of diurnal tidal amplitudes in the MLT region is studied. The effect of diurnal tides on the background wind in the lower thermosphere is also discussed.  相似文献   

3.
The harmonic relationship between the diurnal and semidiurnal tides gives rise to an elementary mathematical relationship that has intriguing consequences for secondary waves produced by non-linear interactions between the diurnal tide and planetary waves. A speculative theory is developed which predicts that, under certain conditions, these secondary waves can be amplified by non-linear interaction with the semidiurnal tide. A peculiar feature of dynamics in the MLT region above Bulgaria is the presence of strong oscillations with periods near 20 and 30 h, especially in the zonal wind component. Observational evidence from a meteor radar at Yambol, Bulgaria suggests that the 20- and 30-h signals are produced as the result of non-linear interactions of the type proposed by the novel theory.  相似文献   

4.
The harmonic relationship between the diurnal and semidiurnal tides gives rise to an elementary mathematical relationship that has intriguing consequences for secondary waves produced by non-linear interactions between the diurnal tide and planetary waves. A speculative theory is developed which predicts that, under certain conditions, these secondary waves can be amplified by non-linear interaction with the semidiurnal tide. A peculiar feature of dynamics in the MLT region above Bulgaria is the presence of strong oscillations with periods near 20 and 30 h, especially in the zonal wind component. Observational evidence from a meteor radar at Yambol, Bulgaria suggests that the 20- and 30-h signals are produced as the result of non-linear interactions of the type proposed by the novel theory.  相似文献   

5.
平流层爆发性增温(SSW)期间,低层大气温度场和风场等的剧烈变化会直接影响潮汐和风剪切作用.此举可能会导致电离层Es的相应变化.本文以2009年1月事件为例,分析了SSW期间Es层的响应.首先,在排除太阳活动和地磁活动对Es层影响的前提下,分析了昆明站附近MLT区域行星波和潮汐波的波动特性,发现此期间存在显著的2日行星波,并伴有日潮汐减弱和半日潮汐增强等波动现象;随后,分析相应时间段内Es层的变化特性发现,重庆和昆明站附近Es层强度明显减弱,且其高度显著抬升.这一现象与低层大气的波动变化具有同步性.最后,通过模拟经典风剪切理论下Es层金属离子的汇聚过程和运动轨迹,再现了SSW期间Es层与低层大气波动的耦合演化过程.该分析结果为研究低层-中层-高层大气的耦合过程提供了一种新的思路.  相似文献   

6.
Behavior of semidiurnal tides in the north and south polar MLT regions simulated by Middle Atmosphere Circulation Model at Kyushu University is described. Summertime enhancement of westward propagating semidiurnal tide with zonal wavenumber s=1 is found, which is consistent with the observed result at the South Pole (Ann. Geophys. 16 (1998) 828). Additional numerical simulations show that the non-migrating semidiurnal tide is mainly generated by the nonlinear interactions between stationary planetary waves with zonal wavenumber s=1 and the migrating semidiurnal tide in the stratosphere and mesosphere as suggested by Forbes et al. (Geophys. Res. Lett. 22(23) (1995) 3247).  相似文献   

7.
Long period variations in the mesosphere wind have been observed for some time by ground-based radars. These planetary scale disturbances have reoccurring periods at or near 5–7, 10, and 16 days and at times dominate the wind field at mesospheric heights. Recently, due to the continuous operation of several of the MLT radars and the availability of measurements from the UARS satellite, it has been possible to compare observations during periods of large planetary wave activity. Wind measurements from four MLT radars; the meteor radars at Durham, NH (43°N,71°W) and Sheffield, UK (53°N,2°W) and MF radars at Urbana, IL (40°N,88°W) and Saskatoon, Canada (52°N,107°W) were compared with the HRDI measurements during intervals when 7-d planetary waves were present. Wind data from the HRDI instrument on UARS has been processed to show the latitudinal structure and the seasonal variation of the planetary scale wind variation. The phases and amplitudes of the waves as determined by both the satellite and the radars are in good agreement. The ground-based measurements show large modulation of tides by these long period components, and also show comparable responses of these low frequency components over thousands of kilometers. The satellite and the ground-based results both indicate a preponderance of wave occurrence during the equinoxes and at preferred latitudes.  相似文献   

8.
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9.
The short-term regional responses of the mesosphere–lower thermosphere (MLT) dynamics over Scandinavia to the exceptionally strong solar storms with their accompanying solar proton fluxes on the Earth in late October 2003 have been investigated using radar measurements at Andenes (69°N, 16°E) and Esrange (68°N, 21°E). Several solar activity storms resulted in solar proton events (SPEs) at this time, but a particularly active period of high proton fluxes occurred between 28 and 31 October 2003. The significant temperature drop (∼25 K), detected by the meteor radar at Andenes at altitude ∼90 km, was in line with the enhancement of the proton fluxes and was caused by the dramatic reduction of the ozone in the high-latitude middle atmosphere monitored by satellite measurements. This exceptionally strong phenomenon in late October 2003 was composed of three geomagnetic storms, with the first one occurring in the daytime of 29 October and the other two storms in the nighttime of 29 and 30 October, respectively. The responses of the prevailing wind and the main tides (24- and 12-h tides) were studied in detail. It was found that the response of the MLT dynamics to the first geomagnetic storm occurring in the daytime and accompanied by solar proton fluxes is very different from those to the second and third geomagnetic storms with onsets during the nighttime. Some physical mechanisms have been suggested in order to explain the observed short-term variability of the MLT dynamics. This case study revealed the impact of the SPEs observed in late October 2003 and the timing of the geomagnetic storms on the MLT neutral wind responses observed over Scandinavia.  相似文献   

10.
With the launch of the TIMED satellite in December 2001, continuous temperature and wind data sets amenable to MLT tidal analyses became available. The wind measuring instrument, the TIMED Doppler Interferometer (TIDI), is operating since early 2002. Its day- and nighttime capability allows to derive tidal winds over a range of MLT altitudes. This paper presents climatologies (June 2002–June 2005) of monthly mean amplitudes and phases for six nonmigrating semidiurnal tidal components between 85 and 105 km altitude and between 45°S and 45°N latitude (westward propagating wave numbers 4, 3, 1; the standing oscillation s0; and eastward propagating wave numbers 1, 2) in the zonal and meridional wind directions.Amplitude errors are 15–20% (accuracy) and 0.8 m/s (precision). The phase error is 2 h. The TIDI analysis agrees well with 1991–1994 UARS results at 95 km. During boreal winter, amplitudes of a single component can reach 10 m/s at latitudes equatorward of 45°. Aggregate effects of nonmigrating tides can easily reach or exceed the amplitude of the migrating tide. Comparisons with the global scale wave model (GSWM) and the thermosphere–ionosphere–mesosphere–electrodynamics general circulation model (TIME-GCM) are partly inconclusive but they suggest that wave–wave interaction and latent heat release in the tropical troposphere both play an important role in forcing the semidiurnal westward 1, westward 3, and standing components. Latent heat release is the leading source of the eastward propagating components.  相似文献   

11.
Semidiurnal tidal features have been examined in the Mesosphere and Lower Thermosphere (MLT) from the long-term (2002–2007) meteor wind data over Maui (20.75°N, 156.43°W). Amplitude and phase obtained from the harmonic analysis exhibit large day to day variability. Mean amplitude obtained from the monthly mean data over the observation period is found to vary within ～8–28 m/s and 10–32 m/s for the zonal and meridional winds, respectively. The amplitude has revealed clear semiannual oscillation (SAO) pattern with maxima during solstices and altitudinal growth in both wind components. Significant resemblance in its variability with other observations carried out from the low latitude sites all over the globe is obtained. Vertical wavelength estimated from the phase gradients exposes large values (>90 km) in all seasons. Contribution of the semidiurnal tide to the total tidal variability in the MLT is found to vary over wide range throughout the year with generally higher influence during winter season over diurnal and terdiurnal components.  相似文献   

12.
The extended Canadian Middle Atmosphere Model (extended CMAM) is a general circulation model, which extends from the surface to about 210 km. Spatial complex spectral analysis is applied to horizontal winds simulated by the extended CMAM to obtain semidiurnal tidal amplitudes and phases (from e5 to w5) in the mesosphere and lower thermosphere (MLT) region. The dominant w2 migrating component and the presence of eight nonmigrating tides (w3, w4, w5, e1, e2, e3, e4 and e5) in the mid-latitudes are identified. Components w1 and s0, which tend to maximize at high latitudes, will be discussed separately in a later paper. The migrating semidiurnal tide (w2) has amplitudes reaching over 20 m s−1 for both zonal and meridional winds in the mid-latitude region. Its form compares well to the published results. The amplitudes of nonmigrating semidiurnal tides are non-negligible compared with the migrating semidiurnal tides. The amplitudes for w3 and e2 exceed 12 and 8 m s−1, respectively.Comparisons are made with four nonmigrating semidiurnal components (w3, w4, e1 and e2) derived from the TIMED Doppler interferometer (TIDI) wind measurements between 85 and 105 km altitude and between 45°S and 45°N latitude. Overall, the basic CMAM and TIDI latitudinal structures of the amplitudes agree well and the agreement between the annual mean amplitudes varies with component. Relative to the TIDI results, the CMAM seasonal variations of w4 are in good agreement, of e2 are in reasonable agreement, of w3 are in partial agreement and of e1 are in poor agreement.The 11 semidiurnal components from the model are superimposed to generate the total semidiurnal winds at Jakarta (6°S, 106°E) and Kototabang (0°, 100°E) and are compared with measurements from two equatorial meteor radar stations at these sites. The relative contributions of components to the reconstructed amplitude vary from month to month. The CMAM reconstructions are generally larger than the radar results by a factor varying between one and two. The phases in the radar data are typically stationary with respect to height, whereas they generally decrease with height in the CMAM reconstruction.  相似文献   

13.
Interactions of tidal constituents and the transfer of energy from the tidal frequencies to other frequencies are investigated using 3-D tidal simulations for the Indonesian seas, focusing on an area of active internal tides. Semidiurnal tides strongly affect diurnal tides; however, semidiurnal tides are essentially unaffected by diurnal tides. The semidiurnal and diurnal constituents interact with each other through non-linear interference, both destructive and constructive. Semidiurnal tides generate harmonics at nearly the diurnal frequency and higher vertical wavenumbers. In Ombai Strait, these harmonics are out of phase with the diurnal tides and interact destructively with the diurnal tides, effectively negating the diurnal response in some locations. However, this is not a general response, and interactions differ between locations. Energy is also transferred from both semidiurnal and diurnal tides to other frequencies across the spectrum, with more energy originating from semidiurnal tides. These energy transfers are not homogeneous, and the spectral responses differ between the Makassar and Ombai Straits, with the region east of Ombai showing a more active surface response compared to a more intense benthic response in Makassar. In deep water away from topography, velocity spectra generally follow the Garrett–Munk (GM) relation. However, in areas of internal tide generation, spectral density levels exceed GM levels, particularly between 4 and 8 cycles per day (cpd), indicating increased non-linear interactions and energy transfer through resonant interactions. The model indicates strong surface trapping of internal tides, with surface velocity spectra having significantly higher energy between 4 and 8 cpd even 100 km away from the prominent sill generating the internal tides.  相似文献   

14.
Cai  Shuqun  Wu  Yuqi  Xu  Jiexin  Chen  Zhiwu  Xie  Jieshuo  He  Yinghui 《中国科学:地球科学(英文版)》2021,64(10):1674-1686
Numerous internal solitary waves(ISWs) have been observed in the southern Andaman Sea. In this study, the two-dimensional Massachusetts Institute of Technology general circulation model is applied to investigate the dynamics of ISWs and explore the effects of the bottom topography and tidal forcing on the generation and propagation of ISWs in the southern Andaman Sea. The results show that the large-amplitude depression ISWs are mainly generated via the oscillating tidal flow over the sill of the Great Channel, and the generation of ISWs is subject to the lee wave regime. The Dreadnought Bank cannot generate ISWs itself; however, it can enhance the amplitudes of eastward-propagating ISWs generated from sill A, owing to constructive interference of internal tide generation between the sill of the Great Channel and the Dreadnought Bank. The eastward-propagating ISWs generated by the eastern shallow sill near the continental shelf can propagate to the shelf, where they evolve into elevation waves because of the shallow water. Sensitivity runs show that both the semidiurnal and diurnal tides over the sill of the Great Channel can generate ISWs in this area. However, the ISWs generated by diurnal tides are much weaker than those generated by semidiurnal tides. Mixed tidal forcing has no significant effect on the generation of ISWs.  相似文献   

15.
A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001 hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed.  相似文献   

16.
Planetary wave activity at quasi 16-, 10- and 5-day periods has been compared at various altitudes through the middle and upper atmosphere over Halley (76°S, 27°W), Antarctica, during the austral winters of 1997–1999. Observational data from the mesosphere, E-region ionosphere and F-region ionosphere have been combined with stratospheric data from the ECMWF assimilative operational analysis. Fourier and wavelet techniques have shown that the relationship between planetary wave activity at different altitudes is complex and during the winter eastward wind regime does not conform to a simple combination of vertical planetary wave propagation and critical filtering. Strong planetary wave activity in the stratosphere can coincide with a complete lack of wave activity at higher altitudes; conversely, there are also times when planetary wave activity in the mesosphere, E-region or F-region has no apparent link to activity in the stratosphere. The latitudinal activity pattern of stratospheric data tentatively suggests that when the stratospheric signatures are intense over a wide range of latitudes, propagation of planetary waves into the mesosphere is less likely than when the stratospheric activity is more latitudinally restricted. It is possible that, on at least one occasion, 16-day planetary wave activity in the mesosphere may have been ducted to high latitudes from the lower latitude stratosphere. The most consistent feature is that planetary wave activity in the mesosphere is almost always anti-correlated to planetary wave activity in the E-region even though the two are in close physical proximity. The oscillatory critical filtering of vertical gravity wave propagation by planetary waves and the re-generation of the planetary wave component at higher altitudes through subsequent critical filtering or breaking of the gravity waves may provide an explanation for some of these characteristics. Alternatively the nonlinear interaction between planetary waves and tides, indicated in the E-region data, may play a role.  相似文献   

17.
Vertical coupling in the low-latitude atmosphere–ionosphere system driven by the 5-day Rossby W1 and 6-day Kelvin E1 waves in the low-latitude MLT region has been investigated. Three different types of data were analysed in order to detect and extract the ∼6-day wave signals. The National Centres for Environmental Prediction (NCEP) geopotential height and zonal wind data at two pressure levels, 30 and 10 hPa, were used to explore the features of the ∼6-day waves present in the stratosphere during the period from 1 July to 31 December 2004. The ∼6-day wave activity was identified in the neutral MLT winds by radar measurements located at four equatorial and three tropical stations. The ∼6-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) were detected in the data from 26 magnetometer stations situated at low latitudes. The analysis shows that the global ∼6-day Kelvin E1 and ∼6-day Rossby W1 waves observed in the low-latitude MLT region are most probably vertically propagating from the stratosphere. The global ∼6-day W1 and E1 waves seen in the ionospheric electric currents are caused by the simultaneous ∼6-day wave activity in the MLT region. The main forcing agent in the equatorial MLT region seems to be the waves themselves, whereas in the tropical MLT region the modulated tides are also of importance.  相似文献   

18.
A numerical simulation of secondary waves generated by nonlinear interaction has been used to interpret the behaviour of planetary waves observed by a meteor radar in the UK (53°27′N, 1°35′W) during the summer of 1992. A new explanation is proposed for the long-period variability of the (3,0) mode quasi-two-day wave in the mesosphere and lower-thermosphere, involving the (2,0) Rossby-gravity mode and pseudo-two-day secondary waves with the same zonal wavenumbers as those of the primary (2,0) and (3,0) modes. These pseudo-two-day secondary waves arise from the nonlinear interaction of the Rossby-gravity modes with long-period oscillations of the zonally averaged flow in the equatorial stratosphere, which can be generated by the interaction between the 10 and 16 day planetary waves. Other maxima existing in the neutral wind power spectra can be identified with various secondary waves originating from nonlinear interaction between the quasi-two-day and long-period planetary waves.  相似文献   

19.
Long-term wind measurements carried out at 6 northern midlatitude sites (Saskatoon, Sheffield, Juliusruh, Collm, Obninsk, Kazan) are investigated to establish a climatology of the semidiurnal tide in the mesopause region for the narrow latitudinal range between 52°N and 56°N. Comparison of zonal and meridional components shows that in general the horizontal components are circularly polarized. Intercomparison of amplitudes and phases generally shows good agreement between the results from the different measuring systems. The results are compared with an empirical model of the semidiurnal tide. The longitudinal variation of the semidiurnal tide is small in summer, but the tidal amplitudes in winter are larger at Saskatoon and Kazan, compared with the results from the other sites. The possible influence of wave–tidal interaction in the stratosphere on the interannual variability of this difference is discussed.  相似文献   

20.
Long-term tendencies in horizontal neutral wind parameters in the southern polar mesosphere/lower thermosphere are presented. The wind data analyzed were obtained from meteor and MF radars situated at Molodezhnaya (45.9°E, 67.7°S), Mawson (62.9°E, 67.6°S) and Davis (78.0°E, 68.6°S). The composite dataset covers years from 1970 to 2006. A Bayesian approach in the form proposed by Wang and Zivot [2000. A Bayesian time series model of multiple structural changes in level, trend and variance. Journal of Business and Economic Statistics 8, 374–386] is used for the trend assessment. This approach allows structural breaks to be identified in the trend parameters (slope, mean or variance of residuals) or demonstrates their absence. The results of our analysis have shown persistence through such structural breaks in trends of the winter and summer prevailing winds and in meridional tidal components. It is demonstrated that the wind parameters exhibit different stable states with transitions between the states. Correlations between the southern polar MLT wind and indices of atmospheric variability (Northern annular mode, Southern annular mode, Multivariate El-Niño/Southern Oscillation Index) were then considered. The results show that statistically significant correlations exist during some periods of observations that do not exist during others.  相似文献   

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