Interannual variability of mesospheric mean winds observed with the MU radar

In an effort to study the interannual variation of mesospheric (65–90 km altitude) mean winds, 10 years (1986–1995) of wind data collected with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E) have been analysed. The analysis reveals that the mean zonal wind circulation in the mesosphere is dominated by an annual variation. The summer westward flow in the mesosphere shows a smooth variation with a peak value in the range 40–60 m/s in June/July. In contrast to the summer westward winds, the winter eastward winds exhibit much more variability. In some years it is found that the winds exceed even 60 m/s and the peak value may occur in any one of the winter months. Scrutiny of the duration of the summer westward winds reveals a two-year periodicity, which has been compared with the quasi-biennial oscillation (QBO) phases at the equator. The search for a dependence of the mean wind on solar activity does not reveal any indications of it. Ten-year averaged winds are compared with the model atmosphere, CIRA-86, values and certain agreements and disagreements are pointed out.     

Midlatitude type 1 echoes observed with the Chung-Li 52 MHz radar

This paper presents observational characteristics of midlatitude ionospheric E region type 1 echoes measured with a 52 MHz Doppler radar located at the National Central University, Chung-Li, Taiwan. Two events, on August 13 and October 15, 1997, are considered which show clear evidence of type 1 echo occurrence with peak Doppler velocities at about −300 m/s and +270 m/s, respectively. On the other hand, interferometry measurements indicate that the drift motion of the scattering region is eastward in both events, at about 80–100 m/s.     

Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

We have observed a number of strong echoes with the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5° and 78.6° with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are shortlived and occur in single 2- or 10-s data dumps. They are offset by 125–130 kHz with respect to the transmitted frequency. In most cases the offset compares well with the frequency of gyro lines in the incoherent-scatter spectrum, as given by the standard linear dispersion relation. But sometimes the measured offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discrepancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming particle beams. A more serious problem with the gyro-line theory is how the line can be excited at altitudes where the collisional damping is substantial. The high intensity and short lifetime of the signal point to a fast-growing plasma instability as the likely excitation mechanism, if the gyro-line interpretation is correct. The cause of the instability could be the same particle beams as those causing the disturbances in the magnetic field. Alternatively, the observations may be interpreted as meteor head echoes. The large Doppler shifts, the short lifetimes and the altitudes of the signals support this explanation. The main difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active conditions. Also, the occurrence of echoes does not seem to follow the expected changes in meteor density. More observations in different conditions are needed to decide between the two interpretations. As it is, we are inclined to believe in the meteor head echo theory, the objections to the gyro-line theory being more fundamental.     

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°2463N–5°3738W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high–resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.     

Some characteristics of atmospheric gravity waves observed by radio-interferometry

Observations of atmospheric acoustic-gravity waves (AGWs) are considered through their effect on the horizontal gradient G of the slant total electron content (slant TEC), which can be directly obtained from two-dimensional radio-interferometric observations of cosmic radio-sources with the Nançay radioheligraph (2.2^°E, 47.3^°N). Azimuths of propagation can be deduced (modulo 180^°). The total database amounts to about 800 h of observations at various elevations, local time and seasons. The main results are:a) AGWs are partially directive, confirming our previous results.b) The propagation azimuths considered globally are widely scattered with a preference towards the south.c) They show a bimodal time distribution with preferential directions towards the SE during daytime and towards the SW during night-time (rather than a clockwise rotation as reported by previous authors).d) The periods are scattered but are larger during night-time than during daytime by about 60%.e) The effects observed with the solar radio-sources are significantly stronger than with other radio-sources (particularly at higher elevations), showing the role of the geometry in line of sight-integrated observations.     

The small-scale structure of VHF mesospheric summer echo layers observed at mid-latitudes

The VHF radar system at Aberystwyth (52.4° N, 4.1° E) has been used to make high-time-resolution, multi-beam observations of mesospheric summer echo layers. These show that the altitude and the sense of vertical movement of the layers can vary over time-scales of minutes and horizontal scales of kilometres. In general, the altitude profiles of signal-to-noise ratio provide evidence of a bifurcated structure with sharp changes in the horizontal wind vector and vertical velocity, and enhanced spectral width occurring at the bifurcation level. The implications of the small-scale structure for studies of the aspect sensitivity of radar returns are discussed, and the changes in wind-field at the bifurcation level are compared with wind corners observed in rocket studies of the mesosphere at polar latitudes.     

Multi-frequency synthetic-aperture imaging with a lightweight ground penetrating radar system

The detection of buried objects, particularly hazardous waste containers and unexploded ordnance (UXO), has gained significant interest in the Unites States in the late 1990s. The desire to remediate the thousands of sites worldwide has become an increasing concern and the application of radar to this problem has received renewed attention. The US Department of Energy's Special Technologies Laboratory (STL), operated by Bechtel Nevada, has developed several frequency-modulated, continuous-wave (FM-CW) ground penetrating radar (GPR) units. To meet technical requirements for higher-resolution data, STL and the University of California, Santa Barbara (UCSB) is investigating advanced GPR hardware, signal processing, and synthetic-aperture imaging with the development of an innovative system. The goal is to design and fabricate a lightweight, battery-operated unit that does not require surface contact, can be operated by a novice user, and can achieve improved resolution. The latter is accomplished by using synthetic-aperture imaging, which forms the subsurface images by fully utilizing the data sequences collectively along a scan path. We also present the backward propagation algorithm as the basic structure of the multiple-frequency tomographic imaging technique, and the conventional fast Fourier transform (FFT) method which can be described as a degenerated case of the model where the computation procedure is approximated under the narrow-beam assumption.     

大气流场的拓扑结构

用简化的大气运动方程,定性分析了大气中几种常见天气系统流场的拓扑结构. 研究表明,气旋反气旋流场与中心点附近的流形相对应;长波演化形成阻塞高压流场,与鞍点和中心点从合并到分离的流形相对应;台风和龙卷风的流场与三维空间中鞍-焦点附近的流形相对应. 研究大气流场的拓扑结构,可以直观清晰地揭示大气运动的形态和形成机理,有助于认识大气运动的规律. 文中讨论基于一定假设,结果与实际大气有差异,因而具有局限性.     

Analysis of waves observed by synthetic aperture radar across ocean fronts

Ocean Dynamics - In this study, synthetic aperture radar (SAR) imaging of waves across ocean fronts was investigated using C-band Sentinel-1 VV-polarized SAR imagery collected over the Yangtze and...     

昆明全天空流星雷达观测中高层大气温度

利用昆明电波观测站（25.6°N,103.8°E）两台不同工作频率的全天空流星雷达在2011年特殊联合观测试验期间的数据,基于Hocking的方法利用不同的温度梯度,在确定了昆明地区中层顶位于流星峰值高度之上的情况下,反演了昆明地区上空88 km和85 km高度的大气温度,并与Aura卫星观测的温度进行比较.对比研究发现,两台流星雷达可以分别正确获得88 km和85 km高度的大气温度,但其中由全球温度梯度模式反演得到的大气温度与卫星观测温度相关性不是很好,而利用卫星观测的温度梯度,两台雷达反演出的大气温度与卫星观测温度存在很好的相关性.结果表明了准确的温度梯度在流星雷达观测大气温度过程中是至关重要的.     

Aircraft measurements of asymmetric temperature microstructure causing azimuth variations of VHF radar echo power

VHF wind-profiling radars often measure a decrease of echo power with zenith angle, which can be explained from in situ measurements of horizontal layering or anisotropy of metre-scale temperature structure in the atmosphere. There can also be an azimuthal variation of echo power, which is increased in an azimuth opposite to the vertical shear vector of horizontal wind. This paper checks if the azimuth variation can also be linked to in situ observations of temperature structure, using aircraft flights in the tropopause region near a VHF radar. At heights where VHF radar measures wind shear and aspect sensitivity, there can be an asymmetry in the probability distribution of horizontal gradient of potential temperature, for horizontal scale of e.g. hundreds of metres. The asymmetry is often of opposite sign for up-shear and down-shear flights, and less when VHF echoes are isotropic instead of aspect sensitive. The range of horizontal scales with asymmetry can be used to distinguish e.g. sheared anisotropic turbulence and Kelvin–Helmholtz instability as causes of azimuthal VHF echo power variations.     

Rapid variations in echo power maps of VHF radar backscatter from the lower atmosphere

For the first time, echo power maps of aspect-sensitive VHF backscatter are shown, with good time and spatial resolutions, for angles 0°–7° from zenith. Sequences of power maps show large changes in appearance over timescales of a few minutes and height intervals of a few hundred metres. Often, individual power maps are consistent with tilted and distorted specular-type scattering layers, rather than anisotropic turbulence, and the direction of maximum echo power is sometimes several degrees off-vertical. Nevertheless, after time-averaging the variable echo-power patterns, the average pattern can become almost circular and centred on zenith, as has been assumed in the past. Echo power maps measured in strong windshear beneath the jet stream show a skewing of the echo power distribution. However, some power maps in the lower stratosphere, despite stronger wind shear, appear more constrained and their maximum echo power remains closer to zenith.     

Reconstruction of linear folding structures with the use of volume balancing

The structure of linear fold zones is considered as a set of objects of several hierarchic levels with specific kinematic models of their formation. The prefolded structure was reconstructed, and recent depths of the sedimentary cover base were predicted on the basis of these notions with the use of the Northwestern Caucasus folding as an example. Fold domains were used as the main objects. The basic parameters of their geometry (dips of axial surfaces, dips of fold system levels, shortening values) measured in natural structures were identified with the deformation ellipsoid. Three kinematic operations: rotation, simple horizontal shearing, and extension, which made it possible to reconstruct the horizontal layering and the position of the profile line segment for each domain. Eleven profiles were reconstructed by the consecutive joining of the prefolded states of the domains. The fold shortenings were obtained for these profiles and their 42 parts. The developed quasi-three-dimensional model of the natural structure demonstrates specific features important for the verification of geodynamic models.     

ST radar observations of atmospheric waves over mountainous areas: a review

Lee and mountain waves are dominant dynamic processes in the atmosphere above mountain areas. ST VHF radars had been intensively used to investigate these wave processes. These studies are summarized in this work. After discussing features of long-period quasi-stationary lee waves, attention is drawn to the frequent occurrence of freely propagating waves of shorter periods, which seem to be more common and characteristic for wave processes generated over mountainous areas. Characteristics of these waves such as their relation to the topography and background winds, the possibility of trapping by and breaking in the tropopause region and their propagation into the stratosphere is investigated. These orographically produced waves transport energy and momentum into the troposphere and stratosphere, which is considered an important contribution to the kinetic energy of the lower atmosphere. The occurrence of inertia-gravity waves in the stratosphere had been confused with lee waves, which is discussed in conclusion. Finally further questions on mountain and lee waves are drawn up, which remain to be solved and where investigations with ST radars could play a fundamental role.     

Studying the fine structure of coherent echo spectra using data from Irkutsk incoherent scatter radar

Studying the processes generating different-scale inhomogeneities is one of the challenging problems of ionospheric physics. Plasma instabilities are one of the physical mechanisms by which small-scale inhomogeneities are formed. The main forms of instability in the ionospheric E-layer are two-stream and gradient-drift ones. The inhomogeneities generated by them lead to an abnormally intense radio scattering of different wavelengths (known as coherent echo (CE) or radio aurora) in the E-layer. Therefore, the method of radiowave backscattering is among the widely used methods for studying such inhomogeneities. The CE phenomenon has been investigated most intensely at high and equatorial latitudes, where the conditions for the CE origination are formed rather regularly. For the last decade, CE has also been intensely studied at midlatitudes, where it is observed less frequently and its formation conditions are less known. In 1998–2006, the purposeful studies of the midlatitude CE peculiarities were performed at the Irkutsk incoherent scatter (IS) radar, with a particular emphasis on its coherent properties. It was for the first time found out that the spectra of some data sets had a fine comb-shaped structure, which generated well-known single-humped CE spectra as a result of statistical averaging. In the scope of this study, unique coherent methods for processing individual data sets of CE signals were developed, making it possible to reveal the peculiarities of unaveraged CE-signal spectra. To describe these peculiarities, we proposed a new model of the inhomogeneity spectrum, which is the superposition of the discrete set of spatial harmonics with close wave numbers. The model was shown to adequately describe the scattered signal characteristics observed experimentally.     

Seismic imaging of complex structures with the CO-CDS stack method

Various seismic imaging methods are introduced to resolve some of the possible ambiguities of seismic interpretation in complex structures. Reducing dependency of imaging techniques on velocity or using diffraction energy for imaging more structural details are the main topics of the imaging research. In this study, we try to improve the seismic image quality in semi-complex structures by combining the common reflection surface (CRS) method with a diffraction based scheme in the common-offset domain. Previously introduced partial CRS and common offset CRS methods exhibited reliable performance in imaging complex media. Here, we were looking for stable and efficient solutions, preserving advantages of the previous methods. Herewith, the proposed operator fits better to diffractions than to reflections. Therefore, we call it the commonoffset common diffraction surface stack (CO CDS). In a previous study, improvement of the quality of seismic image by the CRS method was achieved by combination of the CDS method with the partial CRS. This resulted in the introduction of the partial CDS. Initially, in this study, the common-offset CRS traveltime equation was modified to the common-offset CDS. The hypothetical shot reflector experiment in the CRS method was changed to shot diffraction point experiment. In the introduced operator, two wavefront curvatures, observed at receivers positions, are set equal in order to satisfy the diffraction condition. In the proposed method, we search for accurate attribute sets for each considered offset individually, and then form a new operator by four coherent attributes. Application of the common- offset CDS method on synthetic and field data shows more details of the geological structures with higher quality, while preserving continuity of reflection events. The proposed method is, however, more expensive than the partial and common offset CRS for large dataset.     

The characteristics of mountain waves observed by radar near the west coast of Wales

Radar observations at 46.5 MHz of vertical-velocity perturbations at Aberystwyth (52.4^○N, 4.1^○W) have been used to examine the incidence of mountain waves and their dependence on local topography and the wind vector at low heights. A contrast is drawn between the effects of easterly winds passing over major topographical features to the east of the radar site and those of westerly winds crossing low coastal topographical features to the west. Estimates are made of the vertical flux of horizontal momentum associated with mountain waves, and the general influence of mountain-wave activity on vertical-velocity measurements at the site is assessed.     

Full-correlation analysis of turbulent scattering layers in the mesosphere observed by the MU radar

We have applied a full-correlation analysis technique to the echo power fluctuations observed by the MU radar (35°N, 136°E), and analyzed the horizontal structure of the scattering pattern in the mesosphere as well as their horizontal motions. The velocity of the scattering pattern did not agree with the background wind velocity, but was associated with the horizontal propagating direction of a saturated inertia gravity wave identified in the wind field. The length of the long axis of the characteristic ellipse of the scattering pattern was approximately 50 km, and the direction was almost perpendicular to the propagating direction of the wave. The correlation time of the scattering pattern was approximately 700 s, which is much longer than the lifetime of the isolated turbulence itself. This implies that the observed scattering pattern is associated with a region where the saturated inertia gravity wave generates turbulence.     

VHF radar observations of turbulent structures in the polar mesopause region

The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.