Department of Electronic Engineeing, Wuhan University, Wuhan 430072, China
A substantial progress in the studies of the middle and upper atmosphere in China has been seen. Theoretical and modeling efforts have concentrated on understanding the dynamical and photochemical processes. Observations by Rayleigh and sodium lidars have revealed the characteristics of atmospheric density, temperature and sodium-layer. Many new facilities under construction promise a bright and prosperous future.
Key words Middle and upper atmosphere, Atmospheric dynamics, Photochemistry, Remote sensing
Gaseous envelope above 10 km altitude is nominated as “the middle and upper atmosphere ”, which comprises the stratosphere, mesosphere and thermosphere. As an important link in the solar terrestrial system, it has been attracting a great attention of Chinese scientists. In the past four years, a significant advance has been made in understanding the dynamical and photochemical processes in the huge region. The characteristics on the middle atmospheric wind field over the Chinese area, on the nonlinear interactions among planetary waves, tidal waves and gravity waves, and on the nonlinear propagation of the gravity wave packets are revealed. The essential features of the sporadic Na layers over central China mainland are acquired by a sodium lidar. The variation of atmospheric ozone during total solar eclipse is clarified.
In experimental aspects, the National Natural Science Foundation of China has encouraged the development of ground-based observation facilities to alter the situation that the middle and upper atmospheric data base is sparse in China. Recently a synthetic observation base including two lidars, one VHF radar, one MF radar and one meteor radar is emerging in Wuhan (30.5oN, 114.4oE). The observation base would allow the simultaneous measurements of various atmospheric parameters at the same geographical location. Many valuable studies can be conducted by organizing coordinated campaigns of these instruments. This review addresses the advances in the middle and upper atmospheric studies in China from 1999 to 2002, with an emphasis on the upper stratosphere, mesosphere and lower thermosphere.
II. DEVELOPMENT OF INSTRUMENTATION AND OBSERVATION
Our knowledge on the middle and upper atmosphere depends remarkably on the development level of instrumentation. Scientists and engineers working in this field made a great endeavor to improve the observation ability. A successful acquisition of atmospheric wind and turbulence data by Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS) promoted the development of another VHF Doppler radar located in central China by Wuhan University. It is designed to possess an average power-aperture product of ~1×10‑8 W m2 and scheduled to be put into operation in 2003. An MF radar and a meteor radar dedicated to observing the winds and waves in the mesosphere and lower thermosphere have been set up by Wuhan Institute of Physics and Mathematics, CAS. A new Rayleigh and sodium lidar system developed by Wuhan University has been put into a routine operation since March 2001. Up to now 510 h observation data about the mesospheric Na layer have been acquired. In June 2002 a new telescope with 1036 mm aperture upgraded the detecting ability of the lidar system. Wuhan University has also planned to extend atmospheric trace constituent measurements with a new laser. The temperature and gravity wave activities in the mesospheric region over Beijing were obtained by the tilting filter airglow photometer at Center for Space and Applied Research (CSSAR), CAS. The multichannel photometer and all-sky camera collected a wealthy data sets on auroral features over the Zhongshan Station, Antarctic.
Based on Wuhan University lidar measurements between March and September 2001, Yi et al. (2002) presented the characteristics of sporadic Na layers (Nas layers) over Wuhan (30.5oN, 114.4oE). Nas layers were observed on 29 occasions from 275 h observational data. They tended to show a seasonal variation with a maximum occurrence rate in July. The maximum Na density was 24080 cm-3 which was comparable with those observed at low and high latitudes. Different from the observations at all other sites over the world, most Nas layers over Wuhan tended to occur around the peak altitudes of the normal Nas layers (~92 km). However, a Nas layer was observed at 112.9 km altitude. The formation of the Nas layers over Wuhan generally needed a longer time than those at low and high latitudes. The formation processes usually consisted of consecutive density enhancement bursts. The Nas layers often exhibited broader layer widths than those at low and high latitudes. Li et al.(1999) obtained the background sodium-layer characteristics over Wuhan. The observed results show a good agreement with those theoretically calculated. Ai et al.(1999) investigated the short-term variability of the sodium-layers. It is shown that the distribution of Na density changes apparently. The relative variation in Na density usually reaches 50% within one hour.
A preliminary observation with the MF radar in Wuhan shows that there is an obvious diurnal variation in the lower thermospheric wind field (Li et al., 2002). This is believed to be a manifestation of the diurnal tide activity.
Wu and Xu (2000) discussed vertical wavenumber spectra of the zonal and meridional velocity fluctuations using data measured from 10 chaff rockets and found a spectral slope of -3.0, providing observational evidence of saturated gravity wave spectrum with high resolution in the lower thermosphere. In a subsequent study, Wu et al. (2001) further discussed the mean characteristics of vertical wavenumber spectra of the scalar horizontal wind fluctuations using data measured from 33 chaff rockets. They found that the mean vertical wavenumber spectrum was good consistent with the linear saturation theory in both spectral slope and spectral amplitude. This good agreement provides further support for the linear saturation theory in a mean sense and suggests that saturation processes are present in the horizontal flow and act to produce turbulence. Therefore it is expected that turbulence is enhanced near the polar summer mesopause. In addition, chaff rocket system with very high vertical resolution has also been used to study the space-time structure of turbulence. Wu (1998) calculated spectrum of the vertical velocity fluctuations in an extremely large shear layer and found that the spectrum has spectral lopes of -3.10, -1.65, and -7.11 in the buoyancy subrange, the inertial subrange, and the viscous subrange, respectively, which is in good agreement with the neutral density fluctuations. The relationship between turbulence and gravity wave was also studied. The result indicated that enhanced turbulence was linked well with wave field saturation.
Shen et al.(2001) used the neutral atmospheric data of number density ratio of n(He)/n(N2) and n(O)/n(N2) observed by the satellite AE-D to analyse their variations during magnetic storms in November, 1975. It is indicated that the lifting of the neutral atmosphere with higher proportion of heavier constituents ( such as N2 and Ar) leads to a relative decrease of the concentration of lighter constituents (such as He and O). Comparing with the magnetic quiet day, the ratio of n(He)/n(N2) decreases by about one order of magnitude at 150 km, while n(Ar)/n(N2) increases by a little more than one order of magnitude.
Ma et al.(2001) studied characteristics of temperature at heights of 20-80 km in Chinese area by using the Nimbus-7 SAMS temperature data from 1979 to 1981. The results show obvious difference between the temperature above China and the zonal mean temperature of CIRA-1986 atmospheric model.
Researches on Radio Occultation (RO), a new atmospheric remote sensing technique, have been conducted. The principle of measuring atmospheric temperature, pressure, density and ionospheric electron density and relevant inversion methods for RO were introduced (Zhang et al., 1999). A comparison between the atmospheric densities acquired by the RO and a lidar has been made (Zeng et al., 2001). The influence of the horizontal asymmetry of the ionosphere on RO was analyzed (Zhang et al., 2002). An artificial neural network was applied in the prediction of the ionospheric parameters using the assimilating RO ionospheric data (Zeng et al., 2002).
III. DYNAMICS OF THE MIDDLE AND UPPER ATMOSPHERE
Knowledge of the mean wind is fundamental for studies of atmospheric dynamics since it represents the basic state. Ma and Liao (1999) calculated the mean wind characteristics in the Chinese area at heights of 20-80 km from the Nimbus-7 SAMS temperature data by using a geostrophic balance thermal wind model. It is shown that the mean winds derived from the satellite coincide roughly with those observed by Chinese rockets. There is a significant difference between the zonal mean winds from the satellite and the counterparts from the CIRA-1986.
Based on the observation of Wuhan Meteor radar, the prevailing winds and diurnal tides are studied. The zonal mean wind is usually eastward, but it shows a westward flow for about 20 days in March . The zonal summer maximum exceeds 42 m/s at about 94 km. The southward meridional wind exceeds 18 m/s in June. The diurnal tide propagates upward, which indicates its source is located below 80 km. Zonal and meridional diurnal tidal amplitudes reach their maximum 40 m/s and 50 m/s in March and April, when the mean winds are weak.
Planetary waves can be interpreted as the first zonal Fourier components of a decomposition of the hemispheric wind field. They play an important role in the large-scale dynamics of the middle atmosphere. Hu and Zhang (1999) have investigated the nonlinear interactions between planetary waves in the lower stratosphere by analyzing the winter geopotential height data for 30 hPa at 60°N. It is shown through a bispectral analysis that the nonlinear wave-wave interactions are a commonly-existing phenomenon in the winter stratosphere. Hu et al.(1999) also analyzed the wind data obtained by the MF radar at Yamagawa, Japan. The dynamical spectra show that 2-day wave, diurnal and semidiurnal tides dominate in the mid-latitude summer mesopause region. It is found that there are several triads whose frequencies obey the resonant conditions and their phases are coherent. This suggests the nonlinear interactions among planetary waves, tidal waves and gravity waves.
Gravity waves are ubiquitous in the middle and upper atmosphere. They can explain a large amount of variability in the middle and upper atmosphere and account for the physical causes of some transient phenomena. Zhang and Yi (1999a, 1999b, 1999c) have developed a time-dependent 2-demensional model that can be used for a variety of problems relating to nonlinear wave propagation in a compressible atmosphere. A numerical analysis shows that for an initially given upgoing gravity-wave packet whose disturbance velocity is much less than ambient wind velocity, although there exists nonlinear interaction, during the propagation, the whole wave packet and the wave-associated energy keep moving upward, while the wave front keeps moving downward. Wave-associated perturbation velocity increases with increasing height, and the mean flow shows obvious enhancement when the wave packet passes. After a long time propagation (several periods), wave-associated perturbation and energy can still concentrate in a limited region that is comparable in size to that given initially. The propagation path of wave energy coincides well with the ray path predicted by the linear gravity wave theory, but the magnitude of the wave energy propagation velocity is evidently smaller than the group velocity derived from the linear gravity wave theory. This indicates that once gravity waves are generated, they propagate almost freely along their ray paths, and the nonlinear effect will only lower their propagation velocity. While gravity-wave packets propagate in a nonisothermal atmosphere, the nonlinear propagation paths of wave energy depart evidently from the ray paths derived from the linear gravity wave theory under the WKB approximation. By using this model, the nonlinear propagation of gravity wave packets in a sheared wind field has been examined (Zhang et al., 1999) . It is indicated that the wind shear and nonlinearity act jointly to change the propagation velocity of the wave-associated energy. A breaking process of an upgoing gravity wave packet has been discussed (Zhang and Yi, 1999b). The result exhibits that the height wave breaking occurs is slightly higher than that predicted by the linear saturation theory and wave breaking occurs usually in downstream region of the wave packet.
By a numerical method, Yi (1999a,1999b) has studied the resonant interactions between propagating gravity wave packets. It is shown that through the resonant parametric excitation an upgoing gravity wave packet can cause the growth of two secondary waves from noise level up to a significant intensity in several hours. The primary wave packet is apparently deformed as it decays. The energy transfer among the interacting waves is no longer reversible since their amplitudes are localised. Therefore the characteristic time for the interactions is of a particular significance; it represents a time during which the principal energy transfer arises. Beyond the characteristic time the net energy transfer among the interacting waves becomes rather weak, but the local change in the wave energy densities can be considerable. Only a part of the initial energy of the primary wave packet is transferred to the secondary waves during parametric excitation. The amounts of energy, which each of the two secondary waves extract from the primary wave, are different, exhibiting a parameter preference in the energy transfer. The parametric excitation process can be completed in the propagation time. For the resonant interaction with two gravity wave packets initially having large amplitudes, the evolution rate is faster than that in the parametric excitation. The primary wave packet can lose most of its energy and finally be reduced to small fluctuation. The viscous dissipation not only decreases the wave energies but also strongly affects the local energy transfer among the interacting gravity wave packets.
Data obtained from the mobile SOUSY VHF radar at Andoya/Norway (69°N, 16°E) during the MAC/SINE campaign in summer 1987 have been used to study the short-term variability of tides and mean wind in the polar mesosphere (Yi, 2001). It is shown that the spectral peak corresponding to the semidiurnal tide dominates and persist almost uninterruptedly in time. The spectral peak corresponding to the diurnal tide is usually secondary and interrupted in time. The terdiurnal tide rarely presents a prominent spectral peak. The frequency corresponding to each tidal component often shows a deviation from its definition value. This likely represents a biased or perturbed state of the tide. The frequency fluctuation of the semidiurnal tide is usually smaller than those of the diurnal and terdiurnal tides. At some heights, the time variation of the semidiurnal tide amplitude is similar to that of the 36-h mean wind, which may ascribe to the nonlinear interaction between the tide and planetary waves. But for most heights, there is no clear correlation between the tide and the mean wind. The time variations in either of the zonal and meridional mean winds for two heights with a distance of 2.7 km show a considerable similarity, but the time variation trend in the zonal mean wind is distinct from that in the meridional mean wind, indicating an anisotropy in the variability of the horizontal mean wind. The time variations of the semidiurnal tide amplitude in the zonal wind for two heights with a distance of 2.7 km exhibit a considerable difference, but they present somewhat similarity in the meridional wind. This is a manifestation of the anisotropy and localization in the tidal variability. Since the frequency values of each tidal component in the zonal and meridional winds are usually different, the hodograph of its wind vector with respect to time is no longer a close ellipse in general, but still has a clockwise rotation. The semidiurnal tidal wind vector shows a clockwise rotation with increasing height, indicating a downward phase progression. The diurnal tide wind vector does not display certain rotation with height, implying it is not a travelling wave in the vertical.
The coupling between the dynamics and the chemistry in the gravity wave process is another research direction. Xu et al.(1999) have developed a dynamical-photochemical coupling model of atmospheric gravity waves. The model is used to study the influences of gravity waves on distributions of atmospheric minor species through transportation and the nonlinear photochemical reactions. The calculations indicate that the effect of gravity wave on the distributions of atmospheric trace gases is mainly through the nonlinear photochemical reactions in the mesopause region. The contribution of the nonlinear photochemical reaction caused by gravity waves can exceed the nonlinear transportation and the eddy diffusion. Gravity waves may strongly modify the mean concentrations of atmospheric compounds near the mesopause, especially during nighttime (Hu et al., 1999). Xu (2000) and Xu et al.(2001) investigated the conditions under which gravity waves become unstable due to photochemical heating in the mesopause region. The study shows that the gravity wave growth rate induced by photochemistry is sensitive to the temperature and atomic oxygen concentration profiles. Xu et al.(2000) studied the influence of gravity waves on photochemical heating in the mesopause region. The influence of the variation of background temperature and of atomic oxygen density on the loss of photochemical heating induced by gravity waves is discussed. The results indicate that as background temperature decreases or as the background atomic oxygen density increase, the gravity wave induced loss of photochemical heating increases and the ratio between it and the background photochemical heating rate also increases. Wang et al.(2001) used a 2-D nonlinear gravity wave model to study the propagation of a gravity wave packet produced at tropopause and the gravity wave breaking.
Liang et al.(1999) have studied a propagation mechanism of guided gravity waves invoked by ambient winds. It is indicated that the existence of strong ambient winds make possible the long-distance propagation of medium-scale guided gravity waves. By analyzing the data sets obtained from simultaneous observations with a sodium lidar in Wuhan, China (30.53°N, 114.37°E) and an MF radar in Yamagawa, Japan (31.20°N, 130.62°E), Hu et al. (1999) inferred that the gravity wave activities at the two sites have little connection.
IV. PHOTOCHEMISTRY OF MIDDLE AND UPPER ATMOSPHERE
Wang et al.(1999) simulated the variation in atmospheric ozone and the excited state of oxygen molecular O2 (a1 Δg ) during total solar eclipse at Mohe, China on March 9,1997. They pointed out that 1.27μm airglow radiation characteristics during total solar eclipse is obviously different from that in the twilight. The essential difference is the decrease in the emission altitude. The observation of the total solar eclipse at Mohe using 1.27μm airglow photometer shows that the increase in ozone content between 52 and 67 km is around 50%, which is consistent with the theoretical analysis.
Xu et al. (1999) have studied the influence of gravity waves on photochemical heating in the mesopause region. Their calculation indicates that gravity waves can cause the loss of photochemical heating rate in the mesopause region. It is pointed out that the photochemical heating rate loss invoked by gravity waves increases significantly when the background temperature reduces and the background atomic oxygen density increases.
V. EFFECT OF SOLAR ACTIVITY AND ANTHROPOGENIC PERTURBATION
Qin et al. (1999) discussed the change in the thermospheric composition and density during severe geomagnetic activities. The magnitude of the fluctuation increases as the altitude increases. At around 600km, it can be four times of its initial values. The fluctuation in N2 density is the largest in all the thermospheric compositions. However, the abundance of O (No/Nall) decreases notably during severe geomagnetic activities.
Tschu and Hong (1999) gave a review on aurora research. They presented the history on aurora research, outlined the essential features of the aurora such as height, brightness, form and structure, color and spectroscopy. They also enumerated nowadays scientific issues in aurora research. Yang et al. (1999) investigated the features of aurora at antarctic Zhongshan Station by video graph analysis system of all-sky camera.
VI. DIRECTIONS FOR FURTHER STUDY
The advent of many new observation facilities promises great advances in the studies of the middle and upper atmosphere. New VHF and MF radar systems will enable scientists to probe the wind and waves in the mesosphere and lower thermosphere, to further understand its responses to the various driving forces from the solar activity and anthropogenic perturbations. New lidar systems will allow scientists to study the chemical and physical processes in the mesopause region. In theoretical aspects, Chinese scientists will devote every effort to establishing cause/effect model on space weather events in the middle and upper atmosphere, and to seek their prediction techniques. It is also a scientific objective of Chinese scientists to identify the long-term variations of the middle and upper atmosphere.
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