**ADVANCES ON STUDY
OF MIDDLE AND UPPER ATMOSPHERE AND THEIR COUPLING WITH LOWER ATMOSPHERE**^{}

*
*

*LU Daren,*

Institute of Atmospheric
Physics, CAS, Beijing 100029, China

*YI Fan, *

College of Electronic
Engineering, Wuhan University, Wuhan 430072, China

*XU Jiyao*

Center for Space
Science and Application Research, CAS, Beijing 100080, China

* *and *LI Weiliang*

Chinese Academy of
Meteorological Sciences, China Meteorological Administrationm, Beijing 100081,
China

ABSTRACTIn this paper advances on study of middle and upper atmosphere and their coupling with lower atmosphere in China in recent four years are briefly reviewed. This review emphasized four aspects, i.e. (1) development of instrumentation for middle and upper atmosphere observation; (2) analysis and observation of mid- and upper atmosphere over China; (3) theoretical and modeling study of planetary wave and gravity wave activities in middle atmosphere and their relation to lower atmospheric processes; and (4) study on coupling between the stratosphere and the troposphere.

Key words:middle atmosphere, gravity waves, planetary waves, stratosphere-troposphere exchange (STE), stratospheric ozone

I. INTRODUCTIONStudy on middle and upper atmosphere and their relationship to the lower atmosphere play an important role in understanding the basic processes of the whole atmosphere and their response to global change and multiple applications for prediction and monitoring of the atmospheric situations. In past four years, three aspects in this research field have been conducted by Chinese scientists, i.e., development of instrumentation, middle atmosphere planetary wave propagation and Gravity Waves (GW), observation and analysis of the middle and upper atmosphere, as well as stratosphere-troposphere coupling. In this paper, the activities and some results are briefly introduced.

II. DEVELOPMENT OF INSTRUMENTATION FOR MIDDLE AND UPPER ATMOSPHERE OBSERVATIONSince the knowledge of the middle atmosphere is mainly depending on the appropriate field observation, with space-borne, air borne, and ground-based instrumentations. In past four years, a series of ground-based facilities have been developed and put into operation. The Xianghe VHF/ST radar developed by IAP has been successfully finished and has obtained wind profiles from 2—22 km,

C^{2}_{n }profiles, as well as directional apparent reflectivity from the zenith to 20^{ o }of zenith angles showing Fresnel reflection and turbulent scattering. This VHF/ST radar is the first stage of complete MST radar, which is being progressed. Second VHF radar is being constructed in Wuhan University, which will be operated in 2003. An MF radar dedicated for observing the winds and waves in the mesosphere and lower thermosphere has been set up by Wuhan Institute of Physics and Mathematics since 2002 and quasi-continuous observation has been conducted at Wuhan. A new telescope with 1m aperture has upgraded the detecting ability of the Rayleigh and Sodium Lidar of Wuhan Institute of Physics and Mathematics. In Chinese Antarctic Expedition Station, Zhongshan Station, the multi-channel photometer and all-sky camera for Aurora observation, which are cooperated with Japanese Polar Research Institute, have collected a wealthy of data sets.

III. OBSERVATION AND ANALYSIS OF THE MIDDLE AND UPPER ATMOSPHERE1. Field Observation Activities

LAGEO (Lab. for middle Atmosphere and Global Environment Observation), IAP, CAS initialized quasi-operational ozone sounding over Beijing, China since spring 2001 for twice soundings each week. This activity is collaborated with China Meteorological Administration for advancing operational ozone observation. The Rayleigh and Sodium Lidar system of Wuhan Institute of Physics and Mathematics, CAS is making sodium layer and middle atmospheric density profile observation quasi-continuously. A more powerful Rayleigh and sodium resonance fluorescence Lidar system has been established by Dr. Yi's group of Wuhan University in 2001. During March to Sept. 2001, Yi et al., has made observations of 29 cases of sporadic Na layers in 23 nights out of 39 nights observation .The results reveal the similarity and differences between present observation and other low and high latitude observations.

2. Analysis of Satellite Observations of the Middle and Upper Atmosphere

Shen et al. (2001) used the neutral atmospheric data of number density ratio of n(He)/n(N

_{2}) and n(O)/n(N_{2}) observed by the satellite AE-D to analyze their variations during magnetic storms in November 1975. The results indicate that the lifting of the neutral atmosphere with higher proportion of heavier constituents (such as N_{2}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(N_{2}) decreases by about one order of magnitude at 150 km, while n(Ar)/n(N_{2}) increases by a little more than one order of magnitude.Ma et al. (2001) studied characteristics of temperature at height of 20—80 km over China by using the Nimbus-7 SAMS temperature data from 1979—1981. The results show obvious difference between the temperature above China and zonal mean temperature of CIRA-1986 atmospheric model. The HALOE sunrise data observed by UARS satellite in the summer of 1992—1996 and 1998 are analyzed and interpolated to a 5

^{o}×5^{o}grid region by Jian et al. (2001). The analysis of vertical section of the averaged NO_{x}mixing radio along 30^{o }N and 40^{o }N has been made. The vertical distributions of NO_{x}at different longitudes are compared. The results show that there are two peaks of NO mixing radio at 1 hPa and 1×10^{-5 }hPa, respectively, and a minimal value at 0.1 hPa. NO mixing ratio is very steady between 50 and 0.3 hPa, firmly increasing above 0.1 hPa and undulating in the thermosphere. It also shows that NO_{2}mixing ratio is minimal at 100 and 0.5 hPa, reaching maximum between 5 and 100 hPa, but rapidly increasing below 100 hPa in the troposphere. The position of the maximum is nearly unchanged at different longitudes and a little variance at different latitudes while the lower latitudes get higher maximum position. The NO_{x}and O_{3}distributions have much similar characteristics in the middle atmosphere, especially in the stratosphere, they reach maximum at nearly the same pressure. It seems that they have very close relationship.3. Analysis of Variation Characteristics of Atmospheric Ozone

There are 5 stations over China for monitoring of atmospheric ozone variation. Two of them are used Dobson instruments and started observations since the end of 1950s. From 1979, these instruments are routinely operated at Beijing (39.93

^{o }N 116.40^{o }E) and Kunming (25.02^{o }N, 102.68^{ o }E), respectively. Based on data of total atmospheric ozone observed over these two stations, Bian et al (2002) analyzed the variation characteristics of atmospheric ozone contents, including long-term trend, interannual, seasonal, intraseasonal variations. It is found that, 1) the long-term trend for the period of 1979/1980 – 2000 is –0.642 DU/a and 0.009 DU/a in Beijing and Kunming, respectively. These values of long-term trend are significantly less than the values deduced by other authors (Li & Lu 2002) based on data of shorter time period (e.g. 1980—1991). This difference raises the problem of how to identify the natural multi-year variability and anthropogenic influences; 2) there exist strong intra-seasonal variations, especially in winter, which are comparable to seasonal variations both in Beijing and Kunming; 3) there exists significant QBO signals both in mid-latitude (Beijing) and low latitude (Kunming) which is the main component (combined with long-term trend) of the inter-annual variation.

IV. STUDY ON MIDDLE ATMOSPHERE PLANETARY WAVES AND GRAVITY WAVES (GW)1. Middle Atmosphere Planetary Waves

Atmospheric planetary waves which are mainly produced in the troposphere by dynamical and thermal effects of solar radiation and earth surface characteristics, in certain conditions will propagate to the middle atmosphere and play important roles in regulating middle atmosphere circulation and constituents distribution, such as ozone. This topic is still one of the important research subjects.

To understand the function of the QBO modulation of planetary wave propagation in the middle atmosphere and its effect to the transport of ozone from its low latitude source to high latitudes, Chen and Huang (1999) studied the planetary wave-mean flow interactions during the different stages of the QBO with the quasi-geostrophic zonally–averaged mean flow equations and the linearized primitive equations for planetary waves. In the linear case of steady flows, there is no feedback between the mean flow and the planetary waves. The difference of dissipitating planetary wave propagation between easterly and westerly phases of QBO is chiefly confined to low latitudes. In reality, planetary waves are often neither steady nor conservative. In this situation, there always exists planetary wave-mean flow interaction and coupling. In this case, a noticeable difference of planetary wave propagation emerges clearly at middle and high latitudes with obvious larger upward and equatorward EP fluxes in the easterly phase winter than in the westerly phase winter. By modulating the propagation of planetary waves, the tropical QBO introduces significant variability of the planetary wave amplitudes and the residual circulation in the Northern Hemisphere. The planetary wave amplitudes are shown to be greater during the easterly phase winter than during westerly winter.

By using the transformed Eulerian–mean equations, Chen and Huang (2002) studied the dynamics of planetary waves. It is found from both observations and simulations that in the Northern Hemisphere winter there are two waveguides for meridional propagation of quasi-stationary planetary waves, i.e., the high latitude waveguide and the low latitude waveguide. This is in agreement with theoretical analysis. The convergence of EP fluxes reveals that the stratosphere sudden warning (SSW) is the result of anomalous planetary wave propagation along the high latitude waveguide and its interaction with mean flows. The analysis also shows that the tropical QBO may influence the low latitude waveguide of planetary wave propagation and furtherly modulate the high latitude waveguide through wave-mean flow interactions. The modification of the planetary wave propagation by the equatorial QBO winds is shown to have an important impact on the transport circulation. It is found by model simulation that the meridional transport is amplified during the easterly phase of the QBO. This mechanism may explain the international variability of ozone in the stratosphere at high latitudes.

2. Sources and Mechanisms of Gravity Waves

Gravity Waves (GW) play an important role in the middle atmospheric circulation. It is commonly recognized that sources of the middle atmosphere GW are mainly from the troposphere but the detailed mechanism and quantitative description of the source mechanism are still not very clear. Based on an observation case of strong summer deep convection (hail storm) in mid-latitude China, Chen and Lu (2001) conducted numerical simulation for GW excitation in the stratosphere by the deep convection with a nonhydrostatic compressive model coupled with a bulk cloud microphysics parameterize scheme. The simulation revealed that owing to penetrating convection through the tropopause, three distinct subareas—the wave-energizing, the wave-exciting, and the wave-bearing subareas are found to be responsible for the wave generation. The GW wave-induced momentum flux

_{ }at the tropopause height is about 0.3 N· m^{-2}, which is comparable to those of orographically excited GWs in winter. Since the strong convections are often existed in mid-latitude summer in East Asia, it is anticipated that there exist significant GW sources for this region, not just for tropical area. In fact, GW can also be excited by the stratospheric processes. Based on observation of Stratospheric Sudden Warming (SSW) in Feb. 1979, it was found owing to upward propagation of quasi-stationary planetary waves, strong ageostrophic motion existed in the stratosphere, which caused strong divergence field and strong large amplitude GWs. Huang and Chen (2002) simulated such a mechanism of GW excitation with a linear barotropic spherical-spectral model. Taking the actual geostrophic deviation at 10 hPa on Feb. 22, 1979 as the initial disturbance, the geostrophic adaptation process of geostrophic disturbances, and the excitement and propagation of the large amplitude GWs during SSW are successfully simulated. This simulation reveals that stratospheric geostrophic adaptation process is also a GW excitation mechanism within the middle atmosphere.By using a Full-Implicit-Continuous-Eulerian (FICE) scheme and taking the atmospheric motion equations in spherical coordinates as governing equations, a fully-nonlinear global dynamical model for the middle and upper atmosphere is established, and a reasonable polar boundary is specified. Detailed descriptions of the model formulation and numerical methodology are presented by Zhang et al. (2001). A numerical examination shows that the model is capable of simulating the long time (30 hours) evolution of atmospheric waves. As a further examination of the global dynamical model and a preliminary study of global nonlinear propagation of gravity-wave packets in the middle and upper atmosphere, several more examples of the nonlinear propagation of gravity-wave packets with different properties (e.g. wave vectors, initial disturbance amplitudes, starting positions) are carried out individually. The numerical analysis shows that wave-associated energies propagate almost freely along the ray path derived from linear gravity theory, the nonlinear effect can only lower the magnitude of the wave energy propagation velocities, and the initial disturbance magnitudes have no obvious influences on the propagation paths of wave energy, which is consistent with the result given by two-dimension numerical model. A quantitative comparison of a linear simulation with the linear ray theory is presented, which proves that the present numerical model can precisely simulate the propagation of atmospheric waves, and it is the nonlinearity rather than the model itself that lower the magnitude of the wave energy propagation velocities.

By using an Alternative-Direction-Implicit (ADI) scheme, a numerical model for nonlinear propagation of gravity-wave packets in a three-dimensional compressible atmosphere is developed by Yue and Yi (2001). Results of numerical simulation show that after several periods, the gravity-wave packets propagate steadily upward and keep its shape well. The interior points and boundaries of the simulated region are stable. During the propagation the amplitude of wave-associated perturbation velocities increase with the increasing height. This shows that ADI scheme is reliable for simulating the nonlinear propagation of gravity-wave packets in a three-dimensional compressible atmosphere. From the results, it is also found that under nonlinear condition, a gravity wave packet is still able to keep some characterizations under linear propagation. The nonlinear effect mainly impacts the propagation velocity of gravity-wave packets.

3. Interaction Study

With the German SOUSY radar observation data take from the height range of 78—94.2 km in summer 1987, Xiong and Yi (2001) analyzed the nonlinear wave interaction in the mesopause range (83.4—91.2 km). It is found that 35 h wave, semi-diurnal tides, and inertial-gravity waves at period of about 8.9 h existed at almost all heights in the zonal wind and these frequencies satisfied the resonance condition. Also the 33 h wave semi-diurnal tides and inertial-gravity waves at period of about 19 h in the meridional winds satisfied the resonance condition. By using bispectral analysis, it is found the non-linear wave interactions may exist in the mesopause region which leads to the variation of tide amplitude temporally and spatially.

As for the GW saturation processes, Wu and Xu (2000) discussed vertical wavenumber spectra of the zonal and meridional velocity fluctuation using data measured from 10 chaff rockets data from German scientists 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 and Xu et al. (2001) further discussed the mean characteristics of vertical wavenumber spectra of the scalar horizontal wind fluctuation using data measured from 33 chaff rockets. They found that the mean vertical wavenumber spectrum was 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. We can therefore expect 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.

For the study on the coupling between the dynamics and the chemistry in the gravity wave process is another research direction. Xu et al. have made 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 (Xu 2000). Xu et al. investigated the conditions under which gravity waves become unstable due to photochemical heating in the mesopause region (Xu 2000, Xu & Smith 2001). The study shows that the gravity wave growth rate induced by photochemistry is sensitive to the temperature and atomic oxygen concentration profiles. And the critical vertical wavelength is given. Xu et al. studied the influence of gravity waves on photochemical heating in the mesopause region. The balance of the variation of background temperature and of atomic oxygen density on the loss of photochemical heating induced by gravity waves are discussed. The results indicate that as background temperature decreases or as the background atomic oxygen density increases, the gravity wave induced loss of photochemical heating increases and the ratio between it and the background photochemical heating rate also increases (Xu 2000). Wang et al. (2001) used a 2-D nonlinear gravity wave model to study the propagation of a gravity wave packet produced at the tropopause and the gravity wave breaking.

V. STUDIES ON STRATOSPHERE-TROPOSPHERE EXCHANGE (STE) AND RELATED PROCESSESStudy on stratosphere-troposphere interaction and related processes has been an active research topics owing to significance relating to earth climate change and environmental issues through dynamical, physical, as well as chemical processes.

1. Global Tropopause Climatology and Long-term Variation

Since STE is those processes existed between lower stratosphere and upper troposphere, i.e., above and under the tropopause, the climatology and its spatial and temporal variation of the global tropopause distribution itself become a significant topic. Based on 40 years (1958—1997) NCEP global data, Li et al. (2003) analyzed the characteristics and long-term trend of the global tropopause. It is found that owing to the forcing of both tropical convection and extratropical wave forcing, the seasonal variation of the tropopause height reveals the characteristics of that not just simply as higher in summer and lower in winter. In Northern Hemisphere high latitudes, the annual variation reveals two peaks and two valleys. In Southern Hemisphere high latitudes, the tropopause height in winter is higher than that in summer. All these characteristics are controlled by the general circulation pattern and radiation distribution. It is found from the analysis of the long-term trend of global tropopause height that in certain regions in middle and high latitudes of both hemispheres, long-term trend exists with statistically significance. Also global abrupt change of the long-term trend happened during 1979, i.e., the trend of 1958—1978 is obviously different from that of 1979—1997.

2. Impact of Tropopause Variation on Ozone Distribution in UT/LS

Because most of ozone column resides in the lower stratosphere, small change in ozone abundance could have large impact on climate and the surface UV flux. The tropopause, as a transition or boundary between the well mixed, ozone-poor upper troposphere and the stratified ozone rich lower stratosphere, its variation has direct and evident effect on the distribution of ozone and its column abundance. Li et al. (2002) simulated the impact of tropopause variation on the distribution of ozone in UT/LS using a two-dimensional model. Model results show that the annual cycle of the tropopause height has remarkable impact on the distribution of ozone. The local change of ozone can be more than 10%. The impact is caused mainly by the variation of the horizontal and vertical eddy exchange coefficients in the model due to the annual cycle of the tropopause height. The conclusion is proved by the results of the re-constructed dynamic model. The annual cycle of the tropopause height also remarkably affects the distributions of other trace gases, such as some long-lived species: HCl, HNO

_{3}, NO_{x}etc. and short-lived species: NO_{2}and NO etc. When the tropopause is increased (decreased) by 1 km in winter in northern middle latitudes, model results show that the impact on ozone distribution is evident and the local change of ozone can be more than 6%. However the impact on total ozone is relatively small, the variation is less than 5 Dobson units. This result is less than the statistic results between tropopause height and total ozone (10 to 60 DU per 1 km tropopause change).3.Regional Mass Exchange between Stratosphere and Troposphere over Tibetan andNeighboring AreaBy using daily NCEP data of 1978—1996 and SAGE data of 1988—1993, calculations of mass exchange between stratosphere and troposphere and aerosol and ozone concentration at 100 hPa were made for Tibetan and neighbouring area by Cong et al. (2001). It is found that (1) in summer, mass exchange is predominated by transport from the troposphere to the stratosphere centered at northern Bay of Bengal and southeastern Tibet. In winter, mass exchange is predominated from the stratosphere to the troposphere; (2) 19 year average transport from the troposphere to the stratosphere over this area is about 14.84×10

^{18}kg in summer. There exists a channel in this region for upward mass transport. Owing to pollutants at lower atmosphere at this region, the atmospheric mass transport would increase all aerosol and other pollutants loading near the tropopause of this region, then possibly result in ozone decreasing over Tibetan area.4. Mesoscale Modelling of STE over East Asia

Four synoptic cases of each season with strong cut-off-lows in East Asia were simulated using regional mesoscale model MM5 by Yang and Lu (2003). Their detailed STE dynamical characters were analyzed. The results showed that there existed different mechanisms for cut-off-low induced STE. Turbulent mixing near the jet stream is the primary contributor in winter case. The interaction between the surface front and upper front is the primary contributor in spring case. Strong convection over ocean is the primary contributor in summer case. In autumn, cut-off low induced small but broad area descending of the tropopause plays important role. According to model trial results, it was found that the tropopause was not a simple mass surface but a transition zone. It could be fixed by the variation of flux following the PV. The total upward and downward fluxes in the tropopause are strong for winter and spring cases, while weak for summer and autumn cases. The largest net flux from the stratosphere to the troposphere appears in spring case and with the lowest in summer case. The maximum ratio of upward mass and downward mass appears in summer, showing a strong upward transport of air mass in summer and with weakest in autumn.

5.

Model Simulation of the Effect of NO_{x}Emission from Aircraft on the Distribution of Ozone and the Other Trace GasesModel results show that NO

_{x}emission from aircraft has significant contribution to NO_{x}distribution at 8—15 km in northern middle latitudes by Li et al. (2003). However its impact on ozone distribution is insignificant (less than 4%). Because of the fast development of aviation transportation, the atmospheric effect on aircraft emissions needs paying more attention to. The atmospheric effects are considered when the NO_{x}source from aircraft is lifted and lower 3 km in the model simulation. The results show that the different positions of NO_{x}source can cause large difference of impacts on the distribution of NO_{x}and ozone.

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