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1.
An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (～69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1 km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ～80–87 km of the Southern Hemisphere (SH) to be up to 7.5 K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.  相似文献   

2.
We present results from the Numerical Spectral Model (NSM), which focus on the temperature environment of the mesopause region where polar mesospheric clouds (PMC) form. The PMC occur in summer and are observed varying on time scales from months to years, and the NSM describes the dynamical processes that can generate the temperature variations involved. The NSM simulates the quasi-biennial oscillation (QBO), which dominates the zonal circulation of the lower stratosphere at equatorial latitudes. The modeled QBO extends into the upper mesosphere, due to gravity wave (GW) filtering, consistent with UARS zonal wind and TIMED temperature measurements. While the QBO zonal winds are confined to equatorial latitudes, the associated temperature variations extend to high latitudes. The meridional circulation redistributes the QBO energy—and the resulting temperature oscillations away from the equator produce inter-annual variations that can exceed 5 K in the polar mesopause region, with considerable differences between the two hemispheres. The NSM shows that the 30-month QBO produces a 5-year or semi-decadal (SD) oscillation, and stratospheric NCEP data provide observational evidence for that. This SD oscillation extends in the temperature to the upper mesosphere, where it could contribute to the long-term variations of the region.  相似文献   

3.
A noctilucent cloud is seen at a particular time from a specified place. The journey of the cloud particles from nucleation to observation can be calculated by using a simple model of growth and taking account of the fall speed of the cloud particles. Cloud particles can be backtracked by bringing together growth and fall speed equations and a model of mesospheric winds to find where the particles of a cloud seen at a particular time and place have originated. The wind model that is used here suggests that there is a distinct outer edge to the summertime polar circulation pattern in which water vapour is being carried up from the lower mesosphere to the mesopause. The change in latitude of this outer edge during the summer season may well account for the observed seasonal change in occurrence of mesospheric clouds. Polar mesospheric clouds cause a drying of the upper mesosphere. It is suggested here that diffusion of water vapour dumped at the level of polar mesospheric clouds will take an appreciable time to carry water vapour back up to the mesopause. In consequence, there will be a significant separation between the observed location of a noctilucent cloud and its precursor polar mesospheric cloud.  相似文献   

4.
The 16-day planetary wave in the mesosphere and lower thermosphere   总被引：3，自引：0，他引：3  
A meteor radar located at Sheffield in the UK has been used to measure wind oscillations with periods in the range 10–28 days in the mesosphere/lower-thermosphere region at 53.5°N, 3.9°W from January 1990 to August 1994. The data reveal a motion field in which wave activity occurs over a range of frequencies and in episodes generally lasting for less than two months. A seasonal cycle is apparent in which the largest observed amplitudes are as high as 14 ms−1 and are observed from January to mid-April. A minimum in activity occurs in late June to early July. A second, smaller, maximum follows in late summer/autumn where amplitudes reach up to 7–10 ms−1. Considerable interannual variability is apparent but wave activity is observed in the summers of all the years examined, albeit at very small amplitudes near mid summer. This behaviour suggests that the equatorial winds in the mesopause region do not completely prevent inter-hemispheric ducting of the wave from the winter hemisphere, or that it is generated in situ.  相似文献   

5.
Continuous MF and meteor radar observations allow detailed studies of winds in the mesosphere and lower thermosphere (MLT) as well as temperatures around the mesopause. This height region is characterized by a strong variability in winter due to enhanced planetary wave activity and related stratospheric warming events, which are distinct coupling processes between lower, middle and upper atmosphere. Here the variability of mesospheric winds and temperatures is discussed in relation with major and minor stratospheric warmings as observed during winter 2005/06 in comparison with results during winter 1998/99.Our studies are based on MF radar wind measurements at Andenes (69°N, 16°E), Poker Flat (65°N, 147°W) and Juliusruh (55°N, 13°E) as well as on meteor radar observations of winds and temperatures at Resolute Bay (75°N, 95°W), Andenes (69°N, 16°E) and Kühlungsborn (54°N, 12°E). Additionally, energy dissipation rates have been estimated from spectral width measurements using a 3 MHz Doppler radar near Andenes. Particular attention is directed to the changes of winds, turbulence and the gravity wave activity in the mesosphere in relation to the planetary wave activity in the stratosphere.Observations indicate an enhancement of planetary wave 1 activity in the mesosphere at high latitudes during major stratospheric warmings. Daily mean temperatures derived from meteor decay times indicate that strong warming events are connected with a cooling of the 90 km region by about 10–20 K. The onset of these cooling processes and the reversals of the mesospheric circulation to easterly winds occur some days before the changes of the zonal circulation in the stratosphere start indicating a downward propagation of the circulation disturbances from the MLT region to the stratosphere and troposphere during the stratospheric warming events. The short-term reversal of the mesospheric winds is followed by a period of strong westerly winds connected with enhanced turbulence rates and an increase of gravity wave activity in the altitude range 70–85 km.  相似文献   

6.
First HF radar measurements of summer mesopause echoes at SURA   总被引：3，自引：0，他引：3  
HF sounding of the mesosphere was first carried out at SURA in summer 1994 at frequencies in the range 8–9 MHz using one of the sub-arrays of the SURA heating facility. The observations had a range resolution of 3 km. Almost all measurements indicated the presence of strong radar returns from altitudes between 83 and 90 km with features very similar to VHP measurements of mesopause summer echoes at mid-latitudes and polar mesopause summer echoes. In contrast to VHP observations, HF mesopause echoes are almost always present.  相似文献   

7.
We present time series of January–May mean mesosphere/lower thermosphere (MLT) mean winds and planetary wave (PW) proxies over Europe together with stratospheric stationary planetary waves (SPW) at 50°N and time series of European ozone laminae occurrence. The MLT winds are connected with stratospheric PW and laminae at time scales of several years to decades. There is a tendency for increased wave activity after 1990, together with more ozone laminae and stronger MLT zonal winds. However, possible coupling processes are not straightforward. While mean MLT winds before the 1990s show similar interannual variations than stratospheric PW at 100 hPa, later a tendency towards a connection of the MLT with the middle stratosphere SPW is registered. There is also a tendency for a change in the correlation between lower and middle stratosphere SPW, indicating that coupling processes involving the European middle atmosphere from the lower stratosphere to the mesopause region have changed.  相似文献   

8.
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9.
Based on 5 years of OH imager data between September 2003 and September 2008 over Yucca Ridge Field Station, CO (40.7ºN, 104.9ºW), we presented the variation of gravity wave (GW) occurrence frequency and propagation direction in the upper mesosphere. In summer the GW occurrence frequency was extremely high at above 95% compared to other seasons (around 85%). The GW propagation direction showed a strong northward (poleward) preference in summer and a southward (equatorward) preference in winter. This could be possibly due to ducting of waves in the mesopause thermal structure and wave generation by the strong deep convection located at south side in summer and possible storms located at north side in winter. Westward traveling waves were rare, but eastward were frequent. In addition to seasonal variability, significant interannual variability was also observed.  相似文献   

10.
Continuous wind observations allow detailed investigations of the upper mesosphere circulation in winter and its coupling with the lower atmosphere. During winter the mesospheric/lower thermospheric wind field is characterized by a strong variability. Causes of this behaviour are planetary wave activity and related stratospheric warming events. Reversals of the dominating eastward directed mean zonal winds in winter to summerly westward directed winds are often observed in connection with stratospheric warmings. In particular, the amplitude and duration of these wind reversals are closely related to disturbances of the dynamical regime of the upper stratosphere.The occurrence of long-period wind oscillations and wind reversals in the mesosphere and lower thermosphere in relation to planetary wave activity and circulation disturbances in the stratosphere has been studied for 12 winters covering the years 1989–2000 on the basis of MF radar wind observations at Juliusruh (55°N, since 1989) and Andenes (69°N, since 1998). Mesospheric wind oscillations with long-periods between 10 and 18 days are observed during the presence of enhanced planetary wave activity in the stratosphere and are combined with a reversal of the meridional temperature gradient of the stratosphere or with upper stratospheric warmings.  相似文献   

11.
Studies on the influence of solar activity in 11-year cycle on middle atmospheric thermodynamic parameters, such as temperature, pressure and density, and zonal and meridional wind components over three meteorological rocket launching stations, located in the tropics (Thumba), mid-latitude (Volgograd) and high-latitude (Heiss Island) regions of the northern hemisphere have been carried out. The temperature in all the three regions showed a negative response in the stratosphere and positive association in the mesosphere with the changes in solar activity. The temperature decreases by 2-3% from its mean value in the stratosphere and increases by 4-6% in the mesosphere for an increase in 100 units of solar radio flux. Atmospheric pressure is found to be more sensitive to solar changes. An average solar maximum condition enhances the pressure in the stratosphere by 5% and in the upper mesosphere by 16-18% compared to the respective mean values. Density also showed strong association with the changes in solar activity. Increase in the solar radio flux tends to strengthen winter westerlies in the upper stratosphere over the mid-latitude and summer easterlies in the middle stratosphere over tropics. Larger variability in the zonal wind is noted near stratopause height. Results obtained from the study indicate that there is an external force exerted on the Earth’s atmosphere during the period of high solar activity. These results can be incorporated for further studies on the dynamics of the middle atmosphere in association with the changes in solar activity.  相似文献   

12.
本文利用TIMED卫星搭载的SABER探测仪对全球中间层顶信息进行了研究,包括中间层顶的高度、温度及其季节和纬度变化,并对双中间层顶现象进行了分析.中间层顶的温度约在160～180K之间变化,高度在85～100km内变化,温度和高度都是冬季高夏季低,有着较为一致的变化趋势.中间层顶高纬呈现显著年变化,而低纬和赤道呈现弱的半年变化,南北半球的中间层顶信息有着不对称性.高纬地区的双中间层顶现象十分显著,中间层顶一般会从100km附近迅速降低至85km附近.根据长时间范围内平均的结果显示,北半球的双中间层顶现象在20°N—30°N的中纬范围开始发生,证实了北半球双中间层顶现象不再仅限于极区和中高纬地区.而南半球则仍是在50°S才显著发生双中间层顶现象.我们统计了中高纬地区夏季所有的单个观测剖面并且与当年冬季的平均背景剖面相比较,数据显示,较低的夏季第二中间层顶高度绝大多数比冬季中间层顶低12～16km.  相似文献   

13.
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   

14.
The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.  相似文献   

15.
Observations from the Nimbus 6 pressure modulator radiometer (PMR) have been used to estimate monthly mean planetary wave fluxes of heat and momentum in the stratosphere and mesosphere. While the eddy heat fluxes play an important role in the mean meridional circulation of the winter stratosphere they are shown to be less important in the upper mesosphere. Incorporation of the observed momentum fluxes into the Oxford two-dimensional circulation model has shown that they are incapable of providing the momentum transport necessary to balance the zonal flow accelerations induced by the mean meridional motion. Other unspecified transfer processes represented by Rayleigh frictional damping of the zonal fow are shown to dominate. In contrast the observed fluxes in the stratosphere achieve the necessary redistribution of momentum. Moreover their interannual variability profoundly influences the stratospheric circulation, as demonstrated in the model by the use of two different annual sets of observed momentum fluxes. The desirability of calculating the planetary wave behaviour within the model is indicated.  相似文献   

16.
A simple technique has been used to investigate gravity waves in the Antarctic and Arctic mesosphere. Data from two meteor radars are used, one at Rothera in the Antarctic (68°S) and one at Esrange in the Arctic (68°N). Observations have been made from 2005 to 2008 in the Antarctic and from 2000 to 2008 in the Arctic. The results reveal a seasonal behaviour with a semi-annual cycle. Over both sites wave activity maximises at the solstices and minimises at the equinoxes. Monthly-mean gravity-wave activity increases with height in all seasons except in summer when gravity-wave variances show little or no increase with height at heights below about 90 km. Above about 90 km in summer there is a rapid increase in wave activity. We suggest that this summer-time behaviour is a consequence of rapid wave growth as gravity waves ascend into the more stable air of the lower thermosphere. Differences between the Arctic and Antarctic gravity-wave field are also evident. In particular, a higher level of gravity-wave activity is observed in the Antarctic spring compared to the Arctic spring. We suggest that these inter-hemispheric differences may be the result of differences in the generation of gravity waves in the polar troposphere and stratosphere.  相似文献   

17.
This study first investigates the effect of the Madden-Julian Oscillation(MJO) on the Northern Hemisphere(NH)mesosphere. Both observations and simulations suggest significant cooling in the NH polar mesosphere approximately 35 days after MJO phase 4(P4), which lags the MJO-induced perturbation in the upper stratosphere by 10 days. The enhanced planetary waves(PWs) propagate upward and result in wavenumber-1 pattern temperature anomalies in the mesosphere lagging MJO P4 by 25 days. The anomalous PWs also lead to the weaker eastward zonal wind in the upper stratosphere and lower mesosphere lagging MJO P4 by 30 days. Simultaneously, the weaker westerlies result in weaker climatological westward gravity waves(GWs) in the mesosphere due to critical-level filtering. The mesosphere meridional circulation is suppressed due to both anomalous PWs and GWs, and this suppression causes polar mesospheric cooling lagging MJO P4 by 35 days.  相似文献   

18.
A study on the variability of temperature in the tropical middle atmosphere over Thumba (8 32’ N, 76 52’ E), located at the southern part of India, has been carried out based on rocket observations for a period of 20 years, extending from 1970 to 1990. The rocketsonde-derived mean temperatures over Thumba are corrected prior to 1978 and then compared with the middle atmospheric reference model developed from satellite observations and Solar Mesosphere Explorer (SME) satellite data. Temperature variability at every 1 km interval in the 25–75 km region was analysed. The tropical stratosphere is found to be highly stable, whereas considerable variability is noted in the middle mesosphere. The effect of seasonal cycle is least in the lower stratosphere. Annual and semi-annual oscillations in temperature are the primary oscillations in the tropical middle atmosphere. Annual temperature oscillations are dominant in the mesosphere and semi-annual oscillations are strong in the stratosphere. The stratopause region is noted to be the part of the middle atmosphere least sensitive to the changes in solar activity and long-term variability.  相似文献   

19.
The northern annular mode (NAM) has been successfully used in several studies to understand the variability of the winter atmosphere and its modulation by solar activity. The variability of summer circulation can also be described by the leading empirical orthogonal function (EOF) of geopotential heights. We compare the annular modes of the summer geopotential heights in the northern hemisphere stratosphere and troposphere in the Goddard Institute for Space Studies (GISS) ModelE with those in the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. In the stratosphere, the summer NAM obtained from NCEP/NCAR reanalysis as well as from the ModelE simulations has the same sign throughout the northern hemisphere, but shows greater variability at low latitudes. The patterns in both analyses are consistent with the interpretation that low NAM conditions represent an enhancement of the seasonal difference between the summer and the annual averages of geopotential height, temperature and velocity distributions, while the reverse holds for high NAM conditions. Composite analysis of high and low NAM cases in both model and observation suggests that the summer stratosphere is more “summer-like” when the solar activity is near a maximum. This means that the zonal easterly wind flow is stronger and the temperature is higher than normal. Thus increased irradiance favors a low summer NAM. A quantitative comparison of the anti-correlation between the NAM and the solar forcing is presented in the model and in the observation, both of which show lower/higher NAM index in solar maximum/minimum conditions. The temperature fluctuations in simulated solar minimum conditions are greater than in solar maximum throughout the summer stratosphere.The summer NAM in the troposphere obtained from NCEP/NCAR reanalysis has a dipolar zonal structure with maximum variability over the Asian monsoon region. The corresponding EOF in ModelE has a qualitatively similar structure but with less variability in the Asian monsoon region which is displaced eastward of its observed position. In both the NCEP/NCAR reanalysis and the GCM the negative anomalies associated with the NAM in the Euro-Atlantic and Aleutian island regions are enhanced in the solar minimum conditions, though the results are not statistically significant.  相似文献   

20.
A new two-dimensional zonal model of the stratosphere, based on a formulation in an isentropic framework, with complete chemistry has been coupled with a high resolution detailed microphysical model for polar stratospheric clouds (PSCs). The 2D model chemistry includes all presently known heterogeneous processes on sulfate aerosols and PSCs. The coupling of these two models, with inherently different time scales, is discussed. It is demonstrated that in order to obtain a realistic interrelationship between NOy and N2O an accurate simulation of the sedimentation by PSC particles is necessary. A good agreement of model PSC presence and observations is found for the Antarctic polar winter without the need to impose additional artificial temperature variations in the model. The calculated occurrence of polar stratospheric clouds and resulting heterogeneous chemistry during the Antarctic winter are discussed. Sensitivity of the polar stratospheric chemical composition and cloud formation for different perturbations is investigated by studying the effects of transport across the polar vortex boundary and heterogeneous processing by an enhanced sulfate aerosol load. The importance of including sedimentation for all cases is also discussed.  相似文献   

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