Inorganic bromine in the lower stratosphere

Inorganic bromine content in the lower stratosphere was determined from a series of three balloon flights conducted in 1981 and 1982 from Holloman AFB, New Mexico (33°N). A Direct Flow Sampler (DFS) sampled large quantities of stratospheric air through a 35 cm diameter filter mounted upstream of a blower. Filters were prepared with an inert organic oil and a strong organic base in order to capture both acidic and particulate species. Following flight, the filters underwent an extraction process. Then, aliquots of the solution were subjected to neutron activation analysis. The methodology involved a radiochemical procedure which was developed and tested to measure, simultaneously, both chlorine and bromine content. High-resolution gamma spectrometry was used and bromine content was determined by counting ⁸⁰Br activity. The resulting values for volume mixing ratios of bromine increased from a low of 2.6 pptv at 15 km to a high of 15.6 pptv for the 25–30 km band. The results are shown to be in agreement with earlier measurements by Sedlacek et al. (1984) and with one-dimensional photochemical model predictions by yung et al. (1980).     

Dynamics and the ozone distribution in the winter stratosphere : modelling the inter-hemispheric differences

Results from a two year simulation of a General Circulation Model are used to illustrate the main differences found in the lower stratosphere dynamics and the ozone distribution between the Southern and the Northern Hemispheres in winter.The model extends from ground to mesospheric levels with a spectral horizontal resolution up to isotropic wavenumber 42. It incorporates a fully interactive scheme for the ozone mixing ratio which accounts for photochemical sources and sinks, advection by the model winds and coupling with radiative calculations.The model reproduces the large scale inter hemispheric differences quite well, with a very stable and cold vortex in the Southern Hemisphere and a warmer vortex often distorted in the Northern Hemisphere. It is concluded that due to interactions between dynamics, polar stratospheric cloud formation and chemistry, there is a possibility that some stratospheric ozone depletion could be effective in late winter near the night terminator in the Northern Hemisphere, whereas significant ozone depletion only occurs in early spring in the Southern Hemisphere.The importance of synoptic scale dynamics on the ozone transport between the high latitudes and the equator is also stressed. The model develops tongues of ozone-rich air from the high latitudes which are irreversibly mixed at mid-latitudes with tongues of ozone-poor air from the low latitudes. Similar tongues or filaments are clearly visible in the TOMS satellite data. They result from the activity of medium scale-waves in the Southern Hemisphere, whereas in the Northern Hemisphere the larger scale planetary waves play a major role in their development, and their size and extension are larger. It is concluded that transport of the ozone depletion to the mid-latitudes could be more effective in the Northern than in the Southern Hemisphere.     

Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

The El Chichon volcanic cloud in the stratosphere: lidar observation at Fukuoka and numerical simulation

The stratospheric volcanic cloud from the eruption.of El Chichon, Mexico, on 4 April 1982 was observed routinely by a Nd: YAG lidar system from 18 April 1982 at Kyushu University, Fukuoka, Japan. The observed layers of the cloud above 20 km were in the easterly wind region and those below 20 km were in the westerly region. The main part of the cloud mass was in the upper layer. This upper layer broadened slowly until September 1982, then broadened rapidly and merged with the lower layer as the easterly wind changed to the westerly wind. The vertical eddy diffusion coefficient estimated from the broadening of the upper layer was much smaller than the value usually used in the one-dimensional model calculation of chemical components until September and subsequently remained at about the same value. The increase of the integrated backscattering coefficient (IBC) was about two orders of magnitude larger than the largest increase after volcanic injections for the last 10 years. The IBC reached a maximum value on 3 May and gradually decreased until August 1982, then re-increased until December 1982. The IBC between December 1982 and February 1983 was about the same value as in May 1982. Using the one-dimensional stratospheric sulfate aerosol model simulations it was concluded that to explain the broadening of the upper layer an eddy diffusion coefficient of about 10²cm²s^-1 would be needed in the easterly wind region in summer. It was also concluded that the IBC re-increase was caused after advective horizontal transport from lower to higher latitudes by chemical reactions within the upper layer without meridional diffusion during summer and that the transport was controlled by nucleation, which gives rise to small particles, a decreasing settling velocity of the volcanic cloud and then the cloud being less affected by horizontal transport.     

Ionisation in the Antarctic stratosphere

Results from a partial reflection radar experiment, operated at a frequency of 2.9 MHz at Scott Base, Antarctica, are presented for the time interval from January 1987 to June 1991. It is shown that a layer of ionisation can frequently be identified at scattering heights between 40 and 55 km. Details regarding the maximum density, thickness, and frequency of occurrence of this ionisation layer are presented. From sequential occurrences of the ionisation an approximate life time of 70 min is deduced for the layer. The possibility that the ionisation layer is produced by a flux of relativistic electrons is investigated.     

Long-term tropospheric and lower stratospheric ozone variations from ozonesonde observations

An analysis is presented of the long-term mean pressure latitude seasonal distribution of tropospheric and lower stratospheric ozone for the four seasons covering, in part, over 20 years of ozonesonde data. The observed patterns show minimum ozone mixing ratios in the equatorial and tropical troposphere except in regions where net photochemical production is dominant. In the middle and upper troposphere, and low stratosphere to 50 mb, ozone increases from the tropics to subpolar latitudes of both hemispheres. In mid stratosphere, the ozone mixing ratio is a maximum over the tropics. The observed vertical ozone gradient is small in the troposphere but increases rapidly above the tropopause. The seasonal variation at a typical mid latitude station (Hohenpeissenberg) shows a summer maximum in the low to middle troposphere, shifting to a winter-spring maximum in the upper troposphere and lower stratosphere and spring -summer maximum at 10 mb. The amplitude of the annual variation increases from a minimum in the tropics to a maximum in polar regions. Also, the amplitude increases with height at all latitudes up to about 30 mb where the phase of the annual variation changes abruptly. The phase of the annual variation is during spring in the boundary layer, summer in mid troposphere, and spring in the upper troposphere and lower stratosphere. The annual long-term ozone trends are significantly positive at about + 1.2% yr in mid troposphere (500 mb) and significantly negative at about − 0.6% yr¹ in the lower stratosphere(50mb)     

Simultaneous observation of the quasi-two-day variations in the lower and upper ionosphere over Europe

The large scale character of the observed quasi-two-day fluctuations in the whole ionosphere (from D- uptoF-region maximum) over Europe is shown. The study is based on the lower and upper ionospheric data obtained in Sofia (42.9°, 23.4°E), Ebre Observatory (40.9°N, 0.5°E) and El Arenosillo (37.1°N, 6.7°W) during two summer intervals: June–August 1980 and 1983. The obtained prevailing periods for the F-region fluctuations are 52–55 h and the mean amplitude is higher than 1 MHz. It was found that the fluctuations propagate westward with a mean phase velocity between 4.6 and 6° /h. The quasi-two-day variations in the F-region maximum are probably generated by flucutations in the mesospheric, neutral wind. During the time when well developed quasi-two-day fluctuations exist in the mesospheric neutral wind, similar variations are observed in the lower ionosphere also. Possible mechanisms for generating the D- andF-region electron density fluctuations from these oscillations in the neutral wind are proposed.     

On the association between the QBO and the extratropical stratosphere

Although diagnostic studies and mechanistic model experiments have found that, on average, the polar vortex in northern winter is stronger and colder in the west than in the east years of the equatorial Quasi-Biennial Oscillation (QBO), we show with an expanded data base that the results are not statistically significant. The reason for the insignificance is that in 36% of the winters (13 out of 36) the vortex was warm and weak in the west, and cold and strong in the east years. Only at low activity in the 11-yr solar cycle did the difference between the west (cold) and the east (warm) years become statistically significant. At high solar activity the west years had a warm and the east years a cold polar vortex in the mean. We show this association with the 11-yr solar cycle also in terms of the geostrophic wind.     

Lunar tides in the stratosphere and mesosphere from NIMBUS 6 data

Infra-red radiance data from the NIMBUS 6 satellite have been analysed for lunar tidal variations in temperature at heights from 45 to 80 km. The tide at 45 km agrees well with that previously found from NIMBUS 5 data. The tide at 80 km has an amplitude of about 0.4K, and shows a phase change in the expected sense relative to that at 45 km.     

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s⁻¹ (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s⁻¹ also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere.The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.     

Observations of the annual and semi-annual wave in the stratosphere using Nimbus 5 SCR data

Nimbus 5 SCR radiance data are used, together with 200 mb geopotential data published by the Free University of Berlin, for the two years 1973–1974 to investigate the annual and semi-annual oscillations which occur in the stratospheric zonal mean temperature and geostrophic wind fields. Oscillations in temperature are investigated in both hemispheres, but due to the lack of suitable data only northern hemisphere winds are examined.The results of the analyses are compared to those derived from other globally-less-well-represented sources of data, namely radio sonde and rocket observations. In general good agreement is obtained in the regions of overlap.     

Wave characteristics in the troposphere and stratosphere over the Indian Tropics during the DYANA period

One of the important scientific objectives of the international DYANA campaign was to obtain the characteristics of planetary scale waves in the low-latitude middle atmosphere. India participated in this campaign by way of launching several rockets and high-altitude balloons from a number of locations to determine the vertical structure of different wave modes present during January–March 1990. Rocket launchings were conducted from two stations, namely Thumba (8.5°N, 77.0°E) with M-100 rockets and Balasore (21.5°N, 87.0°E) with RH-200 rockets, while balloons were launched from three stations, i.e. Trivandrum near Thumba, Minicoy (8.2°N, 73.0° E) and Port Blair (11.7°N, 92.7°E). In addition, there were balloon flights from Hyderabad (17.3°N, 78.3°E) and Bhubaneshwar (20.2°N, 85.5°E). The results of the synoptic scale wave activity as obtained from various data sources are given here.Three prominent peaks with wave periods near 6–8 days (short periods), 10–12 days (medium periods) and 30–45 days (long periods) are found to occur at all the stations. The medium- and long-period waves appear to be forced Rossby modes penetrating from midlatitudes while short period waves all have characteristics matching those of mixed Rossby-gravity waves. A very interesting result is the presence of long-period oscillations in the upper stratosphere and mesosphere, with very large amplitude, contrary to earlier observations.     

Aspect sensitivity of VHF scatterers in the troposphere and stratosphere from comparisons of powers in off-vertical beams

The aspect sensitivity of the radar backscatter power at 46.5 MHz has been examined for the troposphere and lower stratosphere. Use is made of the width of the effective backscatter polar diagram, assumed to be Gaussian, derived from the ratios of signal strength for different pairs of beam directions in order to distinguish between anisotropic and isotropic scattering. The results are used to examine the relative contributions of isotropic scatter, anisotropic scatter, and Fresnel reflection or scatter to the signal backscattered in the vertical direction. Furthermore, the change in the scattering characteristics during the passage of a warm front is examined.     

Horizontal movements in the ozone layer

The variations of the column ozone densities at Middle-Asian stations and several others in the former USSR were examined by auto- and cross-correlation analyses. Periodic processes with amplitude of tens of DU and periods of 15 to 25 days occur simultaneously at many stations. The movement of this wave disturbance is directed towards the south-east and can also be seen in the weather maps at 500 hPa and 100 hPa, and the driving force is most probably meteorological air motion.     

Distribution of tropospheric ozone in the tropics from satellite and ozonesonde measurements

Measurements from two independent satellite data sets have been used to derive the climatology of the integrated amount of ozone in the troposphere. These data have led to the finding that large amounts of ozone pollution are generated by anthropogenic activity originating from both the industrialized regions of the Northern Hemisphere and from the southern tropical regions of Africa. To verify the existence of this ozone anomaly over this region of the world, an ozonesonde capability has been established at Ascension Island (8 S, 15 W) since July 1990. According to the satellite analyses. Ascension Island is located downwind of the primary source region of this ozone pollution, which likely results from the photochemical oxidation of emissions emanating from the widespread burning of savannas and other biomass in central and southern Africa. These in-situ measurements confirm the existence of large amounts of ozone in the lower atmosphere. These first ozonesonde profiles suggest that much of the ozone generated over Africa during the ‘burning season’ (primarily July-October) reaches Ascension Island. These high levels of ozone in the lower troposphere become much lower by December. Elevated ozone concentrations in the middle troposphere are once again evident in February, which may be the result of biomass burning emissions being transported from western and northern Africa.