One full-day radar measurement of lower stratospheric winds over Jicamarca

The lower stratospheric wind field is measured simultaneously at several altitudes for 24 h on 23–24 May 1974 using the Jicamarca radar in Peru. Maximum entropy spectral analysis is used to estimate echo power spectra from the measured autocorrelation functions which are truncated beyond the maximum time lag of 6.65 s. The fluctuating wind velocity is resolved into steady and tidal components. A vertical shear of 2–3 m s⁻¹ km⁻¹ is observed throughout the time of observation in the zonal wind. The mean zonal wind is in good agreement with the Lima-Callao radiosonde data obtained in the same month. A very large diurnal component, of amplitude 1–5 m s⁻¹ is observed in the zonal wind in the lower stratosphere. However it appears that a semidiurnal oscillation below the tropopause is predominant.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Vertical winds and turbulence over Thumba

Results of the analysis of barium cloud released over Thumba at an altitude 93 km are presented. In the initial stage, the cloud was in the form of two rings coupled together by a knot and later on the rings were distorted to become elongated loop-like structures. A strong north-south wind shear of 30 m/s/km and a vertical wind gradient are observed which explain the distortion of the initial rings into the loops. The lower loop showed development of turbulence, 280s after the release. However, the upper loop did not show any evidence of turbulence throughout the observed period (10 min), but expanded by wind. The delay of 280 s in the onset of turbulence for the lower loop suggests co-existence of turbulent and non-turbulent regions in the 93–95 km altitude range. The top portion of the upper loop moved upward upto an altitude of about 100km where it recorded maximum upward velocity of 19 ±7 m/s. The values of vertical velocities in the 95–100 km height region coupled with the vertical velocities recorded from other barium clouds released by the same rocket at the lower thermospheric altitudes reveal a wavy profile that is suggestive of the presence of an internal atmospheric gravity wave. The altitude of the turbopause observed in the present experiment is 95 ±2 km, about 10 km lower than that observed earlier. It is suggested that gradients in the vertical wind could cause lowering of turbopause level.     

Variability of winds and temperatures in the lower thermosphere

High-resolution daytime incoherent scatter radar measurements of plasma temperatures and drifts in the ionospheric E-region above Millstone Hill (42.6°N, 71.5°W) have been used to derive horizontal neutral winds and temperatures in the lower thermosphere (105–130 km) during five multi-day campaigns in 1987–1991. The underlying semi-diurnal tidal component has been determined from the observations, with characteristic average amplitudes of 50 ± 15 m/s and 30 ± 10 K. Phase propagation with altitude follows the expected structure of semi-diurnal tidal modes, but reveals complex coupling of tidal modes, particularly above 115 km. Day-to-day variability in the winds and temperatures is large, and the deviations from the semi-diurnal harmonic can exceed 40 m/s and 50 K. No strong correlations have so far been found with geophysical parameters to explain the observed variability.     

Some results on mean tidal structure and day-to-day variability over Arecibo

A long series of monthly 36 h observations, plus a series of observations at the same local time each day over two months at Arecibo, have been analyzed for tidal structure and variability. Some of the results are as follows. (1) A diurnal tide with vertical structure similar to that of the S_1,1 mode dominates the wind field up to heights of the order of 110 km over Arecibo. A semidiurnal oscillation dominates above that height. For non-winter conditions the semidiurnal oscillation in the 80–200 km region closely resembles the S_2,2 mode, though there is the possibility of contributions from higher order modes in the 100–120 km region. (2) Larger semidiurnal amplitudes are observed in the lower thermosphere for winter conditions. The data appears roughly consistent with Bernard's (1979) hypothesis that S_2,2, S_2,4 and S_2,5 modes are thermally excited, with the S_2,4 and S_2,5 modes out of phase in the meteor region in summer and in phase in winter. (3) The day-to-day variability of the tides is at least half the amplitude of the mean oscillations. The maximum wind variability was observed to occur in the 100–110 km region where the diurnal tide is strongly dissipated. (4) The day-to-day deviations in the wind and temperature oscillations from a long term mean at one local time tend to be wave-like structures which are generally correlated from day to day. The structures tend to move upwards, i.e., appear at a later local time, from day to day.     

First low-power VHF radar observations of tropospheric,stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree⁻¹. Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.     

Balloon measurements of stratospheric ion conductivities over the tropics

Conductivity measurements of negative and positive ions were made from about 20 to 35 km by two identical balloon-borne spherical probes at Hyderabad (17.5°N, 78.6°E), India on 22 April 1989 and 22 December 1990. One balloon was launched at 0158 h IST (Indian Standard Time) which reached its ceiling around 0330 h IST. After that time, it floated for about 3 h, 1.5 h before sunrise and 1.5 h after sunrise. Thus it gave data for both day- and night-time conditions at float altitude. The other balloon was launched at 0535 h IST. It gave data for daytime only. Several interesting results have been obtained at the float altitudes. During the night, in the flight of 22 April 1989 the conductivity values of positive ions were found to be about 1.5 times those of negative ions at the float altitude. During the day, in the flight of 22 April 1989, the positive ion conductivity values were found to increase with the increase of solar elevation angle at around 37.5 km altitude. The negative ion conductivity values, however, did not show any day-night variation. In the flight of 22 December 1990, these features were not seen. Instead, a pocket was found where conductivity values were very high (of the order of 10⁻¹¹ mho m⁻¹) at an altitude of about 32.5 km. Also in this flight, the positive ion conductivity was always found to be approximately equal to that of the negative ion conductivity.     

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 measurements of large vertical winds in the upper and lower thermosphere

High resolution vertical wind measurements of the upper and lower thermosphere were made at Poker Flat, Alaska, using a scanning Fabry-Perot spectrometer (FPS). Observations of the λ558 and λ630 nm emissions of atomic oxygen were made on 21 nights and allowed for the simultaneous determination of wind and temperature at altitudes of about 130 and 240 km, respectively. On two occasions, significant upwelling events were measured which lasted between 15 and 25 min. Peak velocities were up to 42 m/s at 130 km and 138 m/s at 240 km. Auroral activity was monitored using a meridian scanning photometer (MSP). On both occasions, the region of upwelling was located on the poleward side of the auroral oval during geomagnetically active conditions. A schematic model is used to describe an event from which the horizontal scale of the upwelling region is estimated to be less than 320 km in the lower thermosphere and less than 800 km in the upper thermosphere.     

Hemispheric synoptic analysis of 95 km winds during the winter of 1983/1984 and comparison with stratospheric parameters

For the winter 1983/1984 the close coupling between stratospheric and mesospheric disturbances is discussed, using derived dynamical quantities of the stratosphere and prevailing winds of the upper mesosphere/lower thermosphere.     

Interannual fluctuations of the stratospheric temperature over the north polar region

The monthly means for the years 1964–1991 of 30 hPa temperatures over the North Pole and averaged over the 70–90°N region are analyzed. A multiple regression model is used to find long-term monthly trends and possible linear associations between these temperatures and the QBO, ENSO, and the 11-yr solar cycle. The model's residuals are examined for detection of other periodic interannual fluctuations in Arctic temperatures.It is found that the interannual variations of temperature at 30 hPa over the Arctic are a superposition of the oscillations due to the QBO, ENSO, the 11-yr solar cycle, and approximate 6-yr periodic fluctuations of unknown origin. The QBO, ENSO, and the solar cycle effects in the Arctic temperature explain about 35% of the total variance of the temperature monthly anomalies. In winter, the QBO, ENSO, and the 11-yr solar cycle signals in the temperature data depend on the phase of the equatorial QBO. The polar vortex seems to be warmer (colder) than normal when the West (East) phase of the equatorial QBO in a period of high solar activity. The monthly temperature trends over the Arctic show seasonal variations with positive trends in February and March. The year-round trends (sum of the monthly trends) are about −0.5 K per decade.     

The effects of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes

The influence of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes is investigated. A 3-dimensional neutral wind model is developed and used together with an atmospheric model in a gravity-wave ray-tracing analysis. It is demonstrated that the thermospheric wind can act as a filter for waves travelling at unfavorable angles to the mean flow, via the mechanisms of reflection and critical coupling. This wind filtering action rotates clockwise diurnally through 360° in the northern hemisphere. Observational evidence is presented which supports these predictions. Extensive modelling indicates that (a) faster and longer period waves are least affected by the neutral winds and (b) fixed-height (e.g. HF Doppler) observations of medium scale gravity waves is only likely to be possible for waves generated locally (within 500–1000 km). Waves generated at greater distances are probably dissipated before reaching the observation region.     

Some direct comparisons of mesospheric winds observed at Kyoto and Adelaide

Direct comparisons have been made of the prevailing and tidal wind fields observed in the 80–100 km height region using data obtained with a meteor radar at Kyoto (35°N, 136°E) and a partial reflection spaced antenna system at Adelaide (35°S, 138°E). Data taken with a partial reflection system at Townsville (19°S, 147°E) has also been included so that the latitudinal variations of the tidal structures could be taken into account. The comparisons extend over periods of up to one month duration centered on the equinoxes of 1979 and the January solstice of 1980. They show that there are often significant differences in the tidal amplitudes and phases observed at Kyoto and Adelaide, despite their near geographic conjugacy, probably indicating the presence of antisymmetrical tidal modes. The diurnal tide is appreciably stronger at Adelaide on the average, than at Kyoto, whereas the semi-diurnal amplitudes are on the average greater at Kyoto.     

Comparison of geostrophic and observed winds in the upper mesosphere over Saskatoon,Canada

Thicknesses derived from SAMS (Stratospheric and Mesospheric Sounder, on-board Nimbus 7) and 30 mbar height analyses based on radiosondes have been combined to give monthly mean height charts of the upper mesosphesre, (0.01 mbar level, approximately 78–80 km). Four years of data are available, 1979–1982.Based on this material, the geostrophic winds were derived for the grid point 50°N, 110°W and compared with winds observed over Canada (52°N, 107°W) using the Saskatoon medium frequency (MF) radar. Agreement, especially in the summer months, was very good, however, significant ageostrophy was evident in autumnal and winter months. Possible causes are confluence associated with large scale circulations and gravity wave drag.     

Non-homogeneous structure of neutral winds in the lower thermosphere during the MAC/EPSILON campaign

The paper presents the results of an investigation of the height variations of dynamic processes in the 80–110 km height region, carried out in Kazan, U.S.S.R. (56°N, 49°E) by the radiometeor method during the MAC/EPSILON campaign. Experimental results show that the largest values of vertical wind gradients, as well as zonal and meridional temperature gradients can be found at heights of ~ 83 km. At heights of 80 ⩽ h ⩽ 100 km, we can observe energy absorption of IGW and tides which are the major sources of turbulent energy in the above-mentioned height interval. Using the effects of IGW energy absorption, values of the turbulent eddy diffusion coefficient K_l ranging from 1600 to 4400 m²/s were calculated for October 1987. The energy dissipation rate ϵ was estimated to be from 0.1 to 0.4 W/kg.     

Vertical penetration of planetary waves into the lower ionosphere

Earlier work which provided evidence for coupling between pressure variations in the stratosphere and lower ionosphere in winter has been extended. Day-to-day changes in the height of fixed electron density isopleths in the E-region at a middle latitude often exhibit quasi-oscillations with amplitudes between 2 and 10km and periods between 5 and 30 days. It is found that the correlation between these oscillations and corresponding variations in the height of winter isobaric surfaces in the stratosphere, resulting from the presence of planetary-scale waves, is sometimes good and sometimes poor. Examination of the type of wave disturbance in the stratosphere and of the stratospheric zonal wind profiles suggests that the conditions for stratosphere-ionosphere coupling are met only when well-defined planetary waves of increasing amplitude with height are seen in the lower stratosphere and when the stratospheric zonal wind pattern is favourable to the vertical propagation of such waves.     

The impact of gravity waves on nitric oxide in the lower thermosphere

Observations of nitric oxide (NO) by the Solar Mesosphere Explorer (SME) during equinox indicate a lower-thermosphere equatorial minimum which is at variance with theoretical predictions. To address this discrepancy a zonally averaged model of the thermosphere and upper mesosphere is used to evaluate the influence of a latitude variation in turbulence. Five numerical simulations were performed with different latitude structures of eddy diffusion (K_T), ranging from uniform in latitude, peaks at low, mid-, or high-latitude, to a hemispherically asymmetric distribution. A local increase in eddy diffusion causes the lower thermosphere to cool and induces a latitude pressure gradient that drives horizontal and vertical winds. The circulation, turbulent transport and temperature dependent chemistry act to change the distribution of species. Comparison of the model predictions of NO with SME data, and simulated wind and temperature structure with empirical climatology, indicates a preference for a midlatitude peak in K_T.     

Interactions between diurnal tides and gravity waves in the lower thermosphere

Recent progress on interactions between breaking gravity waves and the diurnal tide in the upper mesosphere and lower thermosphere is reviewed, mainly based on the recent results of our numerical models.     

First results with the Adelaide VHF radar: spaced antenna studies of tropospheric winds

The first results from a VHF radar of the ST type located at Buckland Park near Adelaide, Australia (35°S, 138°E), are presented. The radar is designed to be versatile and can be used to measure velocities in the lower atmosphere using both the spaced antenna (SA) and Doppler beam-swinging (DBS) techniques. Here studies of irregularities and motions made with the spaced antenna technique are discussed. It is shown that the scale of the diffraction pattern formed by the backscattered radiation varies with altitude, with the mean pattern scale being smaller in the troposphere than in the stratosphere. The observations are consistent with the backscattered energy decreasing as a function of off-vertical angle by 1.5 dB per degree in the troposphere and by about 2.8 dB per degree in the lower stratosphere. An intercomparison of zonal velocities measured with the SA and DBS methods shows good agreement. In May and August 1984 an extensive comparison was made between the velocities measured by the SA method and winds determined from over 80 balloon-borne radiosondes released from Adelaide Airport, situated some 36 km to the south of the radar. The velocities were compared on a statistical basis and showed excellent agreement, although the SA speeds tended to be 1–2 m s⁻¹ smaller in magnitude than the radiosonde velocities. Overall, the rms differences between the two sets of measurements was only 3–4ms⁻¹ throughout the troposphere, a result which is consistent with the random errors inherent in each technique, as well as the spatial separation between the radar and balloon observations. The utility of the SA method for meteorological observations is illustrated by a study of both the horizontal and vertical wind fields during the passage of a cold front made in November 1984. The high time resolution available with the radar allows detailed studies of the development of the pre-frontal jet, the wind convergence into the front and associated vertical motions.