Analysis and interpretation of airglow and radar observations of quasi-monochromatic gravity waves in the upper mesosphere and lower thermosphere over Adelaide,Australia (35°S, 138°E)

Observations of wave-driven fluctuations in emissions from the OH Meinel (OHM) and O₂ Atmospheric band were made with a narrow-band airglow imager located at Adelaide, Australia (35S, 138E) during the period April 1995 to January 1996. Simultaneous wind measurements in the 80–100 km region were made with a co-located MF radar. The directionality of quasi-monochromatic (QM) waves in the mesopause region is found to be highly anisotropic, especially during the solstices. During the summer, small-scale QM waves in the airglow are predominately poleward propagating, while during winter they are predominately equatorward. The directionality inferred from a Stokes analysis applied to the radar data also indicates a strong N–S anisotropy in summer and winter, but whether propagation is from the north or south cannot be determined from the analysis. The directionality of the total wave field (which contains incoherent as well as coherent features) derived from a spectral analysis of the images shows a strong E–W component, whereas, an E–W component is essentially absent for QM waves. The prevalence of QM waves is also strongly seasonally dependent. The prevalence is greatest in the summer and the least in winter and correlates with the height of the mesopause; whether it is above or below the airglow layers. The height of the mesopause is significant because for nominal thermal structures it is associated with a steep gradient in the Brunt-Väisälä frequency that causes the base of a lower thermospheric thermal duct to be located in the vicinity of the mesopause. We interpret the QM waves as waves trapped in the lower thermosphere thermal duct or between the ground and the layer of evanescence above the duct. Zonal winds can deplete the thermal duct by limiting access to the duct or by negating the thermal trapping. Radar measurements of the prevailing zonal wind are consistent with depletion of zonally propagating waves. During winter, meridional winds in the upper mesophere and lower thermosphere are weak and have no significant effect on meridionally propagating waves. However, during summer the winds in the duct region can significantly enhance ducting of southward propagating waves. The observed directionality of the waves can be explained in terms of the prevailing wind at mesopause altitudes and the seasonal variation of distant sources.     

MF radar observations of mean winds over Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E)

Continuous MF radar measurements of mesospheric mean winds are in progress at the observatories in Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E). The observations at Yamagawa and Wakkanai were started in August 1994 and September 1996, respectively. The real-time wind data are used for the study of major large scale dynamic features of the middle atmosphere such as mean winds, tides, planetary waves, and gravity waves, etc. In the present study of mean winds, we have utilized the data collected until June 1999, which include the simultaneous observation period of little more than two and a half years, for the two sites. The database permits us to draw conclusions on the characteristics of mean winds and to compare the mean wind structure over these sites. The mean prevailing zonal winds at both sites are dominated by westward/eastward motions in summer/winter seasons below 90 km. Meridional circulation at meteor heights is generally southward during most times of the year and it extends to lower mesospheric heights during summer also. The summer westward jet at Wakkanai is consistently stronger than those at Yamagawa. However, the winter eastward winds have identical strength at both locations. Meridional winds also show larger values at Wakkanai. The mean wind climatology has been examined and compared with the MU radar observations over Shigaraki (34.9°N, 136.1°E). The paper also presents the results of the comparison between the MF radar winds and the latest empirical model values (HWM93 model) proposed by Hedin et al. (1996. Journal of Atmospheric and Terrestrial Physics 58, 1421–1447). Hodograph analyses of mean winds conducted for the summer and winter seasons show interesting similarities and discrepancies.     

Comparison of ground-based OH temperature data measured at Irkutsk (52°N, 103°E) and Zvenigorod (56°N, 37°E) stations with Aura MLS v3.3

Data about the variations of mesopause temperature (～87 km) obtained from ground-based spectrographic measurements of the OH emission (834.0 nm, band (6-2)) at Irkutsk and Zvenigorod observatories were compared with satellite data on vertical temperature distribution in the atmosphere from Aura MLS v3.3. We analyzed MLS data for two geopotential height levels: 0.005 hPa (～84 km) and 0.002 hPa (～88 km) as the closest to OH height (～87 km). We revealed that Aura MLS temperature data have lower values than ground-based (cold bias). In summer periods, that difference increases. Aura cold biases compared with OH(6-2) at Irkutsk and Zvenigorod were calculated. For the 0.002 hPa height level, the biases are 10.1 and 9.4 K, and for 0.005 hPa they are 10.5 and 10.2 K at Irkutsk and Zvenigorod, respectively. When the bias is accounted for, an agreement between Aura MLS and OH(6-2) data obtained at both Irkutsk and Zvenigorod is remarkable.     

MST radar investigation on inertia-gravity waves associated with tropical depression in the upper troposphere and lower stratosphere over Gadanki (13.5°N, 79.2°E)

Continuous measurements of 3-dimensional winds, spectral parameters, and tropopause height for ～114 h during the passage of a tropical depression using mesosphere–stratosphere–troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) are discussed. The spectral analysis of zonal and meridional winds shows the presence of inertia-gravity wave (IGW) with the dominant periodicity of 56 h and intrinsic period of 27 h in the upper troposphere and lower stratosphere (UTLS). The strengthening of easterly jet and associated wind shears during the passage of the depression is one of the causative mechanisms for exciting the IGW. A well-established radar method is used to identify the tropopause and to study its response to the propagating atmospheric disturbances. The significance of the present study lies in showing the response of tropopause height to the IGW during tropical depression for the first time, which will have implications in stratosphere–troposphere exchange processes.     

Short-term changes in the aerosol characteristics at Kharagpur (22°19′N, 87°19′E) during winter

In-situ measurements of number density, size distribution, and mass loading of near-surface aerosols were carried out at Kharagpur, a site on the eastern part of Indo-Gangetic Plains during the winter month of December 2004. The data have been used to investigate wintertime characteristics of aerosols and their effects on the occurrence of haze. The aerosol number density is found to be of the order of 10⁹ m^?3 and mass loading is ~265±70 μg m^?3 (5–8 times that reported from south Indian sites). The diurnal patterns and day-to-day variations in aerosol number density and mass loading are closely associated with atmospheric boundary layer height. During haze events, the number density of submicron particles is found to be 2–5 times higher than that during non-hazy period. This could be attributed to the enhanced concentration of anthropogenic aerosols, low atmospheric boundary layer height/ventilation coefficient and airflow convergence.     

Seasonal and inter-annual variability in alkalinity in Liverpool Bay (53.5° N, 3.5° W) and in major river inputs to the North Sea

A critical factor controlling changes in the acidity of coastal waters is the alkalinity of the water. Concentrations of alkalinity are determined by supply from rivers and by in situ processes such as biological production and denitrification. A 2-year study based on 15 cruises in Liverpool Bay followed the seasonal cycles of changing concentrations of total alkalinity (TA) and total dissolved inorganic carbon (DIC) in relation to changes caused by the annual cycle of biological production during the mixing of river water into the Bay. Consistent annual cycles in concentrations of nutrients, TA and DIC were observed in both years. At a salinity of 31.5, the locus of primary production during the spring bloom, concentrations of NO _x decreased by 25 ± 4 μmol kg⁻¹ and DIC by 106 ± 16 μmol kg⁻¹. Observed changes in TA were consistent with the uptake of protons during primary biological production. Concentrations of TA increased by 33 ± 8 μmol kg⁻¹ (2009) and 33 ± 15 μmol kg⁻¹ (2010). The impact of changes in organic matter on the measured TA appears likely to be small in this area. Thomas et al. (2009) suggested that denitrification may enhance the CO₂ uptake of the North Sea by 25%, in contrast we find that although denitrification is a significant process in itself, it does not increase concentrations of TA relative to those of DIC and so does not increase buffer capacity and potential uptake of CO₂ into shelf seawaters. For Liverpool Bay historical data suggest that higher concentrations of TA during periods of low flow are likely to contribute in part to the observed change in TA between winter and summer but the appropriate pattern cannot be identified in recent low-frequency river data. On a wider scale, data for the rivers Mersey, Rhine, Elbe and Weser show that patterns of seasonal change in concentrations of TA in river inputs differ between river systems.     

Gravity wave intensity and momentum fluxes in the mesosphere over Shigaraki, Japan (35°N, 136°E) during 1986–1997

Averaged seasonal variations of wind perturbation intensities and vertical flux of horizontal momentum produced by internal gravity waves (IGWs) with periods 0.2/1 h and 1/6 h are studied at the altitudes 65/80 km using the MU radar measurement data from the middle and upper atmosphere during 1986/1997 at Shigaraki, Japan (35°N, 136°E). IGW intensity has maxima in winter and summer, winter values having substantial interannual variations. Mean wave momentum flux is directed to the west in winter and to the east in summer, opposite to the mean wind in the middle atmosphere. Major IGW momentum fluxes come to the mesosphere over Shigaraki from the Pacific direction in winter and continental Asia in summer.     

Temperature trends in the middle atmosphere as seen by historical Russian rocket launches: Part 2, Heiss Island (80.6°N, 58°E)

Long-term changes of the temperature of the middle atmosphere are investigated using a data bank obtained by Russian rocketsondes at Heiss Island (80.6°N, 58°E). The major interest of the data series is that it is one of the longest and uninterrupted records obtained at high latitudes in the northern hemisphere over 25 years, from 1969 to 1994. Previous estimates using this dataset has shown the largest trends. The revised analysis performed here took into account all possible discontinuities in the data series, such as a change in the time-of-measurement, T₀, and in the type of sensor. For this purpose, some data were filtered out, and a statistical model based on multiple regression analyses included step functions to take into account such discontinuities. The temperature responses to different sources of variability (solar activity, volcanic aerosols) were retrieved for summer and winter periods. The response to the 11-year solar cycle in the winter period is found to be largely positive in the stratosphere (∼+4 K) and largely negative in the mesosphere (∼−8 K), with a smaller and opposite response in summer. This response depends on the phase of the QBO, as already shown by previous studies. The response to volcanic aerosols is found to be significantly positive in the upper mesosphere, in good agreement with numerical simulations and with observations above France. The long-term trend resulting from this reanalysis indicates a cooling of the middle atmosphere, increasing with altitude from −2 K/decade at 40 km to a maximum of −6 K/decade around 65 km. This result is slightly larger than the trend observed at mid-latitude but quite smaller than previous estimates.     

Long-term changes of the upper stratosphere as seen by Japanese rocketsondes at Ryori (39°N, 141°E)

Wind and temperature profiles measured routinely by rockets at Ryori (Japan) since 1970 are analysed to quantify interannual changes that occur in the upper stratosphere. The analysis involved using a least square fitting of the data with a multiparametric adaptative model composed of a linear combination of some functions that represent the main expected climate forcing responses of the stratosphere. These functions are seasonal cycles, solar activity changes, stratospheric optical depth induced by volcanic aerosols, equatorial wind oscillations and a possible linear trend. Step functions are also included in the analyses to take into account instrumental changes. Results reveal a small change for wind data series above 45 km when new corrections were introduced to take into account instrumental changes. However, no significant change of the mean is noted for temperature even after sondes were improved. While wind series reveal no significant trends, a significant cooling of 2.0 to 2.5 K/decade is observed in the mid upper stratosphere using this analysis method. This cooling is more than double the cooling predicted by models by a factor of more than two. In winter, it may be noted that the amplitude of the atmospheric response is enhanced. This is probably caused by the larger ozone depletion and/or by some dynamical feedback effects. In winter, cooling tends to be smaller around 40–45 km (in fact a warming trend is observed in December) as already observed in other data sets and simulated by models. Although the winter response to volcanic aerosols is in good agreement with numerical simulations, the solar signature is of the opposite sign to that expected. This is not understood, but it has already been observed with other data sets.     

Characteristics of the semidiurnal tide in the MLT over Maui (20.75°N, 156.43°W) with meteor radar observations

Semidiurnal tidal features have been examined in the Mesosphere and Lower Thermosphere (MLT) from the long-term (2002–2007) meteor wind data over Maui (20.75°N, 156.43°W). Amplitude and phase obtained from the harmonic analysis exhibit large day to day variability. Mean amplitude obtained from the monthly mean data over the observation period is found to vary within ～8–28 m/s and 10–32 m/s for the zonal and meridional winds, respectively. The amplitude has revealed clear semiannual oscillation (SAO) pattern with maxima during solstices and altitudinal growth in both wind components. Significant resemblance in its variability with other observations carried out from the low latitude sites all over the globe is obtained. Vertical wavelength estimated from the phase gradients exposes large values (>90 km) in all seasons. Contribution of the semidiurnal tide to the total tidal variability in the MLT is found to vary over wide range throughout the year with generally higher influence during winter season over diurnal and terdiurnal components.     

Meteor wind radar observations of tidal amplitudes over a low-latitude station Trivandrum (8.5°N, 77°E): Interannual variability and the effect of background wind on diurnal tidal amplitudes

Based on the horizontal winds measured using SKiYMET meteor wind radar during the period of June 2004–May 2007, the seasonal and interannual variability of the diurnal and semidiurnal amplitudes and phases in the mesospheric and lower thermospheric (MLT) region over a low-latitude station Trivandrum (8.5°N) are investigated. The monthly values of amplitudes and phases are calculated using a composite day analysis. The zonal and meridional diurnal tidal amplitudes exhibit both annual and semiannual oscillations. The zonal and meridional components of semidiurnal tide show a significant annual oscillation. The phase values of both diurnal and semidiurnal tides exhibit annual oscillation above 90 km. The effect of background wind in the lower atmosphere on the strength of diurnal tidal amplitudes in the MLT region is studied. The effect of diurnal tides on the background wind in the lower thermosphere is also discussed.     

Gravity waves in Fabry–Perot measurements made at Mt. John (44°S,170°E), New Zealand

Observations of winds in mesospheric airglow layers have been made at Mt. John (44°S,170°E), New Zealand for some years. We present a modelling study of airglow emissions which shows that the properties of wind detection based on airglow emission means that high-frequency gravity waves are effectively filtered from the wind spectrum observed. This filtering means that any waves with periods of the order of hours should be detectable in the record (as they will not be hidden in the noise of the higher-frequency waves ubiquitous at these heights). One example of such a wave is shown. As part of the analysis, we show that because the airglow layers differ in width, some waves might be observed in only one airglow layer, even when present in both.     

Tidal and low-frequency currents off the Jaguaribe River estuary (4° S, 37° 4′ W), northeastern Brazil

This article characterizes the spatial and temporal current variations, in the subtidal and tidal ranges, during the rainy and dry seasons, at the continental shelf off the Jaguaribe River, through measurements of continuous current field data from an acoustic Doppler current profiler (ADCP) mooring during 124 days, from June 12 to October 14, 2009. To support this dataset, we collected corresponding data from a meteorological station located at the estuary. The spatial variation showed that highest current speeds occur near the coast, with an offset of a NNW coastal jet, decreasing intensity, monotonically, towards offshore up to 0.1 ms^?1. In the rainy season, small inversions of the wind field were observed, lasting 2 to 3 days on average and were accompanied by the direction of surface currents only. In the dry season, the period of reversal of wind fields and currents lasted 14 and 35 h, respectively. The analysis of empirical orthogonal functions in rainy and dry seasons showed that the continental shelf is predominantly barotropic, where the second and third modes explained only 7% of the total variance, during the dry season. The tidal currents are more intense in the direction normal to the coast, showing a semidiurnal tidal regime. Energy distribution between tidal currents and currents of longer periods showed that for the component parallel to bathymetry, subtidal frequency currents are dominant, contributing to more than 70% of the variance. For the normal component to the coastline bathymetry, there is a significant increase of power concerning tidal currents, at all depths, so they contribute with about 55% of the total variance.     

Analysis of gravity waves in the tropical middle atmosphere over La Reunion Island (21°S, 55°E) with lidar using wavelet techniques

The capabilities of the continuous wavelet transform (CWT) and the multiresolution analysis (MRA) are presented in this work to measure vertical gravity wave characteristics. Wave properties are extracted from the first data set of Rayleigh lidar obtained between heights of 30 km and 60 km over La Reunion Island (21°S, 55°E) during the Austral winter in 1994 under subtropical conditions. The altitude-wavelength representations deduced from these methods provide information on the time and spatial evolution of the wave parameters of the observed dominant modes in vertical profiles such as the vertical wavelengths, the vertical phase speeds, the amplitudes of temperature perturbations and the distribution of wave energy. The spectra derived from measurements show the presence of localized quasi-monochromatic structures with vertical wavelengths <10 km. Three methods based on the wavelet techniques show evidence of a downward phase progression. A first climatology of the dominant modes observed during the Austral winter period reveals a dominant night activity of 2 or 3 quasi-monochromatic structures with vertical wavelengths between 1/2 km from the stratopause, 3/4 km and 6/10 km observed between heights of 30 km and 60 km. In addition, it reveals a dominant activity of modes with a vertical phase speed of –0.3 m/s and observed periods peaking at 3/4 h and 9 h. The characteristics of averaged vertical wavelengths appear to be similar to those observed during winter in the southern equatorial region and in the Northern Hemisphere at mid-latitudes.     

Estimation of equatorial wave momentum fluxes using MST radar winds observed at Gadanki (13.5°N, 79.2°E)

A method of computing the vertical flux of zonal momentum (associated with equatorial waves) from the zonal and vertical components of the winds measured by the Indian MST radar at Gadanki (13.5°N, 79.2°E) is presented. The application of the method to the radar data gives flux values of 16×10⁻³, 8.0×10⁻³ and 5.5×10⁻³ m² s⁻² for slow Kelvin (12-day period), fast Kelvin (5.33-day period) and Rossby-gravity (RG) (3.43-day period) waves, respectively, in the upper troposphere. These flux values compare quite well with the values 4×10⁻³ m² s⁻² and 1×10⁻³ m² s⁻² obtained from radiosonde zonal wind and temperature data by Wallace and Kousky, 1968for slow Kelvin and RG waves, respectively. An estimate of the error in the fluxes gives a value of ∼ 1.2×10⁻³ m² s⁻².