Horizontal Propagation Direction Research Articles

Occurrence characteristics of mesospheric gravity waves at 51 °N

Abstract Observations of the OH(8-3) band rotational temperature have been carried out from Calgary (51 °N, 114 °W), Canada, from 1988 to 1990. The measurements have been taken in orthogonal scanning mode and the data set has been analyzed to study salient features of gravity waves in the mesospheric region from this mid-latitude station. Rotational temperature data from 36 nights showed distinct gravity wave activity during this period. The gravity wave characteristics studied include the dominant period, horizontal structure speed, implied horizontal wavelength and horizontal component of direction of propagation. The preferred direction in the horizontal wave propagation at Calgary is towards the north-west. A comparison of the observed horizontal propagation directions with the permitted directions, using model wind profiles for Calgary, shows good agreement. This indicates that the upward flow of wave energy could be modified by the background wind.

Mesospheric gravity waves at Saskatoon (52°N), Kyoto (35°N), and Adelaide (35°S)

The gravity waves with periods of 10 min to 8 hours in the mesosphere (67–80 km) are studied using observations from the Adelaide MF radar (35°S), Saskatoon MF radar (52°N), and the middle and upper atmosphere (MU) radar at Shigaraki, near Kyoto (35°N). The seasonal variations of the gravity wave intensity deduced from the horizontal wind velocities showed semiannual variations with solstitial maxima and equinoctial minima at all three locations, but the month of maximum intensity in the year depends on the locations and wave frequency. The kinetic energy due to gravity waves showed similar amplitude between Kyoto and Adelaide, but smaller at Saskatoon, which agrees well with the results of the preliminary comparison using a limited frequency range and limited observation periods by Nakamura et al. [1993a, b]. The horizontal propagation direction of these gravity waves showed small seasonal variations and significant dependency with wave periods at Saskatoon but showed more frequent variation without a large frequency dependency at Kyoto and Adelaide. These characteristics indicate that at the lower latitudes gravity wave activity is larger, as pointed out by Tsuda et al. [1994] from various observational techniques. Also, the frequent changes of the gravity wave intensities at the lower latitudes suggest that there are strong influences of the background wind variation in the middle atmosphere such as semiannual oscillation, mesospheric semiannual oscillation and other wind variations in the low latitudes.

Simultaneous observations of mesospheric gravity waves with the MU radar and a sodium lidar

Simultaneous observations of mesospheric gravity waves have been carried out using meteor wind measurements with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (34.9°N, 136.1°E), and density perturbations of the sodium lidar at Hachioji, Tokyo, Japan (35.6°N, 139.4°E). The study utilizes 7 hours of data collected on the night of December 15–16, 1993, during a time period when a fairly monochromatic gravity wave was dominant. Using hodograph analysis, the dominant gravity wave was found to exhibit a vertical wavelength of 16 km, an intrinsic period of 9.1 hours, and a horizontal wavelength of about 1900 km. The horizontal propagation direction of the gravity wave was determined from the phase relations between the horizontal wind components and the temperature perturbations at Shigaraki. The wave propagated southward, being almost orthogonal to the baseline between Shigaraki and Hachioji. Employing the dispersion and polarization relations for linear gravity waves, the wave‐induced neutral density perturbations from the MU radar observations were estimated. A comparison with the corresponding density perturbations derived from the sodium density measurements showed good agreement. The amplitudes of the neutral density perturbations observed at both locations, which are separated by 310 km, were similar, with a maximum perturbation of ∼7% and a good correlation of phase. Time variations of the hourly variance of the density perturbations also agreed quite well between the two independent determinations, which again supports the view that the radar and the lidar detected the same gravity wave.

Differential Polarized Reflectance Spectroscopy at Air/Water and Air/Metal Interfaces

We report the direct measurement of differential polarized reflectance spectra for samples at air/water and air/metal interfaces. The central component for these measurements is a polarization-division interferometer (PDI). This interferometer uses an in-house-designed beamsplitter constructed in-house from a BaF2 polarizer and a matching substrate. In conjunction with a linear polarizer in front of the source and two rooftop mirrors, one in each arm of the interferometer, the PDI divides the input beam into two orthogonal linear polarization components, recombines them for interference at the beamsplitter, and directs the output beam at 90° to the direction of the input beam. Collimated light rays exiting the interferometer are focused by an f/5 lens and bent from the horizontal propagating axis by a ZnSe wedge to give an angle of incidence of ∼75°, at the sample. The reflected rays are brought back to the horizontal propagation direction by another ZnSe wedge and focused onto the detector by an f/1 lens. The interferogram obtained in this manner represents the differential polarized reflectance interferogram whose cosine Fourier transform directly gives the differential polarized reflectance spectrum. Thus, these spectra were obtained without need for the commonly used photoelastic modulator. Representative spectra for monolayers on a water surface and for samples on a gold surface are presented.

Periods of planetary waves in geomagnetic variations

Abstract. Periods of planetary waves, especially the 10- and 16-day waves, were found in Fourier analyses of 10-year geomagnetic time series from two mid-latitude stations in the northern hemisphere. This suggests that planetary waves influence geomagnetic variations. Cross-spectral analysis of magnetic time series from seven stations located at around 50°N at the beginning of 1979, when a 16-day wave occurred in the stratosphere, also shows a 16-day oscillation. However, study of the phases does not reveal the horizontal direction of wave propagation. Furthermore, the temporal variations of the 16-day oscillation in magnetic time series are presented as dynamic spectra and the results are compared with global investigations of geopotential height data at 1 hPa (around 48 km) with respect to the 16-day wave for the same time interval. In some cases this comparison suggests a clear correlation between geomagnetic variations and planetary waves as well as a propagation of the 16-day wave up to the dynamo region (100-170 km).

Periods of planetary waves in geomagnetic variations

Periods of planetary waves, especially the 10- and 16-day waves, were found in Fourier analyses of 10-year geomagnetic time series from two mid-latitude stations in the northern hemisphere. This suggests that planetary waves influence geomagnetic variations. Cross-spectral analysis of magnetic time series from seven stations located at around 50°N at the beginning of 1979, when a 16-day wave occurred in the stratosphere, also shows a 16-day oscillation. However, study of the phases does not reveal the horizontal direction of wave propagation. Furthermore, the temporal variations of the 16-day oscillation in magnetic time series are presented as dynamic spectra and the results are compared with global investigations of geopotential height data at 1 hPa (around 48 km) with respect to the 16-day wave for the same time interval. In some cases this comparison suggests a clear correlation between geomagnetic variations and planetary waves as well as a propagation of the 16-day wave up to the dynamo region (100-170 km).

Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 2. Characteristics of gravity waves

This paper discusses the characteristics of gravity waves in the equatorial region revealed by analyzing radiosonde measurements of wind velocity and temperature fluctuations at 0–35 km, with a height resolution of 150 m, made every 5–7 hours between February 27 and March 22, 1990, in East Java, Indonesia. We conducted hodograph analysis to delineate vertical and horizontal propagation characteristics and found that most gravity waves were generated in the middle of the troposphere and that they propagated upward into the stratosphere. The amplitudes of wind velocity and temperature fluctuations due to gravity waves were larger in the stratosphere than in the troposphere. The direction of horizontal propagation of gravity waves was rather uniformly distributed in the troposphere, but it became eastward in the lower stratosphere, being opposite to that of the mean winds because of quasi‐biennial oscillation. The vertical wavenumber spectra of wind velocity were described fairly well by a saturated model spectrum, although their amplitudes were smaller in the stratosphere. A typical vertical wavelength was 2–2.5 km, while the horizontal phase velocities were 5–7 and 12 m/s in the troposphere and stratosphere, respectively. The amplitudes of small‐scale gravity waves were significantly larger in the equatorial stratosphere than at middle latitudes. Time‐height variation of the wind velocity variance due to gravity waves showed a clear correlation with that for high relative humidity, which implies that cloud convection is an important mechanism of gravity wave generation in the equatorial region.

A statistical study of the structure, saturation and sources of inertio-gravity waves in the lower stratosphere observed with the MU radar

Using wind data monitored by the MU radar at Shigaraki, Japan (35°N, 136°E) in the three years of 1986–1988, a statistical analysis is made of inertio-gravity waves (IGWs) with short vertical wavelengths (⩽ 4 km) and long ground-based periods (⩾ 10 h) which are dominant in the lower stratosphere in all seasons. The intensity of the IGWs is at its peak in winter when a strong subtropical westerly wind jet is situated over Japan. The zonal component of vertical momentum flux associated with the IGWs in winter is mostly negative and the magnitude decreases with altitude, suggesting a strong interaction between IGWs and the background wind field. The parameters of the wave structure are estimated by a hodograph analysis, taking into consideration the effect of vertical shear of the background wind that is not negligible in winter. The obtained parameters, such as the vertical and horizontal energy propagation directions, vertical and horizontal wavelengths, and intrinsic period, are compared between two periods with a strong subtropical jet including winter and with a weak jet including summer. The degree of saturation of IGWs, defined as the ratio of amplitude of the horizontal perturbation velocity parallel to the horizontal wavenumber vector to the intrinsic horizontal phase velocity, is also statistically and directly examined using the results of wave parameter estimation. It is shown that the degree of saturation is larger in winter than in summer and that there are few oversaturated IGWs even in winter. Moreover, in order to examine the generation mechanisms of the IGWs, an analysis is made from two points of view: geostrophic adjustment of the westerly wind jet and the topographic effect. The meridional propagation direction of IGWs is examined in a section of latitude and altitude relative to the jet axis using ECMWF operational data. Most of the IGWs observed in the 12–18 km height region (above the ground) in winter propagate meridionally toward the jet axis, indicating that geostrophic adjustment at least just as the jet axis is not the main generation mechanism of the IGWs. On the other hand, the characteristics of intensive IGWs propagating westward relative to the background wind in the 18–22 km height region in winter are in good accord with mountain waves excited in strong westerly winds near the surface.

Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 2. Propagation direction

We studied the characteristics of dominant component of gravity waves (λz ∼ 10 km) in the vertical profile of the wind velocities observed in the mesosphere (at around 70–75 km) with the middle and upper atmosphere (MU) radar in the four observation campaigns (June 1987, July 1990, October 1986, and January/February 1991) and the monthly mesosphere observations carried out 4–5 days a month in December 1985 to December 1988. By assuming linear dispersion relations of gravity waves, both vertical and horizontal propagation directions of gravity waves were determined. Most of the dominant waves propagated upward, except that they sometimes propagated downward mainly in summer. A typical vertical wavelength and intrinsic period of the dominant gravity waves were 4–15 km and 4–15 hours, respectively. The mean values of the vertical wavelength, horizontal wavelength, and intrinsic periods were 8.2 km, 1200 km, and 10.2 hours, respectively, which did not show significant seasonal variations. Most of the waves were found to be dissipated by the eddy diffusion of 15–100 m2/s, which does not agree with MF radar observations (10‐ to 120‐min observed periods) but is rather consistent with sodium lidar observations (36–300 m2/s). The mean horizontal phase velocity was 33 m/s. The horizontal propagation direction was generally eastward throughout a year except for the westward propagation in early winter (November to December). This suggests that the dominant gravity waves are less sensitive to a change in the mean zonal wind direction in the middle atmosphere than the short‐period gravity waves (2 hours to 5 min). Momentum flux carried by the dominant gravity waves was estimated to be 0.5–1 m2/s2, which was generally smaller than the momentum flux of gravity waves with periods ranging from 8 hours to 5 min. The amplitude of the momentum flux of the dominant gravity waves had an autumn maximum, while the momentum flux with periods of 8 hours to 5 min had equinoctial minima. Thus in September and October the momentum flux due to the dominant gravity waves became significant. We further pointed out that the possibility that / depends on the wave periods, and therefore the major frequency component of is not easily determined from simple dispersion relation only.

Climatological statistics of stratospheric inertia‐gravity waves deduced from historical rocketsonde wind and temperature data

Ten years of rocketsonde observations of wind and temperature in the 28–57 km height range at 12 stations (spanning 8°S to 76°N) were analyzed. The aim was to determine the geographical and seasonal variation of statistics relating to the propagation of inertia‐gravity waves. As noted by earlier investigators, there is a clear tendency for the rocketsonde hodographs to display clockwise rotation with height in the extratropical northern hemisphere. This is consistent with the notion that the variations are dominated by inertia‐gravity waves with upward energy propagation. By using the wind and temperature data simultaneously it was possible to determine a dominant direction of horizontal wave propagation for each profile. This quantity has an impressive seasonal variation in mid‐latitudes, with strong eastward propagation apparent in summer and generally westward propagation in winter. This seasonal cycle is consistent with theoretical notions of how the mean flow ought to affect wave propagation. Two stations within 10° of the equator are included in this analysis. The results at these near‐equatorial stations contrast strongly with those at higher latitudes. There is a clear tendency for the hodographs at these stations to display quite linear polarization (rather than the systematic rotation generally seen at higher latitudes). The wind variations also show a remarkable tendency to align themselves in the zonal direction. When the wind and temperature variations are used together, it can be shown that the variations seen are consistent with a clear dominance of eastward propagation.

Gravity waves in the mesosphere observed with the middle and upper atmosphere radar

We have observed wind motions from 60 to 90 km altitudes with the middle and upper atmosphere (MU) radar during daylight hours (0800–1600 LT) from October 13 to 31, 1986. Gravity waves with fairly sinusoidal vertical structure were evident on 16 days of the 19 days of observations. They were characterized by a typical vertical wavelength of 5–15 km and intrinsic periods centered at about 8.6 hours. The propagation velocity of these waves was determined using the linear gravity wave theory. All of the waves propagated downward and had an equatorward component of the meridional propagation. The median direction of horizontal propagation was slightly east of south, and the mean horizontal phase speed was 35.3 m/s. The vertical wave number spectra of horizontal components of mesoscale wind fluctuations agree well with the theoretical spectrum of saturated gravity waves. At frequencies from 1 × 10−5 to about 4 × 10−5 c/s frequency spectra of vertical and radial wind components had logarithmic slopes of 1/3 and −5/3, respectively, which agree fairly well with a model gravity wave spectrum. The effects of Doppler shifting on frequency spectra are most obviously recognized at large frequencies near the Brunt Väisälä frequency. We have also determined the upward flux of horizontal momentum induced by waves with periods from 10 min to 8 hours and further estimated the westward and northward body force of 5.1 and 4.0 m/s/d, respectively.

Gravity Wave Propagation Characteristics (60–120 km) as Determined by the Saskatoon MF Radar (Gravnet) System: 1983–85 at 52°N, 107°W

Abstract Gravity waves (GW) have been detected and their characteristics measured by observations with the Saskatoon multiple bistatic system, Gravnet. Data are available from 50 days for two height ranges 64–97 km, ∼100–115 km, and for the four seasons of 1983–85. Wave characteristics include horizontal wavelength, phase velocity, period, and amplitude. Background wind data allows the corresponding intrinsic parameters to be calculated; many waves are Doppler shifted to near their critical levels. Altitude and seasonal variations in the GW characteristics are shown. The strongest variation is in the horizontal direction of wave propagation, with southward directions dominating, but significant eastward(westward) fluxes in summer(winter) below 100 km.

Semidiurnal internal tide in JASIN: Observations and simulation

Temperature was observed in the upper 80 m by moored thermistor chains at three locations in Rockall Channel west of Scotland. Isotherms were interpolated, and a 1‐week period of exceptionally energetic tidal oscillations was analyzed. The moored array (horizontal separations ranging from 6 to 20 km) was used as an antenna to determine the dominant horizontal wavelength and direction of propagation of the internal tide within the array. Rockall Bank, 100 km to the southeast, was identified as the source of the internal tide. The semidiurnal internal tide generated by the interaction of the surface tide with Rockall Bank was simulated by use of a model due to Prinsenberg and Rattray. The model predicts generation of the internal tide at the shelf break and propagation seaward as energetic beams which lie along internal wave characteristics. Some energy is trapped near the surface in association with the pycnocline. There is substantial structure in the velocity and vertical displacement fields. Most of the total energy of the internal tide is in the first vertical mode. However, at particular depths, vertical modes as high as 4 (horizontal wavelengths of 25 km) dominate. The high degree of spatial variability in the modeled internal tide illustrates the potential for error when basing a description of the tide on sparse observations. There is good agreement between the modeled vertical displacements and the 1‐week period of energetic oscillations of isotherm depth observed at the moorings. In addition, there is good agreement between the model and the tidal velocity variance measured during the same week at depths ranging from 10 to 1000 m and distances ranging from 50 to 130 km from Rockall Bank. Richardson numbers associated with the vertical shear of the modeled internal tide range down to values less than 2.

Propagation characteristics of hydromagnetic waves in various layers of the ionosphere

This paper derives the basic propagation characteristics of hydromagnetic waves in various layers of the ionosphere. It is shown that propagation in the upper ionosphere and the F 2 layer is largely isotropic. In the lower region of the ionosphere there are two possible modes of propagation, both being anisotropic. Propagation characteristics of waves in this lower region, however, are relatively independent of the direction of horizontal propagation. Calculations of intrinsic wave attenuation show that ducted propagation of Pc 1 signals over appreciable horizontal distances may only take place in the upper layers of the ionosphere.

Propagation of radiation from a finite beam or source through an anisotropic random medium

In a previous paper, we solved the problem of propagation of a plane wave in an anisotropic random medium. The characteristic correlation distances in the horizontal propagation direction were assumed to be large compared to the radiation wavelength divided by 2π, (λ̄/2π), while in the vertical direction, these distances were, roughly speaking, assumed to be of the same order or smaller than λ̄/2π. For this medium, we derive here the equation governing the propagation of the coherence function for a finite beam or source of characteristic dimension a. This equation is then averaged over the vertical direction and solved. The solution is given in terms of a definite integral over the horizontal spectrum of the index-of-refraction field. The point source solution (a → 0) and the plane-wave case (a → ∞) are given as special cases. Explicit solutions are given for the propagation of an acoustic field in the ocean under conditions for which the spectrum satisfies a minus two power law.

Propagation of radiation from a finite beam or source through an anisotropic random medium

In a previous paper we solved the problem of propagation of a plane wave in an anisotropic random medium. The characteristic correlation distances in the horizontal propagation direction were assumed to be large compared to the radiation wavelength, while in the vertical direction these distances were assumed to be of the same order or smaller than the wavelength. For this medium, we derive here the equation governing the propagation of the coherence function for a finite beam or source. This equation is then averaged over the vertical direction and solved. The solution is given in terms of a definite integral over the horizontal spectrum of the index of refraction field. The point source solution and the plane wave case are given as special cases. Explicit solutions are given for the propagation of an acoustic field in the ocean under conditions where the spectrum is dominated by the effects of internal waves.

Diurnal variations of F-region waves

An hour by hour study of F-region waves by means of radio Doppler-sounders has been made over many days. The horizontal and vertical trace (phase) velocities, horizontal direction of propagation, and power spectrum densities of 10–100 min period F-region waves reveal strong patterns of diurnal variation. In February and March the direction of horizontal propagation determined from an array of cw Doppler-sounders in the New York City area, is toward the south during the day with components of motion toward the north at night. In June and July the directions are more consistently toward east both day and night. The waves are dispersed with short periods fastest. Phase velocities, for any period, increase to a maximum at night. The diurnal change in velocity is nearly an order of magnitude in the winter, but some-what less in the summer. The velocity of the waves varies directly with the diurnally-varying height of observation of the fixed-frequency Doppler-sounder array. The results suggest a time-variation of wave velocity as well as a height gradient in vertical trace velocity. A discussion of alternate possible wave types is given.

Spin modulation of high‐latitude hiss measured by an electric dipole

Natural radio noise received by the ISIS satellites in the polar and auroral regions often exhibits spin modulation. For noise in the whistler‐mode frequency domain observed on seven high‐latitude passes, the data indicate that temporal‐spatial variations in the level of the noise source are not important on a time scale of one spin period, and thus the dependence of the signal on the attitude of the receiving dipole antenna can be studied. Relative variations in level during one half of a spin period have been synthesized by simply convolving the attitude dependence of the antenna impedance with a spatial distribution of electric vectors. The resonance cone angle, corresponding to the infinity in the refractive index, has been predicted to an accuracy of 3°, while the theoretical amplitude and shape of the modulation agree well with what is observed. It is found that longitudinal electric waves yield better agreement than transverse electromagnetic waves. In some cases, preferred horizontal propagation directions can be roughly identified. The computations confirm the relevance of established theory for the antenna impedance and the ion sheath.

On long waves with cross-wind in an atmosphere

Asymptotic equations of long waves in a compressible medium of infinite height with arbitrary density and wind profiles, including cross-wind, are systematically derived to provide the mathematical description of certain pressure-front phenomena observed in the atmosphere. It is assumed that the final state of the wave is a plane wave moving with nearly constant velocity, even though its velocity field has a component transverse to the direction of propagation. The coefficients of the asymptotic long-wave equations are found to depend on the equilibrium profiles of the density and of the velocity component in the direction of propagation of the wave, but not on the profile of the transverse velocity. Any horizontal direction of propagation is found to be possible, even for waves of permanent type.