Yasuhiro Murayama

Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 1. Equatorial waves and diurnal tides

This paper describes the characteristics of the mean winds, equatorial waves with periods ranging from 4 to 20 days, and diurnal tides determined by analyzing the profiles of wind velocity, temperature, and humidity obtained every 5–7 hours in the height range up to about 35 km with a height resolution of 150 m during an observation campaign conducted February 27 to March 22, 1990, in East Java, Indonesia. The structures of the mean winds in the troposphere and lower stratosphere seemed to be affected by the Australian monsoon and the quasi-biennial oscillation, respectively. Frequency spectra indicated that the equatorial waves as well as the diurnal tides were dominant below about 25 km, while gravity waves with periods shorter than 4 days became more significant above 25 km. A 7-day oscillation showing an antiphase relation between the eastward and northward components and exhibiting large amplitudes was observed in the lower troposphere. The time-height variations of the activity of this 7-day oscillation were clearly correlated with a region of high relative humidity. Perturbations in the zonal wind and temperature with wave periods varying from 15 to 17 days were also enhanced in the troposphere, while Kelvin waves with periods of about 7 and 20 days were detected in the lower stratosphere, and activity near the tropopause was conspicuously enhanced. We found that the 20-day Kelvin wave greatly modified the structure of the tropopause, such as the minimum temperature, the tropopause height, and the values of the Brunt-Vaisala frequency squared N2, which further suggests the effects of Kelvin waves on the transportation of tropospheric water vapor into the stratosphere and on the downward mixing of stratospheric minor constituents into the troposphere. The observed profiles of the diurnal oscillation were compared with those of a numerical model assuming only migrating tides, which reasonably agreed above about 25 km. Below 25 km, however, the observed amplitudes were 1–1.5 m/s, exceeding those of the model about 10 times. Moreover, the phase profiles involved fluctuations with small vertical scales, suggesting interference by many nonmigrating tides with short vertical wavelengths.

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.

Seasonal variation of gravity wave activity in the lower atmosphere observed with the MU radar

In this paper we analyzed the seasonal variation of gravity wave activity in the troposphere and lower stratosphere (5–25 km) as revealed by monthly observations with the MU radar between December 1985 and December 1989. In the lower stratosphere, wind velocity variance due to gravity waves with periods of 5 min-21 hours showed a clear annual variation with a maximum in winter and a minimum in summer, agreeing fairly well with the seasonal variation of the jet stream intensity. This suggests that the excitation of gravity waves is closely related to the behavior of the background mean winds. The specific kinetic energy per unit volume of short-period gravity waves (ground-based period of 5 min-2 hours) was enhanced in winter and spring, its peak in the profile being located near the peak of the jet stream (∼12 km). On the other hand, the kinetic energy of the long-period (2–21 hours) component showed clear annual variation with a winter maximum and a summer minimum in the middle troposphere and lower stratosphere. The kinetic energy of the long-period component was smaller in the lower stratosphere than in the troposphere, which did ot seem to penetrate the lower stratosphere. In the short-period component the ratio of the horizontal to the vertical wind variances near the jet peak height was observed to be large in winter and small in summer, probably due to the seasonal variation of the Brunt-Vaisala frequency. The wind variance ratio for the long-period component in the upper troposphere and lower stratosphere was greatly reduced in winter, possibly due to the Doppler shifting of gravity waves. The vertical flux of zonal momentum for the 5 min- to 21-hour period component showed negative values in the upper troposphere and lower stratosphere, implying that the upward propagating gravity waves in the lower stratosphere mostly traveled westward relative to the background wind. We did not find any significant seasonal variations in the vertical flux of meridional momentum.

Variations of the gravity wave characteristics with height, season and latitude revealed by comparative observations

Abstract This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E). The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream. We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers. Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions. The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.

MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes

Abstract The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the waves vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak ( m s −1 ) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.

Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

[1] The vertical coupling of the stratosphere-mesosphere system through quasi-stationary and traveling planetary waves during the major sudden stratospheric warming (SSW) in the Arctic winter of 2003/2004 has been studied using three types of data. The UK Met Office (UKMO) assimilated data set was used to examine the features of the global-scale planetary disturbances present in the winter stratosphere of the Northern Hemisphere. Sounding the Atmosphere using Broadband Emission Radiometry (SABER) satellite measurements were used as well for extracting the stationary planetary waves in the zonal and meridional winds of the stratosphere and mesosphere. Radar measurements at eight stations, four of them situated at high latitudes (63–69N) and the other four at midlatitudes (52–55N) were used to determine planetary waves in the mesosphere-lower thermosphere (MLT). The basic results show that prior to the SSW, the stratospheremesosphere system was dominated by an upward and westward propagating � 16-day wave detected simultaneously in the UKMO and MLT zonal and meridional wind data. After the onset of the SSW, longer-period (� 22–24 days) oscillations were observed in the zonal and meridional MLT winds. These likely include the upward propagation of stationary planetary waves from below and in situ generation of disturbances by the dissipation and breaking of gravity waves filtered by stratospheric winds. Citation: Pancheva, D., et al. (2008), Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004, J. Geophys. Res., 113, D12105, doi:10.1029/2007JD009011.

A comparison of mean winds and gravity wave activity in the northern and southern polar MLT

Mean winds and waves observed in the mesosphere and lower thermosphere with MF radars located at Davis (69oS, 78oE) and Poker Flat (65oN, 147oW) are compared. Measurements covering the pe- riod from 1999 to mid 2000 show differences in the strength of the horizontal wind fields. In the southern hemisphere the zonal and meridional winds reach their maximum values near the summer solstice, but are de- layed by 2-3 weeks in the northern hemisphere. Gravity wave variances also show significant differences, as do the strength of vertical velocities.

Equivalent electron densities at reflection heights of tweek atmospherics in the low-middle latitude D-region ionosphere

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20—28 el./cm3 at ionospheric reflection heights of 80—85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

Comparison of wind measurements between Yamagawa MF Radar and the MU Radar

A new MF radar was installed at Yamagawa (31.20°N, 130.62°E), Japan in August, 1994 by the Communications Research Laboratory, and the first comparison of wind measurements taken with the Yamagawa MF radar and meteor wind measurements with the MU radar at Shigaraki (34.85°N, 136.10°E) are carried out. In spite of their spatial separation of approximately 650 km, reasonable agreement between wind velocities by the two radars at 80–92 km in altitude is confirmed, except for small scale perturbations that are probably due to gravity waves enhanced for the period from 17 h LT on September 13 to 7 h LT on September 14, 1994. It is found that the winds of the MF radar above 92 km tend to be smaller than the winds obtained with the MU radar. Receiver saturation and interference are possible causes of the reduced wind velocities observed with the MF radar. The height structures of semidiurnal tides are also compared. The amplitude and phase of zonal components agree at most heights between the two radars. For the meridional component the amplitudes fluctuate above 88 km with a significant difference and the phase from the MU radar leads that of the MF radar by less than several hours at 86 km to 92 km. An enhanced gravity wave seems to pass over the MU radar almost from north to south, resulting in a contamination of the meridional wind component of the semidiurnal variation.

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

Abstract We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e -folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.