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N. M. Gavrilov and S. Fukao

Abstract

A theoretical model of an ensemble of harmonics of internal gravity waves (IGWs) propagating from random sources in the atmosphere is used to explain the seasonal variations of IGW intensity observed at different altitudes in the lower and middle atmosphere with the Japanese middle and upper atmosphere radar. Calculations reproduce the seasonal cycles of IGW amplitudes having a maximum in winter and a minimum in summer in the upper troposphere and having maxima in winter and summer and minima at equinoxes in the mesosphere. The seasonal behavior at different altitudes appears to be produced by the seasonal variations of the background wind and temperature, which influence the IGW generation, propagation, and dissipation.

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H. Luce, G. Hassenpflug, M. Yamamoto, S. Fukao, and K. Sato

Abstract

Kelvin–Helmholtz (KH) instability is likely one of the most important sources of clear-air turbulence in the lower atmosphere. It produces billows, which mix and transport heat and materials vertically in the stably stratified atmosphere. Billows can also dissipate energy; therefore they can affect the larger-scale dynamics. While only a few direct observations have been reported in the tropopause region, in this work the authors report very detailed observations of billow structures around 16-km altitude, in the upper part of the jet stream. Observations were made with very high frequency (VHF)-band mid- and upper-atmosphere (MU) radar (Shigaraki, Japan; 34.85°N, 136.10°E) whose height resolution was improved with a range-imaging technique. KH billow structures were observed for at least 2 h and were found to have horizontal wavelengths of about 5.3 km and vertical extents between 0.5 and 1.0 km. Analysis of wind and temperature profiles measured by radiosondes launched from nearby meteorological stations indicated the presence of nearly monochromatic disturbances, likely due to a dominant inertia–gravity wave (IGW) superimposed on the background wind field. The presence of the IGW was also confirmed by analysis of wind profiles measured by the MU radar just before the KH billows were detected by the observations in range-imaging mode. The IGW, with vertical and horizontal wavelengths of about 3.5 and 600 km, respectively, may have been a direct radiation from the jet stream, as suggested by recent works, and likely played a major role in the onset of the observed KH instability.

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H. Luce, T. Takai, T. Nakamura, M. Yamamoto, and S. Fukao

Abstract

Humidity is, among other things, a key parameter in the evolution of atmospheric dynamics and in the formation of clouds and precipitation through latent heat release. The continuous observation of its vertical distribution is thus important in meteorology. In the absence of convection, humidity in the lower troposphere is distributed into nearly horizontally stratified layers. The thin humidity gradients at the edges of these layers are known to be the main cause of very high-frequency (VHF) stratosphere–troposphere (ST) radar backscatter in the lower troposphere. This property has been experimentally demonstrated many times in the literature from comparisons between balloon measurements and low-resolution radar observations. In the present work, original results of comparisons between Raman lidar measurements of water vapor and middle- and upper-atmosphere (MU) radar measurements of echo power using a range-imaging technique are shown at high spatial and temporal resolutions (∼50 m, ∼20 s). Other tremendous advantages of such comparisons are the simultaneity, time continuity, and colocalization of the lidar and radar measurements. The results show that the radar can be used for continuously monitoring the thin positive and negative gradients of humidity when operated in range-imaging mode. With additional information from balloon measurements, it would be possible to retrieve humidity profiles in the lower troposphere at an unprecedented vertical and time resolution.

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H. Luce, S. Fukao, M. Yamamoto, C. Sidi, and F. Dalaudier

Abstract

For many years, mesosphere–stratosphere–troposphere (MST) radar techniques have been used for studying the structure and dynamics of the lower and middle atmosphere. In particular, these instruments are unique tools for continuously monitoring vertical and horizontal components of the atmospheric wind at high spatial and temporal resolutions. From the very beginning, many studies have been carried out analyzing the reliability of the MST radar wind measurements and their accuracy. However, until now, very few studies have been presented confirming the high performances of the VHF Middle and upper Atmospheric (MU) radar of Japan (35°N, 136°E) for measuring the wind field. The present paper thus gives original comparisons between horizontal velocities measured by MU radar and by instrumented balloons using global positioning system (GPS) radiosondes. Twelve radiosondes were successfully used during the French–Japanese MU Radar Temperature Sheets and Interferometry (MUTSI) campaign (10–26 May 2000, Japan). They were launched about 30 km westward from the radar site, hung below capesphere-type balloons. During the campaign, two sets of radar parameters with oblique beams directed 10° and 15° off zenith at 150-m and ∼2-min resolutions were used. For both configurations, a very good agreement between the two kinds of measurements was found, indicating that both wind profiles are not affected by systematic measurement biases. Moreover, the standard deviation of the differences is less than 2.6 m s−1 using all radar data within a range height of 2–20 km and less than 1.5 m s−1 for a radar signal-to-noise ratio larger than 0 dB in oblique directions and a horizontal radar-balloon distance smaller than 50 km. Two cases of significant differences (10–15 m s−1) around the jet-stream altitude could qualitatively be explained by spatial and temporal variability of the wind field during the passage of a warm front.

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H. Luce, S. Fukao, F. Dalaudier, and M. Crochet

Abstract

In the present paper, strong isotropic radar echo enhancements are shown that are related to the existence of nearly statically neutralized layers (40–120 m thick) observed with high-resolution (10 cm) temperature measurements performed during the Middle and Upper Atmosphere Radar, Temperature Sheets and Interferometry (MUTSI) campaign (8–26 May 2000, Japan). These events have been observed in the immediate vicinity of the tropopause, just above a jet stream maximum. They most likely result from strong turbulent mixing generated by shear instabilities and could be one of the mechanisms responsible for the generation of strong temperature gradients (temperature gradient sheets) at the mixed layer edges. Further investigations will be necessary to evaluate the occurrence of this neutralization mechanism and its contribution to the generation of strong temperature gradient sheets within the free atmosphere.

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S. Fukao, M. F. Larsen, M. D. Yamanaka, H. Furukawa, T. Tsuda, and S. Kato

Abstract

Analysis of vertical velocity measurements made for four days each month over the period from 1986 to 1988 by the MU radar in Japan shows a reversal in direction near the peak in the zonal wind profile during the winter months. More specifically, the reversal is noted during periods when the peak horizontal wind speeds 60 m s−1. The vertical velocities associated with the circulation have magnitudes of 10-20 cm s−1, and the depth of the circulation is of the order of several kilometers. In 6 out of 14 cases when the feature was observed, the direction of the vertical circulation, although not the magnitude, could be explained by adiabatic ascent or subsidence along the average potential temperature surface slopes for the observation intervals. The direction of the circulation was such that it would tend to produce cooling and heating for the ascent and subsidence, respectively, that would tend to strengthen or at least maintain the jet. In the remaining eight cases, the direction of the vertical circulation could not be explained by the slope of the time-averaged potential temperature surfaces alone since the combination of the horizontal winds and the slopes of the isentropic surfaces would have led to a prediction of a circulation directly opposed to that observed. Thus, either the local tendency in the time-averaged potential temperature must have been significant, structure with scales smaller than the rawinsonde station separation must have been present, or diabatic effects may have played a role in the dynamics of the vertical velocity feature.

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T. Tsuda, T. Inoue, S. Kato, S. Fukao, D. C. Fritts, and T. E. VanZandt

Abstract

We present vertical wavenumber spectra of mesoscale wind fluctuations using data observed in the troposphere, lower stratosphere and mesosphere by the MU radar at 35°N in Japan in October 1986 and June 1987, as well as lower stratospheric spectra obtained by the Arecibo UHF radar at 18°N in Puerto Rico in June 1983. These spectra are much more homogeneous than previously available spectra since all of the data were observed by the same radar technique, the data in the different atmospheric regions were taken essentially simultaneously, and all of the spectra were analyzed using very similar methods. In the large-wavenumber ranges of the observed spectra, the asymptomatic slopes and amplitudes agree well with the saturated gravity wave spectral model developed by Dewan and Good (1986) and Smith et al. (1987), which has a slope of −3 and a spectral amplitude proportional to the buoyancy frequency squared. The good agreement between the model spectrum and the observed spectra from different altitudes, different reasons, and two different stations located at 35° and 18°N suggests that the model is essentially correct, in spite of the heuristic nature of some of its assumptions.

The spectral densities of the zonal and meridional components are similar at large wavenumbers, while the meridional spectrum has larger energy density at small wavenumbers where the spectrum is not saturated. The dominant vertical scales of the gravity wave field in the mesosphere, lower stratosphere, and troposphere are estimated to be >10 km, 2.2 to 3.3 km, and ≥3.3 km in october and ≥4.5 km in June, respectively, consistent with determinations from previous studies.

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M. F. Larsen, S. Fukao, O. Aruga, M. D. Yamanaka, T. Tsuda, and S. Kato

Abstract

Vertical-velocity measurements made by a direct vertical-beam method are compared to vertical velocities derived from VAD (velocity-azimuth display) measurements over a 27-h period. The results indicate that the two types of measurements in regions where the scatter is isotropic agree well. The largest discrepancies occur in the regions characterized by strong stratification and anisotropic or aspect-sensitive scatter. Although there are various assumptions inherent in the VAD calculations of the vertical velocities, indications are that the source of error is the aspect sensitivity, which produces effective off-vertical pointing angles in the vertical beam when the reflectivity layers are tilted out of the horizontal plane. However, other additional sources of bias or error cannot be excluded.

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T. E. VanZandt, S. A. Smith, T. Tsuda, T. Sato, S. Fukao, S. Kato, and D. C. Fritts

Abstract

We present in this paper a study of the azimuthal anisotropy of the motion field observed during a six-day campaign in March 1986 using the MU radar in Shigaraki, Japan. The radial wind velocity was observed at 20° zenith angle, at every 30° of azimuth during four days, and at every 45° during two days. A jet stream was present during the entire six days. The average radial velocity variance from 10.4 to 19.2 km was calculated every four minutes and then averaged over 20 minutes or one hour.

The average variance was found to be a strong function of both azimuth and time. The azimuthal variations were analyzed in terms of the mean and the first and second harmonics. The mean is proportional to the kinetic energy per unit mass of the radial wind fluctuations, and the first harmonic is proportional to the vertical flux of horizontal momentum per unit mass. The strong azimuthal variation was usually dominated by the second harmonic; i.e., with two peaks, but was occasionally dominated by the first harmonic, with one peak. The phase of the first harmonic was usually westward, but the phase of the second harmonic was quite variable.

It was shown by a development of gravity wave theory that all of the observed azimuthal variations could probably be caused by a gravity wave field whose parameters vary with time.

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Tri W. Hadi, T. Horinouchi, T. Tsuda, H. Hashiguchi, and S. Fukao

Abstract

Characteristics of sea-breeze circulation over the tropical site of Jakarta, Indonesia, have been documented based on analyses of satellite images and data from long-term L-band boundary layer radar measurements carried out at Serpong (6.4°S, 106.7°E). Inspection of satellite imagery reveals that a sea-breeze front develops well along the northern coastal plain of West Java and propagates inland until its structure is deformed over complex topography. It is found that the sea-breeze signal detected by the boundary layer radar is most well defined during the dry season months of July–October. In all of these months, radar observations indicate a late afternoon intensification of sea-breeze flow in the 0.5–0.8-km height range between 1700 and 1800 LT, which is not elucidated upon by surface measurements. The effect of weather conditions on the sea-breeze pattern is investigated by using a cloudiness index derived from data of incoming solar radiation. The results show that sea-breeze intrusion over the radar site occurs earlier during more cloudy days, whereas the intensity of sea-breeze circulation weakens accordingly. In the rainy season months of January and February, diurnal wind variation is characterized by daytime onshore flow enhancement, which is not likely attributed to sea-breeze circulation.

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