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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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