Simultaneous Measurements of Clear Air Turbulence at the Tropopause by High-power Radar and Instrumented Aircraft

Browning, K. A.; Watkins, C.D.; Starr, J. R.; McPherson, Annika

doi:10.1038/2281065a0

Cited by 26 publications

(9 citation statements)

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“…The radar system here uses a relatively small sampling volume (i.e., narrow beam width and short pulse length) and ought to be less severely affected. Several early studies using UHF radar systems with range-height displays [Hicks, 1969;Browning et at., 1970;Dutton and Panofsky, 1970, and ref-erences therein], aircraft [Roach, 1969;Axford, 1970Axford, , 1973, and visual observations [Ludtam, 1967;Young, 1971] reported Kelvin-Helmholtz instabilities (KHI) and/or breaking waves above 10 km, in the region of strong wind shear above the jet stream. The generation of KH!…”

Section: Discussionmentioning

confidence: 99%

The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

Worthington¹,

Thomas²

1996

Radio Science

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Radar measurements have often been used to estimate gravity wave momentum flux profiles in the troposphere, lower stratosphere, and mesosphere. The reliability of the measurements in the troposphere and lower stratosphere is investigated, with a comparison of three techniques employing various radar beam directions and zenith angles, including the standard "symmetric-beam" method. The study uses the 46.5 MHz MST radar system at Aberystwyth, Wales (52.4øN, 4.1øW). For short-period waves, errors in momentum flux profiles arising, for example, from differing data quality in the two radial beams sometimes prove difficult to eliminate in the symmetric-beam method. For long-period waves, including inertia-gravity waves, problems arise from the vertical wind measurement which is implicit in any estimate of momentum flux. Furthermore, a difference is sometimes found between the echo powers of the two symmetric radial beams, and this fluctuates with the horizontal wind perturbations from long-period waves. The possible influence in this power difference of tilted anisotropic scattering layers, which might also bias radial velocity measurements, is discussed. A momentum flux method which measures the vertical wind with a vertical beam, and the horizontal wind with a pair of radial beams, avoids some of the problems at both short and long periods. Agreement of this method with the symmetric-beam results would allow improved confidence in momentum flux profiles. 1. Introduction Early estimates of the energy and momentum fluxes associated with gravity waves suggested that their effects at upper heights could be important [e.g., Gossard, 1962; Hines, 1972]. However, the role of such waves in balancing the momentum and energy budgets of the mesosphere awaited studies such as those of Houghton [1978] and Lindzen [1981]. Interest in the momentum budget at lower heights was inspired by the observation of substantial fluxes associated with mountain waves [e.g., Lilly and Kennedy, 1973] and the realization that the flux divergence could affect the large-scale circulation at the lower heights [Lilly, 1972]. Observational techniques to measure the vertical transport of horizontal momentum have included the use of aircraft-mounted systems [e.g., Lilly and Kennedy, 1973; Bougeault et al., 1993; Alexander and Pfister, 1995], radiosonde flights [e.g., Mobbs and Rees, 1989; Shutts et al., 1994], and mesospherestratosphere-troposphere (MST) radar [e.g., Fukao et al., 1988a; Fritts et al., 1990]. The symmetric-beam radar method of Vincent and Reid [1983] has been applied to measure momentum flux in both the lower atmosphere [e.g., Fukao et al., 1988a; McAfee et al., 1989; $ato, 1990, 1993; Fritts et al., 1990; Thomas et al., 1992; Prichard and Thomas, 1993] and the mesosphere [e.g., Tsuda et al., 1990; Reid and Vincent, 1987]. In this method, the momentum flux in a vertical plane is obtained using two radial radar beams in that plane, pointing at symmetric zenith angles +0 and -0: uw --r}ø -r2-ø 2 sin 20 (1) where u, w represent the horizont...

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Section: Discussionmentioning

confidence: 99%

The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

Worthington¹,

Thomas²

1996

Radio Science

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“…RoÈ ttger and Schmidt (1979) use a VHF radar to detect the ®ne structure of KHI in echo-power measurements near a height of 4.5 km. An impression of the development of tilted structures caused by wave breaking or KHI development, from Browning and Watkins (1970), is reproduced in Fig. 7a.…”

Section: Instability Of Long-period Wavesmentioning

confidence: 97%

“…7a, b. a Schematic representation of various stages in the development of Kelvin-Helmholtz instability, showing the formation of tilted structures, after Browning and Watkins (1970). b Model of tilted layers to explain the powerdi erence e ects in Figs.…”

Section: Instability Of Long-period Wavesmentioning

confidence: 99%

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Long-period unstable gravity-waves and associated VHF radar echoes

Worthington

Thomas

1997

Ann. Geophys.

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Abstract. VHF atmospheric radar is used to measure the wind velocity and radar echo power related to longperiod wind perturbations, including gravity waves, which are observed commonly in the lower stratosphere and tropopause region, and sometimes in the troposphere. These wind structures have been identi®ed previously as either inertia-gravity waves, often associated with jet streams, or mountain waves. At heights of peak wind shear, imbalances are found between the echo powers of a symmetric pair of radar beams, which are expected to be equal. The largest of these power di erences are found for conditions of simultaneous high wind shear and high aspect sensitivity. It is suggested that the e ect might arise from tilted specular re¯ectors or anisotropic turbulent scatterers, a result of, for example, Kelvin-Helmholtz instabilities generated by the strong wind shears. This radar power-di erence e ect could o er information about the onset of saturation in long-period waves, and the formation of thin layers of turbulence.

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“…Some success in detecting CAT has been achieved with ground-based ultrasensitive radars, and new insights have been attained into the nature of turbulence in the free atmosphere with this technique (Browning et al, 1970;Atlas et al, 1970).…”

Section: Introductionmentioning

confidence: 99%

Satellite radiances and clear air turbulence probabilities

Woods

Panofsky

1973

Boundary-Layer Meteorol

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Abstract. Radiance gradients recorded by Nimbus III and IV, over the United States and the Atlantic Ocean, respectively, are well-related to probability of clear air turbulence. Making use of satellite data only, it is possible to avoid CAT almost entirely. In general, radiosonde-measured temperature gradients are not as well correlated with CAT as satellite radiance gradients; but if the satellite track is parallel to the isotherms, radiance gradients along the satellite track are negligible; and temperature gradients at constant pressure are better indicators of CAT unless the satellite instrument scans at right angles to the track.

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Simultaneous Measurements of Clear Air Turbulence at the Tropopause by High-power Radar and Instrumented Aircraft

Abstract: Large amplitude Kelvin-Helmholtz billows observed by radar at a height of almost 11 km are shown to have been associated with clear air turbulence which produced a vertical aircraft acceleration of 0.65g.

Cited by 26 publications

References 4 publications

The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

Long-period unstable gravity-waves and associated VHF radar echoes

Satellite radiances and clear air turbulence probabilities

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