Microscale ordered motions and atmospheric structure associated with thin echo layers in stably stratified zones

Abstract

Abstract. The interpretation of ultra-high resolution radar observations of thin clear-air echo strata is made with the aid of fine-scale aircraft measurements. The echo layer, generally comprising two sub-strata each 5 m thick and spaced 7-10 m apart, is found within a 10-20 m deep section of a strong inversion where the thermal stability and shear are maximized, and the Richardson number is close to 0.25. Mechanical turbulence is restricted entirely to this layer; the variance of the N-S velocity component, av 2, is the strongest, consistent with the orientation of the shear vector in this stratum. Spectra and cospectra of a 9-s slant run through the echo stratum show remarkably ordered motions. A strong negative peak of <vw> covariance at 80-m scale, accompanied by a zero of <uw> covariance and bulges in the longitudinal (v) and vertical (w) velocity spectra, is identified with breaking KelvinHelmholtz waves oriented in the N-S direction along the shear vector. A synthesis of the temperature and velocity structures from measurements along the flight path confirms the ordered motion deduced from the spectra and reveals a group of K-H waves of 80-m length and 10-m height at the height of the radar echo. Microscale K-H ripples of 3-4 m length are also deduced to be present in the 0.5 m thick interfacial region where the thermal gradient and shear are strongly enhanced by the larger shearing K-H wave.Two possible sources of the echoes are proposed: (1) scatter from fully developed turbulence within the interfacial zone in an inertial subrange falling entirely in sub-meter scales; and (2) the incoherent summation of specular reflections from properly oriented portions of the microscale K-H ripples. While the authors favor the first of these mechanisms, both require stringent conditions (rf the physical microstructure which are beyond the available observations. Fossil turbulence is precluded as an echo mechanism.

Kelvin-Helmholtz waves observed by aircraft at different stages of their life-cycle in a low-level inversion

“…Table III lists phase angles obtained in the same frequency bands between the vertical component of air velocity and a number of variables. As shown by Stewart (1969), a phase angle PhwT between the vertical component of air velocity and temperature close to *90" is a good indicator of neutral (non-amplifying) waves whereas a phase angle PhwT in the range of 180" suggests unstable (amplifying) waves (Metcalf and Atlas, 1973). The observations along Legs 13 and 15 show values which are as much as 27" and 46" apart from the *90" to be expected in neutral waves.…”

“…The propagation and trapping of nearly-neutral wave motions has been observed within a marine inversion layer by Metcalf (1975) and at the base of a strong morning inversion by Mascart et al (1977), using instrumented aircraft. The development of breaking gravity waves has been studied by Metcalf and Atlas (1973) and Kerman (1974); in particular, these authors analysed to what extent waves and small-scale turbulence are mutually interactive.…”

The observed generation and breaking of atmospheric internal gravity waves over the ocean

“…When the waves co-exist with turbulence in the atmospheric surface layer, spectral and co-spectral analysis seems to be one of the powerful tools to detect waves and their harmonics. This procedure has been used by Axford (197 l), Metcalf and Atlas (1973), Caughey and Readings (1975) and Metcalf (1975) among others. In the following sections, analysis of the data in the frequency domain is carried out to establish the presence of the waves and to determine the characteristic features.…”

“…Several studies of internal gravity waves (Axford, 1970,197 1, for example) have been at very high altitudes, carried out to understand the contribution of the waves to clear-air turbulence. Metcalf and Atlas (1973) and Metcalf (1975) using aircraft-mounted instruments, studied waves at low-level inversions with the base of the inversions a few hundred meters above the surface. Caughey and Readings (1975) described waves within 183 m above the earth's surface during nocturnal inversions over land.…”

Asymmetry of atmospheric microstructure over synoptic scales

“…Effects of tilted specular scatterers are considered also by Metcalf and Atlas (1973) and Metcalf (1975), for FM-CW radar. A few studies report aspect sensitivity for wavelengths much smaller than that of VHF radar (e.g.…”