Acoustic echo-sounding techniques and their application to gravity-wave, turbulence, and stability studies

Abstract

Abstract. This paper is a brief summary of recent experimental studies conducted by the WPL staff in order to: (a) compare acoustic echo strengths with those predicted from measured turbulence intensities and scatter theory; (b) develop optimum experimental geometries for Doppler work, and (c) use the acoustic echo-sounder as a quantitative tool in studies of gravity wave dynamics in, and immediately above, the stable planetary boundary layer. We find that the observed acoustic echo strengths are roughly an order of magnitude greater than those predicted theoretically. This discrepancy might be in part due to partial reflection although the comparison is somewhat clouded by uncertainties in our knowledge of the equipment characteristics, propagation losses, etc. Comparisons between Doppler and in situ wind measurements give confidence in the Doppler results, but further experimentation and comparisons are needed. Preliminary use of acoustic Doppler data in a case study of gravity-wave dynamics in the planetary boundary layer has yielded boundary-layer wind speed and direction profiles which give insight into the mechanisms responsible for the wave generation. The Doppler data yield estimates of the wave associated momentum fluxes (~ a few dyn cm -z) as well. The results derived from the acoustic techniques are quite encouraging, but thus far remain unsubstantiated by independent wind and flux measurements.

Acoustic sounding of land and sea breezes

“…Equation (1) has been investigated by several authors (Beran et al, 1973;Asimakopoulos et af., 1975). However, serious problems exist in relating radar returns to atmospheric thermal structure in a quantitative way.…”

“…In the past few years, Sodars have been widely used for remote sensing of the atmospheric lower layers, based on the backscattering of sonic waves by inhomogeneities in the refractive index of air (Beran et al, 1973;Hersten et al, 1973;Coulman, 1973 ;Frisch and Clifford, 1974). However, when applying this methodology to the lower atmosphere, it is difficult to interpret the signal received in terms of thermal structure.…”

An Autonomous Doppler Sodar Wind Profiling System

“…A small but measurable fraction of the transmitted pulse is scattered by 180°and can, therefore, be detected by a monostatic system. A thorough analysis of acoustic scattering, based on the theory of Tatarskii (1971), may be found in, for example, Ottersten et al (1974) and Beran et al (1973). The parameters that are relevant to the present discussion are expressed in the normalized echo-strength equation (Beran et al 1973)…”

“…The present understanding of the factors that govern acoustic scattering by atmospheric turbulence relies on a number of assumptions regarding the spatial dependence of the bulk temperature variation within the scattering volume (Beran et al 1973). The bulk temperature variability is quantified as the temperature structure functions C 2 T ,…”

Turbulent Layers Associated with a Critical Level in the Planetary Boundary Layer

“…As was described before, numerous internal gravity waves can be seen in the stable PBL. Beran et al (1973) reported that the predominant horizontal wavelength (*) extends from a few to 10km. The phase velocity extends from a few to 20m• sec-1.…”