Observations of Helmholtz Waves in the Lower Atmosphere with an Acoustic Sounder

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Abstract
Acoustic probing of the lower atmosphere (<150 m) reveals structures that appear similar to those of instability waves produced by wind shear at the stable interface of a temperature inversion. The acoustic sounder was located in the vicinity of a meteorological tower 152 m in height. Profiles of wind velocity and temperature were taken during the acoustic sounder operation. Regions of enhanced thermal stability and wind shear produce strong echoes which the acoustic sounder maps on a height vs time facsimile record. In this paper we limit our discussion of those echo returns that have the characteristic appearance of Helmholtz waves. Richardson numbers calculated from the tower measurements over the layer thickness as determined from the acoustic sounder returns appear to he of the order of ½, while sub-strata embedded within the layer thickness exist where the Richardson number drops near the theoretically predicted critical value of ¼. In addition, measurements of the wavelengths associated with the “breaking” phenomenon conform to the theoretically predicted range of values.

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