Quantitative Analysis and Interpretation of Dot Echoes Observed with a Doppler Sodar

Abstract

While some people involved in the acoustic remote sensing field are aware of the possibility of receiving dot echoes from nonatmospheric targets, most of the papers available in the scientific literature dealing with this phenomenon associate them to atmospheric targets, such as clusters of water vapor inhomogeneity, thermodynamical processes of condensation and reevaporation of water vapor, anisotropic irregularities localized in thin layers, etc. At present, dot echoes are defined by their appearance on the echogram and are not differentiated by causative processes. As such, they share similar characteristics, such as being randomly distributed and having a time length that is similar to the time length of the emitted tone. In this paper dot echoes conforming to this definition are investigated through the analysis of the signal in both the time and frequency domain. The timescale of a dot signature along with the configuration of the sodar system provide an upper limit to the size of the targets producing these echoes. The spectral characteristics and the first and second momenta of the echoes are compared with clear-air echoes as well as with echoes produced by pilot balloons released from nearby sodar antennas. The conclusion is that the dot echoes analyzed in this paper are reflections from birds and are not due to atmospheric effects.