Nonturbulent layers in polar summer mesosphere 1. Detection of sharp gradients using wavelet analysis

The polar mesosphere is characterized by a very high radar backscattering c oefficient. Both radar and in situ rocket spectra indicate that two qualita tively different scattering mechanisms are operating: one turbulent and one edge-like. Fourier analysis methods applied to in situ data have been very useful in showing that spectra are steeper than the canonical -5/3 turbule nt form. Such steep spectra accompany the edge-like structures. However, si nce Fourier methods assume that the energy is distributed throughout a give n window, they are not very useful for describing a finite number of sharp edges in a data set, which is the normal situation. Here we use the techniq ues of wavelet analysis to partition a rocket data set taken during a polar mesosphere summer echo event characterized by the sounding system radar in Norway. We find that the edges can be either isolated in space or collocat ed with turbulence and, likewise, that turbulent regions can be found witho ut steep edges. Our partitioning verifies previous notions concerning the i nterpretation of shallow and steep spectra. The first clear evidence for th eoretically predicted viscous-convective and viscous-diffusive subranges is also presented. In a companion paper [Alcala and Kelley, this issue] we us e the well-known scattering properties of our wavelet basis functions to ca lculate the scattering from the region sampled by the rocket probe in a man ner that accounts for multiple edges in the same scattering volume.