On the reality of upper mesospheric/lower thermospheric turbulent “eddies”

Abstract

Abstract. The structure of atmospheric turbulence and its relationship to the refractive index discontinuities responsible for the scattering of radio waves are discussed. Emphasis is placed more on the nature of the intermittent scatterers observed at upper mesospheric/lower thermospheric altitudes than on volume scattering. The preferred model is of transient patches composed of vortex strings. An overview of the methods used to estimate the rate of dissipation of turbulent energy using MF radar data which emphasizes the advantages of imaging Doppler interferometry is presented, together with a review of pertinent laboratory investigations of coherent structures. An attempt is made to remove the contamination by gravity waves from imaging Doppler interferometry fluctuating velocity data. Emphasis is given to the fact that all radar measurements of turbulent intensity to date are upper estimates, not only because of the presence of gravity wave fluctuations contaminating the data, but also because the turbulence itself is not stationary and homogeneous but intermittent in both space and time. The intermittent nature of the turbulence, in which the refractive index discontinuities responsible for the radar backscatter are associated with decaying, homogeneous turbulent coherent structures, is in contrast to the theory of volume scattering, which assumes the radar pulse volume to be partially filled with stationary, homogeneous turbulence.

Diurnal variations in the rate of dissipation of turbulent energy in the equatorial upper mesosphere–lower thermosphere

“…Because of their inability to track scatterers from frame to frame, Roper and Brosnahan [1997] estimated the lifetime of individual scatterers to be some 100 s, about half the Brunt‐Väisälä period. The intermittency in scatterer appearance is well illustrated in Figure 11 of Roper [1998], in which the number of echoes per 102.4 s radar frame within 2° of zenith at 84 km are plotted from 1300 to 1416 hours on 10 April. In some of those frames, there are no identifiable scatterers; in others, as many as eight.…”

“… Roper [1998] reviewed the established methods for medium frequency (MF) radar measurements of ɛ, the rate of dissipation of turbulent energy, with particular emphasis being placed on the structure function formulation used by Roper [1966] in the analysis of radio meteor data. In this formulation, ɛ estimates are based on the measurement of line of sight velocities separated in space.…”

“…Note that a correction is applied to u β to minimize the influence of gravity wave contamination as proposed by Roper [1998]. This correction relies on the universally observed gravity wave energy spectrum which, when transformed to length space, yields where v is the wave amplitude, or …”

“…The goodness of fit of the straight line is also calculated, and used to determine whether or not to subtract the gravity wave amplitude (weighted by the significance of fit) from the measured velocity determined from all scatterers within 3° of zenith. An example of this fit appears as Figure 12 of Roper [1998]. Figure 2 presents histograms of the distribution of R 2 , the square of the correlation coefficient between the data and the straight line fit.…”

“…The kinematic viscosities used here, as with the Brunt‐Väisälä frequency ω, are from the COSPAR '86 model. ɛ min has been considered by several authors [see, e.g, CIRA, 1986; Hall et al , 1998; Roper , 1998] to be the dimensionally correct where υ is the kinematic viscosity and ω is the Brunt‐Väisälä frequency. A formulation consistent with the work of Roper [2000] and the equations used here can be derived from Roper's estimate of the turbulent diffusivity as amended in section 3, viz., with kinematic viscosity υ substituted for K , yielding A turbulence Reynolds number where ɛ is the measured rate of dissipation of turbulent energy, should have a value of 1 at the turbopause.…”