W. K. Hocking

On the extraction of atmospheric turbulence parameters from radar backscatter Doppler spectra—I. Theory

Abstract A theory is developed for the extraction of r.m.s. velocities of scatterer motions from spectra measured with a Doppler backscatter radar. The effects of finite beam-widths, finite pulse lengths, beam broadening, shear broadening, and other such spectral ‘contaminants’ are considered. It is shown that these ‘contaminants’ can play a major role in determining the measured spectral widths (and, equivalently, the signal fading time), and so must be properly considered if the spectral widths are to be used to extract the r.m.s. motions of the scatterers. It is also shown that these r.m.s. motions can be used to estimate turbulence intensities, in those cases where turbulence is the dominant cause of the scatterer motions.

Seasonal variability of vertical eddy diffusivity in the middle atmosphere: 1. Three‐year observations by the middle and upper atmosphere radar

The vertical eddy diffusivity K due to atmospheric turbulence with spatial scales of 100–102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from January 1986 to December 1988. The method of analysis follows Lilly et al. [1974], Sato and Woodman [1982], and Hocking [1983a, 1985, 1988], and the contamination due to beam broadening, vertical shear, and transience has been removed. Although observations for horizontal wind speeds larger than approximately 40 m/s, such as occur near the tropopause jet stream in winter, have been omitted because of excessive beam broadening, sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper troposphere and lower stratosphere (6–20 km altitude) and for the mesosphere (60–82 km altitude). The monthly median of K shows a local maximum near the tropopause jet stream altitude. It becomes larger in the mesosphere, increasing gradually with height. Maxima of K are observed in winter near the tropopause and in summer in the mesosphere, and the seasonal variability of K reaches approximately an order of magnitude. A semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons.

Meteor decay times and their use in determining a diagnostic mesospheric Temperature‐pressure parameter: Methodology and one year of data

Ambipolar diffusion coefficients in the meteor region have been measured using meteor decay times determined with the CLOVAR radar [Hocking, 1997]. This parameter may then be used to infer information about atmospheric temperatures or pressures in that region. In particular, we show annual variations in the parameter . Our procedures, together with the underlying assumptions, are described, and our results are compared to the CIRA (1986) empirical model as well as other experimental data. These successful measurements have been possible because of our use of a new 5-element interferometer which has minimal inter-element coupling and has an angular location accuracy of better than ±2°, together with new algorithms which introduce important corrections to the decay time height profile. We concentrate in this paper on monthly averages, but higher temporal resolutions are possible.

An improved interferometer design for use with meteor radars

The measurement of the directions of radio meteors with an interferometric system is beset by two problems: (1) The ambiguity in the measured directions for antennas spaced by more than λ/2 and (2) the effects of mutual impedance when the antennas are spaced at λ/2 and less to avoid these ambiguities. In this paper we discuss the effects of mutual impedance between spaced antennas and describe an interferometer which both minimizes these effects and avoids the ambiguities associated with spacings larger than λ/2. We have modeled a version of this design numerically and show that under ideal conditions an interferometer of total span 4.5λ can yield directions accurate to about 0.3° with a signal-to-noise ratio of 20 dB. Finally, we have tested the design with observations from the 1996 Geminid and 1997 Quadrant meteor showers and find that even without a ground plane, the interferometer provides unambiguous directions to an accuracy of the order of 1.5°.

Temperatures Using radar‐meteor decay times

Experimental studies of the temperature and pressure dependence of the ambipolar diffusion coefficient in the mesopause region have been undertaken by studying meteor trail decay times with radars at a variety of sites in North America, with latitudes between 75N and 35N. The site at Resolute Bay, Canada, has proven especially useful, due to the wide range of mesopause temperatures experienced at that site between summer and winter. Theoretical predictions have been confirmed, and an algorithm is outlined which permits meteor decay times to be used to determine absolute measurements of mesospheric temperatures.

Turbulence in the region 80–120 km

Abstract Measurements of turbulent energy dissipation rates and eddy diffusion coefficients have been collated, and mean height profiles of fundamental turbulence parameters in the region 80–120 km are presented.

Absolute reflectivities and aspect sensitivities of VHF radio wave scatterers measured with the SOUSY radar

Abstract By accurately calibrating the SOUSY radar in West Germany it has been made possible to measure absolute values of effective reflection coefficients and turbulence structure constants. Some typical values of these parameters as a function of altitude are presented. Such profiles are presented for both a vertically directed beam, and also for two beams directed 7° off-vertical. Comparisons of powers on the vertical and off-vertical beams show that scatter became more aspect sensitive at the tropopause and in the lower stratosphere, but, unexpectedly, scatter was observed to become considerably more isotropic in the higher regions of the stratosphere (above 15–18 km) on this occasion. An enhancement of signal from the tropopause occurred not only on the vertical beam, but also on the off-vertical beams. Comparisons of signal strengths scattered from the mesosphere and measured with the vertical and off-vertical beams showed that for the present observations mesospheric scatter was close to isotropic. The backscatter cross-sections at VHF have been compared with other measurements at medium and high frequencies at other locations, and these comparisons help set some limits on the scales of turbulent and specular scatterers in the mesosphere.

Simultaneous and colocated observation of winds and tides by MF and meteor radars over London, Canada (43°N, 81°W), during 1994–1996

Simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85–94 km height range from July 1994 to June 1996 over London, Ontario, Canada. Results have been obtained for every month of the year. From these comparisons it is concluded that, in general, the MF spaced antenna technique and the meteor method both provide reliable means for synoptic studies of neutral air motions in the height range 85–94 km, at timescales of greater than 12 hours, and therefore are valuable tools in middle atmospheric research. However, we do find that some parameters seem to be estimated with greater precision than others. In particular, the measurements of the zonal long-term wind variations, and the semidiurnal tide in both the zonal and meridional directions, seem particularly robust and reliable, while there is somewhat less consistency between measurements of the zonal diurnal tide and the meridional monthly mean winds. The former problem is very likely to be an artifact of the strong diurnal variation in meteor count rates. However, we cannot claim that our meridional monthly mean wind agreement is always good, and these discrepancies deserve further investigation.

Global-scale tidal variability during the PSMOS campaign of June-August 1999: interaction with planetary waves

During the PSMOS Global-scale tidal variability experiment campaign of June 1-August 31, 1999, a network of radars made measurements of winds, waves and tides in the mesosphere/lower-thermosphere region over a wide range of latitudes. Clear evidence was found that fluctuations in tidal amplitudes occur on a global scale in both hemispheres, and that at least some of these fluctuations are periodic in nature. Modulation of the amplitude of the 12 h tide was particularly evident at periods of 10 and 16 days, suggesting a non-linear interaction with planetary waves of those periods to be responsible. In selected cases, the secondary waves predicted from non-linear theory could be identified and their zonal wave numbers determined. In some, but not all, cases the longitudinal structure of the secondary waves supports the theory of planetary-wave/tidal interaction being responsible for the observed tidal modulation. It was noted also that beating between a 12.4-lunar and the solar tide could produce a near 16-day modulation of the 12 h tide amplitude that is frequently observed in late summer.

Observation and measurement of turbulence m the middle atmosphere with a VHF radar

Abstract The intensity of atmospheric turbulence in the middle atmosphere has been investigated by utilizing the spectral half-widths of echoes received by a VHF radar. The work was performed using the SOUSY 53 MHz radar in West Germany and this is the first time that the technique has been tested in such detail at VHF. It was found that broadening of the spectrum due to both finite beam half-width and vertical shears in the horizontal wind played a major role in determining the spectral half-widths observed. Extraction of information concerning turbulence requires careful removal of these effects, and is often not possible. For any radar and for any given wind profile there is a lower limit to the turbulent energy dissipation rate e which can be measured and this lower limit is described for various radars. Some actual measurements of mesospheric turbulent energy dissipation rates are presented.