MST radar networks and campaigns session summary and recommendations

Topics addressed included: determination of sources of propagation and dissipation of atmospheric disturbances and waves; measurement parameters; comparison of different method and instruments; suitable combinations of instruments; information exchange; and training courses

The relation of gravity waves and turbulence in the mesosphere

Since researchers couldn't prove that the mesospheric turbulence layers are generated by the simultaneously existing short-period gravity waves, they invoked other generation mechanisms than wave breaking. Possible mechanisms like lateral convection (Rottger 1980a), quasi-geostrophic flows at mesoscales (Lilly, 1983) or vortical modes of motion as seen in the ocean (Muller and Pujalet, 1984) could be candidates. Researchers are inclined to see a connection of these layers or laminae with very-long-period internal waves because of the periodicity in their vertical structure and their long mean persistency. Rottger (1980b) had proposed that such structures are due the modulation of the me an temperature and wind profiles by internal waves. The superposition of random or short-term wave-induced wind and temperature fluctuations with the background profile, modulated by very-long-period waves (quasi-inertia waves) then would yield the observed effects, and could explain the vertical periodicity, the long-term mean persistency as well as some short-term variability of their intensity

Determination of the Brunt-Vaisala frequency from vertical velocity spectra

Recent work on the spectra of vertical velocity oscillations due to gravity waves in the troposphere, stratosphere and the mesosphere has revealed a typical feature which we call the Brunt-Vaisala cutoff. Several observers noticed a spectral peak near the Brunt-Vaisala frequency. This peak often is characterized by a very steep slope at the high frequency part, but a fairly shallow slope towards lower frequencies. Some example spectra of stratosphere observations are given. This distinct spectral shape (most clear at the upper height 22.5 km) can be explained by the fact that the vertical velocity amplitudes of atmospheric gravity waves increase with frequency up to their natural cutoff at the Brunt-Vaisala frequency. The measurement of the frequency of the peak in a vertical velocity spectrum was found to yield most directly the Brunt-Vaisala-frequency profile. Knowing the Brunt-Vaisala frequency profile, one can deduce the potential temperature profile, if one has a calibration temperature at one height. However, even the uncalibrated profile will be quite useful, e.g., to determine fronts (defined by temperature inversions) and the tropopause height. This method fails for superadiabatic lapse rates when the Brunt-Viasala frequency is imaginary. The application of this method will also be difficult when the wind velocity is too high, causing the Doppler effect to smear out the total spectrum and blur the Brunt-Vaisala cutoff. A similar deficiency will also appear if the gravity-wave distribution has a maximum in wind direction

On the varying slope of velocity spectra

Spectra of zonal, meridional and vertical wind velocity, measured during a 24 hour period with the spaced-antenna technique indicate quite a variable slope as a function of height. It is found that the spectral slope (1h to 24h) of all three components correlates with the mean horizontal wind velocity. A possible conclusion is that the frequency dependence of power density of horizontal and vertical fluctuation component apparently depends on the mean wind velocity. However, the vertical spectra at periods larger than about 1 hour can also be influenced by spillover (due to finite radar antenna beam width) from the horizontal fluctuation component or by a Doppler shift

Comparison of reflectivity and wind profiles measured on 46.8 MHz and 430 MHz at the Arecibo Observatory

Comparisons of troposphere and stratosphere radar experiments at ultrahigh frequency (UHF) and very high frequency (VHF) were done at the Arecibo Observatory in April 1980 with the 430 MHz and 46.8 MHz radar. The velocity profiles measured on both frequencies with the Doppler beam swinging mode were compared. In general, the velocity profiles were equivalent. The VHF profile, however, shows more fluctuations with height than the UHF profile, although the latter was recorded with 150 m resolution instead of 300 m resolution on VHF

The use of the experimentally deduced Brunt-Vaisala frequency and turbulent velocity fluctuations to estimate the eddy diffusion coefficient

The determination of the turbulent energy dissipation rate or the eddy diffusion coefficient from radar observations can be done through the turbulence refractive index structure constant, deduced from calibrated echo power measurements, or through the turbulent velocity fluctuations, deduced from the echo spectrum width. Besides the radar parameters, power and spectrum width, the first approach needs knowledge of profiles of temperature and electron density in the mesosphere and the fraction of the radar volume filled with turbulence. The latter approach needs knowledge of the temperature profile, namely, the Brunt-Vaisala frequency. The use of this latter approach is demonstrated

The influence of velocity variability on the determination of wind profiles

High sensitivity radars allow the determination of velocity estimates at time resolutions down to one minute or better. Because of the variability introduced to the mean wind due to turbulence and waves, the high resolution profiles may not be too useful for forecasting applications, although they yield the most realistic estimate of the instantaneous wind profile. Profiles of wind speed and direction, vertical velocity and echo power, which were deduced in real-time on 23 August 1981 with the spaced antenna drift mode of the SOUSY-VHF-Radar are shown. Whereas these profiles were measured within 1 minute, the operating routine allowed the selection of variable (longer) measuring periods, and one has to search for the optimum duration of the data averaging period. A high time resolution wind vector diagram is given which gives an idea of the temporal variability. The data were obtained with the spaced antenna technique, which allows a good estimate of the horizontal wind without having to correct for the vertical velocity component. The wind vectors specifically indicate a quasi-periodic variation in direction. This is assumed to be due to gravity waves since the vertical velocity also shows periodical variations with the same period. The consistency of these spaced-antenna VHF radar results along with the radiosonde data convinced researchers that the method is quite suitable for wind profiling applications

Relationship of strength of turbulence to received power

Because of contributions due to reflection, the determination of the turbulence refractive index structure constant may be affected. For pure scattering from turbulence in the inertial subrange, the radar echo power can be used to calculate the refractive index structure constant. The radar power is determined by a convolution integral. If the antenna beam is swung to sufficiently large off-zenith angles ( 12.5 deg) so that a quasi-isotropic response from the tail ends of the Gaussian angular distribution can be anticipated, the evaluation of the convolution integral depends only on the known antenna pattern of the radar. This procedure, swinging the radar beam to attenuate the reflected component, may be called angular or direction filtering. The tilted antenna also may be pick up reflected components from near the zenith through the sidelobes. This can be tested by the evaluation of the correlation function. This method applies a time domain filtering of the intensity time series but needs a very careful selection of the high pass filters

Morphology of the scattering target: Fresnel and turbulent mechanisms

Further studies of VHF radar signals from the troposphere and stratosphere revealed not only scattering from isotropic turbulence at scales of half the radar wavelength but also partial or Fresnel reflection or scattering from horizontally stratified temperature discontinuities. Proof for this observation was given by the large spatial and temporal coherence of radar signals. Thin structures, particularly in the stratosphere, may be persistent over some ten seconds, which is longer than the coherence time of 3 m scale turbulence in the stratosphere. The vertical thickness of the structures was estimated to be much thinner than 150 m. Observations over a longer time period indicate that these fine scale structures or sheets are clumped together forming patches or ensembles of mostly downward sloping structures

Increase of antenna area instead of transmitter power, part 6.3A

An extension of the antenna area may be preferred to an increase of transmitter power, when it is considered that reflection often dominates the scatter contribution at near zenith angles. It is noticed that an increase of the antenna area A, linearly increases the contribution of reflection. This most likely occur at vertical incidence, since the mean generalized refractive index gradient (M) is largest in the vertical direction. The altitude ranges at which the echo power gets weak and we have to consider improvements of sensitivity are mostly larger than 5 to 8 km. It follows that the considerations are valid up to antenna diameters close to 200 m for wavelengths of 6 m. On the other hand the reflected component has to be larger than the scattered component which only holds for near zenith angles