AN IMPROVED INTERPRETATION OF VHF OBLIQUE RADAR ECHOES BY A DIRECT BALLOON C-N(2) ESTIMATION USING A HORIZONTAL PAIR OF SENSORS

Successful comparisons between VHF oblique (15 degrees) radar echo pro files and reconstructed ones deduced from high-resolution temperature balloon measurements were presented by Luce et al. [1996]. The method was based on the evaluation of the three-dimensional isotropic refract ive index spectrum at the Bragg wavelength from the available temperat ure profiles (''spectral method''). However, the isotropic hypothesis is questionable, especially in regions where temperature sheets [Dalau dier et al., 1994] are observed. Indeed, the associated anisotropic te mperature fluctuations should contribute to the one-dimensional vertic al temperature spectrum at small scales. In the present paper, another method, less sensitive to anisotropic contaminations, is used. This m ethod is based on an estimation of the temperature structure constant C-T(2) from variances of horizontal differences of temperature measure d by two high-resolution sensors 1 m apart horizontally (''horizontal variance method''). It is shown that the main differences between the C-T(2) estimations obtained from the two methods are mainly observed a t high resolution in stable regions where few turbulent fluctuations i n the temperature field are observed. However, the two methods give ap proximately equivalent results at the radar range resolution (600 m) a nd the quality of the comparisons with the radar observations is sligh tly improved with the horizontal variance method. In order to demonstr ate the full advantages of this technique, the use of radars with more powerful capabilities is suggested for future investigations.