International audienceAn empirical technique for retrieving profiles of the square of the Brunt-Väisälä frequency, ?B2, from MST radar return signal power is presented. The validity of the technique, which is applied over the altitude range 1.0-15.7km, is limited to those altitudes at which the humidity contributions to the mean vertical gradient of generalised potential refractive index, M, can be ignored. Although this is commonly assumed to be the case above the first few kilometres of the atmosphere, it is shown that humidity contributions can be significant right up to the tropopause level. In specific circumstances, however, the technique is valid over large sections of the troposphere. Comparisons of radar- and (balloon-borne) radiosonde-derived ?B2 profiles are typically quantitatively and qualitatively well matched. However, the horizontal separation between the radar and the radiosondes (which were launched at the radar site) increases with increasing altitude. Under conditions of mountain wave activity, which can be highly localised, large discrepancies can occur at lower-stratospheric altitudes. This demonstrates the fact that radiosonde observations cannot necessarily be assumed to be representative of the atmosphere above the launch site
