Radio acoustic measurement of fog-capping thermal inversions

The radio acoustic sounding system (RASS) is a remote technique for measuring temperature profiles in the lower troposphere1,2. In essence, it consists of a powerful acoustic source beaming a short burst of sinusoidal waves towards the zenith. The speed of this pulse in its upwards path is proportional at every height to the square root of the local temperature. This speed is continuously measured from the ground by means of a Doppler radar. The radar echo is due to the change in refractive index of air compressed by the acoustic wave. The faint echo is enhanced by choosing an acoustic wave in Bragg resonance with the radio wave. The record of the measured sound speed displayed as a function of delay from the start of the acoustic beam leads to the acquisition of the temperature vertical profile. We now describe the use of such a system at metre-wavelengths in the study of fog-capping thermal inversions at a site in the Po Valley.