Sound Propagation in the Nocturnal Boundary Layer

An experimental study of sound propagation near the ground in stable, nighttime conditions was performed in conjunction with the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). Low-frequency sound transmissions were continuously recorded at microphones out to a distance of 1.3 km from a loudspeaker during CASES-99 intensive observation periods (IOPs) 6 and 7. Fading episodes in the received signal energy of 10 to 20 dB, lasting several minutes to an hour, were frequently observed. Strong discrete events, such as the density current and solitary wave of IOP 7, were found to have significant effects on acoustical signals, although substantial variability in received sound energy often occurred outside such events. Sound propagation model predictions demonstrate that wind and temperature data from a tall tower, such as the CASES-99 60-m tower, can be used to predict the momentary variations in a 50-Hz sound signal with good success. Tethersonde and rawinsonde data are generally too infrequent to model many of the strong variations present in the signal. The sensitivity of sound waves to changes in nocturnal boundary layer structure could allow development of new remote sensing methods.