Acoustic methods for the remote probing of the lower atmosphere

The potential usefulness of acoustic methods for the remote probing of the lower atmosphere is reviewed. Starting with a comparison of the effects of temperature, wind, and humidity fluctuations upon the refractive index of air to electromagnetic and acoustic waves, it is shown that the fluctuations in acoustic refractive index may be expected to be about 1000 times stronger than in the radio case. The opportunities for passive and for line-of-sight remote acoustical sensing of the troposphere offered by this relatively strong interaction are briefly identified. Since the scattered power is proportional to the square of the refractive index fluctuations, the scatter of acoustic waves may be expected to be roughly one million times stronger than for radio waves. Based on the theoretical work of Kallistratova (but including the effects of atmospheric absorption), the system parameters required for effective acoustic echo-sounding of the lower atmosphere are deduced. It is concluded that the acoustic sounding technique could be developed to monitor, to heights of at least 1500 meters, 1) the vertical profile of wind speed and direction, 2) the vertical profile of humidity, 3) the location and intensity of temperature inversions, 4) the three-dimensional spectrum of mechanical turbulence, and 5) the three-dimensional spectrum of temperature inhomogeneity (i.e., of optical refractive index fluctuation). Typical time and height resolutions for the proposed acoustic echo-sounders could be of the order 10 seconds and 10 meters; the spatial wave number explored could range from about 10^-2m^-1to about 400 m^-1.