SIGNAL POWER FOR RADIO ACOUSTIC SOUNDING OF TEMPERATURE - THE EFFECTS OF HORIZONTAL WINDS, TURBULENCE, AND VERTICAL TEMPERATURE-GRADIENTS

A simple expression for the expected average signal power of a radio acoustic sounding system (RASS) comprising a monostatic pulsed Doppler radar and a continuous-wave broadbeam acoustic source is developed. The effects of horizontal winds, atmospheric turbulence, and vertical temperature gradients are included. Under ideal conditions (i.e., in the absence of winds, turbulence, and gradients) the received signal power is a maximum and, for a broadband acoustic source, is predicted to be proportional to the radar range resolution and inversely proportional to the acoustic bandwidth and square of the range R. Turbulence-induced distortion of the acoustic wave fronts yields the expected R-26/5 range dependence, provided winds are light. This result is based on the assumption that the turbulence strength does not vary with range. For the more realistic case of turbulence concentrated primarily within the boundary layer (R less-than-or-equal-to 1 km) a much weaker range dependence at longer ranges is predicted. A flattening of the acoustic wave fronts by temperature gradients typical of unstable midday conditions results in an R-6 range dependence, provided winds are light. Turbulence effects are predicted to dominate at shorter ranges (R less-than-or-equal-to 8 km) and gradient effects at longer ranges. The expected compensation of power loss associated with the wind-induced lateral displacement of the acoustic wave fronts by the turbulence- and gradient-induced broadening of the displaced RASS focal spot is also predicted.