The Importance of Moisture Fluctuations and Wind Shear in Acoustic Backscatter in GATE

Abstract

We explain, using in situ data, some of the mechanisms contributing to acoustic backscatter measured using an active, vertically pointing echosonde (acoustic sounder) mounted aboard ship in the tropical Atlantic. Averaged tethered-sonde profiles indicate that the substantial wind shear through the low-level stable layer in the wake of precipitating convection contributes to the acoustic backscatter from this layer. The undisturbed top of the mixed layer shows much weaker shear and no echo, although the average stabilities are nearly the same in the undisturbed and wake cases. Combining theory with data provides an explanation of the importance of moisture in acoustic backscatter. In the suppressed mixed layer (little or no precipitating convection) the contribution to the backscatter due to moisture (Ce2) approaches a factor of 3 greater than temperature fluctuations alone (CT2. In the disturbed boundary layer (organized convection) and moderately disturbed boundary layer (isolated convection), the moisture contributes less than 20%. Except near the top and near the bottom of the suppressed mixed layer, the co-fluctuations of moisture and temperature (Cet contribute immaterially. The categorized profiles of CT2, Ce2 and Cet do not exhibit clear tendencies toward Z?4/3 slopes predicted by other authors.