Acoustic tomographic measurements in the atmospheric surface layer

Over the past years, the acoustic travel time tomography was used to observe meteorological fields in the near surface boundary layer. In this contribution different possible field of application, as well difficulties of the implementation of operational data availability were discussed.

The acoustic tomography is introduced as an experimental technique for remote monitoring of spatially averaged meteorological quantities, such as the virtual temperature and the horizontal wind speed. This ground-based remote sensing technique uses the nearly horizontal propagation of sound waves in the atmospheric surface layer. Here the acoustic travel time tomography was applied by measuring the travel time at defined propagation paths between several sound sources and receivers. The resulting sound speed data were used to obtain estimates of the meteorological parameters.

Several measuring configurations were realised during a field experiment in summer 2002. On the one hand an arrangement with a maximally spatial extension, on the other hand an array with an optimal resolution were applied. The results of the different configurations were compared with conventional measuring systems and the advantages and limitations were pointed out.

A couple of essential difficulties of the measuring method are taken under examination. One problem is the separation of the different influences on the measured sound velocity. Thereto several splitting methods are tested and their limitations are discussed. As well, the impact of the atmospheric composition and the temperature dependence of the specific heat ratio of humid air on the speed of sound and as a result on the recalculated temperature were examined. In addition, some improvements on the measuring system, which allow the application under outdoor conditions, were introduced.

Finally, some future applications of the acoustic tomography are presented, e.g. the detection of internal gravity waves or coherent structures in the near-surface layer as well the design and development of an indoor tomographic system applicable e.g. inside wind tunnels or atmospheric chambers.