Radio acoustic sounding of the planetary boundary layer

Abstract

Refractive index variations of the atmosphere caused by sound waves are used as tracers for radar waves in order to determine the sound velocity and thus the (virtual) temperature remotely. This combined usage of acoustic and electromagnetic waves—called ‘‘radio acoustic sounding system (RASS)’’—has gained considerable interest, as sensitive radar wind profilers have been used for one decade in atmospheric research and monitoring. The RASS function can easily be added to radar wind profilers with the help of sound sources at the radar site. Due to the sound attenuation, the range of most systems is limited to the typical depth of the planetary boundary layer, which is particularly important for the exchange of mass, energy, and momentum between atmosphere and surface. Due to refinements of the signal analysis, a measurement accuracy comparable to conventional in situ sensors has recently been achieved. In contrast to in situ measurements, RASS has the great advantage to provide continous and simultaneous data from many altitudes. RASS cannot only deliver accurate temperature profiles but also profiles of the mean wind and of high-frequency fluctuations of the vertical wind component. Currently a RASS-based method is being developed for high-resolution horizontal wind measurements.