Acoustic remote sensing study of the influence of internal gravity waves on the lower atmosphere
Final Report Abstract
The main objective of the project was the exploration of the influence of intemal gravity waves (IGWs) on turbulent regimes inside a stable atmospheric boundary layer (ABL) using acoustic tomography. An advantage of the acoustic tomographic measurements in the ABL over point measurements is the capability of obtaining the spatial characteristics of the organized structures whose direction of propagation usually does not coincide with that of the mean wind velocity. To investigate the statistical characteristics of the IGW induced meso-scale fluctuations of acoustic signal parameters and meteorological quantities the entire project included three experiments: The first experiments were carried out in summer and autumn 2004 separately by the Russian and the German project group. Also a common experiment was organised in summer 2005 in Russia. From the measurements of acoustic travel time fluctuations (periods of 1 min-l h) with distant receivers, the temporal fluctuations of the effective sound speed are retrieved along different ray paths. By using a coherence analysis of the fluctuations near spatially distanced ray turning points, the internal wave-associated fluctuations are filtered and their spatial characteristics are estimated. The calculated coherences show an existence of wavelike fluctuations, which are typical for IGWs and eddy structures generated due to different types of the IGWs instabilities. A set of dominant periods was found both in the variations in acoustic signal parameters, like travel time and azimuth of propagation of the signal's wave front, and of parameters of turbulence. The turbulent fluxes of momentum and sensible heat are modulated in time and space by spatially extended and highly coherent structures, such as IGWs. Furthermore, the capability of a special method of acoustic tomography in estimating the vertical wind shear was demonstrated in the project. The remote sensing of wind shear and its fluctuations is of great importance for aviation meteorology but also for the simulation of sound propagation at longer distances. A number of episodes was observed, when the enhancement in the vertical gradients of wind speed up to their critical values was accompanied by an enhancement of the intensity of turbulence. These situations indicate that the local instabilities of IGWs can be a possible source of the intermittency of small-scale turbulence under conditions of stable stratifications of the ABL. The results of the project demonstrate the influence of anisotropic coherent structures on the energy transport during nighttime in the stable ABL. In the future, an improved modeling of the dynamics of IGWs and eddy structures is possible in the stably stratified ABL taking into account their interaction with small-scale turbulence. This knowledge of acoustic wave and meteorological parameter statistics influenced by IGWs is needed for an enhanced forecasting of the atmospheric structure as well as noise contamination under conditions of a typical nighttime ABL.
