Interpretation of sodar measurements: relevant properties of the atmosphere's planetary boundary layer

Abstract

Assuming a Kolmogorov spectrum of turbulence, the acoustic scattering cross section may be written as a function of incident wavelength, scattering angle, and the magnitude of the root-mean-square turbulent temperature and velocity fluctuations. In the “surface” layer SL (max ≃ 3–30 m height) of the atmosphere the statistical properties of turbulent velocity, temperature, humidity, and pressure fluctuations above many surface types are well understood for a wide variety of meteorological conditions. Hence, it is possible using sodar to derive unique, quantitative measurements of SL atmospheric turbulence. Detailed information about planetary boundary layer PBL (max ≃ 300 m–2 km height) turbulence is not required in order to make continuous sodar measurements of vertical profiles of turbulence related atmospheric structure and winds (from signal Doppler shifts). Sodar is, thus, now routinely used, e.g., for air pollution related, micrometeorological research, and wind-shear detection measurements. Our perception of many complicated PBL structural features and processes has been greatly expanded by the now familiar vertical-time section sodar records of various “layers” and thermal convection. However, except for “unstable” conditions of free convection, SL turbulence models are not generally applicable for the quantitative interpretation of PBL sodar data. In order to realize the full indirect sounding potential of sodar during “stable” atmospheric conditions, it is necessary to determine the relationship(s) between observed “macroscale” features such as multiple layers (with and without waves), intermittant “patches” of turbulence and “bursts” of vertical momentum or heat transport and the space-time dependent properties of the turbulence in such situations. This paper first reviews the application of contemporary SL and PBL models to the interpretation of sodar measurements. Functional relationships between sodar observed and model implicit parameters are summarized. Finally, the application of sodar for development and verification of selected atmospheric models is discussed. [Research supported by EPA Grant R800397.]