Mean Vertical Motions in the PBL Measured by Doppler Sodar: Accuracy, Ambiguities, Possible Improvements

Accurate measurements of vertical wind velocity w by sodar are useful for investigating dynamic phenomena in the PBL such as convection, subsidence, divergence, and vertical movements associated with local circulation. Moreover, its statistics may be used to derive some micrometeorological parameters. When very low vertical movements need to be measured the estimation accuracy of this parameter becomes crucial. A method is proposed to estimate the accuracy of the mean vertical velocity as measured by sodar. This method is used to evaluate the improvement that can be achieved if the loudspeaker transfer function is used to correct the spectra. The transfer function is obtained by averaging the spectra obtained when the sodar is passively recording the background environmental acoustic noise. Results show that the sodar system can be used to measure mean vertical velocities averaged over periods of 3 h or more with an uncertainty better than 0.05 m s21, providing that a two-step procedure is used to retrieve the Doppler shift. The transfer function correction improves the accuracy by a percentage between 12% and 20%, depending on the averaging period. The improvement may even be better if care is used in designing the antennas so that the influence of atmospheric parameters on the electroacoustic transducer decreases and their alignment is suitably accurate.