High accuracy direct detection Doppler wind lidar measurements using the molecular double edge technique

The double edge lidar technique for measuring the wind uses the molecular signal backscattered from the atmosphere. We measure the wind as the average drift velocity of the molecular motion. The molecular signal is spectrally broadened by Doppler shifts due to the random thermal motion of molecules and by Brillouin scattering. The double edge technique uses two high spectral resolution edge filters which are located in the wings of the Rayleigh-Brillouin profile. A Doppler shift produces a positive change in signal for one edge filter with respect to its initial value. For the other edge the corresponding signal change is opposite in sign and equal in magnitude. Relatively large changes in measured signal are observed for small frequency shifts due to the steep slope of the edge. In general the edge filter measurement depends upon the magnitude of the atmospheric backscattered molecular and aerosol signals. Thus the molecular and aerosol signal would have to be spectrally separated, measured separately and treated independently in the analysis. We can, however, locate the edge filter measurement in a crossover region, where the fractional change in the measured molecular and aerosol signals are equal for a given frequency shift. That is, the measurement sensitivities are equal for the molecular and aerosol portions of the signal. The aerosol signal then acts in an identical manner to the molecular signal and the measurement is desensitized to the effects of aerosol scattering.