Lidar determination of the frequency of variations of the boundary-layer top

The frequency of thermals up- and downdraft, during the day, and that of gravity waves, during night, are retrieved by applying a fast Fourier transform to the temporal evolution of the boundary-layer height as obtained by lidar measurements. The principal components of each obtained spectrum of frequency are related to the dominant processes occurring at the convective and nocturnal boundary-layer tops. The formation of lee-waves systems during special meteorological conditions determined the oscillation of the boundary-layer height. Fluctuations at the nocturnal boundary-layer height can occur even if the conditions through the layer depth are statically stable. These oscillations are principally due to wind shear and buoyancy (gravity) waves at the altitude of the nocturnal boundary-layer top.