Internal gravity waves convectively forced in the atmospheric residual layer during the morning transition

Generation of internal gravity waves in the boundary layer is investigated from observations. Simultaneous measurements from a 2 mu m Doppler lidar and a 0.5 mu m backscatter lidar are combined to analyse the occurrence, or not, of internal gravity waves in the residual layer during the morning transition on two days, 10 and 14 June 2005. Three cases are studied for illustrating three different flow configurations in the residual layer: no wave, evanescent wave and propagating wave. Comparison of the three cases suggests two necessary conditions for the generation of gravity waves: a stably stratified residual layer and a convective boundary layer with mechanical forcing frequencies less than the Brunt-Vaisala frequency. The horizontal wind shear probably plays a role in the dynamics of the waves, but, in the cases analysed, it is not sufficient alone to generate the observed waves through the obstacle effect. In the case of wave propagation, the waves tilt upstream and against the wind shear, with a typical horizontal wavelength and a line phase direction with respect to the vertical of 2.4 km and 32 degrees, respectively. Unexpectedly, we found that measurements of the wave-associated vertical velocity and the displacement of tracers (0.5 mu m depolarization ratio or 2 mu m backcatter, both indicative of relative humidity fluctuations) are in phase. Possible explanations include: (i) aerosol particles are not passive with respect to temperature or water vapour fluctuations; or (ii) a nonlinear wave-turbulence interaction is at work and needs further investigation. Copyright (C) 2011 Royal Meteorological Society