Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. (2011) Internal gravity waves convectively forced in the atmospheric residual layer during the morning transition Fabien Gibert, a * Nicolas Arnault, a Juan Cuesta, a Riwal Plougonven b and Pierre H. Flamant a a Laboratoire de M´ et´ eorologie Dynamique (LMD)/Institut Pierre Simon Laplace, Ecole Polytechnique, Palaiseau, France b Laboratoire de M´ et´ eorologie Dynamique (LMD)/Institut Pierre Simon Laplace, Ecole Normale Sup´ erieure, Paris, France *Correspondence to: F. Gibert, Laboratoire de M´ et´ eorologie Dynamique (LMD)/Institut Pierre Simon Laplace, Ecole Polytechnique, Palaiseau, France. E-mail: gibert@lmd.polytechnique.fr Generation of internal gravity waves in the boundary layer is investigated from observations. Simultaneous measurements from a 2 μm Doppler lidar and a 0.5 μm 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–Va¨ısala 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 ◦ , respectively. Unexpectedly, we found that measurements of the wave-associated vertical velocity and the displacement of tracers (0.5 μm depolarization ratio or 2 μm 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 Key Words: convective boundary layer; depolarization ratio; gravity wave; lidar; residual layer; thermal forcing Received 15 December 2010; Revised 30 March 2011; Accepted 5 April 2011; Published online in Wiley Online Library Citation: Gibert F, Arnault N, Cuesta J, Plougonven R, Flamant PH. 2011. Internal gravity waves convectively forced in the atmospheric residual layer during the morning transition. Q. J. R. Meteorol. Soc. DOI:10.1002/qj.836 1. Introduction The residual layer (RL) is usually defined as a neutrally stratified layer resulting from the decay of turbulence in the formerly convective boundary layer (CBL) (Stull, 1988). Almost isolated from the ground by the nocturnal stable layer (NSL), the RL is usually not affected by turbulent transport of surface-related properties but by subsidence and generally by the shear-related burst of turbulence (Mahrt, 1999; Yi et al., 2001). In this study we investigate RL–CBL interactions during the morning transition when the RL is progressively eroded by the CBL. Exchanges between RL and CBL are of particular interest for air quality studies and for a general understanding of the vertical mixing and the budget Copyright c  2011 Royal Meteorological Society