  Convection over a wavy heated bottom wall in the air flow has been studied in
experiments with the Rayleigh number $\sim 10^8$. It is shown that the mean
temperature gradient in the flow core inside a large-scale circulation is
directed upward, that corresponds to the stably stratified flow. In the
experiments with a wavy heated bottom wall, we detect large-scale standing
internal gravity waves excited in the regions with the stably stratified flow.
The wavelength and the period of these waves are much larger than the turbulent
spatial and time scales, respectively. In particular, the frequencies of the
observed large-scale waves vary from 0.006 Hz to 0.07 Hz, while the turbulent
time in the integral scale is about 0.5 s. The measured spectra of these waves
contains several localized maxima, that implies an existence of waveguide
resonators for the large-scale standing internal gravity waves. For
comparisons, experiments with convection over a smooth plane bottom wall at the
same mean temperature difference between bottom and upper walls have been also
conducted. In these experiments various locations with a stably stratified flow
are also found and the large-scale standing internal gravity waves are observed
in these regions.
