Numerical experiments of stratified turbulence are conducted with a high-resolution nonhydrostatic Boussinesq model to investigate the energy spectrum produced by synoptic scale forcings. The spectral slope obtained in our experiment with planetary rotation is close to -3 in the synoptic scales and -5/3 in the mesoscales, as observed in the troposphere. While the vortical modes are predominant in the -3 spectral range, magnitude of the divergent modes is greater than that of the vortical ones in the -5/3 range. The downscale energy cascade associated with the divergent modes plays an important role in the formation of the mesoscale kH-5/3 spectrum. In experiments without planetary rotation, the obtained spectral slope is steeper and energy transfer to larger vertical wavenumbers is increased.