by a Hasegawa-Mima like equation. The zonal flow excitation is studied through coupling of coherent modes. The drift wave is considered to be a monochromatic pump wave which may couple to the zonal flow through symmetric sidebands and this gives a system of four coupled waves [@a15]- [@a21]. The corresponding system may be reduced to a fifth order dispersion relation for the zonal flow. It was shown in Ref. [@a20] that the coherent mode coupling models agree well with the corresponding model derived using the wave kinetic equation approach.

To estimate the zonal flow growth rate and real frequency the mode coupling saturation [@a45] level is employed. In Ref. [@a372] simulations of ITG modes in CLM were performed and a good agreement between the simulations and the experiments were found on the saturation level. This particular finding validates the use of the mode coupling saturation level used in this study.

However, the efforts of comparing analytical estimations with measurements are rather limited. The effects of coupling between the acoustic modes and the zonal flows are mostly unexplored as well. It has been indicated in gyrokinetic turbulence simulations that small density gradient scale lengths ($L_n$) increases the transport [@a371] which encourages 