Secondary waves generated by breaking of convective gravity waves in the mesosphere and their influence in the wave momentum flux

Gravity waves generated by convective clouds are numerically simulated using a two-dimensional mesoscale model that extends to the upper mesosphere (physical model top of 105 km), and characteristics of the waves are investigated before and after wave breaking occurred in the mesosphere. Gravity waves generated by convective clouds have wide spectrum corresponding to their sources, but large portions of the waves with short horizontal and vertical wavelengths are reflected, trapped, and filtered out before they reach to the upper mesosphere. After wave breaking occurs in the mesosphere (z = 65-85 km), secondary waves are generated by momentum forcing due to breaking of the primary waves. The mesospheric forcing has a wide spectrum with strong power not only for components at which the primary waves have strong power but also for components with short zonal wavelengths and periods. Interaction between upward-propagating primary waves (UPW) and downward-propagating secondary waves (DSW) are investigated below the mesospheric forcing region by comparing the result from an additional simulation with a lower model top (z = 60 km) that excludes wave breaking in the mesosphere. It was found that DSWs increase spectral powers for both short and long horizontal and vertical wavelengths and for westward-propagating components that are mostly filtered in the UPWs. For a given easterly basic-state wind in the middle atmosphere, DSWs produce negative momentum flux at wide phase speeds, and this cancels a large portion of the positive momentum flux by the UPWs. Consequently, the magnitude of the positive momentum flux decreases and the negative momentum flux increases, which is caused by DSWs. This change reduces the gravity wave drag significantly below the mesospheric forcing region. This result implies that current gravity wave parameterizations without considering DSWs may overestimate the momentum flux and resultant drag and suggests the need to include the effects of secondary waves in the parameterizations of gravity waves.