Impacts of gravity wave drag induced by cumulus convection in the middle atmosphere

Impacts of gravity wave drag induced by cumulus convection (GWDC) in the middle atmosphere are investigated using Whole Atmosphere Community Climate Model (WACCM) developed at National Center for Atmospheric Research. For this, spectral GWDC parameterization is implemented into WACCM. The spectral GWDC (SGWDC) parameterization uses analytic three-dimensional cloud-top wave momentum flux spectrum as its reference-level spectrum. For the calculation of the vertical evolution of the spectrum, Lindzen-type method is used. To examine impacts of GWDC, we perform two 12-yr simulations driven by climatological boundary conditions: one without the SGWDC parameterization (control simulation) and one with the parameterization (SGWDC simulation). In the SGWDC simulation, cloud-top wave momentum fluxes are strong mainly in the tropics and mid-latitude storm track regions in winter hemisphere. In the SGWDC simulation, the magnitude of zonal-mean zonal momentum forcing by convective internal gravity waves is about 13–27 (37–50) m s^–1 day^–1 in the mesosphere in January (July). In the stratosphere, forcing is strong near the equatorial stratopause, and its magnitude is about 1–5 m s^–1 day^–1 in the solstice seasons. In the SGWDC simulation, model biases in the zonal-mean zonal wind and temperature with respect to observations are alleviated in most regions except near polar mesopause. Near the equatorial stratopause where excessive easterly appears in the control simulation, the GWDC is eastward, along with the enhancement of eastward forcing by nonorographic gravity waves. As a result, the semi-annual oscillation with more realistic westerly phase is produced in the SGWDC simulation. In the equatorial lower stratosphere, zonal-mean zonal wind in the SGWDC simulation does not exhibit the quasi-biennial oscillation, but it has a strong interannual variability, which does not appear in the control simulation. This variability is due to planetary-wave breaking and meridional and vertical advections induced by including the SGWDC parameterization in model as well as the parameterized GWDC itself.