Momentum flux spectrum of convectively forced internal gravity waves and its application to gravity wave drag parameterization, Part II: Impacts in a GCM (WACCM)

Impacts of a spectral parameterization of gravity wave drag (GWD) induced by cumulus convection (GWDC) in the NCAR Whole Atmosphere Community Climate Model (WACCM1b) are investigated. In the spectral GWDC parameterization, reference wave momentum flux spectrum is launched at cloud top and analytically calculated based on the physical properties of convection and the large-scale flow. The cloud-top wave momentum flux is strong mainly in the Tropics and midlatitude storm-track regions, and exhibits anisotropy and spatiotemporal variability. The anisotropy and variability are determined by the distributions and variations of convective activities, the moving speed of convection, and horizontal wind and stability in convection regions. Zonal-mean zonal GWDC has a maximum of 13-27 (37-50) m s⁻¹ day⁻¹ in the mesosphere in January (July). Impacts of GWDC on zonal wind appear mainly in the low to midlatitudes of the upper stratosphere and mesosphere. In these regions, biases of zonal wind with respect to observation are reduced more than 50% through the GWDC process. In contrast to zonal wind, impacts of GWDC on temperature occur mainly in the mid- to high latitudes. Through the analysis of forcing terms in the zonal wind and temperature equations, it is found that impacts of GWDC result from interaction among wave forcing terms (resolved wave forcing, parameterized background GWD, and GWDC) and meridional circulations induced by the wave forcing terms. With regard to tropical variability, when GWDC is included, the model produces the stratospheric semiannual oscillation with more realistic amplitude and structure and stronger interannual variabilities in the lower stratosphere. These enhanced variabilities are caused by resolved wave forcing and meridional circulations.