Simulation of the quasi-biennial oscillation using a fully 3D transient gravity-wave parameterization

Using a 3-dimensional (3D) Lagrangian ray-tracing approach, a realistic gravity-wave (GW) parameterization has been developed: Multi-Scale Gravity Wave Model (MS-GWaM). It is a unique and useful tool to simulate and study the 3D transient dynamics of GWs and their interactions with various meteorological phenomena. We implement MS-GWaM into the ICOsahedral Non-hydrostatic model (ICON) and conduct a simulation of the quasi-biennial oscillation (QBO). A particular focus of the study is on the effect of oblique propagation and transient dynamics of GWs on the simulated QBO dynamics. Source spectra of GWs in MS-GWaM are calculated online using the convective latent heat modeled by ICON's cumulus parameterization, as the QBO dynamics is sensitive to the source spectra. In the GW-source scheme a tuning parameter, the areal fraction of convective cells in a model grid cell, is used to produce a reasonable QBO period. In the simulation result, the amplitude of the QBO is realistic in the middle stratosphere but underestimated in the lower stratosphere. The easterly QBO wind is shorter in height than that of the observed QBO. It is notable that in the solstice seasons when the convective activity is maximal off the equator (~8°), GWs with relatively large horizontal wavelengths tend to propagate equatorward from the active convection region. This oblique propagation leads to an effective coupling between the GWs and the QBO. Another simulation is performed using the same experimental setup except that in MS-GWaM the horizontal propagation is neglected and the steady-state assumption is used, as in conventional GW parameterizations. The QBO in this simulation exhibits a large difference from that using the 3D transient MS-GWaM: The QBO has a much longer period (~4 years) and its easterly phase descends a bit less. QBO jets tend to form off the equator in the solstice seasons, centered on the latitudes of active convection.