Towards a direct simulation of a full cycle of the quasi-biennial oscillation in a global storm-resolving model

This study presents the very first attempt to directly simulate a full cycle of the quasi-biennial oscillation (QBO) in a global storm-resolving model (GSRM) that explicitly resolves deep convection and gravity waves instead of parameterizing them. Using the ICOsahedral Nonhydrostatic (ICON) model with a horizontal and vertical resolution of about 5 km and 400 m, respectively, we show that a GSRM in the convective gray zone is in principle capable of simulating the basic dynamics that lead to a QBO-like oscillation of the zonal wind in the tropical stratosphere. ICON shows overall good fidelity in simulating the downward propagation of QBO jets in the upper tropical stratosphere, which happens also for the right reasons. In the lower stratosphere, however, ICON does not simulate the downward propagation of the QBO jets to the tropopause, predominantly due to a pronounced lack of planetary-scale wave forcing. As a consequence, the QBO jets degrade with increasing simulation time and lose strength substantially. We show that the lack of planetary-scale wave forcing in the lower stratosphere is caused by a lack of planetary-scale wave momentum fluxes entering the stratosphere, which are about 20%–40% too weak. We attribute this lack of planetary-scale wave momentum flux to a substantial underestimation of the spatio-temporal variability of tropical deep convection in general and convectively coupled equatorial waves in the tropical troposphere in particular. While conventional general circulation models can compensate for a lack in resolved wave forcing by tuning the parameterized gravity wave forcing, GSRMs no longer have this tuning screw, making their QBO more susceptible to being influenced by tropospheric mean state biases. We thus conclude that simulating a realistic spatio-temporal variability of tropical convection is currently the main roadblock towards simulating a reasonable QBO in GSRMs.