Henning Franke

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This study presents the first attempt to directly simulate a full cycle of the quasi-biennial oscillation (QBO) in a global storm-resolving model (GSRM) that explicitly simulates deep convection and gravity waves instead of parameterizing them. Using the Icosahedral Nonhydrostatic (ICON) model with horizontal and vertical resolutions of about 5 km and 400 m, respectively, we show that an untuned state-of-the-art global storm-resolving model is already on the verge of simulating a QBO-like oscillation of the zonal wind in the tropical stratosphere for the right reasons. ICON shows overall good fidelity in simulating the QBO momentum budget and the downward propagation of the QBO jets in the upper QBO domain (25 km–35 km). In the lowermost stratosphere, however, ICON does not simulate the downward propagation of the QBO jets to the tropopause. This is the result of a pronounced lack of QBO wave forcing, mainly on planetary scales. We show that the lack of planetary-scale wave forcing in the lowermost stratosphere is caused by an underestimation of the planetary-scale wave momentum flux entering the stratosphere, which is too weak by 20%–40%. We attribute this lack of planetary-scale wave momentum flux to a substantial lack of convectively coupled equatorial waves in the tropical troposphere. Therefore, we conclude that in the present global storm-resolving model, simulating a realistic spatio-temporal variability of tropical deep convection, in particular convectively coupled equatorial waves, is currently the main roadblock towards simulating a reasonable QBO.