Models with a well-represented stratosphere have been shown to increase the predictability on weather and climate scales through stratosphere-troposphere coupling. However, stratospheric phenomena, such as the Quasi-biennial Oscillation (QBO) or stratospheric ozone, are still poorly represented in current atmospheric models. This study characterizes and improves stratospheric simulations with the next generation weather/climate model ICON-ART, and gives insights into the dynamical and chemical (stratospheric ozone) interactions in the polar stratosphere.
The climatology of a low resolution version of ICON-ART is examined and validated against ERA-Interim data. The sub-grid scale orographic (SSO) gravity wave drag is found unconstrained in the upper stratosphere, leading to warm biases in the Northern Hemisphere (NH) winter polar stratosphere. After reducing the SSO gravity wave drag, the modelled NH polar stratosphere has a more realistic mean state. At a low resolution, the model also fails to simulate the QBO-like oscillation that is observed in the equatorial zonal wind and is dominated by easterlies.
In order to explore dynamical interactions between the stratospheric zonal winds in the tropics and the polar stratosphere, a QBO-like oscillation is imposed in ICON by relaxation using ERA-Interim winds (QBO-relaxation). ... mehrThen, the Holton-Tan relationship, a phenomenon in which equatorial zonal winds synchronize with the NH winter polar stratosphere, is generally reproduced in the QBO-relaxation simulations. Based on idealized experiments, I found that the polar stratosphere is sensitive to the QBO phases and to relaxation height ranges. When the modelled equatorial zonal wind in the upper stratosphere is relaxed towards ERA-Interim, it results in (1) a colder NH polar stratosphere, for which the modulation of planetary wave meridional propagation is responsible, and (2) a Holton-Tan relationship where the timing depends on the relaxation depth.
Furthermore, two stratospheric ozone representations in ICON-ART are compared: (1) interactive-ozone considers ozone-meteorology coupling and (2) non-interactive-ozone uses the climatological ozone of the interactive-ozone simulation. Climatological ozone is a default in many climate models. When ozone is calculated interactively, the model simulated a colder polar stratosphere with a delayed polar vortex breakup in the Southern Hemisphere. Depending on the background zonal wind, planetary waves are found to feedback onto the changes in the polar stratospheric temperature either negatively or positively.