Resolution Dependence and Rossby Wave Modulation of Atmospheric Rivers in an Aquaplanet Model
This study examines atmospheric rivers (ARs) in aquaplanet simulations run at different horizontal resolutions. More frequent long-lasting ARs are found when the resolution is increased, and this can be largely explained by changes in Rossby wave activity.
This research suggests that atmospheric river (AR) frequency is largely modulated by Rossby wave activity, measured by the local finite-amplitude wave activity diagnostic. This diagnostic can be used to interpret changes in AR frequency, or even predict such changes, particularly when required data for detecting and identifying AR events are unavailable.
In this study, we examine atmospheric rivers (ARs) in a set of aquaplanet simulations using the Model for Prediction Across Scales (MPAS) dynamical core run at multiple horizontal resolutions, namely 240 km, 120 km and 60 km. As the resolution becomes finer, we find more long-lasting ARs, as well as a general increase in the local finite-amplitude wave activity of upper-tropospheric absolute vorticity (LWA), a good measure for Rossby wave phase and amplitude. We find a strong relationship between ARs and the equatorward component of LWA, consistent with the notion that ARs are largely driven by planetary-scale troughs (low pressure). A logistic regression model is used to quantify the probability of AR occurrence based solely on LWA. The model is skillful in predicting AR occurrence at a given longitude and time, and it is also able to explain most of the increase in AR frequency with resolution.