Simulation of Atmospheric Rivers: Dynamical Modulation and Resolution Dependence

Atmospheric rivers (ARs) are examined in a set of aquaplanet simulations using the Model for Prediction Across Scales (MPAS) non-hydrostatic dynamical core run at multiple horizontal resolutions, namely 240 km, 120 km and 60 km. As the resolution is increased, there is an increase in the occurrence of long-lasting ARs. At the same time there is also an increase in the local finite-amplitude wave activity of upper-tropospheric absolute vorticity (LWA), a proxy for Rossby wave amplitude that is closely linked with wave breaking. Consistent with the notion that changes in ARs are driven by mid-latitude dynamics, a strong relationship is identified between ARs and the equatorward component of LWA. A logistic regression model is used to quantify the probability of AR occurrence based solely on LWA and explains most of the change in AR frequency with resolution. LWA is a diagnostic that may be easily applied to the broadly available output of CMIP6 and other model simulations, thus enabling scientists to infer AR and wave breaking characteristics. AR characteristics, in particular, require higher resolution moisture and winds at multiple levels that are not always easily available.