Convectively Coupled Equatorial Waves in ICON

We show how in the Sapphire configuration of the Icosahedral Non-hydrostatic Model (ICON) the representation of Convectively Coupled Equatorial Waves (CCEWs) is sensitive to the fall speeds of rain and ice, and how reducing these fall speeds can lead to a better representation of CCEWs in ICON. In particular, reductions in the fall speeds of rain and ice lead to more active convectively coupled Kelvin, Inertio-Gravity, and Mixed Rossby-gravity waves and, at the same time, less active convectively coupled Equatorial Rossby waves and Tropical Depressions.

We then explore how changes in these fall speeds upscale from the kilometer scales to the synoptic and planetary scales of CCEW, finding that this up-scaling is mediated by the frequency of occurrence of shallow, stratiform, and deep convection. Reducing the fall speed of rain and ice leads to increases in the frequency of occurrence of, respectively, shallow and stratiform convection profiles and, at the same time, leads to decreases in the frequency of occurrence of deep convection profiles. We argue that changes in these profiles are reflected in the ability of the model to develop the tilt and top-heaviness of CCEWs, which ultimately leads to their better representation.

Our findings suggest a physical representation of CCEWs within ICON and provide further support for the classification of CCEWs into two distinct categories, a gravity wave group and a moisture mode group, each associated with distinct convective profiles and with distinct propagation mechanisms.