Storm-Resolving Model ICON at the Air-Sea Interface: Insights into Momentum Dynamics and Parameterization Challenges

Storm-resolving models, such as the ICON model at 5 km resolution, are transforming our understanding of the Earth’s climate system by explicitly resolving key small-scale processes. This study highlights the dual nature of this modeling revolution: the advantages of improved representation of subgrid-scale dynamics and the challenges posed by existing parameterizations in capturing air-sea interactions.
On the one hand, a detailed momentum analysis of equatorial boundary layer winds using the coupled storm-resolving model ICON reveals dynamics that deviate from traditional assumptions. We identify two persistent wind patterns—zonal and meridional—governed by SST-driven pressure gradients, vertical turbulent flux, and horizontal momentum transport. These transport terms, largely overlooked in conventional models, and resolving the fine-scale interaction between SST gradients and boundary layer dynamics play a decisive role in shaping surface winds. A revised wind model, incorporating these findings, demonstrates strong agreement with storm-resolving model outputs.
On the other hand, storm-resolving models expose limitations in parameterizations of small-scale processes at the air-sea interface. For instance, the surface exchange coefficients—such as drag (c_D) and heat exchange (c_H)—are shown to be inadequate under low-wind regimes, leading to biases in surface pressure distribution and convection patterns. Using the ICON atmosphere-land-only "OptiFlux" configuration, we demonstrate that even small adjustments to these coefficients can substantially improve the representation of surface fluxes, strengthen pressure gradients, and enhance atmospheric dynamics.
These two aspects of this study illustrate the transformative potential and pressing challenges of storm-resolving models in further research.