A Particle-in-Cell Wave Model for Efficient Sea-State Estimates in Earth System Models—PiCLES

Ocean surface waves have been demonstrated to be an important component of coupled Earth System Models (ESMs), influencing atmosphere-ocean momentum transfer; ice floe breakage; CFC, carbon, and energy uptake; and mixed-layer depth. Modest errors in sea state properties do not strongly affect the impacts of these parameterizations. The modest data and accuracy needed contrast sharply with the high computational costs of spectral wave models in next-generation ESMs, which can very easily exceed the cost of the ocean model component. We establish an alternative, cost-efficient prototype wave modeling framework for air-sea and ice-ocean interactions, enabling the routine use of sea state-dependent air-sea coupling in future ESMs. In contrast to spectral models, the Particle-in-Cell for Efficient Swell (PiCLES) wave model is customized for coupled atmosphere-ocean-sea ice modeling. Combining Lagrangian wave growth solutions with the Particle-In-Cell method leads to a model that periodically projects wave information onto any convenient grid and scales in an embarrassingly parallel manner. The set of equations solves for the growth and propagation of a parametric wave spectrum's peak wavenumber vector and total wave energy, which reduces the state vector size by a factor of 50–200 compared to the standard resolution of spectral models. PiCLES's current computational costs in idealized wind-sea simulations are about one order of magnitude faster than established wave models used in ESMs, with sufficient accuracy in bulk sea-state variables relevant for coupling. PiCLES is compared to WAVEWATCH III in efficiency and accuracy in idealized cases.