Development of a River Dynamical Core for E3SM to simulate compound flooding on Exascale-class heterogeneous supercomputers

Physically-consistent quantification of flood risks in global models requires kilometer scale flood simulations using 2D physics schemes, both of which are unavailable in the current generation Earth System Models. In this work, we have developed the River Dynamical Core (RDycore), which is an open-source, 2D shallow water equation library for the Energy Exascale Earth System Model (E3SM). RDycore uses PETSc and libCEED libraries to run efficiently on CPUs and GPUs. RDycore was validated for analytical, manufactured, and a well-studied dam break problem. For a problem with 471 million grid cells, RDycore achieves a speedup of 6.6x and 7.6x on GPUs compared to CPUs on Perlmutter and Frontier supercomputers, respectively. The one-way E3SM-RDycore coupling is demonstrated by performing multiple 5-day flooding simulations during Hurricane Harvey driven by five precipitation datasets. The work presented here is the foundational step in providing hardware and algorithmic portability for simulating kilometer-scale river dynamics within E3SM.