Design Rationale of the JcupLT Coupling Library: Lessons Learned from Jcup Development and Applications

Takashi Arakawa

Coupling libraries are essential infrastructure for multi-component simulations in weather, climate, and earth system modeling. Jcup is a coupling library developed since 2007 and applied to a wide range of coupled simulations, including atmosphere–ocean coupling, land surface modeling, seismic–structural coupling, and AI-integrated simulations. Through nearly two decades of development and application, both the strengths and limitations of Jcup’s design have become clear. This paper documents the design rationale of JcupLT, a successor coupling library that inherits the proven design principles of Jcup while addressing the performance and usability issues identified through practical applications. The name JcupLT stands for both Lightning and Lightweight, reflecting its design goals of high speed and low overhead. We describe the development history and applications of Jcup, identify its strengths and limitations, and explain how each design decision in JcupLT was motivated by practical experience.