An accurate understanding of Earth system change mechanisms and predicting their impacts relies on the development of Earth system models.Compared to global models,Regional Earth System Models (RESMs) concentrate more on medium-to small-scale processes and their regional impacts within specific areas,featuring higher spatial resolution and more detailed physical processes.RESMs enable coupled simulations of multi-layered Earth interactions,thereby enhancing the ability to reproduce,analyze,and forecast extreme climate events.Consequently,the development and application of RESMs hold significant scientific and practical importance for addressing various climate change-related challenges and assisting in prediction and decision-making across multiple fields,including disaster prevention and mitigation,water resources management,agriculture,energy,environmental conservation,and resource exploitation.The concept of RESMs was initially proposed by Giorgi around 1995.Over the past three decades,their development has primarily followed two approaches.The first approach,known as independent development,involves coupling regional weather/climate models with specialized models tailored to specific application goals.This method aims to broaden the application scope of regional models in specific fields based on reliable atmospheric,land surface,and oceanic descriptions.Typically,specialized models are directly coupled with regional weather/climate models,resulting in relatively simple model structures and limited functions.Representative models include WRF-Hydro,PFWRF,WRF-HMS,RegCM-FVCOM,CWRF-FVCOM,WRF-Crop,WRF-CMAQ,and WRF-Chem.The second approach,holistic integration,seeks to construct a comprehensive model of coupled multi-sphere processes for digital twin regional Earth systems.This approach aims to create a unified and coordinated framework that emphasizes deep integration among various models.Such an approach not only requires technical compatibility among the models but also demands theoretical and methodological innovations to better simulate and understand the complex dynamics and interactions within Earth systems.Representative models of this approach include RegCM-ES,TerrSysMP,ROM,and R-CESM.Irrespective of the development approach,RESMs exhibit the following common characteristics:(i) Multi-layer coupling:RESMs provide a more detailed representation and online coupling of land surface and ocean processes compared to regional weather/climate models.They integrate biogeochemical,hydrological,human activity,and atmospheric chemistry processes,enabling a comprehensive understanding and simulation of the dynamic relationships among various Earth system components.Through the use of couplers,RESMs achieve flux coupling and interactions across different spatiotemporal scales,thereby enhancing the precision of simulations of natural cycles and the impacts of human activities on these cycles.(ii) Higher spatial resolution:RESMs can simulate small-scale processes,explicitly representing atmospheric convection,boundary layer processes,oceanic mesoscale eddies,complex vegetation structures on land surfaces,and changes in land use.These capabilities lead to more accurate simulations and predictions of extreme weather and climate events and their effects on local environments.(iii) Integration of data assimilation:The initial state in RESMs involves multiple processes across different layers,and the initial value of any variable can influence the entire model.Assimilating observational data from multiple sources and layers into the model’s initial state not only reduces the initial errors of related processes but also minimizes error propagation throughout the entire system,thereby shortening the model’s spin-up time and enhancing simulation accuracy.In light of the overview provided,this study advocates for the integration of interdisciplinary research efforts through open-source collaboration to expedite the development of RESMs in China.There is an urgent need to conduct interdisciplinary research utilizing the newly established model,with a particular focus on interactions among multi-layer and multi-scale processes.Additionally,efforts should be directed towards establishing a regional digital twin platform for monitoring and early warning based on high-resolution RESMs.Such platform could play a crucial role in disaster prevention and mitigation in critical regions and support vital decision-making processes.