Regionally Refined Model of E3SM Version 2: Overview of Atmosphere, Land, and River

The US DOE Energy Exascale Earth System Model (E3SM) project aims to develop a high resolution, fully coupled Earth system model for climate simulations. In E3SM version 1, we established that the regionally refined model (RRM) is an economical (~8x less computational cost compared to the globally uniform counterpart) and effective tool for high resolution model development and studies. We adopted the RRM approach for E3SM version 2 to develop high-resolution (25 km) coupled simulations over North America (NA). Recently, we have completed the CMIP6 DECK (Diagnosis, Evaluation, and Characterization of Klima) and historical simulations with the E3SMv2 NARRM - A first-of-kind climate production simulations using RRM.

In the present study, we will share important lessons we learned when applying RRM for a standard climate model release, including the strategy of hybrid time steps to achieve improved climate representations at the high-resolution patch without sacrificing the overall global performance. Besides confirming that the global mean climate and climate sensitivity are similar for the NARRM and its parallel low-resolution model, we will highlight the benefits of refining the horizontal resolution at North America in the atmosphere, land, and river components, such as orographic precipitation, stratocumulus clouds near California coastal regions, shortwave cloud forcing, and aerosols. Lastly, we will discuss potential approaches for future improvements.