Understanding Cloud and Convective Characteristics in Version 1 of the E3SM Atmosphere Model

Abstract

This study provides comprehensive insight into the notable differences in clouds and precipitation simulated by the Energy Exascale Earth System Model Atmosphere Model version 0 and version 1 (EAMv1). Several sensitivity experiments are conducted to isolate the impact of changes in model physics, resolution, and parameter choices on these differences. The overall improvement in EAMv1 clouds and precipitation is primarily attributed to the introduction of a simplified third-order turbulence parameterization Cloud Layers Unified By Binormals (along with the companion changes) for a unified treatment of boundary layer turbulence, shallow convection, and cloud macrophysics, though it also leads to a reduction in subtropical coastal stratocumulus clouds. This lack of stratocumulus clouds is considerably improved by increasing vertical resolution from 30 to 72 layers, but the gain is unfortunately subsequently offset by other retuning to reach the top-of-atmosphere energy balance. Increasing vertical resolution also results in a considerable underestimation of high clouds over the tropical warm pool, primarily due to the selection for numerical stability of a higher air parcel launch level in the deep convection scheme. Increasing horizontal resolution from 1°to 0.25°without retuning leads to considerable degradation in cloud and precipitation fields, with much weaker tropical and subtropical short-and longwave cloud radiative forcing and much stronger precipitation in the intertropical convergence zone, indicating poor scale awareness of the cloud parameterizations. To avoid this degradation, significantly different parameter settings for the low-resolution (1°) and high-resolution (0.25°) were required to achieve optimal performance in EAMv1. Plain Language Summary The Energy Exascale Earth System Model (E3SM) is a new and ongoingU.S. Department of Energy (DOE) climate modeling effort to develop a high-resolution Earth system model specifically targeting next-generation DOE supercomputers to meet the science needs of the nation and the mission needs of DOE. The increase of model resolution along with improvements in representing cloud and convective processes in the E3SM atmosphere model version 1 has led to quite significant model behavior changes from its earlier version, particularly in simulated clouds and precipitation. To understand what causes the model behavior changes, this study conducts sensitivity experiments to isolate the impact of changes in model physics, resolution, and parameter choices on these changes. Results from these sensitivity tests and discussions on the underlying physical processes provide substantial insight into the model errors and guidance for future E3SM development. Key Points:• CLUBB along with the companion changes in EAMv1 primarily account for the overall improvements in clouds and precipitation simulation • Underestimate of coastal Sc in EAMv1 is due to CLUBB and model tuning; increased vertical resolution partially offsets this degradation • The poor scale awareness of EAMv1 r...

Aerosols in the E3SM Version 1: New Developments and Their Impacts on Radiative Forcing

“…The comparison suggests that the different ∆R is primarily driven by differences in the CTRL warm cloud radiative forcing R , although there are different sensitivities between N c , N d , or R . The lowered base R of warm clouds in EAMv1 likely comes from changes in model vertical resolution, cloud treatments, and/or model calibration against satellite‐based observations for all clouds (Rasch et al, ; Xie et al, ). The increased ∆R in EAMv1, compared to CAM5.3, is primarily due to stronger sensitivity to N d .…”

An Overview of the Atmospheric Component of the Energy Exascale Earth System Model

“…Remaining biases in these high‐latitude cloud systems are still associated with treatments of heterogeneous ice nucleation and the WBF process discussed by Tan et al () and Zhang et al (). CAM5 tropical CRE biases appear centered on regions of deep convection, while EAM low‐latitude biases are located primarily in regions of trade cumuli and summertime stratocumulus (Xie et al, ). Experimental versions of EAM now exist with substantial improvements for these features in EAM.…”

“…The CAM lineage used 18 layers in CCM2 (Hack et al, ), 27 in CAM3 (Collins et al, ), 30 in CAM4 (Neale et al, ) and CAM5 and CAM6 has added two extra model layers at the tropopause, but the surface layer has remained constant at approximately 100 m over all these generations. Recent changes in the model physics described below (with an extensive discussion of consequences in Xie et al, ) have reduced the model sensitivity to vertical resolution, although there is still some dependence. EAMv1 uses a traditional hybridized sigma pressure vertical coordinate.…”

The Summertime Precipitation Bias in E3SM Atmosphere Model Version 1 over the Central United States

“…The aerosol processes are treated with the four‐mode version of the Modal Aerosol Module (Liu et al, ). More details of the model physics can be found in Xie et al () and Rasch et al (). EAMv1 uses the Spectral Element dynamical core (Dennis et al, ; Taylor & Fournier, ) and has two sets of standard globally uniform horizontal‐resolution configuration: 1° (ne30) and 0.25° (ne120).…”

“…Department of Energy Community Atmosphere Model version 5.3 (Neale et al, ), Energy Exascale Earth System Model Atmosphere Model version 1 (EAMv1) has implemented a number of new features in the physical parameterizations and supports two different standard horizontal resolutions: 1° (ne30) and 0.25° (ne120; Xie et al, ; Golaz et al, ; Rasch et al, ). While most physical parameterization settings are identical, a few cloud‐related parameters need to be retuned to optimize the simulation of global climatology for different resolutions because of the poor scale awareness of EAMv1 physics (Xie et al, ). The summertime precipitation over the Central United States, however, was not among the top metrics for model tuning during the EAMv1 development and has not been closely studied.…”