Better calibration of cloud parameterizations and subgrid effects increases the fidelity of the E3SM Atmosphere Model version 1

Abstract

Abstract. Realistic simulation of the Earth's mean-state climate remains a major challenge, and yet it is crucial for predicting the climate system in transition. Deficiencies in models' process representations, propagation of errors from one process to another, and associated compensating errors can often confound the interpretation and improvement of model simulations. These errors and biases can also lead to unrealistic climate projections and incorrect attribution of the physical mechanisms governing past and future climate change. Here we show that a significantly improved global atmospheric simulation can be achieved by focusing on the realism of process assumptions in cloud calibration and subgrid effects using the Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1). The calibration of clouds and subgrid effects informed by our understanding of physical mechanisms leads to significant improvements in clouds and precipitation climatology, reducing common and long-standing biases across cloud regimes in the model. The improved cloud fidelity in turn reduces biases in other aspects of the system. Furthermore, even though the recalibration does not change the global mean aerosol and total anthropogenic effective radiative forcings (ERFs), the sensitivity of clouds, precipitation, and surface temperature to aerosol perturbations is significantly reduced. This suggests that it is possible to achieve improvements to the historical evolution of surface temperature over EAMv1 and that precise knowledge of global mean ERFs is not enough to constrain historical or future climate change. Cloud feedbacks are also significantly reduced in the recalibrated model, suggesting that there would be a lower climate sensitivity when it is run as part of the fully coupled E3SM. This study also compares results from incremental changes to cloud microphysics, turbulent mixing, deep convection, and subgrid effects to understand how assumptions in the representation of these processes affect different aspects of the simulated atmosphere as well as its response to forcings. We conclude that the spectral composition and geographical distribution of the ERFs and cloud feedback, as well as the fidelity of the simulated base climate state, are important for constraining the climate in the past and future.

Cloud Phase Simulation at High Latitudes in EAMv2: Evaluation Using CALIPSO Observations and Comparison With EAMv1

“…Conversely, while recalibrated parameters for ZM scheme also increase ice cloud and decrease liquid cloud, simulated total cloud cover is increased as shown in Figure 8d. Reduced convective autoconversion efficiency and decreased ice particle size detrained from deep convection probably prolong the lifetime of ice clouds (Ma et al., 2022). Note that the decrease of liquid cloud due to the modified WBF process scaling factor and ZM tuning is largely canceled out by the introductions of the new dCAPE_ULL trigger and the minimum CDNC.…”

Effective radiative forcing of anthropogenic aerosols in E3SM version 1: historical changes, causality, decomposition, and parameterization sensitivities

“…6. These parameterization changes were applied during the E3SMv1 development and some of them have been further tuned for an alternate configuration of E3SM (Ma et al, 2022).…”

“…Note that the 0.2 m s −1 lower bound might be too high to be applied to all conditions, but removing it in E3SMv1 will degrade the cloud simulation if the model is not further tuned. In newer versions of E3SM(Ma et al, 2022;Golaz et al, 2022), this lower bound is reduced to 0.1 m s −1 , in combination with other tunings in the cloud and turbulence parameterizations.2 CLUBB calculates the vertical diffusion transport of other quantities, including water vapor, cloud hydrometeors (excluding cloud droplet number), and other trace gases.…”

Aircraft Observations of Turbulence in Cloudy and Cloud‐Free Boundary Layers Over the Western North Atlantic Ocean From ACTIVATE and Implications for the Earth System Model Evaluation and Development

“…The weaker turbulence coming from CLUBB as used in CAM6 might be rectified from a revision of its parameters. Ma et al (2021) recently performed a retuning of the parameters used to define the width of the bivariate normal PDFs as well as the effects of the turbulence skewness Sk w in the determination of quantities in the development of the E3SM EAMv2. This retuning was performed using theoretical considerations to improve the simulation of mostly subtropical stratocumulus decks.…”

“…Both of these models use the higher-order scheme CLUBB that unifies boundary layer turbulence, shallow convection, and cloud macrophysics. The main difference in the CLUBB implementation between these two models is that EAMv2 recently underwent a retuning of the parameters used to define the width of the binormal PDFs as well as the effects of the turbulence skewness Sk w in the determination of quantities (Ma et al, 2021). This retuning was undertaken to improve the simulation of the subtropical stratocumulus decks.…”