Importance of Ice Nucleation and Precipitation on Climate with the Parameterization of Unified Microphysics Across Scales version 1 (PUMASv1)

Abstract

Abstract. Cloud microphysics is critical for weather and climate prediction. In this work, we document updates and corrections to the cloud microphysical scheme used in the Community Earth System Model (CESM) and other models. These updates include a new nomenclature for the scheme, and the ability to run the scheme on Graphics Processing Units (GPUs). The main science changes include removing an ice number limiter and associated changes to ice nucleation, adding vapor deposition onto snow, and introducing an implicit numerical treatment for sedimentation. We also detail the improvements in computational performance that can be achieved with GPU acceleration. We then show the impact of these scheme changes on (A) mean state climate, (B) cloud feedback response to warming and (C) aerosol forcing. We find that corrections are needed to the immersion freezing parameterization without a limit on ice number. We also find that the revised scheme produces less liquid and ice, but that this can be adjusted by changing the loss process for cloud liquid (autoconversion). Furthermore, there are few discernible effects of the PUMAS changes on cloud feedbacks, but some significant reductions in the magnitude of Aerosol Cloud Interactions (ACI). Small cloud feedback changes appear to be related to the implicit sedimentation scheme, with a number of factors affecting ACI.

On the links between ice nucleation, cloud phase, and climate sensitivity in CESM2

“…Our findings suggest that CESM2's ice nucleation updates raised ECS primarily by correctingpartially inadvertently -much of the cloud phase bias present in CESM1 . Our findings are consistent with results from a new version of CESM2's microphysics with revised ice nucleation (Gettelman et al, 2022), which did not lead to substantial change in ECS. Correcting the ice limit issue in a reduced-resolution CESM2 version had reduced cloud feedback strength (Zhu et al, 2022), yet we find this does not occur in the standard-resolution model.…”

“…Reversion to CESM1's ice nucleation scheme was found to undo most of the feedback difference causing the ECS jump (Gettelman et al, 2019), while correcting the bug considerably lowered ECS in a reduced-resolution version of CESM2 (Zhu et al, 2022). However, an update to CESM2's microphysics that replaced the ice nucleation scheme while removing the bug only weakly affected ECS (Gettelman et al, 2022). The connection between ice nucleation and ECS remains poorly understood.…”

Acceleration of the Parameterization of Unified Microphysics Across Scales (PUMAS) on the Graphics Processing Unit (GPU) With Directive‐Based Methods

“…The default method used for sedimentation in PUMAS is an explicit method and it contains sub‐steppings to ensure numerical stability. There is another option to calculate the sedimentation process implicitly, which does not require sub‐steppings to maintain the numerical stability (Gettelman et al., 2023). It was originally derived from the Geophysical Fluid Dynamics Laboratory (GFDL) microphysics (Harris et al., 2020; Zhou et al., 2019) and later improved for better accuracy by Guo et al.…”

“…CAM6 uses the Morrison‐Gettelman two‐moment cloud microphysics parameterization (MG2, Bacmeister et al., 2014; Gettelman & Morrison, 2015; Gettelman et al., 2020; Sun et al., 2019; Zheng et al., 2016) by default and the MG3 parameterization can be used alternatively to additionally simulate the graupel or hail (Gettelman et al., 2019). The MG parameterization has been renamed to PUMAS to reflect the broader contributions from the research community (Gettelman et al., 2023). Note that the same PUMAS code base can be configured for either the MG2 or MG3 parameterization.…”

On the links between ice nucleation, cloud phase, and climate sensitivity in CESM2

“…Reversion to CESM1's ice nucleation scheme was found to undo most of the feedback difference causing the ECS jump (Gettelman et al, 2019), while correcting the bug considerably lowered ECS in a reduced-resolution version of CESM2 (Zhu et al, 2022). However, an update to CESM2's microphysics that included removal of the bug only weakly affected ECS (Gettelman et al, 2022), and the connection between ice nucleation and ECS remains poorly understood.…”