Combining long-term observations and Lagrangian case studies to evaluate stratiform cloud precipitation processes in climate models

Abstract

A leading objective of current climate modeling efforts is narrowing uncertainty in predictions of how much surface air temperatures increase from a doubling of carbon dioxide concentration as a measure of Earth’s equilibrium climate sensitivity (ECS). Results from the Intergovernmental Panel on Climate Change (IPCC) Sixth Coupled Model Intercomparison Project (CMIP6) indicate that the range of predicted equilibrium climate sensitivity reported across current international climate models is greater than in recent previous generations of models. Efforts to identify differences in model physics behind the increased range of predicted equilibrium climate sensitivity have pinpointed differing shallow cloud responses to warming as a key driver of equilibrium climate sensitivity range, including both warm and mixed-phase conditions modulated by precipitation at mid and high latitudes.

Here we posit that DOE ARM long-term observations are uniquely capable for constraining shallow warm and cold cloud precipitation processes. Because precipitation occurrence and intensity is furthermore of central importance to both warm and cold cloud system evolution of mesoscale state and albedo, we propose a two-pronged effort: (i) robustly evaluate cloud base precipitation rates using long-term observations at multiple sites, and (ii) derive an ensemble of aerosol-aware Lagrangian case study ensembles for large-eddy simulation (LES) and climate models in single-column model (SCM) mode. We propose to focus the Lagrangian case studies on cold-air outbreaks , which represent a key cloud type that undergoes mixed-phase precipitation-modulated transitions from high to low albedo states, and has been identified as deficient in climate models generally. These two prongs represent modular elements of a long-term strategy to evaluate and improve the shallow cloud physics in NASA’s ModelE3 climate model, and advance understanding of how uncertainties in the physics of shallow cloud precipitation processes impact its predictions of equilibrium climate sensitivity.

Project objectives can be summarized as follows:

1. Long-term observations will be used to evaluate ModelE3 precipitation processes over all shallow cloud conditions at three sites: the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) Andenes site, the North Slope of Alaska (NSA) Utqiagvik site, and the Eastern North Atlantic (ENA) site. Model evaluation will be enabled by the Earth Model Column Collaboratory (EMC²), an open-source ground-based radar and lidar simulator.
2. Aerosol-aware Lagrangian LES/SCM case studies will be derived for three observed cold-air outbreak events observed at each site (COMBLE, NSA, and ENA). The cold-air outbreak case study ensemble will allow us to efficiently diagnose ModelE3 behavior in SCM mode and provide a testbed for improvements to aerosol-cloud interactions and precipitation physics. We will also generate LES and SCM input and output files that conform to emerging international formats for community use.
3. Process-oriented analyses of long-term data (results of objective 1) and SCM case study performance (including results of objective 2) will be probed to identify differences responsible for extratropical cloud feedback processes that dominate equilibrium climate sensitivity spread across ModelE3 configurations. Satellite observations and SCM case studies from other sources will also be used at this step.

The project structure is designed to develop component data sets and codes that will be flexibly applicable to other community models.