Building a Comprehensive Library of Cloud-Resolving Simulations to Study Marine Cloud Brightening across a Spectrum of Environmental Conditions

Research on Marine Cloud Brightening (MCB) shows the potential to cool the planet, yet uncertainties exist in predicting its efficacy within global climate models (GCMs), largely due to limitations in capturing complex low marine cloud mechanisms and associated aerosol-cloud interactions. As these clouds' evolution and their response to aerosols are sensitive to ambient environmental conditions, it becomes imperative to determine different responses across a spectrum of conditions. In this study, we introduce an innovative approach aimed at encompassing the wide array of conditions prevalent in low marine cloud regions through the creation of a comprehensive library of observed environmental conditions. This approach is subsequently employed to select a representative selection of cases for process model studies, such as Large Eddy Simulations (LES).

Utilizing European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis, version 5 (ERA5) wind data, we generate over 2000 Lagrangian isobaric (950 hPa) boundary layer forward trajectories for specific locations within the Northeast Pacific Ocean's stratocumulus deck region during the summer seasons of 2018 to 2021. Thereafter, meteorological, cloud, and aerosol variables from reanalysis and satellite data are compiled along the trajectories. By using critical cloud-controlling variables (CCVs) (e.g., along-trajectory means, and differences between the beginning and end of the trajectory for wind speed, mixing ratio, subsidence, and estimated inversion strength), we employ Principal Component Analysis (PCA) to reduce the dimensionality of the data. This technique illustrates that three principal components capture 62% of the variability among CCVs. Notably, PCA facilitates the efficient selection of LES cases that span the observed CCV, aerosol, and cloud phase space.

Utilizing the PCA results, we identify more than 50 distinct cases representing a diverse array of environmental conditions. These cases will be used to initiate 2-day realistic, high-resolution, large-domain LES experiments, thereby simulating a spectrum of aerosol-cloud interactions under observed as well as perturbed aerosol conditions. Our LES incorporates a prognostic aerosol scheme, accounting for aerosol budget tendencies such as coalescence and interstitial scavenging, surface sources, and entrainment from the free troposphere. The LES is forced with meteorological data as well as an accumulation-mode aerosol number concentration calculated from the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) masses of aerosol species and their assumed particle size distributions. Such a large number of simulations will enable us to synthesize valuable statistics to assess how well LES can simulate the cloud lifecycle under the 'best estimate' environmental conditions, and how sensitive the simulated clouds are to variations in these driving fields. This ongoing procedure holds the promise of enhancing our ability to evaluate the efficacy of intentional MCB under a range of representative conditions. As we continue to conduct LES experiments, its outcomes will contribute to advancing our understanding of MCB's potential impact as a climate intervention method.