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Using water stable isotopes to quantify the roles of entrainment, drizzle, and aerosols in determining marine stratocumulus properties

Active Dates 9/1/2023-8/31/2026
Program Area Atmospheric System Research
Project Description
Using water stable isotopes to quantify the roles of entrainment, drizzle, and aerosols in determining marine stratocumulus properties

Principal Investigator: Lisa Welp, Purdue University

Co-Investigator: Patrick Chuang, University of California Santa Cruz

Low-level marine stratocumulus clouds reflect solar radiation and cool the Earth’s climate. The behavior of these clouds play an important role in Earth’s climate sensitivity because the spatial coverage, structure, and radiative properties may change due to anthropogenic forcing. Accurately predicting the behavior of the stratocumulus-topped boundary layer (STBL) is difficult to model, requiring knowledge of moisture, energy, and aerosol budgets. The microphysical influence of aerosols influences shortwave radiative effects through modifying cloud liquid water path, drop number density, and droplet size distributions. In turn, the entrainment of warm, dry, free tropospheric air and drizzle formation mediates the response of stratocumulus clouds to aerosols, controls the aerosol budget of the clouds, and determines how sensitive the radiative changes of stratocumulus clouds are to aerosols. Radar observations have shown that low marine stratocumulus clouds precipitate frequently, but conventional surface observing methods are inadequate to quantify these drizzle rates, largely due to evaporation between the cloud and the surface. This light precipitation is important, even if it does not reach the surface, for understanding STBL energy budgets, cold pool formation, and aerosol scavenging. Additionally, it is still not clear the most appropriate way to relate entrainment rates with the vertical and horizontal structure of the entrainment interface layer. While large eddy simulation (LES) models are extremely informative for relating turbulence at different scales to entrainment rates, assumptions that are made, like the bulk microphysics and sub-grid scale mixing schemes, are inherently uncertain with consequences for the simulated entrainment rates. Additional constraints are needed to improve observational-based estimates of entrainment and drizzle fluxes in the context of aerosol-cloud interactions.

The central goal of this research is to improve understanding of warm boundary layer processes and their sensitivity to aerosols by measuring entrainment and drizzle fluxes in STBLs and quantifying their drivers and radiative impact using ARM observations. In this study, we add stable water isotopes of the atmosphere to further constrain humidity budgets and to separate drizzle from the entrainment mixing with dry free tropospheric air. This will use data collected during the summer 2023 SCILLA airborne campaign near San Diego, CA that is part of the DOE ARM EPCAPE deployment. We will relate the estimated entrainment and drizzle fluxes with simultaneous measurements of cloud microphysical properties to achieve the following proposed project objectives.
1.Improve understanding of entrainment and drizzle processes in the STBL by separating their influences on the cloud moisture and energy budgets.
2.Relate cloud top meteorological and aerosol conditions to spatially- and temporally-averaged entrainment and drizzle fluxes.
3.Improve predictions of cloud radiative properties due to entrainment and drizzle imprinting on the microphysical structure in the STBL cloud layer.

At the completion of this project, we will have improved our understanding of the function of the entrainment interface layer in modulating exchange of energy, moisture, and aerosol between the lower free troposphere and the STBL in warm boundary-layer clouds (ASR topic area #3 in this FOA). We will improve quantitative understanding of the roles of entrainment, drizzle, and aerosols in determining cloud radiative properties and improving satellite retrievals. These advancements are key to reducing uncertainty in aerosol-cloud interactions and cloud feedbacks in climate sensitivity.
Award Recipient(s)
  • Purdue University (PI: Welp, Lisa)