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Investigating the overlooked longwave impacts of mineral dust on warm boundary-layer clouds

Active Dates 9/1/2023-8/31/2026
Program Area Atmospheric System Research
Project Description
Investigating the overlooked longwave impacts of mineral dust on warm boundary-layer clouds

Principal Investigator: Adeyemi Adebiyi, University of California – Merced

Unfunded Collaborators: Yan Feng, Argonne National Laboratory; Peter Bogenschutz and Xue Zheng, Lawrence Livermore National Laboratory

Mineral dust accounts for about two-thirds of the masses of all aerosols and absorbs about a third of the solar radiation in the atmosphere. Therefore, the interactions between dust and radiation are important for our understanding of the Earth's radiative budget and for reducing the overall uncertainty associated with the estimates of how our climate will change in the future. Despite the importance of mineral dust, it remains unclear whether the effective dust radiative effect is to warm or cool the global climate system. While the sign of dust radiative effects depends on the vertically resolved dust absorption properties, it also depends on the accurate representation of all the pathways of dust interactions. These pathways of dust-radiation interactions primarily include the direct radiative effect, whereby dust directly scatters and absorbs shortwave (SW) and longwave (LW) radiation, and dust semi-direct radiative effect (SDE), which describes the adjustment of dust radiative effects due to the cloud response to dust absorption. Explanation of dust SDE mostly utilizes the mechanisms previously developed for other absorbing aerosols, such as black carbon. However, unlike black carbon, mineral dust has a significant influence on LW radiation because its sizes span more than three orders of magnitude in diameter (0.1 µm to 100 µm). To make matters worse, most climate models missed about three-quarters of dust particles larger than 5 µm diameter that are present in the atmosphere. Because these coarse dust particles absorb and scatter a substantial fraction of the dust LW radiation, their severe underestimation in models is problematic, as it suggests that models may be missing important mechanisms that are not part of the conventional explanation used to describe dust SDE since dust LW is substantially more than previously estimated and its significance on dust SDE cannot be ignored.

Therefore, the central goal of this proposed research is to investigate how largely overlooked pathways of interaction associated with dust semi-direct effect influence our understanding of the Earth's climate system. Specifically, we hypothesize that the absorption of LW radiation by mineral dust substantially changes the boundary-layer clouds, contributes to its dust SDE, and influences the overall radiative budget. To obtain a more accurate estimate of dust SDE, our proposed research seeks to accomplish the following scientific objectives: (1) Determine the relationship between warm clouds and dust absorption properties, influenced by changes in coarse dust load, dust-cloud vertical configurations, and surface types. (2) Identify the mechanisms by which warm clouds are semi-directly influenced by changes in dust LW radiation due to coarse dust aerosols and the variability in dust vertical distribution. (3) Quantify dust semi-direct effect that accounts for the overlooked dust absorption of LW radiation.

Our approach will leverage the ARM observations at two permanent sites (Eastern North Atlantic, ENA, and Southern Great Plains, SGP) that are ideally suited to study dust SDE over land and ocean environments. We propose to estimate the relationship between warm clouds and mineral dust by conducting composite analyses that partition the ARM observations into cases that characterize changes in dust absorption properties due to coarse dust load, dust-cloud vertical configurations (whether dust is above or below clouds), and dust vertical distributions (described by its geometric thickness and separating distance from clouds). A similar analysis will also be conducted over the broader North Atlantic Ocean, incorporating ground and satellite observations. These observational analyses will inform model simulations to identify mechanisms by which warm clouds are semi-directly influenced by dust LW radiation and obtain more accurate estimates of dust SDE. This will include using ARM-informed observations in a radiative transfer model and the single-column model (SCM) functionality of the DOE’s Energy Exascale Earth System Model (E3SMv2) to understand dust-cloud interactions at single locations over ENA and SGP and by following the dust-cloud evolution over North Atlantic Ocean. These experiments will also be for cases examining changes in radiative fluxes and cloud properties as a function of changes in dust absorption due to coarse dust load, dust-cloud vertical configurations, and dust vertical distribution. Finally, we propose to improve the representation of coarse dust in E3SMv2 by introducing a new "super-coarse" mode with diameters between 10µm and 40µm in the E3SMv2 aerosol module. Consequently, we will run regionally refined simulations that account for dust LW radiation and conduct sensitivity to coarse dust load and sizes in the estimates of dust SDE over the North Atlantic Ocean.
Award Recipient(s)
  • University of California, Merced (PI: Adebiyi, Adeyemi)