Breakout Summary Report

Breakout Description

The Warm Boundary-Layer Processes Working Group focuses on understanding and model representation of processes controlling the structural and radiative properties of clouds, aerosols, and their interactions with the underlying surface in the lowest few kilometers of the atmosphere. Research areas include: 1) characterization of boundary-layer and cloud dynamics; 2) cloud and aerosol microphysics and their interactions; 3) factors influencing cloud formation; and 4) radiative processes that together influence the vertical transfers of energy, moisture, and atmospheric components.

Main Discussion

Warm Boundary-Layer Processes (WBLP) Work Group breakout hosted talks and discussions related to boundary-layer turbulence, thermodynamics structures, cloud and drizzling microphysical processes and their interaction with aerosols and the underlying surface properties.

The session started with a talk from the ARM infrastructure team to provide updates about data products relevant to WBLP, followed a mixture of lightning talks (5-min each) and long talks (10-min each). A total of 15 talks were grouped into three sub-sessions, focusing on 1) boundary-layer structure and interactions with land surface, 2) cloud dynamics and microphysics, and 3) aerosol-cloud interactions. The majority of speakers this year are early career scientists and 1/3 are female. Since most talks were well rehearsed, the scheduled time for discussions was sufficient for the group to engage speakers and facilitate follow-up offline discussions.

Key Findings

Key points for each sub-session are summarized below:

Boundary-layer structure and interactions with land surface:

• The coupling between clouds and planetary boundary layer (PBL) affects the stability and diurnal cycle of PBL. Neglecting cloud coupling could lead to an underestimation of aerosol-cloud interactions. [Su et al.]


• The mechanism and sources of the water vapor increase observed during the afternoon-to-evening transition were investigated. The comparison of mixing diagrams between observations and single-column simulations nicely revealed additional information on the source of model discrepancy. [He et al.]


• LES single columns have been used to inform observational practices in studying the heat and moisture budgets of the convective boundary layer. Several recommendations have been made, including normalizing entrainment fluxes, extrapolating surface fluxes, and using at least 10-min averaging window. [Tessa et al.]


• Automated Large Eddy Simulations over land with coupled Land Surface Model are introduced and evaluated using ARM SGP observations and LASSO simulations. This model will be installed on the ARM Cumulus supercomputer to perform high-resolution simulations. [Raghunathan et al.]


• A moisture-vertical velocity quadrant analysis technique has been used to evaluate the shallow convection parameterization assumptions such as the double Gaussian PDF used in CLUBB and SHOC. Results show that a double Gaussian PDF can represent dry downdrafts well but not the moist downdrafts associated with overshooting. [Xiao et al.]


• Using 10,000 individual life cycles of shallow cumulus clouds from LES for cases during the HI-SCALE field campaign, the time tendency of cloud size is derived by the Lotka-Volterra equation as a predator-prey system for cloud growth. Such an approach is expected to be included into the subgrid parameterization to represent the cloud life cycle effect more accurately. [Chen et al.]

Cloud dynamics and microphysics:

• Drivers of cloud condensation nuclei (CCN) at the ENA site are studied. Local aerosol emissions significantly impact the observed aerosol properties and tend to have smaller particle sizes. Positive CCN advection was balanced by precipitation scavenging on monthly timescales, and sub-monthly changes in CCN are governed by cloud processes both at the site and upstream of it. [Ghate et al.]


• A 7-year data set (2016-2022) of radiative fluxes at 1-minute time resolution has been built for the ENA site, using RRTM and data sets of water vapor, cloud and drizzle water path, below-cloud drizzle diameter, liquid water content, and rain rate. The evaluation has been performed and uncertainty is characterized. This data set allows us to characterize radiation budget and cloud radiative effects at both surface and top of the atmosphere. [Cadeddu et al.]


• Salt-water plumes were injected near the ocean surface in PINACLES runs to assess the efficacy of different plume injection strategies. It is found that these active plumes may loft less quickly due to negative buoyancy from droplet evaporation, and the plume area at 600 m increases with injection rate. These active plumes lead to larger droplet concentration (increasing with injection rate) and smaller cloud droplets but have little/no changes in sub-cloud layer turbulence and cloud water mass. [Dhandapani et al.]

Aerosol-cloud interactions:

• Aitken activation and turbulent and convective fluxes within the boundary layer restore accumulation-mode aerosols against losses to precipitation. Large-scale ascent moistens and brightens clouds while Aitken buffering acts to sustain brighter, more homogeneous clouds. [Blossey et al.]


• Based on observations and simulations, it is found that mesoscale cellular convection (MCC) cell size significantly regulates aerosol-induced cloud albedo via its effect on cloud water adjustment. Cloud scale and inhomogeneity are non-monotonically related, and the relationship is modulated by cloud fraction. [Zhou et al.]


• A novel causal framework has been applied to study aerosol-cloud interactions using ARM observations during the ACE-ENA campaign. It is found that in non-drizzling clouds, vertical velocity at cloud base is mediating the influence of cloud number concentration and of cloud effective radius on future liquid water path. This mediation is almost absent in the drizzling cloud. In drizzling clouds, cloud number concentration is strongly related to effective radius such that liquid water content remains constant over time, indicating that the cloud is in a dynamical steady state. [DeCaria et al.]


• Aerosol injection time does not affect stratocumulus-to-cumulus transition notably. To cool the earth climate, an efficient way is to inject more aerosols to a clean MBL at the initial stage. [Zhang et al.]

Issues

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Decisions

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