A challenging aspect of conducting airborne in situ observations of the atmosphere is how to optimize flight plans for specific objectives and constraints associated with weather and flight ...restrictions. For aerosol-cloud interaction research, two recent campaigns utilized a “stairstepping” approach whereby an aircraft conducts level legs at various altitudes while moving forward with each subsequent leg: the 2019 MONterey Aerosol Research Campaign (MONARC) over the northeast Pacific and the 2020–2022 Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) over the northwest Atlantic. We examine the homogeneity of several atmospheric variables both vertically and horizontally in the marine boundary layer with a focus on the sub-cloud environment. In well-mixed boundary layers, there was generally good horizontal and vertical homogeneity in potential temperature, winds, water vapor mixing ratio, various trace gases, and many aerosol variables. Selected aerosol variables exhibited the most variability owing to sensitivity to humidity and near-cloud conditions (supermicrometer aerosol concentrations), coastal pollution gradients (e.g., organic aerosol mass), and small spatial scale phenomena such as new particle formation (aerosol number concentration for particles with diameter >3 nm). Illustrative cases are described when stairstepping can pose issues requiring extra caution for data analysis: (i) poor vertical mixing and layers decoupled from those below; (ii) multiple cloud layers; (iii) fluctuating cloud base/top and boundary layer top heights; and (iv) horizontal variability across specific features leading to sharp gradients such as right near coastlines and over the Gulf Stream with strong sea surface temperature changes. Results from this study provide a guide both for future studies aiming to examine these mission datasets and for designing new airborne campaigns.
Cloud processing is known to generate aerosol species such as sulfate and secondary organic aerosol, yet there is a scarcity of airborne data to
examine this issue. The NASA Aerosol Cloud meTeorology ...Interactions oVer the western ATlantic Experiment (ACTIVATE) was designed to build an
unprecedented dataset relevant to aerosol–cloud interactions with two coordinated aircraft over the northwestern Atlantic, with aerosol mass spectrometer data used from four deployments between 2020–2021 to contrast aerosol composition below, in (using a counterflow virtual impactor)
and above boundary layer clouds. Consistent features in all time periods of the deployments (January–March, May–June, August–September) include
the mass fraction of organics and relative amount of oxygenated organics (m/z 44) relative to total organics (f44) increasing in droplet
residuals relative to below and above cloud. Detailed analysis comparing data below and in cloud suggests a possible role for in-cloud aqueous
processing in explaining such results; an intriguing aspect though requiring more attention is that only approximately a quarter of the cloud
cases (29 of 110) showed higher organic mass fractions either below or above cloud. Of those 29 cases, the majority (25) showed higher organic mass
fraction below cloud base where the cloud processing signature is presumably more evident as compared to above cloud. These results are consistent
with the few past studies analyzing droplet residuals pointing to higher organic enrichment than in adjacent cloud-free areas. The data findings are
important as other datasets (e.g., reanalysis) suggest that sulfate is both more abundant than organics (in contrast to this work) and more closely
related to drop number concentrations in the winter when aerosol–cloud interactions are strongest. Here we show that organics are more abundant than
sulfate in the droplet residuals and that aerosol interaction with clouds potentially decreases particle hygroscopicity due to the increase in
organic:sulfate ratio for droplet residuals relative to surrounding cloud-free air. These results are important in light of the growing importance
of organics over the northwestern Atlantic in recent decades relative to sulfate owing to the success of regulatory activity over the eastern United States to cut sulfur dioxide emissions.
Process modeling of Aerosol‐cloud interaction (ACI) is essential to bridging gaps between observational analysis and climate modeling of aerosol effects in the Earth system and eventually reducing ...climate projection uncertainties. In this study, we examine ACI in summertime precipitating shallow cumuli observed during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE). Aerosols and precipitating shallow cumuli were extensively observed with in‐situ and remote‐sensing instruments during two research flight cases on 02 June and 07 June, respectively, during the ACTIVATE summer 2021 deployment phase. We perform observational analysis and large‐eddy simulation (LES) of aerosol effect on precipitating cumulus in these two cases. Given the measured aerosol size distributions and meteorological conditions, LES is able to reproduce the observed cloud properties by aircraft such as liquid water content (LWC), cloud droplet number concentration (Nc) and effective radius reff. However, it produces smaller liquid water path (LWP) and larger Nc compared to the satellite retrievals. Both 02 and 07 June cases are over warm waters of the Gulf Stream and have a cloud top height over 3 km, but the 07 June case is more polluted and has larger LWC. We find that the Na‐induced LWP adjustment is dominated by precipitation feedback for the 2 June precipitating case and there is no clear entrainment feedback in both cases. An increase of cloud fraction due to a decrease of aerosol number concentration is also shown in the simulations for the 02 June case.
Plain Language Summary
Aerosol‐cloud‐interaction (ACI) regulates the energy budget of the Earth and poses the largest uncertainty in climate projection. Particularly, ACI of low clouds is poorly understood and causes the spread of Earth System Models (ESMs) in predicting cloud and climate responses to aerosol changes. Process studies have shown a nonlinear cloud water amount and cloud fraction adjustments due to aerosol changes via precipitation and cloud top entrainment, which are not often captured correctly in ESMs. This study explores the physical mechanisms of ACI in marine low clouds with a focus on precipitating low clouds using a cloud process model and unprecedented field campaign measurements of meteorology states, cloud properties, and aerosols collected during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment. We show that the aerosol‐induced cloud water amount adjustment is dominated by changes in precipitation and there is no clear entrainment feedback in both cases. Our findings can help improve the representation of ACI within precipitating marine low clouds in ESMs.
Key Points
Aerosol‐cloud interactions in precipitating shallow cumuli are investigated using large‐eddy simulations (LES) and observations
LES show that aerosol‐induced cumulus cloud water adjustment is dominated by precipitation with no clear entrainment feedback
An increase in cloud fraction in response to aerosol number concentration decrease is shown in the precipitating cumuli
