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1. Environmental Effects on Aerosol-Cloud Interaction in non-precipitating MBL clouds over the Eastern North Atlantic

   Zheng, X. ( January 2021 , ACP)

   null (Ed.)

   Over the eastern north Atlantic (ENA) ocean, a total of 21 non-drizzling single-layer marine boundary layer (MBL) stratus and stratocumulus cloud caseperiods are selected in order to investigate the impacts of the environmental variables on the aerosol-cloud interaction (ACI_r) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during the period 2016 – 2018. The ACI_r represents the relative change of cloud-droplet effective radius r_e with respect to the relative change of cloud condensation nuclei (CCN) number concentration (N_CCN) in the water vapor stratified environment. The ACI_r values vary from -0.004 to 0.207 with increasing precipitable water vapor (PWV) conditions, indicating that r_e is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher N_c and PWV. The environmental effects on ACI_r are examined by stratifying the data into different lower tropospheric stability (LTS) and vertical component of turbulence kinetic energy (TKE_w) regimes. The higher LTS normally associates with a more adiabatic cloud layer and a lower boundary layer and thus results in higher CCN to cloud droplet conversion and ACI_r. The ACI_r values under a range of PWV double from low TKE_w to high TKE_w regime, indicating a strong impact of turbulence on the ACI_r. The stronger boundary layer turbulence represented by higher TKE_w strengthens the connection and interaction between cloud microphysical properties and the underneath CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size distribution spectra, and consequently results in an enlargement of r_e. The enhanced N_c conversion and condensational growth induced by more intrusions of CCN effectively decrease r_e, which jointly presents as the increased ACI_r. The TKE_w median value of 0.08 m^2 s^(-2) suggests a feasible way in distinguishing the turbulence-enhanced aerosol-cloud interaction in non-drizzling MBL clouds. 

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2. Investigation of aerosol–cloud interactions under different absorptive aerosol regimes using Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) ground-based measurements

   https://doi.org/10.5194/acp-20-3483-2020  

   Zheng, Xiaojian ; Xi, Baike ; Dong, Xiquan ; Logan, Timothy ; Wang, Yuan ; Wu, Peng ( January 2020 , Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The aerosol indirect effect on cloud microphysical and radiative propertiesis one of the largest uncertainties in climate simulations. In order toinvestigate the aerosol–cloud interactions, a total of 16 low-level stratuscloud cases under daytime coupled boundary-layer conditions are selectedover the southern Great Plains (SGP) region of the United States. Thephysicochemical properties of aerosols and their impacts on cloudmicrophysical properties are examined using data collected from theDepartment of Energy Atmospheric Radiation Measurement (ARM) facility at the SGP site. The aerosol–cloud interaction index (ACIr) is used to quantify the aerosol impacts with respect to cloud-droplet effective radius. The mean value of ACIr calculated from all selected samples is0.145±0.05 and ranges from 0.09 to 0.24 at a range of cloudliquid water paths (LWPs; LWP=20–300 g m−2). The magnitude of ACIr decreases with an increasing LWP, which suggests a diminished cloud microphysical response to aerosol loading, presumably due to enhanced condensational growth processes and enlarged particle sizes. The impact of aerosols with different light-absorbing abilities on the sensitivity of cloud microphysical responses is also investigated. In the presence of weak light-absorbing aerosols, the low-level clouds feature a higher number concentration of cloud condensation nuclei (NCCN) and smaller effective radii (re), while the opposite is true for strong light-absorbing aerosols. Furthermore, the mean activation ratio of aerosols to CCN (NCCN∕Na) for weakly (strongly) absorbing aerosols is 0.54 (0.45), owing to the aerosol microphysical effects, particularly the different aerosol compositions inferred by their absorptive properties. In terms of the sensitivity of cloud-droplet number concentration (Nd) to NCCN, the fraction of CCN that converted to cloud droplets (Nd∕NCCN) for the weakly (strongly) absorptive regime is 0.69 (0.54). The measured ACIr values in the weakly absorptive regime arerelatively higher, indicating that clouds have greater microphysicalresponses to aerosols, owing to the favorable thermodynamic condition. Thereduced ACIr values in the strongly absorptive regime are due to the cloud-layer heating effect induced by strong light-absorbing aerosols. Consequently, we expect larger shortwave radiative cooling effects from clouds in the weakly absorptive regime than those in the strongly absorptive regime. 

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3. What Are the Similarities and Differences in Marine Boundary Layer Cloud and Drizzle Microphysical Properties During the ACE‐ENA and MARCUS Field Campaigns?

   https://doi.org/10.1029/2022JD037109  

   Marcovecchio, Alexa. R. ; Xi, Baike ; Zheng, Xiaojian ; Wu, Peng ; Dong, Xiquan ; Behrangi, Ali ( September 2023 , Journal of Geophysical Research: Atmospheres)

   This study compares macrophysical and microphysical properties of single‐layered, liquid‐dominant MBL clouds from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) (above 60°S) and the ARM East North Atlantic (ENA) site during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE‐ENA) field campaign. A total of 1,136 (16.5% of clouds) and 6,034 5‐min cloud samples are selected from MARCUS and ARM ENA in this study. MARCUS clouds have higher cloud‐top heights, thicker cloud layers, larger liquid water path, and colder cloud temperatures than ENA. Thinner, warmer MBL clouds at ENA can contain higher layer‐mean liquid water content due to higher cloud and ocean surface temperatures along with greater precipitable water vapor (PWV). MARCUS has a higher drizzle frequency rate (71.8%) than ENA (45.1%). Retrieved cloud and drizzle microphysical properties from each field campaign show key differences. MARCUS clouds feature smaller cloud droplets, whereas ENA clouds have larger cloud droplets, especially at the upper region of the cloud. From cloud top to cloud base, drizzle drop sizes increase while number concentrations decrease. Drizzle drop radius and number concentration decrease from cloud base to drizzle base due to net evaporation, and MARCUS' lower specific humidity leads to a higher drizzle base than ENA. The broader surface pressure and lower tropospheric stability (LTS) distributions during MARCUS have demonstrated that there are different synoptic patterns for selected cases during MARCUS with less PWV, while ENA is dominated by high pressure systems with nearly doubled PWV.

    

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4. Cloud Processes and the Transport of Biological Emissions Regulate Southern Ocean Particle and Cloud Condensation Nuclei Concentrations

   Sanchez, K. J. ; Roberts, G. C. ; Saliba, G. ; Russell, L. M. ; Twohy, C. ; Reeves, M. ; Humphries, R. S. ; Keywood, M. D. ; Ward, J. P. ; McRobert, I. M. ( January 2021 , Atmospheric chemistry and physics)

   null (Ed.)

   Long-range transport of biogenic emissions from the coast of Antarctica, precipitation scavenging, and cloud processing are the main processes that influence the observed variability in Southern Ocean (SO) marine boundary layer (MBL) condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations during the austral summer. Airborne particle measurements on the HIAPER GV from north-south transects between Hobart, Tasmania and 62°S during the Southern Ocean Clouds, Radiation Aerosol Transport Experimental Study (SOCRATES) were separated into four regimes comprising combinations of high and low concentrations of CCN and CN. In 5-day HYSPLIT back trajectories, air parcels with elevated CCN concentrations were almost always shown to have crossed the Antarctic coast, a location with elevated phytoplankton emissions relative to the rest of the SO in the region south of Australia. The presence of high CCN concentrations was also consistent with high cloud fractions over their trajectory, suggesting there was substantial growth of biogenically formed particles through cloud processing. Cases with low cloud fraction, due to the presence of cumulus clouds, had high CN concentrations, consistent with previously reported new particle formation in cumulus outflow regions. Measurements associated with elevated precipitation during the previous 1.5-days of their trajectory had low CCN concentrations indicating CCN were effectively scavenged by precipitation. A coarse-mode fitting algorithm was used to determine the primary marine aerosol (PMA) contribution which accounted for < 20% of CCN (at 0.3% supersaturation) and cloud droplet number concentrations. Vertical profiles of CN and large particle concentrations (Dp > 0.07µm) indicated that particle formation occurs more frequently above the MBL; however, the growth of recently formed particles typically occurs in the MBL, consistent with cloud processing and the condensation of volatile compound oxidation products. 

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5. Cloud Microphysical Effects of Aerosol Particle Sources and Marine Boundary Layer Processes

   Sanchez, Kevin J ( January 2017 , UC San Diego Electronic Theses and Dissertations)

   Marine boundary layer (MBL) clouds are an important, though uncertain, part of Earth’s radiative budget. Previous studies have shown sources of aerosol particles in the remote MBL consist of primary sea spray, the oxidation of organic and inorganic vapors derived from the ocean, entrainment from the free troposphere, and anthropogenic pollution. The potential for these particles to become cloud condensation nuclei (CCN) varies largely dependent on their hygroscopic properties. Furthermore, when clouds form, physical processes can alter the optical properties of the cloud. This dissertation aims to identify variations in aerosol sources that affect MBL CCN concentrations and physical processes throughout the cloud lifetime that influence cloud optical properties. Ambient measurements of marine particles and clouds were made throughout two campaigns in the north Pacific and four campaigns in the north Atlantic. Both clean marine and polluted clouds were sampled. In addition, dry MBL particles were measured to identify their chemical composition and size distribution, which is necessary to identify their potential to be CCN active. The organic hygroscopicity influenced CCN concentrations and cloud optical properties significantly for particles that were mostly organic, such as ship stack and generated smoke particles. For a typical range of organic hygroscopicity the amount of reflected solar radiation varied by 2-7% for polluted conditions and less than 1% for clean conditions. Simulated droplet spectral width was shown to be more representative of observations when using a weighted distribution of cloud base heights and updraft velocities, and increased the cloud reflectivity up to 2%. Cloud top entrainment and decoupling of the MBL were found to account for a decrease in cloud radiative forcing. Cloud top entrainment was corrected for homogeneous entrainment and accounted for a decrease in radiative forcing of up to 50 Wm-2. Clustering of individual marine aerosol particles resulted in the identification of particle types derived from dimethyl-sulfide (DMS) oxidation. Two particle types were identified to come from DMS oxidation products and accounted for approximately 25% and 65% of CCN at 0.1% supersaturation during the winter and summer, respectively. One of the particle types was found to be entrained from the free troposphere. 

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