A multi-year data set on aerosol-cloud-precipitation-meteorology interactions for marine stratocumulus clouds

Airborne measurements of meteorological, aerosol, and stratocumulus cloud properties have been harmonized from six field campaigns during July-August months between 2005 and 2016 off the California coast. A consistent set of core instruments was deployed on the Center for Interdisciplinary Remotely-Piloted Aircraft Studies Twin Otter for 113 flight days, amounting to 514 flight hours. A unique aspect of the compiled data set is detailed measurements of aerosol microphysical properties (size distribution, composition, bioaerosol detection, hygroscopicity, optical), cloud water composition, and different sampling inlets to distinguish between clear air aerosol, interstitial in-cloud aerosol, and droplet residual particles in cloud. Measurements and data analysis follow documented methods for quality assurance. The data set is suitable for studies associated with aerosol-cloud-precipitation-meteorology-radiation interactions, especially owing to sharp aerosol perturbations from ship traffic and biomass burning. The data set can be used for model initialization and synergistic application with meteorological models and remote sensing data to improve understanding of the very interactions that comprise the largest uncertainty in the effect of anthropogenic emissions on radiative forcing. © The Author(s) 2018.

On the effect of dust particles on global cloud condensation nuclei and cloud droplet number

Athanasios Nenes

Aerosol-cloud interaction studies to date consider aerosol with a substantial fraction of soluble material as the sole source of cloud condensation nuclei (CCN). Emerging evidence suggests that mineral dust can act as good CCN through water adsorption onto the surface of particles. This study provides a first assessment of the contribution of insoluble dust to global CCN and cloud droplet number concentration (CDNC). Simulations are carried out with the NASA Global Modeling Initiative chemical transport model with an online aerosol simulation, considering emissions from fossil fuel, biomass burning, marine, and dust sources. CDNC is calculated online and explicitly considers the competition of soluble and insoluble CCN for water vapor. The predicted annual average contribution of insoluble mineral dust to CCN and CDNC in cloud-forming areas is up to 40 and 23.8%, respectively. Sensitivity tests suggest that uncertainties in dust size distribution and water adsorption parameters modulate the contribution of mineral dust to CDNC by 23 and 56%, respectively. Coating of dust by hygroscopic salts during the atmospheric aging causes a twofold enhancement of the dust contribution to CCN; the aged dust, however, can substantially deplete in-cloud supersaturation during the initial stages of cloud formation and can eventually reduce CDNC. Considering the hydrophilicity from adsorption and hygroscopicity from solute is required to comprehensively capture the dust-warm cloud interactions. The framework presented here addresses this need and can be easily integrated in atmospheric models. Copyright 2011 by the American Geophysical Union.

Blackwell Publishing Ltd2011

Aerosol-cloud drop concentration closure in warm cumulus

Athanasios Nenes

Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE). An "aerosol-cloud" closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm-3, drives large microphysical differences (250-2300 cm-3) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties (effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosol's effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study. Copyright 2004 by the American Geophysical Union.

2004

Impact of biomass burning on cloud properties in the Amazon Basin

Athanasios Nenes

We used a one-dimensional (1-D) cloud parcel model to assess the impact of biomass-burning aerosol on cloud properties in the Amazon Basin and to identify the physical and chemical properties of the aerosol that influence droplet growth. Cloud condensation nuclei (CCN) measurements were performed between 0.15% and 1.5% supersaturation at ground-based sites in the states of Amazonas and Rondônia, Brazil during several field campaigns in 1998 and 1999 as part of the Large-Scale Biosphere Atmosphere (LBA) Experiment in Amazonia. CCN concentrations measured during the wet season were low and resembled concentrations more typical of marine conditions than most continental sites. During the dry season, smoke aerosol from biomass burning dramatically increased CCN concentrations. The modification of cloud properties, such as cloud droplet effective radius and maximum supersaturation, is most sensitive at low CCN concentrations. Hence, we could expect larger interannual variation of cloud properties during the wet season that the dry season. We found that differences between CCN spectra from forested and deforested regions during the wet season are modest and result in modifications of cloud properties that are small compared to those between wet and dry seasons. Our study suggests that the differences in surface albedo, rather than cloud albedo, between forested and deforested regions may dominate the impact of deforestation on the hydrological cycle and convective activity during the wet season. During the dry season, on the other hand, cloud droplet concentrations may increase by up to 7 times, which leads to a model-predicted decrease in cloud effective radius by a factor of 2. This could imply a maximum indirect radiative forcing due to aerosol as high as ca. -27 W m-2 for a nonabsorbing cloud. Light-absorbing substances in smoke darken the Amazonian clouds and reduce the net radiative forcing, and a comparison of the Advanced Very High Resolution Radiometer (AVHRR) analysis and our modeling studies suggests that absorption of sunlight due to smoke aerosol may compensate for about half of the maximum aerosol effect. Sensitivity tests show that complete characterization of the aerosol is necessary when kinetic growth limitations become important. Subtle differences in the chemical and physical makeup are shown to be particularly influential in the activation and growth behavior of the aerosol. Knowledge of the CCN spectrum alone is not sufficient to fully capture the climatic influence of biomass burning.

2003

Impact of biomass burning on cloud properties in the Amazon Basin

Athanasios Nenes

2003

Aerosol-cloud drop concentration closure in warm cumulus

Athanasios Nenes

2004