Roy K. Woods

RACORO EXTENDED-TERM AIRCRAFT OBSERVATIONS OF BOUNDARY LAYER CLOUDS

A first-of-a-kind, extended-term cloud aircraft campaign was conducted to obtain an in situ statistical characterization of continental boundary layer clouds needed to investigate cloud processes and refine retrieval algorithms. Coordinated by the Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF), the Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign operated over the ARM Southern Great Plains (SGP) site from 22 January to 30 June 2009, collecting 260 h of data during 59 research flights. A comprehensive payload aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft measured cloud microphysics, solar and thermal radiation, physical aerosol properties, and atmospheric state parameters. Proximity to the SGPs extensive complement of surface measurements provides ancillary data that support modeling studies and facilitates evaluation of a variety of surface retrieval algorithms. The five-...

Differentiating natural and anthropogenic cloud condensation nuclei in the California coastal zone

Aerosol samples were collected and cloud condensation nuclei (CCN) concentrations at five supersaturations were measured along and off the central California coast within the cloud-topped, marine boundary layer from aircraft flights during August 2007. Receptor modelling has been applied to estimate the natural versus anthropogenic source contribution of cloud condensation nuclei in this region, a region of climatically important marine stratocumulus. The results suggest that anthropogenic CCN accounted for about 50% of the CCN active at 0.3% supersaturation in this region during the measurement period.

Surface and Airborne Measurements of Organosulfur and Methanesulfonate Over the Western United States and Coastal Areas.

This study reports on ambient measurements of organosulfur (OS) and methanesulfonate (MSA) over the western United States and coastal areas. Particulate OS levels are highest in summertime, and generally increase as a function of sulfate (a precursor) and sodium (a marine tracer) with peak levels at coastal sites. The ratio of OS to total sulfur (TS) is also highest at coastal sites, with increasing values as a function of Normalized Difference Vegetation Index (NDVI) and the ratio of organic carbon to elemental carbon. Correlative analysis points to significant relationships between OS and biogenic emissions from marine and continental sources, factors that coincide with secondary production, and vanadium due to a suspected catalytic role. A major OS species, methanesulfonate (MSA), was examined with intensive field measurements and the resulting data support the case for vanadiums catalytic influence. Mass size distributions reveal a dominant MSA peak between aerodynamic diameters of 0.32-0.56 μm at a desert and coastal site with nearly all MSA mass (≥ 84%) in sub-micrometer sizes; MSA:non-sea salt sulfate ratios vary widely as a function of particle size and proximity to the ocean. Airborne data indicate that relative to the marine boundary layer, particulate MSA levels are enhanced in urban and agricultural areas, and also the free troposphere when impacted by biomass burning. Some combination of fires and marine-derived emissions leads to higher MSA levels than either source alone. Finally, MSA differences in cloud water and out-of-cloud aerosol are discussed.

Stratocumulus Cloud Clearings and Notable Thermodynamic and Aerosol Contrasts across the Clear-Cloudy Interface

Data from three research flights, conducted over water near the California coast, are used to investigate the boundary between stratocumulus cloud decks and clearings of different sizes. Large clearings exhibit a diurnal cycle with growth during the day and contraction overnight and a multiday life cycle that can include oscillations between growth and decay, whereas a small coastal clearing was observed to be locally confined with a subdiurnal lifetime. Subcloud aerosol characteristics are similar on both sides of the clear–cloudy boundary in the three cases, while meteorological properties exhibit subtle, yet important, gradients, implying that dynamics, and not microphysics, is the primary driver for the clearing characteristics. Transects, made at multiple levels across the cloud boundary during one flight, highlight the importance of microscale (~1 km) structure in thermodynamic properties near the cloud edge, suggesting that dynamic forcing at length scales comparable to the convective eddy scale may be influential to the larger-scale characteristics of the clearing. These results have implications for modeling and observational studies of marine boundary layer clouds, especially in relation to aerosol–cloud interactions and scales of variability responsible for the evolution of stratocumulus clearings.

On the presence of giant particles downwind of ships in the marine boundary layer

This study examines large oceangoing ships as a source of giant cloud condensation nuclei (D_p > 2 µm) due to wake and stack emissions off the California coast. Observed particle number concentrations behind 10 ships exceeded those in “control” areas, exhibiting number concentration enhancement ratios (ERs) for minimum threshold diameters of ~2, ~10, and ~20 µm as high as 2.7, 5.5, and 7.5, respectively. ER decreases with increasing downwind distance and altitude. ER becomes better correlated with ship size variables (gross tonnage, length, and beam) as the minimum size threshold increases from 2 to 20 µm, whereas ship speed has a less distinct relationship with ER. One case study of a container ship shows that there are higher concentrations of sea-salt tracer species behind it relative to adjacent control areas. These results have implications for cloud properties and precipitation in marine boundary layers exposed to ship traffic.

Relationships between giant sea salt particles and clouds inferred from aircraft physicochemical data

NASA [NNX14AM02G]; Office of Naval Research [N00014-10-1-0811, N00014-11-1-0783, N00014-10-1-0200, N00014-04-1-0118, N00014-16-1-2567]

Contrasting cloud composition between coupled and decoupled marine boundary layer clouds

Marine stratocumulus clouds often become decoupled from the vertical layer immediately above the ocean surface. This study contrasts cloud chemical composition between coupled and decoupled marine stratocumulus clouds for dissolved nonwater substances. Cloud water and droplet residual particle composition were measured in clouds off the California coast during three airborne experiments in July–August of separate years (Eastern Pacific Emitted Aerosol Cloud Experiment 2011, Nucleation in California Experiment 2013, and Biological and Oceanic Atmospheric Study 2015). Decoupled clouds exhibited significantly lower air-equivalent mass concentrations in both cloud water and droplet residual particles, consistent with reduced cloud droplet number concentration and subcloud aerosol (D_p > 100 nm) number concentration, owing to detachment from surface sources. Nonrefractory submicrometer aerosol measurements show that coupled clouds exhibit higher sulfate mass fractions in droplet residual particles, owing to more abundant precursor emissions from the ocean and ships. Consequently, decoupled clouds exhibited higher mass fractions of organics, nitrate, and ammonium in droplet residual particles, owing to effects of long-range transport from more distant sources. Sodium and chloride dominated in terms of air-equivalent concentration in cloud water for coupled clouds, and their mass fractions and concentrations exceeded those in decoupled clouds. Conversely, with the exception of sea-salt constituents (e.g., Cl, Na, Mg, and K), cloud water mass fractions of all species examined were higher in decoupled clouds relative to coupled clouds. Satellite and Navy Aerosol Analysis and Prediction System-based reanalysis data are compared with each other, and the airborne data to conclude that limitations in resolving boundary layer processes in a global model prevent it from accurately quantifying observed differences between coupled and decoupled cloud composition.

CASPER: Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research

CapsuleCASPER objective is to improve our capability to characterize the propagation of radio frequency (RF) signals through the marine atmosphere with coordinated efforts in data collection, data analyses, and modeling of the air-sea interaction processes, refractive environment, and RF propagation.

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.