Patrick Y. Chuang

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Analysis of the Influence of Film-Forming Compounds on Droplet Growth: Implications for Cloud Microphysical Processes and Climate

Abstract Decades of cloud microphysical research have not provided conclusive understanding of the physical processes responsible for droplet spectral broadening. Numerous mechanisms have been proposed—for example, entrainment mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing of CCN, and radiative cooling—all of which are likely candidates under select conditions. In this paper it is suggested that variability in the composition of CCN, and in particular, the existence of condensation inhibiting compounds, is another possible candidate. The inferred potential abundance of these amphiphilic film-forming compounds (FFCs) suggests that their effect may be important. Using a cloud parcel model with a simplified treatment of the effect of FFCs, it is shown that modest concentrations of FFCs (on the order of 5% of the total aerosol mass) can have a marked effect on drop growth and can cause significant increases in spectral dispersions. Moreover, it is shown that FFCs may, in s...

Kinetic limitations on droplet formation in clouds

The ‘indirect’ radiative cooling of climate due to the role of anthropogenic aerosols in cloud droplet formation processes (which affect cloud albedo) is potentially large, up to −1.5 W m−2 (ref. 1). It is important to be able to determine the number concentration of cloud droplets to within a few per cent, as radiative forcing as a result of clouds is very sensitive to changes in this quantity, but empirical approaches are problematic. The initial growth of a subset of particles known as cloud condensation nuclei and their subsequent ‘activation’ to form droplets are generally calculated with the assumption that cloud droplet activation occurs as an equilibrium process described by classical Köhler theory,. Here we show that this assumption can be invalid under certain realistic conditions. We conclude that the poor empirical correlation between cloud droplet and cloud condensation nuclei concentrations is partly a result of kinetically limited growth before droplet activation occurs. Ignoring these considerations in calculations of total cloud radiative forcing based on cloud condensation nuclei concentrations could lead to errors that are of the same order of magnitude as the total anthropogenic greenhouse-gas radiative forcing.

CCN measurements during ACE-2 and their relationship to cloud microphysical properties

Measurements of cloud condensation nuclei (CCN) concentration at 0.1% supersaturation were made onboard the CIRPAS Pelican over the northeast Atlantic during June and July, 1997, in the vicinity of Tenerife, Spain, as part of the second Aerosol Characterization Experiment (ACE-2). The average CCN concentration (N ccn ) in the marine boundary layer for clean air masses was 27±8 and 42±14 cm’3 for cloudy and clear conditions, respectively, consistent with measurements made near the British Isles and close to Tasmania, Australia, during ACE-1 for similar conditions. A local CCN closure experiment was conducted. Measured N ccn is compared with predictions based on aerosol number size distributions and size-resolved chemical composition profiles determined from measurements and the literature. A sublinear relationship between measured and predicted N ccn ,N ccn ~N0.51 , was found. This result is consistent with some previous studies, but others have obtained results much closer to the expected 1 : 1 relationship between measured and predicted N ccn . A large variability between measured and predicted N ccn was also observed, leading to the conclusion that, for 95% of the data, the predictions agree with measurements to within a factor of 11. Relationships between belowcloud N ccn and aerosol accumulation mode concentration, and in-cloud cloud droplet number

Aerosol-cloud relationships in continental shallow cumulus

Aerosol-cloud relationships are derived from 14 warm continental cumuli cases sampled during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Cloud droplet number concentration is clearly proportional to the subcloud accumulation mode aerosol number concentration. An inverse correlation between cloud top effective radius and subcloud aerosol number concentration is observed when cloud depth variations are accounted for. There are no discernable aerosol effects on cloud droplet spectral dispersion; the averaged spectral relative dispersion is 0.30 ± 0.04. Aerosol-cloud relationships are also identified from comparison of two isolated cloud cases that occurred under different degrees of anthropogenic influence. Cloud liquid water content, cloud droplet number concentration, and cloud top effective radius exhibit subadiabaticity resulting from entrainment mixing processes. The degree of LWC subadiabaticity is found to increase with cloud depth. Impacts of subadiabaticity on cloud optical properties are assessed. It is estimated that owing to entrainment mixing, cloud LWP, effective radius, and cloud albedo are decreased by 50–85%, 5–35%, and 2–26%, respectively, relative to adiabatic values of a plane-parallel cloud. The impact of subadiabaticity on cloud albedo is largest for shallow clouds. Results suggest that the effect of entrainment mixing must be accounted for when evaluating the aerosol indirect effect.

Airborne Phase Doppler Interferometry for Cloud Microphysical Measurements

Conducting accurate cloud microphysical measurements from airborne platforms poses a number of challenges. The technique of phase Doppler interferometry (PDI) confers numerous advantages relative to traditional light-scattering techniques for measurement of the cloud drop size distribution, and, in addition, yields drop velocity information. Here, we describe PDI for the purposes of aiding atmospheric scientists in understanding the technique fundamentals, advantages, and limitations in measuring cloud microphysical properties. The performance of the Artium Flight PDI, an instrument specifically designed for airborne cloud measurements, is studied. Drop size distributions, liquid water content, and velocity distributions are compared with those measured by other airborne instruments.

Coupling between Land Ecosystems and the Atmospheric Hydrologic Cycle through Biogenic Aerosol Pathways

AUTHOR AFFILIATIONS: BARTH, SUN, WIEDINMYER, KARL, KIM, LEVIS, MAHOWALD, MOORE, NANDI, NEMITZ, POTOSNAK, SMITH, AND STROUD—National Center for Atmospheric Research, Boulder, Colorado; MCFADDEN—University of Minnesota, Saint Paul, Minnesota; CHUANG—University of California, Santa Cruz, Santa Cruz, California; COLLINS—Texas A&M University, College Station, Texas; GRIFFIN—University of New Hampshire, Durham, New Hampshire; HANNIGAN—University of Colorado, Boulder, Colorado; LASHER-TRAPP—Purdue University, West Lafayette, Indiana; LITVAK—University of Texas, Austin, Texas; NENES—Georgia Institute of Technology, Atlanta, Georgia; RAYMOND—Bucknell University, Lewisburg, Pennsylvania; STILL—University of California, Santa Barbara, Santa Barbara, California CORRESPONDING AUTHOR: Dr. Mary Barth, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307 E-mail: barthm@ucar.edu

Worldwide data sets constrain the water vapor uptake coefficient in cloud formation

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought.

Effect of Aerosol on Cloud–Environment Interactions in Trade Cumulus

AbstractThis study examines the role of aerosol in mediating interactions between a warm trade cumulus cloud system and the environment that spawns it. Numerical simulations of the observed and well-studied Rain in Cumulus over the Ocean (RICO) field experiment are performed. The results draw on simulations of 34-h duration so as to avoid conclusions based on transients. Simulations show that, on average, aerosol-perturbed clouds are initially deeper and more vigorous but that after about 14 h there is a reversal in this trend, and unperturbed clouds deepen relative to the perturbed clouds. Differences in cloud depth are about 100 m, and differences in vertical velocity variance are about 30%. After about 20 h, most cloud fields are statistically similar with the exception of rain rate and optical depth, which are lower and higher, respectively, in the high-aerosol conditions. By sampling the model output at various points in the cloud system evolution, the mechanisms responsible for the initial differenc...

In Situ, Airborne Instrumentation: Addressing and Solving Measurement Problems in Ice Clouds

A meeting of 31 international experts on in situ measurements from aircraft was held to identify unresolved questions concerning ice formation and evolution in ice clouds, assess the current state of instrumentation that can address these problems, introduce emerging technology that may overcome current measurement issues and recommend future courses of action that can improve our understanding of ice cloud microphysical processes and their impact on the environment. The meeting proceedings and outcome has been described in detail in a manuscript submitted to the Bulletin of the American Meteorological Society (BAMS) on March 24, 2011. This paper is currently under review. The remainder of this summary, in the following pages, is the text of the BAMS article. A technical note that will be published by the National Center for Atmospheric Research is currently underway and is expected to be published before the end of the year.