James G. Hudson

Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-20±4 W m^(−2)) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

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 ...

Intercomparison Study of the Size-Dependent Counting Efficiency of 26 Condensation Particle Counters

ABSTRACT Particle detection efficiency curves for 26 condensation particle counters were determined during a calibration workshop in preparation for the Aerosol Characterization Experiment 1 (ACE1). Three different types of commercially available particle counters, the ultrafine condensation particle counter (TSI-3025) and the condensation particle counters (TSI-3010 and TSI-3760 or TS1-7610) were investigated at default temperature and flow settings as well as for other flow rates and temperature differences between the saturator and the condenser. Furthermore, the pulse-height-analysis ultrafine condensation particle counter and a TSI-3010 modified to achieve a higher temperature difference were calibrated. In this study, the large number of particle counters investigated provided the opportunity to obtain a more statistically significant picture of the performance of different particle counters for different operating conditions.

An Instantaneous CCN Spectrometer

Abstract A thermal gradient diffusion cloud chamber with a supersaturation field that increases along the path of the flow of sample is used as a cloud condensation nucleus (CCN) spectrometer. The CCN spectrum is derived from the final droplet size distribution which has been determined to be related to the nucleus critical super-saturation based on routine calibration procedures using laboratory-produced monodisperse salt particles. The instrument is compared against a previously reported CCN counter on natural and laboratory-produced aerosol and against an absolute CN counter on the monodisperse aerosol. Preliminary results of atmospheric measurements are given. This device produces a CCN spectrum over the entire useful range of interest for cloud physics (0.01% to 1% supersaturation). More than 40 channels of resolution over this range are simultaneously provided. Data can be collected on a continuous basis for integration times as short as 2 seconds. The instrument has obtained data from four differen...

Characterization of Combustion Aerosols for Haze and Cloud Formation

Aerosols resulting from the combustion of acetylene, wood, and JP-4 aviation fuels have been characterized in both the laboratory and the larger field scales by activity as cloud condensation nuclei (CCN), the total particle or condensation nuclei (CN) count, ion chromatography (IC) on filter samples, and morphology by scanning electron microscopy. The CCN/CN ratio for a given aerosol sample is a quantitative indicator of the ability of a combustion aerosol to become involved in atmospheric removal by nucleation scavenging. On both the laboratory and the field scales, this ratio was in the range 0.2 to 1.0 for the wood combustion aerosol, 0.2 to 0.5 for the acetylene case, and only 0.01 to 0.03 for JP-4. The CCN/CN ratios are identical for both the field and laboratory studies, implying that laboratory studies of CCN activities can be justifiably extrapolated to field studies. Aging and size-classified nucleation studies are also reported. 20 refs., 9 figs., 3 tabs.

Maritime-Continental Drizzle Contrasts in Small Cumuli

Abstract Continuous aircraft measurements of cloud condensation nuclei (CCN) were made during 16 summertime flights in eastern Florida. The air masses were divisible into maritime and continental regimes that respectively corresponded to wind direction—easterly (onshore) and westerly (offshore). Throughout these small cumulus clouds there were consistently higher concentrations of smaller droplets in the continental air. There was much more drizzle (diameter > 50 μm) in the maritime clouds where drizzle was associated with larger mean cloud droplet (2–50-μm diameter) sizes, higher concentrations of large cloud droplets, and greater amounts of cloud droplet liquid water. An apparent cloud droplet mean size threshold for the onset of drizzle was almost never exceeded in the continental clouds but was often exceeded in the maritime clouds, especially at higher altitudes. All together these results demonstrate that higher CCN concentrations suppressed drizzle.

Cloud condensation nuclei near marine stratus

Extensive airborne measurements of cloud condensation nucleus (CCN) spectra and concentrations of total particles, or condensation nuclei (CN), below, in, and above the stratus cloud decks off the southern California coast point to important aerosol-cloud interactions. Consistently low CCN concentrations below cloud appear to be due to cloud scavenging processes which include Brownian coagulation, nucleation, coalescence, and drizzle. The higher CCN and CN concentrations above cloud are associated with ambient ozone concentrations which suggest a link with continental, probably anthropogenic, sources, even at distances of 500 km from the California coast.

Droplet spectral broadening in marine stratus

Abstract Broadening of the cloud droplet (diameter 50 μm, i.e., drizzle). Broad droplet spectra were most closely associated with drizzle drops. Both the concentration, C, and slope, k, of the cloud condensation nuclei (CCN) spectra were theoretically found to affect droplet spectral width. For individual cloud parcels a higher C and lower k each contributed to broader droplet spectra. When mixing among cloud parcels with different updrafts was considered, the predictions deviated especially at larger mean droplet diameters. Variations in updraft velocity result in differences in droplet concentrations ...

Cloud Condensation Nuclei

Abstract The state of knowledge of the particles upon which liquid droplets condense to form atmospheric water clouds is presented. The realization of cloud condensation nuclei (CCN) as a distinct aerosol subset originated with the cloud microphysical measurements and theoretical insights of Patrick Squires 40 years ago. He helped originate and continue the development of CCN counters and made significant CCN measurements for more than 25 years. Recognition of the importance of CCN has expanded from warm-rain efficiency to aerosol scavenging, cloud radiative properties and other topics. In spite of a promising beginning and much encouragement over the years, CCN knowledge has increased minimally. Significant uncertainties about global climate change cannot be reduced without expansion of the knowledge base of CCN.

The Impact of Ship-Produced Aerosols on the Microstructure and Albedo of Warm Marine Stratocumulus Clouds: A Test of MAST Hypotheses 1i and 1ii

Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.