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James G. Hudson

Abstract

As part of the Cooperative Convective Precipitation Experiment (CCOPE) Aerosol Measurement Program, airborne grab samples and simultaneous nearby continuous surface measurements of CCN spectra were compared. A strong correlation was found between these measurements for samples gathered up to the altitude of cloud base. This implies that the surface CCN measurements made in CCOPE represent the input CCN spectra to the clouds. These results considerably enhance the potential usefulness of surface CCN measurements and therefore CCN measurements in general.

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James G. Hudson

Abstract

Measurements of the spectra of cloud condensation nuclei (CCN) within and near the boundaries of clouds are presented. Some of the in-cloud measurements excluded the nuclei within cloud droplets; these are interstitial CCN while other measurements included all nuclei inside the cloud (total CCN). The difference between these spectra can be used to determine which nuclei are involved in cloud droplets. The vast majority of the data indicate that droplets are more likely to be grown on nuclei with lower critical supersaturations as predicted by classical theory. This technique is introduced as a possible tool for investigating the effects of mixing on the evolution of the droplet spectra.

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James G. Hudson

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.

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James G. Hudson

Abstract

Comparisons between cloud-base CCN concentrations and cloud droplet concentrations in stratus clouds over San Diego and 100 km out to sea showed a positive correlation. The supersaturation in these clouds, as derived from the matching of the CCN spectra and the droplet spectra, is consistent with the concept of a moderate updraft intermediate between the supersaturation of a cumulus cloud and a fog.

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James G. Hudson

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 different airborne platforms as well as from a mobile surface platform. The number of channels allows the CCN spectrum to be displayed differentially instead of the usual cumulative display traditionally used in cloud physics.

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James G. Hudson

Abstract

Measurements of fog drop size distribution, cloud condensation nuclei and fog condensation nuclei were made at several locations along the U.S. Pacific Coast. Wide variations in fog microstructure were associated with and appeared to be mostly due to variations in nucleus concentration. Simultaneous drop and nucleus measurements were used to determine the effective supersaturations (S) in fog which wore found to be .≲0.10%.

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James G. Hudson
and
Patrick Squires

Abstract

For certain field and laboratory investigations, it would he useful to have apparatus capable of yielding a continuous, instantaneously recorded measurement of the concentration of cloud nuclei. A description of such a device is given, in which some of the problems of static diffusion chambers have, been mitigated. It is shown that clear plateaus exist in the deduced cloud nucleus concentration when plotted against the time the sample spends in the chamber, or against size of droplets detected, or sample flow rate. Transient supersaturations have been eliminated and the sample has been confined to a zone of precisely known supersaturation. It is shown that depletion of supersaturation by growing droplets does not occur for count rates as high as 1500 nuclei sec−1.

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James G. Hudson
and
Patrick Squires

Abstract

A slightly improved version of the continuous flow diffusion cloud chamber which has been described earlier (Hudson and Squires, 1973) has been rotated so that the plates are vertical instead of horizontal, the air flow remaining horizontal. Without other modification, its count agrees with that of the improved horizontal version to within about 1% over the supersaturation range 1.0–0.5%. The vertical mode of operation has certain advantages over even the improved horizontal chamber; in particular, it extends downward the supersaturation range over which reliable results can be obtained. Satisfactory plateaus can be obtained at least down to a supersaturation of about 0.1%. Two vertical chambers have been found to agree to about 1% in the supersaturation range 1.0–0.25%, and to about 10% down to a supersaturation of 0.1%.

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James G. Hudson
and
Seong Soo Yum

Abstract

No abstract available.

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James G. Hudson
and
Seong Soo Yum

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.

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