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U. Katz and W. C. Kocmond

Abstract

The activation supersaturation of several artifically prepared NaCl aerosols was measured in the laboratory using a thermal gradient diffusion chamber. Size distributions of these aerosols and total particulate concentrations were determined with a Whitby Aerosol Analyzer and a Gardner Small Particle Detector. From a combination of these data, the relationship was derived between supersaturation and necessary minimum size of the particles for nucleation. This experimentally obtained correlation indicates that, at a given supersaturation, a NaCl particle, has to be two to three times larger than theory predicts in order to be active as a cloud condensation nucleus. Possible explanations for the discrepancy are discussed.

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W. C. Kocmond and E. J. Mack

Abstract

Simultaneous airborne observations of Aitken and cloud nuclei upwind and downwind of Buffalo, N.Y., were made in an effort to determine the effects of air pollution on condensation nucleus concentration. The data show significant increases over background concentrations of both Aitken and cloud nuclei (at 0.3% supersaturation) immediately downwind of pollution sources, and, also, that a secondary maximum in cloud nucleus concentration usually occurs about 10–15 mi farther downwind. In spite of these large increases in the total number of nuclei downwind of an industrial-urban complex such as the Niagara Frontier, the concentration of cloud and Aitken particulates approaches upwind background levels within 20–50 mi of the sources.

Attempts to define the role (if any) of air pollution in cloud microstructure were confined to a single set of airborne observations. The limited drop-size distribution and cloud nucleus data were not sufficient to justify firm conclusions, and additional measurements are recommended.

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J. E. Jiusto, R. J. Pilié, and W. C. Kocmond

Abstract

Analytic and experimental investigations were conducted to examine the concept of modifying fog with hygroscopic material. An approximate equation was derived that is useful in estimating the feasibility of such applied problems. The combined results show that it is possible to improve visibility in warm fog by seeding with micron-size salt particles (NaCl). The visibility in laboratory fog produced in a 600-m3 chamber was increased by factors of 3–10, with as little as 1.7 mg m−3 of NaCl being effective. Only a modest reduction (<1%) in ambient relative humidity by the giant salt particles is necessary to cause substantial evaporation of the fog droplets. Extrapolation of these results suggests that clearing a suitable landing zone for aircraft should not involve prohibitive amounts of properly sized seeding material.

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R. J. Pilié, E. J. Mack, W. C. Kocmond, W. J. Eadie, and C. W. Rogers

Abstract

Extensive measurements were made of the microphysics of valley fog in the Chemung River Valley near Elmira, New York. This paper discusses data on drop size distributions, drop concentrations, liquid water contents, and haze and cloud nucleus concentrations obtained on eight fog nights.

The behavior patterns of the microphysical variables were found to be extremely consistent. Shallow ground fog usually occurs prior to the formation of deep valley fog. The data show that ground fog is characterized by droplet concentrations of 100 to 200 per cubic centimeter in the 1 to 10 μm radius range with mean radii of 2 to 4 μm. As deep fog forms aloft, droplet concentration near the surface decreases to less than 2 cm−3 and the mean radius increases from 6 to 12 μm. Droplets of radii <3 μm disappear. Thereafter, droplet concentration and liquid water content increase gradually until the first visibility minimum at the surface when typical values range from 12 to 25 cm−3 and 50 to 150 mg cm−3, respectively. The small droplets reappear at first visibility minimum. Subsequently, bimodal drop size distributions occur in approximately half of the fogs with one mode at 2–3 μm radius and a second mode between 6 and 12 μm. Aloft, drop size distributions become narrower and the mean radius decreases with both increasing altitude and increasing age of the fog. The cloud nucleus concentration active at S = 3.0% is usually between 800 and 1000 cm−3 near the surface and decreases to 500–800 cm−3 at 300 m.

It is argued from the data that supersaturation in the thin ground fog exceeds that in deep fog. The initial surface obscuration in deep fog appears to be due to droplets that form aloft and are transported downward into unsaturated air by turbulent diffusion. New droplets are apparently not generated near the surface until after the first visibility minimum.

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R. J. Pilié, E. J. Mack, W. C. Kocmond, C. W. Rogers, and W. J. Eadie

Abstract

Extensive measurements were made of micrometeorological variables associated with eleven fogs in the Chemung River Valley near Elmira, N.Y. Temperature was measured at five levels on a 17 m tower, dew point at three levels, wind speed and direction at two levels, and net radiation and vertical wind at one level. Visibility was measured at three locations, and dew deposition and evaporation at one location near the surface. Vertical temperature distributions were also measured using an aircraft. The microphysical variables are discussed in Part II of this paper.

Consistent patterns of behavior of all micrometeorological variables were observed. The formation of ground fog may be explained by radiational cooling of the surface and associated low-level heat exchange. To explain observed temperature behavior (maximum cooling rate near 100 m in the 6 h preceeding fog) and the initial formation of a thin fog layer slightly below that level, it seems necessary to invoke Defant's model of valley circulation. Radiation divergence at the fog layer aloft then produces an inversion near the fog top and unstable conditions at lower levels. The fog base therefore propagates downward.

Dew deposition is responsible for formation of a low-level dew point inversion before fog forms, a necessary condition for initial fog formation aloft. The inversion breaks as fog forms and dew weight is constant from that time until sunrise. Evaporation of dew after sunrise maintains saturation throughout the fog depth as fog temperature increases, and is therefore responsible for fog persistence. Dissipation of fog occurs when evaporation rate is no longer adequate to maintain saturation.

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R. J. Pilié, E. J. Mack, C. W. Rogers, U. Katz, and W. C. Kocmond

Abstract

This paper summarizes the results of seven field expeditions aboard the Naval Postgraduate School's R/V Acania, designed specifically to study the formation of marine fog along the California coast. On the basis of observations and analyses, physical models have been formulated for the formation and persistence of at least four different types of marine fog which occur off the West Coast: 1) fog triggered by instability and mixing over warm water patches; 2) fog developed as a result of lowering (thickening) stratus clouds; 3) fog associated with low-level mesoscale convergence; and 4) coastal radiation fog advected to sea via nocturnal land breezes. In addition, it has been found that the triggering of embryonic fogs and further downwind development produces a synoptic-scale fog-stratus system and is responsible for redevelopment of the unstable marine boundary layer after Santa Ana events.

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