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Alfred H. Woodcock

Abstract

Observations, on the island of Oahu, Hawaii, of unusual rains and clouds in marine air during fresh easterly winds show that rain can form within and fall continuously from a shallow layer of warm stratocumulus in about 5 to 13 minutes. This time estimate is shown to be dependent upon where, in the rapidly moving marine cloud system, an island effect initiating raindrop growth is assumed to begin. The clouds are associated with frontal passage nearby, and they form light rains of about 1 to 8 mm/h, averaging 3.5. On the windward shore the clouds produce sparsely distributed showers, suggesting a cell-like structure of the rain development processes over the sea, whereas a few kilometers inland over the Koolau Mountain Range they produce continuous rain and then quickly dissipate. The raindrop-generating processes seem to occur largely over the island rather than over the windward sea, as has been previously suggested concerning the more common orographic showers from the northeasterly tradewind cumuli. The unusually short raindrop-formation times revealed are thought to require some modification of current ideas about the collision-coalescence process of raindrop growth in these oceanic clouds. It is suggested that turbulence in the clouds caused by passage over Oahu may accelerate the collision-coalescence process and thus reduce the raindrop growth times.

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Alfred H. Woodcock

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Alfred H. Woodcock

During 23 hours of fresh to strong winds in December 1975, air flowed rapidly and continuously out of a drill hole in the top of the summit cone of Mauna Kea volcano, Hawaii. Measurements made during this outflow indicate that the air entered the mountain dry and cold, but flowed out relatively wet and warm, resulting in an average latent- and sensible-heat loss from the cone interior of about 116 W · m−2. A sensitive vane anemometer, and thermistor and mercury-in-glass thermometers, were used to make these observations.

Published observations made during moderate winds in this and a second drill hole had revealed relatively low air and heat flow rates, alternating daily into as well as out of the cone, with outflow generally during the day and inflow largely at night. The diurnal differences in the flow direction suggested that the well-known, semidiurnal atmospheric-pressure changes were the main cause of the air “breathing” within the cone. The latent-heat outflow in moderate winds was about 4 W · m−2.

The continuous outflow observations presented here indicate that wind speed has a marked if not dominant effect on the airflow and heat flow from the Mauna Kea summit cones, and that the resulting cooling during one day of strong winds can equal that of ten or more days of lower winds. This intense local cooling may explain the long survival of permafrost on Mauna Kea, and underscores the potential of air-land interaction in altering the internal air pressure and heat and water distribution in the cinder cones of Mauna Kea and perhaps in other volcanoes as well.

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Alfred H. Woodcock

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Alfred H. Woodcock

Abstract

New observations confirm the formation of advective fogs over the western approaches to the Cape Cod Canal. The large air-water temperature differences required for the formation of these fogs are shown to occur when the colder Cape Cod Bay waters enter the canal, apparently in response to wind effects in producing upwelling. A simple means of anticipating the formation of these fogs is suggested.

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Alfred H. Woodcock

Notes by Gardner Emmons about the initiation of low advective fogs on Cape Cod are presented. Subsequent measurements made in these fogs confirm his suggestion that mixing and temperature changes associated with tidal currents account for the fog. Puzzling temperature measurements that are at apparent variance with the mixing theory of fog formation are presented. It is proposed that these temperature discrepancies are due to the effects of water vapor condensation on the sea water surface.

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Alfred H. Woodcock

Abstract

Water distribution in marine advection fogs, as a function of droplet size, is estimated from the weights of the salt particles found when fog droplets are evaporated, and from the weights of the salt particles found in fog-free air over the sea. It is assumed that the salt concentration in the droplets is that to be expected after condensational growth of salt particles of about 10−13–10−10 g (0.4–4 µm diameter) for several hours in water-saturated air. The curves representing the water distribution so derived are shown to he similar to curves derived by other workers from measurements of the sizes and numbers of marine fog droplets, thus indicating a major role for salt particles in the initiation of some fogs at or near 100% relative humidity.

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Alfred H. Woodcock

Abstract

Marshall and Palmer (1948) have shown that, for rains of a given intensity, there is a definite distribution curve of number of raindrops of a particular range of diameter. The writer has indicated here that the chlorinity of rains also varies with rain intensity. Recently obtained data, concerning atmospheric sea salt, are presented in the form of distribution curves. These curves show the number of sea-salt particles sampled at different altitudes, of a given weight range, plotted against the weight. A computation is made, using a salt-particle distribution curve obtained at cloud levels, in which water is added to each particle until it reaches an assumed chlorinity for a given rain intensity. Each particle is thereby increased in size and becomes a drop of a new weight. The distribution curves of these computed drops are compared to the observed distribution curves of Marshall and Palmer, for various rain intensities, and are found to be remarkably similar. This result implies that, in the process of growth, the droplets containing each salt particle grow to raindrop size through coalescence with much more numerous and relatively non-saline cloud droplets.

The numbers of droplets in cumulus clouds over the sea are compared to the numbers of condensation nuclei in the sub-cloud layer and to the number of larger sea-salt particles.

A method of sampling the large sparsely-distributed salt particles in the atmosphere is described briefly.

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Alfred H. Woodcock

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Alfred H. Woodcock and Henry Stommel

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