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  • Author or Editor: Arthur L. Rangno x
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Arthur L. Rangno

Abstract

Images of frozen drops with pieces missing were collected on two days of airborne sampling in shallow supercooled stratiform frontal clouds in the coastal waters of Washington State. In those limited regions where ice appeared to be newly formed, ice fragments with rounded portions accounted for about 5% of the total ice particle concentrations. These results are in rough agreement with the body of literature on laboratory experiments concerning the freezing of drops in free fall that have suggested a modest, though not insignificant, role for the fragmentation of freezing drops on total ice particle concentrations when larger supercooled drops are present.

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Peter V. Hobbs and Arthur L. Rangno

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Peter V. Hobbs and Arthur L. Rangno

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Peter V. Hobbs and Arthur L. Rangno

Abstract

Extremely high ice particle concentrations developed rapidly in the ascending tops of maritime cumulus congestus clouds after drizzle drops had already formed below this level by the collision–coalescence mechanism. In one building cloud with a top temperature no colder than −8°C, the ice particle concentrations increased from 0 to >350 L−1 within 9 min. In another cloud with a top temperature no colder than −13°C, the ice particle concentrations increased from ≤1 to ∼1100 L−1 within 12 min. Subsequently, the ice particle concentrations in these clouds decreased, even though the cloud top temperature of one of the clouds continued to decrease to −23.5°C.

The mechanism responsible for these prodigious increases in ice particle concentrations is not clear. The concentrations built too fast to be explained by the riming-splintering mechanism as it is presently formulated. It is suggested that high ice particle concentrations might form in localized pockets of high supersaturation with respect to water.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

Measurements and observations have been made on the development of ice in 90 cumulus (cumulus and cumulonimbus) and 72 stratiform (altocumulus, altostratus, nimbostratus, stratocumulus, and stratus) clouds. Ice particle concentrations significantly in excess of those to be expected from ice nucleus measurements (i.e., ice enhancement) were measured in 42 of the cumuliform and 36 of the stratiform clouds. For the complete data set, and for cloud top temperatures (TT) between −6° and −32°C, the maximum concentrations of ice particles (I max in L −1) in the clouds were essentially independent of TT(r=0.32). However, I max was strongly dependent on the broadness of the cloud droplet size distribution near cloud top. If the breadth of the droplet size distribution is measured by DT, such that the cumulative concentration of droplets with diameters ≥DT exceeds a prescribed value, then for −32≤TT≤−6°C:where n=8.4 and DO=18.5 μm for the cumuliform clouds and n=6.6 and DO=19.4 μm for the stratiform clouds.

When DT>D 0 and TT≤−6°C, initial concentrations of ice were intercepted near the tops of clouds in the form of clusters ∼5–25 m wide. These clusters form strands of ice which, with increasing distance from cloud top, widen and merge and may eventually appear as precipitation trails below cloud base.

In light of these findings, it is postulated that ice enhancement is initiated during the mixing of cloudy and ambient air near the tops of clouds and that it is postulated with the partial evaporation and freezing of a small fraction (∼0.1%) of the droplets approximately >20 μm in diameter. Contact nucleation might be responsible for the freezing of these droplets. Under suitable conditions, this primary mechanism for ice enhancement may be augmented by other ice-enhancement mechanisms (e.g., ice splinter production during riming, and crystal fragmentation).

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