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  • Author or Editor: Andrew G. Detwiler x
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Andrew Detwiler
Hillyer G. Norment


The M-meter is designed to measure total mass concentration of hydrometeors from aircraft aloft. Its essence is a free-rotating, vaned disk with face normal to the freestream; rotation is driven by airflow through the vanes. Hydrometeors collected on the disk surface are expelled by being flung tangentially from its periphery. This expulsion causes a reduction of equilibrium rotation rate that is proportional to freestream hydrometeor mass concentration.

A prototype was carried under the wing of a research airplane as it probed precipitating clouds at levels from just above to just below the melting layer. M-meter measurements of hydrometeor mass concentrations are demonstrated to be in qualitative agreement with independent measurements.

This unique, simple, rugged instrument with high sampling volume probably can be refined to provide accurate in situ measurement of cloud plus precipitation water mass—a capability sorely needed. It warrants additional research and development, which is not planned by those involved at present. This note is for the purpose of stimulating further interest in this promising concept.

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Rand E. Feind
Andrew G. Detwiler
, and
Paul L. Smith


Comparisons are made between liquid water concentration (LWC) readings obtained from a Johnson–Williams (J–W) cloud water meter and a King (Commonwealth Scientific and Industrial Research Organisation) liquid water probe, both mounted on the armored T-28 research aircraft during penetrations of springtime convective storms in Oklahoma and Colorado. The King probe readings are almost always higher, being up to twice those of the J–W instrument in clouds with narrower cloud droplet spectra. In clouds with broader droplet spectra, the ratio often climbs to three or greater. The King probe responds partially to drops larger than cloud droplet size, and also to some ice particles, so its reading can be higher than the cloud LWC present. However, this and earlier comparisons by others indicate that the primary reason for this discrepancy is that the J–W probe often underestimates the cloud LWC due to incomplete response to larger cloud droplets. Thus, published studies involving cloud LWC in convective storms based on readings of the T-28 J–W probe have often overestimated the effects of entrainment and precipitation scavenging on depletion of updraft liquid water, particularly in those areas characterized by clouds with broad droplet size spectra.

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