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Andrew Detwiler

Abstract

Indirect justification for extrapolating the Goff-Gratch formula for saturation vapor pressure over liquid water to temperatures as low as −60°C has been obtained during a recent study of ice nucleation on aerosol particles in a laboratory cloud chamber.

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Andrew Detwiler

Abstract

No abstract available.

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Andrew Detwiler and Bernard Vonnegut

Abstract

Experiments conducted in a laboratory cloud chamber at −15°C show no significant initiation of freezing in supercooled cloud droplets when they are penetrated by 5.3 MeV alpha particles emitted by a 500 μCi 210>Po source.

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Andrew Detwiler and Haesook Cho

Abstract

A rough analysis shows that it may be economically feasible to reduce space heating costs during the cold season in the northern United States by modifying naturally-occurring cloud cover, or by artificially forming clouds in otherwise clear skies.

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Andrew Detwiler and V. Ramaswamy

Abstract

Results from one-dimensional cirrus cloud model simulations in the absence of upward velocities are used to show that the growth/sublimation of the ice particles in the cloud, and the fact that they are falling, can be important factors in determining the net heating rate in the air through which these clouds settle. The vertical profiles of the heating rate inside the cloud are stretched as a result of the settling of the cloud. Results for clouds at various altitudes in both midlatitude and tropical atmospheres are compared.

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Andrew Detwiler and Andrew J. Heymsfield

Abstract

An analysis of aircraft-measured data obtained in the lower portion of a High Plains thunderstorm anvil is presented. A “wind shadow” is still evident 5 to 7 core diameters downstream of the storm core. The wind fluctuations are predominantly horizontal on large scales and isotropic on small scales. Little evidence for gravity waves is found in this convectively neutral region of the anvil. Small-scale turbulence is encountered sporadically along cross-anvil penetrations. Weak zones of smooth cloud-edge downdraft are found along the lateral boundaries. The power spectra of the wind components is shallower than the −5/3 value predicted for an inertial subrange turbulent cascade at the smallest scales resolved (<2 km). Lightning is encountered/triggered by the aircraft twice in different relatively turbulent regions far from the storm core where the temperature is −35°C and small graupel is present.

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Andrew Detwiler and Hillyer G. Norment

Abstract

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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Andrew G. Detwiler and Bernard Vonnegut

Abstract

The ice saturation ratio at which 1% of aged silver iodide and lead iodide aerosol particles nucleate ice from moist air is observed to depend on temperature. Between roughly −30 and −67°C the threshold for both aerosol types rises slowly with decreasing temperature in agreement with a simple classical nucleation theory. Between −6 and −30°C the threshold for the silver iodide aerosol rises more rapidly than predicted by simple theory while the threshold for lead iodide decreases.

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Andrew Detwiler, Darin Langerud, and Tracy Depue

Abstract

Daily observations of cloud condensation nuclei (CCN) were made for three summer months in 2005 at a site in rural western North Dakota. The goal was to define the natural background CCN population characteristics and to lay the groundwork for investigating the potential impact of intentionally modifying clouds in this region using hygroscopic cloud-seeding techniques. Concentrations of CCN active at ∼0.5% supersaturation, averaged over several midday hours on each day, ranged from less than 200 to more than 1700 cm−3. This is similar to variability in CCN concentrations that have been observed in past studies in other rural areas of the central and northern high plains of the United States. At this site, only 2 out of 17 days with active convection at that site were characterized by concentrations of less than 300 cm−3 active at 0.5% supersaturation, indicating that the region is characterized by typically continental CCN populations on most convective days. Operational seeding might be more effectively conducted if CCN population characteristics could be forecast based on source regions for air forecast to arrive in a particular region on a particular day. However, back-trajectory calculations were found to have limited use for predicting CCN concentrations based on prior history of the air arriving at this observation site during this period.

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Patrick C. Kennedy and Andrew G. Detwiler

Abstract

An armored T-28 research aircraft made direct observations of the hydrometeors present at approximately the −3°C temperature level in the inflow region of a multicell thunderstorm. During the penetration, both the Colorado State University (CSU)–University of Chicago and Illinois State Water Survey (CHILL) 11-cm-wavelength dual-polarization research radar and the Denver, Colorado, Front Range Airport (KFTG) Weather Surveillance Radar-1988 Doppler (WSR-88D) were scanning this storm. Polarimetric radar indications of hail (high reflectivity and low differential reflectivity) appeared near the surface in the echo core adjacent to the aircraft track approximately 6 min after the T-28's inflow transit. Radial velocity data from the KFTG radar were combined with those recorded at CSU–CHILL to synthesize the airflow fields in the storm around the time of the T-28 penetration. Hail trajectories were initiated from a location at which the T-28 encountered a burst of approximately 1-cm-diameter, low-density graupel particles within the general storm inflow region. Forward-time trajectory calculations indicated that these graupel particles subsequently grew slightly into small hailstones and ended up within a few kilometers of the near-surface polarimetric radar hail-signature location. Trajectories computed backward in time imply that these hail embryos originated aloft in the forward portion of the echo complex. These are the first quantitative, direct in situ observations of recirculating precipitation becoming embryos for hail development.

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