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  • Author or Editor: J. Hallett x
  • Journal of Applied Meteorology and Climatology x
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T. C. Foster
and
J. Hallett

Abstract

The production of ice crystals as a result of the expansion and cooling of moist air was investigated by laboratory experiment. In particular, the warmest expanded air temperature that produces crystals was sought as a function of the initial temperature. The results fit the standard theory of homogeneous nucleation of water droplets, as long as the droplets remain at a cold enough temperature for sufficient time to from by homogeneous nucleation. Experiments were also carried out with “dry” air (dewpoint less than −40°C). Quantitatively different results were obtained, namely, that much colder expanded air temperatures were required to produce crystals with essentially no variation in numbers of crystals produced as the initial temperature varied. These results are also consistent with the same homogeneous nucleation theory. These idea were applied to the production of aircraft-produced ice particles by means of the adiabatic expansion and cooling that occur near the propeller blades of the aircraft; this mechanism is sufficient for the production of such particles in some flying situations. In particular, the difference from one aircraft to another seems a less important factor than the variations in Right conditions for a given aircraft. Situations that require large thrust from the propellers (for example, climbing, icing, or flying at very slow speed with flaps down) are most likely to produce ice particles and should be avoided in all cloud passes made when repenetration is intended. Various actual cases of aircraft produced ice were examined, and in some the same larger-than-average thrust condition was met.

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R. G. Oraltay
and
J. Hallett

Abstract

Melting, freezing, and evaporation of individual and aggregates of snow crystals are simulated in the laboratory under controlled temperature, relative humidity, and air velocity. Crystals of selected habit are grown on a vertical filament and subsequently melted or evaporated in reverse flow, with the velocity adjusted for appropriate fall speed to reproduce conditions of the melting layer. Nonequilibrium conditions are simulated for larger melting ice particles surrounded by smaller drops at a temperature up to +5°C or growth of an ice crystal surrounded by freezing ice particles down to −5°C. Initial melting of well-defined faceted crystals, as individuals or in combination, occurs as a water layer >10 μm thick. For larger (>100 μm) crystals the water becomes sequestered by capillary forces as individual drops separated by water-free ice regions, often having quasiperiodic locations along needles, columns, or arms from evaporating dendrites. Drops are also located at intersections of aggregate crystals and dendrite branches, being responsible for the maximum of the radar scatter. The drops have a finite water–ice contact angle of 37°–80°, depending on ambient conditions. Capillary forces move water from high-curvature to low-curvature regions as melting continues. Toward the end of the melting process, the ice separating the drops becomes sufficiently thin to fracture under aerodynamic forces, and mixed-phase particles are shed. Otherwise ice-free drops are shed. The melting region and the mechanism for lowering the melting layer with an increasing precipitation rate are associated with smaller ice particle production capable of being lofted in weaker updrafts.

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D. Lamb
,
K. W. Nielsen
,
H. E. Klieforth
, and
J. Hallett

Abstract

Investigations of the structure and organization of synoptic-scale storms over the Sierra Nevada Mountain Range during two successive winters (1971–73) were made with a modified B-26 aircraft. Measurements of liquid water content, temperature and dew point were made along horizontal traverses in a vertical plane oriented roughly perpendicular to the main crest and extending from Lake Tahoe to Sacramento, Calif. It is shown that the spatial distribution of liquid water is linked to the gross terrain features, as is the surface distribution of precipitation. The main centers of cloud liquid water content tend to form 40–75 km upwind of the main crest in highly convective cells.

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K. A. Browning
,
J. Hallett
,
T. W. Harrold
, and
D. Johnson

Abstract

Attempts have been made to obtain samples of freshly fallen hailstones from severe storms in Oklahoma with the purpose of studying the nature and extent of spongy ice within natural hail. Interception by automobile of radar echoes with Ze > 105 mm6 m−3 has been found to provide a workable technique for collecting large hailstones as they fall to the ground. Observations suggest that the regions of highest reflectivity were associated more closely with the falls of large hail than with the accompanying heavy rain.

Immediate sectioning of the freshly fallen hailstones revealed the presence of thin shells of spongy ice in many of the larger stones. Calorimetric analyses gave liquid water contents of up to 12 ± 4% of the mass of the stones. Some of the hailstones were aspherical owing to preferential melting of the regions of spongy ice during fall. In the case of hailstones that were stored at sub-freezing temperatures, spongy ice shells could often still be identified from the presence of millimeter size cavities embedded within ice composed of large crystals.

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