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Franco Prodi

Abstract

A membrane filter technique for precisely locating chlorides on any prepared ice surface has been tested on hailstone cross sections. Chloride patterns, different in intensity in the various growth stages of the hailstones, were detected along the grain boundaries. To relate the environmental growth conditions of the hailstones to the chloride distributions, the filter technique has been applied to deposits of ice grown by accretion in a wind tunnel from droplets of known NaCl concentrations. The results show a dependence of the chloride distributions on time and temperature of storage: initially chlorides are uniformly distributed in the lattice structure and subsequently segregate to the grain boundaries during storage at temperatures close to 0°C.It is suggested that the technique, especially when performed on freshly fallen hailstones, may contribute in inferring growth conditions of the hailstones in the parent cloud and their trajectories, provided that a realistic model of the chloride concentration in the liquid water of the hailcloud is outlined.

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Franco Prodi

Abstract

Local density measurements were performed in the internal structure of hailstones by means of photometric observations on x-ray micrographs of thin slices. Comparison with the results of measurements performed by the same technique on ice artificially grown by accretion shows that dry growth near the net limit or dry growth wetted in a final stage is probably the most common condition of growth in natural hailstones. The technique is also suggested as a method for obtaining accurate quantitative data an air bubble size and concentrations which are needed in relating structure to growth conditions,. As a cross check, measurements of the total density by the usual immersion technique were also performed on hailstones from the same hail-fall as those examined by the x-ray absorption technique.

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Franco Prodi

Abstract

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Endre Wirth
and
Franco Prodi

Abstract

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Laura Levi
and
Franco Prodi

Abstract

The dependence of crystal orientation on air and deposit temperatures in ice accreted in the dry growth regime has been re-examined by producing accretions over a wide range of these temperatures and critically examining the structural aspects of the deposits, such as surface roughness. It is shown that the variation in the preferred c-axis orientation with the air and deposit temperatures during growth is an intrinsic effect of she freezing mode of ice, rather than an effect of surface roughness.

The Fϕ distributions of the angle ϕ between the accretion radius and the crystal c-axis, and those of its component angles F(η) and F(), have been obtained. It is shown that for cloud temperature Ta > −18°C, the F(ϕ) distributions have a main maximum at small angles. For −18°C ≥ Ta ≥ −23°C, the location of the maximum depends strongly on the deposit temperature Td reaching values near 45° for Td < −10°C. For Ta < −23°C the distributions show a large disorder at Td ≥ −5°C, but a maximum about 45° when Td is below this value.

Sharp secondary peaks frequently appear at the side of or superposed to, the main maxima. The application of the χ2-test to the distributions shows that secondary maxima may not be used to establish a more precise definition of Ta and Td . They may on the other hand be a consequence of the shape of the lobes, which may determine the splitting of the main maxima.

The application of the results to natural hailstone analysis is discussed.

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Griffith Morgan
and
Franco Prodi

Abstract

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Gianni Santachiara
,
Franco Belosi
, and
Franco Prodi

Abstract

This paper addresses the problem of the large discrepancies between ice crystal concentrations in clouds and the number of ice nuclei in nearby clear air reported in published papers. Such discrepancies cannot always be explained, even by taking into account both primary and secondary ice formation processes. A laboratory experiment was performed in a cylindrical column placed in a cold room at atmospheric pressure and temperature in the −12° to −14°C range. Supercooled droplets were nucleated in the column, in the absence of aerosol ice nuclei, by injecting ice crystals generated outside in a small syringe. A rapid increase in the ice crystal concentration was observed in the absence of any known ice multiplication. The ratio between the mean number of ice crystals in the column, after complete droplet vaporization, and the number of ice crystals introduced in the column was about 10:1. The presence of small ice crystals (introduced at the top of the column) in the unstable system (supercooled droplets) appears to trigger the transformation in the whole supercooled liquid cloud. A possible explanation could be that the rapidly evaporating droplets cool sufficiently to determine a homogeneous nucleation.

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Franco Prodi
,
Vittorio Prodi
, and
Gaetano Fiore

Abstract

Microchemical analysis and x-ray micrographs were performed on thin slices of ice cylinders artificially grown in a tunnel by accretion of supercooled droplets of contaminated water. Characteristic distribution patterns of contaminants were determined for various types of growth, with reference to contaminant transport that can take place in bulk ice. The results, if extended to natural hail, can be very helpful in inferring the environmental growth conditions.

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Franco Prodi
,
Alvise Moretti
, and
Orazio Sturniolo

Abstract

A 35.8-GHz scatterometer was tested and used for backscattering gain measurements of spherically shaped artificial hydrometeors varying in composition and size: homogeneous ice with 1.7 ≤ r ≤ 15 mm, different volume fractions of air bubbles embedded in a matrix of ice with 4.9 ≤ r ≤ 15 mm, and two-layered material (air core surrounded by a layer of ice with 1 ≤ r core ≤ 13.5 mm and external radius r ext = 15 mm; water core surrounded by a layer of ice with 0 ≤ r core ≤ 9 mm and r ext = 15 mm; ice core surrounded by a liquid water film with 1.7 ≤ r core ≤ 15 mm; and ice core surrounded by a layer of ice containing air bubbles with 3 ≤ r core ≤ 10 mm and r ext = 15 mm). Numerical computations at the wavelength of 8.37 mm were performed for the same hydrometeors using a Mie computer code and a two-layered code in both of which the refractive index of the corresponding statistically homogeneous mixture was computed using the second-order Maxwell-Garnett and the Bruggeman theories.

Laboratory measurements showed the scatterometer's marked sensitivity to such characteristic optical variations of the hydrometeors as air bubble content, hoarfrost, and liquid film formation, air and water core. A comparison to numerical computations shows good agreement for two-layered spheres, excepting ice shells containing air bubbles, and for small-sized homogeneous ice spheres. In the presence of air bubbles, the comparison indicates the need for a more accurate experimental procedure in the measurement of air volume fraction and the need to know both air bubble size distribution and gradient in the ice matrix.

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Federico Porcù
,
Leo Pio D’Adderio
,
Franco Prodi
, and
Clelia Caracciolo

Abstract

Coalescence and breakup of drops are recognized as the main mechanisms determining raindrop size distributions on the ground. Full knowledge of these processes is hindered by the challenging difficulties both in the laboratory and tunnel experiments and during observations in the open air.

In real rain breakup is mainly due to collision between drops of different sizes (collisional breakup) and occurs when the collisional kinetic energy (CKE) is not absorbed by the colliding drops. In this work, the authors observe and measure the dependence on altitude of the occurrence of collisional breakup in real rainfall events, and then estimate the corresponding limit terminal velocities of drops and their size when breakup significantly takes place.

Data from Pludix, an X-band microwave disdrometer, were collected at three locations at different elevations: collisional breakup position in the power spectrum of Pludix increases toward higher frequencies with increasing altitude. Terminal velocities and sizes of the drops at breakup were determined consequently, with drop sizes resulting in 4.55 ± 0.35, 4.02 ± 0.32, and 3.16 ± 0.3 mm for altitudes of 15, 950, and 3300 m MSL, respectively. The authors computed the CKE of the colliding drops at the breakup, finding an upper limiting value of about 1.22 × 10−5 J for all three altitudes. This shows that most dominant collisional breakup signature occurs at similar CKE values for all three locations, corresponding to different drop diameters at different altitudes because of the effect of air density on the drop terminal velocity.

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