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T. B. Low
and
Roland List

Abstract

The collision, coalescence and breakup of single raindrop pairs were studied at terminal velocities and laboratory pressure (100 kPa) in 761 collision experiments (out of 14 000 attempts). Six size combinations were used with drop pair diameters of [0.18;.0.0395 cm], [0.40; 0.0395 cm], [0.44; 0.0395 cm], [0.18; 0.0715 cm], [0.18; 0.10 cm] and [0.30; 0.10 cm]. For averaging purposes the experiments were repeated over one hundred times for each pair.

The new coalescence efficiencies and fragment size distributions in breakup turned out to be consistent with those of McTaggart-Cowan and List (1975b) and permitted the combination of the two data sets into a single data bank spanning essentially the entire range of raindrop sizes.

The analysis addressed three main geometric shapes formed by the drops after initial contact, namely, filaments, sheets and disks, and the fragment size distributions after breakup. Significant collisional growth, i.e., coalescence, occurred only when drops <0.06 cm in diameter were struck by larger ones. An empirical equation involving collision kinetic (CKE) and surface tension energies was developed to approximate the observed coalescence efficiencies.

Breakup fragment size distributions normally show two or three peaks, one close to the size of the large drop of the collision pair, one at times (for filaments) reflecting the small drop, and the third centered at sizes below the small drop diameter. At high energy collisions involving larger drops the mechanism most favorable for coalescence was the disk shape because with its high deformation it is able to dissipate the most energy either through air drag or by internal viscosity through oscillations. The lowest collision energy for breakup is required for filaments; more is needed for sheets and most for disks.

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T. B. Low
and
R. List

Abstract

No abstract available.

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T. B. Low
and
Roland List

Abstract

The experimental drop collision/breakup results of Low (1977) and Low and List (1982) and McTaggart-Cowan and List (1975b), taken at laboratory pressure and terminal drop speeds, were parameterized for future use in cloud and precipitation modeling. The primary analyses of the 10 representative raindrop pairs were based on the three main geometric shapes generally assumed by the drop pairs after their initial contact and before breakup (or coalescence): filaments, sheets and disks. Relationships for the average total fragment number for each category are given. The fragment number distributions resulting from the collisions in each classification were fitted as sums of normal and log-normal distributions with the parameters of each distribution being related to the drop sizes and physical quantities derived from them (like the collision kinetic energy, CKE).

Each collision was then weighted according to the individual contribution and summed to give the probability of occurrence of each breakup type. The weighting functions were based on the CKE of each pair as determined in the center of drop mass frame. With the newly established coalescence efficiencies for raindrop pairs by Low and List (1982) the collision breakup equations were expanded into general overall equations for all drop pairs as expected in natural rain.

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Roland List
,
T. B. Low
, and
J. D. McTaggart-Cowan

Abstract

The purpose of this study is to assess the entrainment by rain of chaff which is used to track air motions by radar. Experiments are described where 214 individual water drops with diameters of 4.9 mm and falling at 78% of terminal velocity collide with single strands of (cylindrical) chaff fibres (diameters of 25 μm and lengths of 10.7 cm), which were falling freely at the time of collision. The length of the fibres investigated is adequate to be used by 10 cm wavelength tracking radars. On the average the water drops carried the chaff over a distance of 4.5 cm from the point of original contact. The actual distance of carry depends on the initial point of contact with respect to both the drop and the fibre; it is greatest for centered collisions.

A simple model is outlined on the basis of the equal but opposite drag forces the chaff experiences within the drop and within the air. Extrapolations for the carrying distance were then made for drops of various sizes, falling at terminal velocities, and drop spectra exhibiting a given Marshall–Palmer distribution. The main conclusion is that the average increase in the downward motion of the chaff due to rain is quite small as compared with the free fallspeed of the chaff and can be neglected in practical applications to the tracking of air motions by radar.

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R. E. Stewart
,
R. W. Crawford
,
N. R. Donaldson
,
T. B. Low
, and
B. E. Sheppard

Abstract

Precipitation and environmental conditions occurring during accretion in Canadian east coast winter storms are described and investigated. Accretion is generally associated with snow, freezing rain, and ice pellets within saturated conditions. Precipitation types are sometimes invariant but usually evolve during individual accretion events. Accretion events are also generally associated with moderate wind speeds (average of 7.5 m s−1) and warm temperatures (between −1° and 0°C are most common). Remote sensing of particle shapes and terminal velocities are capable of identifying some of the features of these precipitation types. Model calculations indicate that a detailed understanding of precipitation characteristics, such as the nature of wet snow, is needed to accurately simulate accretion.

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B. E. Sheppard
,
R. E. Stewart
,
G. A. Isaac
, and
T. B. Low

Abstract

Atmospheric temperature profiles and integrated water vapor and liquid are retrieved from ground-based microwave radiometric measurements using both nonlinear optimal estimation (NLOE) and statistical inversion (SI) methods. The results obtained from both methods are compared with collocated radiosonde observations during the Canadian Atlantic Storms Program field project in 1986. In general, the NLOE was superior to the SI method when clouds with high liquid water contents or when precipitation was present. Under these conditions, temperature profiles derived using NLOE had smaller root-mean-square differences from radiosonde observations than those retrieved using SI. Also, the overestimation of integrated vapor retrieved using the SI method was eliminated using the NLOE method. The radiometric observations were used in two case studies of winter cyclonic storms striking Atlantic Canada.

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