Search Results

You are looking at 1 - 3 of 3 items for :

  • Author or Editor: T. B. Low x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
T. B. Low
and
R. List

Abstract

No abstract available.

Full access
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.

Full access