Breakup In Collisions Research Articles

Ice Multiplication by Breakup in Ice–Ice Collisions. Part II: Numerical Simulations

AbstractIn Part I of this two-part paper, a formulation was developed to treat fragmentation in ice–ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin–two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere.Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions >0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice–ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%–98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication.Breakup in collisions of snow (crystals >~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%–90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%–40%.

Ice Multiplication by Breakup in Ice–Ice Collisions. Part I: Theoretical Formulation

Abstract For decades, enhancement of ice concentrations above those of active ice nucleus aerosols was observed in deep clouds with tops too warm for homogeneous freezing, indicating fragmentation of ice (multiplication). Several possible mechanisms of fragmentation have been suggested from laboratory studies, and one of these involves fragmentation in ice–ice collisions. In this two-part paper, the role of breakup in ice–ice collisions in a convective storm consisting of many cloud types is assessed with a modeling approach. The colliding ice particles can belong to any microphysical species, such as crystals, snow, graupel, hail, or freezing drops. In the present study (Part I), a full physical formulation of initiation of cloud ice by mechanical breakup in collisions involving snow, graupel, and/or hail is developed based on an energy conservation principle. Theoretically uncertain parameters are estimated by simulating laboratory and field experiments already published in the literature. Here, collision kinetic energy (CKE) is the fundamental governing variable of fragmentation in any collision, because it measures the energy available for breakage by work done to create the new surface of fragments. The developed formulation is general in the sense that it includes all the types of fragmentation observed in previous published studies and encompasses collisions of either snow or crystals with graupel/hail, collisions among only graupel/hail, and collisions among only snow/crystals. It explains the observed dependencies on CKE, size, temperature, and degree of prior riming.

MASS AND ENERGY CORRELATIONS IN FAST TERNARY BREAK-UP OF 197Au + 197Au SYSTEM

Detailed characteristics of a new reaction mechanism of fast ternary break-up in collisions of a heavy nucleus-nucleus system 197 Au + 197 Au at 15 MeV/nucleon, studied in experiments at the INFN LNS Laboratory in Catania by using the multidetector array CHIMERA are presented. Results of an analysis of mass and energy correlations complement our earlier report that demonstrated the mechanism of aligned re-separation of the Au + Au system into three massive fragments in semi-peripheral collisions.

Evolution of grains in a turbulent solar nebula

In accretion disk models of the solar nebula, turbulence is driven by convective instability. This mechanism requires high opacity, which must be provided by solid grains. Evolution of the grain size distribution in a turbulent disk is computed numerically, using realistic collisional outcomes and strengths of grain aggregates, rather than an arbitrary “sticking efficiency.” The presence of turbulence greatly increases the rate of grain collisions; the coagulation rate is initially much greater than in a nonturbulent disk. Aggregates quickly reach sizes ∼0.1–1 cm, but erosion and breakup in collisions prevent growth of larger bodies for plausible aggregate impact strengths. These aggregates are too small to settle to the plane of the disk, and planetesimal formation is impossible as long as the turbulence persists. However, the opacity of the disk is reduced by aggregate formation; some combinations of opacity law and surface density produce an optically thin disk, cutting off turbulent convection. The disk may experience alternating periods of turbulence and quiescence, as grains are depleted by coagulation and replenished by infall from the presolar cloud. Planetesimals can form only during the quiescent intervals; it is argued that such episodes were rare during the lifetime of the accretion disk.