Ice Multiplication by Breakup in Ice–Ice Collisions. Part I: Theoretical Formulation (original) (raw)
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Ice Multiplication by Breakup in Ice–Ice Collisions. Part II: Numerical Simulations
Journal of the Atmospheric Sciences
In 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 a...
Colliding Ice Crystals in Turbulent Clouds
Journal of the Atmospheric Sciences
Collisions, resulting in aggregation of ice crystals in clouds, is an important step in the formation of snow aggregates. Here, we study the collision process by simulating spheroid-shaped particles settling in turbulent flows and by determining the probability of collision. We focus on platelike ice crystals (oblate ellipsoids), subject to gravity, and to the Stokes force and torque generated by the surrounding fluid. We also take into account the contributions to the drag and torque due to fluid inertia, which are essential to understand the tendency of crystals to settle with their largest dimension oriented horizontally. We determine the collision rate between identical crystals, of diameter 300 μm, with aspect ratios in the range 0.005 ≤ β ≤ 0.05, and over a range of energy dissipation per unit mass, ε, 1 ≤ ε ≤ 250 cm2 s−3. For all values of β studied, the collision rate increases with the turbulence intensity. The dependence on β is more subtle. Increasing β at low turbulence ...
Monthly Weather Review, 2004
A revised approach to cloud microphysical processes in a commonly used bulk microphysics parameterization and the importance of correctly representing properties of cloud ice are discussed. Several modifications are introduced to more realistically simulate some of the ice microphysical processes. In addition to the assumption that ice nuclei number concentration is a function of temperature, a new and separate assumption is developed in which ice crystal number concentration is a function of ice amount. Related changes in ice microphysics are introduced, and the impact of sedimentation of ice crystals is also investigated.
Some effects of cloud turbulence on water–ice and ice–ice collisions
Atmospheric Research, 1998
Formation of relative velocities between low-density ice particles and the surrounding air within a turbulent medium is analyzed. Turbulence is assumed to be homogeneous and isotropic. Both the inertial and viscous turbulent ranges are taken into account when describing the turbulent flow characteristics. It is shown that the inertia of drops and ice particles falling within a turbulent flow leads to the formation of a significant relative velocity between particles, which can be Ž . greater especially for those with small sedimentation velocity than gravity-driven velocity difference. It is also shown that for ice particles, the turbulence effects related to particles' inertia are much more pronounced than for water drops. An increase in relative velocity between particles leads to an increase in the swept volume and the collision kernel. As a result, a marked increase in the rates of riming and ice-ice interactions is expected. q 1998 Elsevier Science B.V. All rights reserved.
A Review of Ice Particle Formation Models
Modelling of ice clouds in the atmosphere is in general a more complex task than modelling their liquid water counterparts, owing to the plethora of ice crystal nucleation processes and their non-linear dependence on surrounding conditions. Accurate modelling of ice clouds plays an important role in weather prediction and climatology, particularly in their contribution to greenhouse effect, global warming and precipitation processes, and the impact of aviation on the environment. In this paper, we review different ice particle formation models, focusing on the underlying assumptions, advantages and limitations of each scheme.
Theoretical model of the Bergeron–Findeisen mechanism of ice crystal growth in clouds
Atmospheric Environment, 2004
A numerical study of growth rate of ice particles in an array of water droplets (Bergeron-Findeisen mechanism) has used the method of electrostatic image charges to determine the vapour field in which a particle grows. Analysis of growth rate in various conditions of relevance to clouds has shown that it is proportional to liquid water content and to ice particle size, while it is inversely proportional to cloud droplet size. The results show that growth rate is enhanced by several percent relative to the usual treatment in which vapour is assumed to diffuse from infinity towards a growing ice particle. The study was performed for ice particles between 25 and 150 mm radii, water droplet sizes between 6 and 20 mm diameter and a wide range of liquid water contents. A study was also made to determine the effect of reducing the vapour source at infinity so that the droplets alone provided the vapour for particle growth. A parameterisation of ice particle growth rate is given as a function of liquid water content and ice particle and droplet sizes. These studies are of importance to considerations in thunderstorm electrification processes, where the mechanism of charge transfer between ice particles and graupel could take place. r
A new theory of heterogeneous ice nucleation for application in cloud and climate models
Geophysical Research Letters, 2000
A new formulation is presented of the thermodynamical theory of heterogeneous ice crystal nucleation in clouds by freezing. This theory unifies and explains the empirical ice nuclei dependence on temperature and supersaturation, predicts crystal formation via condensation-freezing at a subsaturation over water. The theory also explains observations of high nucleation rates and crystal concentrations at warm(-5 >-12 øC)temperatures when the splintering mechanism may be not effective. This theory can be applied to parameterizations for use in cloud and climate models.
Atmospheric Research, 2014
An improved approach for cloud droplet activation process parameterization is proposed that can utilize the empirically determined hygroscopicity information and practically limit the sizes of newly activated droplets. With the implementation of the improved approach in a cloud model, the aerosol effects on ice microphysics in convective cloud and precipitation development under different thermodynamic conditions is investigated. The model is run for four different thermodynamic soundings and three different aerosol types, maritime (M), continental (C) and polluted (P). Warm rain suppression by increased aerosol (i.e., CCN) is clearly demonstrated when weakly convective warm clouds are generated but the results are mixed when relatively stronger convective warm clouds are generated. For one of the two soundings that generate strong convective cold clouds, the accumulated precipitation amount is larger for C and P than for M, demonstrating the precipitation enhancement by increased CCN. For the maritime cloud, precipitation is initiated by the warm rain processes but ice hydrometeor particles form fast, which leads to early but weak cloud invigoration. Another stronger cloud invigoration occurs later for M but it is still weaker than that for C and P. It is the delayed accumulation of more water drops and ice particles for a burst of riming process and the latent heat release during the depositional growth of rimed ice particles that invigorate the cloud strongly for C and P. For the other sounding where freezing level is low, ice particles form fast for all three aerosol types and therefore warm rain suppression is not clearly shown. However, there still is more precipitation for C and P than for M until the accumulated precipitation amount becomes larger for M than for C near to the end of the model run. The results demonstrate that the precipitation response to aerosols indeed depends on the environmental conditions.
Atmospheric and Oceanic Science Letters, 2012
This paper outlines a one-dimensional, heightdependent bin model with detailed microphysical processes in which ice splinters are produced by a riming process. The model is then applied to simulate the shift of particle size distribution effected by the secondary ice production process within clouds with different generating cells and cloud top temperatures. The result of model simulations reveals the general effects of cloud updrafts on increasing ice particle concentration by extending the residence time of ice particles in clouds and providing sufficiently large supercooled water droplets. The rimesplintering mechanism is more effective in clouds with lower ice seeding rates than those with higher rates. Evolutions of hydrometeor size distribution triggered by the rime-splintering mechanism indicate that the interaction between large ice particles and supercooled water drops adds a "second maximum" to the primary ice spectra.
Effect of aerosols on freezing drops, hail and precipitation in a mid-latitude storm
Journal of the Atmospheric Sciences, 2015
A midlatitude hail storm was simulated using a new version of the spectral bin microphysics Hebrew University Cloud Model (HUCM) with a detailed description of time-dependent melting and freezing. In addition to size distributions of drops, plate-, columnar-, and branch-type ice crystals, snow, graupel, and hail, new distributions for freezing drops as well as for liquid water mass within precipitating ice particles were implemented to describe time-dependent freezing and wet growth of hail, graupel, and freezing drops. Simulations carried out using different aerosol loadings show that an increase in aerosol loading leads to a decrease in the total mass of hail but also to a substantial increase in the maximum size of hailstones. Cumulative rain strongly increases with an increase in aerosol concentration from 100 to about 1000 cm−3. At higher cloud condensation nuclei (CCN) concentrations, the sensitivity of hailstones’ size and surface precipitation to aerosols decreases. The phys...