Ice Multiplication by Breakup in Ice–Ice Collisions. Part II: Numerical Simulations (original) (raw)
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Ice Multiplication by Breakup in Ice–Ice Collisions. Part I: Theoretical Formulation
Journal of the Atmospheric Sciences, 2017
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....
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.
In the present study, the Weather Research and Forecasting (WRF) model was used to simulate the features associated with a severe thunderstorm over India while examining the sensitivity of the simulation to three microphysical (MP) schemes (WDM6, Thompson and Morrison). The model simulated results (e.g., surface temperature, relative humidity, pressure, reflectivity and rainfall) for all sensitivity experiments are compared with observations (e.g., AWS, TRMM and DWR). There are major differences in the simulations of the thunderstorm among the MP schemes. The Morrison scheme simulates CAPE, surface properties, wind speed, vertical velocity, reflectivity and precipitation reasonably well, compared to other MP schemes, though there are some uncertainties. Therefore, an attempt is made to improve the simulation through modifications in the Morrison scheme. Different heterogeneous ice nucleation formulations have been tested into the Morrison double-moment bulk cloud MP scheme. We hypothesize that the improvement in cloud ice generation and its subsequent influence in cloud microphysics and dynamics through latent heat release may eventually lead to an improvement in thunderstorm simulation. The results demonstrate that the modification in the microphysical scheme better reproduces CAPE, wind speed, maximum reflectivity, vertical velocity and cloud hydrometeors (ice and mixed-phase processes) than the default Morrison and other schemes and compared to observations. The modified MP-scheme produces greater latent heating due to deposition in the upper troposphere and gives rise to increased updraft. This seems to be one of the most responsible processes that enhance the intensity of the storm compared to existing microphysical schemes. This study therefore provides a framework for the improvement of thunderstorm simulation through the modification of the cloud ice parameterization of the model.
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 ...
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...
Precipitation is fundamental to the hydrological cycle. There are two possible mechanisms for its formation in clouds generally: the "warm-rain process" and the "ice crystal process". This study uses a microphysically advanced aerosol-cloud (AC) model to understand the contributions from the warm (from the warm-rain process) and cold (from the ice crystal process) components of the surface precipitation in a pair of contrasting convective storms that are cold-based (STEPS) and slightly warm-based (MC3E). Tagging-tracer techniques enabled analysis of microphysical pathways leading to the simulated precipitation. The cold components of graupel and rain mass are higher than the corresponding warm components in both simulations of the STEPS and MC3E storms. About ~80% of accumulated surface precipitation is predicted to originate from ice-crystal process in both cases. In sensitivity tests with lowering of cloud base to warmer levels near the ground, the origin of mo...
A wintertime case study on the impact of ice particle habits on simulated clouds and precipitation
Atmospheric Research, 2006
Ice particles are considered in prevalent cloud microphysical parametrisation schemes in a simplified way by assigning them certain fixed forms, whereas in the atmosphere they occur in various different shapes. In this study, the sensitivity of cloud and precipitation properties to the selected interpretation of the ice particle shape is investigated with the help of simulations of a wintertime weather episode using the mesoscale weather forecast model 'Lokal-Modell' of the DWD. The results show that the interpretation of the particular ice particle type has a clear impact on the local surface precipitation rates while the area mean surface precipitation is only slightly affected. Most striking, however, is the influence on the simulated masses of cloud water in the atmosphere. To avoid the prescription of a fixed ice particle shape, a parametrisation variant is presented to account for the variation of ice particles' shape according to their growth history.
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.
The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds
Journal of the Atmospheric Sciences, 2015
The rapid glaciation of tropical cumulus clouds has been an enigma and has been debated in the literature for over 60 years. Possible mechanisms responsible for the rapid freezing have been postulated, but until now direct evidence has been lacking. Recent high-speed photography of electrostatically suspended supercooled drops in the laboratory has shown that freezing events produce small secondary ice particles. Aircraft observations from the Ice in Clouds Experiment–Tropical (ICE-T), strongly suggest that the drop-freezing secondary ice production mechanism is operating in strong, tropical cumulus updraft cores. The result is the production of small ice particles colliding with large supercooled drops (hundreds of microns up to millimeters in diameter), producing a cascading process that results in rapid glaciation of water drops in the updraft. The process was analyzed from data collected using state-of-the-art cloud particle probes during 54 Learjet penetrations of strong cumulu...
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.