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Chengzhu Zhang and Jerry Y. Harrington

Abstract

The uptake of water vapor excess by ice crystals is a key process regulating the supersaturation in cold clouds. Both the ice crystal number concentration and depositional growth rate control the vapor uptake rate and are sensitive to the deposition coefficient . The deposition coefficient depends on temperature and supersaturation; however, cloud models either ignore or assume a constant .

In this study, the effects of on crystal growth and homogeneous freezing of haze solution drops in simulated cirrus are examined. A Lagrangian parcel model is used with a new ice growth model that predicts the deposition coefficients along two crystal growth axes. Parcel model results indicate that predicting can be critical for predicting ice nucleation and supersaturation at different stages of cloud development. At cloud base, model results show that surface kinetics constrain the homogeneous freezing rate primarily through the growth impact of small particle sizes in comparison to the mean free path. The deposition coefficient has little effect on homogeneous freezing rates, because the high cloud-base supersaturation produces near unity. Above the cloud-base nucleation zone, decreasing supersaturation causes to decrease to values as low as 0.001. These low values of lead to higher steady-state supersaturation. Also, the low values of produce substantial impacts on particle shape evolution and particle size, both of which are dependent on updraft strength.

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Chengzhu Zhang and Jerry Y. Harrington

Abstract

A model for kinetically limited vapor growth and aspect ratio evolution of atmospheric single ice crystals is presented. The method is based on the adaptive habit model of J. Chen and D. Lamb but is modified to include the deposition coefficients through a theory that accounts for axis-dependent growth. Deposition coefficients are predicted for each axis direction based on laboratory-determined critical supersaturations and therefore extends the adaptive habit approach and the capacitance model to low ice supersaturations. The new model is used to simulate changes in single-crystal primary habit in comparison to a hexagonal growth model. Results show that the new model captures the first-order features of axis-dependent, kinetically limited growth. The model reproduces not only the strong reductions in growth as supersaturations decrease but is also able to reproduce the near cessation of minor axis growth as saturations decline. While the new model reproduces the qualitative features of kinetically limited growth, relative errors are generally between 5% and 20% but can become larger than 50%. Parcel model simulation comparisons show that the new growth method reproduces the general features of axis-dependent growth in a changing temperature environment. The method also produces relatively accurate estimates of mass evolution with spherical particles, indicating that it may have broad applicability. Although the model compares well to a detailed method, uncertainties remain in the knowledge of surface kinetics that future studies need to unravel.

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Jerry Y. Harrington, Kara Sulia, and Hugh Morrison

Abstract

It is common for cloud microphysical models to use a single axis length to characterize ice crystals. These methods use either the diameter of an equivalent sphere or mass–size equations in conjunction with the capacitance model to close the equations for ice vapor diffusion. Single-axis methods unnaturally constrain growth because real crystals evolve along at least two axis directions. Thus, they are unable to reproduce the simultaneous variation in mass mixing ratio, maximum dimension, and mass-weighted fall speeds. While mass–size relations can at times capture the evolution of one of these with relatively low errors, the other properties are generally under- or overpredicted by 20%–40%. Part I of this study describes an adaptive habit method that evolves two axis dimensions, allowing feedbacks between aspect ratio changes and mass mixing-ratio evolution. The adaptive habit method evolves particle habit by prognosing number and mass mixing ratios along with two axis length mixing ratios. Compared with a detailed Lagrangian bin representation of ice habit distribution evolution in a parcel framework, the bulk method reproduces the ice mass mixing ratio, mean axis lengths, and mass-weighted fall speeds generally to within less than 5% relative error for layered and deeper mixed-phase clouds.

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Christopher M. Hartman and Jerry Y. Harrington

Abstract

The effects of solar heating and infrared cooling on the vapor depositional growth of cloud drops, and hence the potential for collection enhancement, is investigated. Large eddy simulation (LES) of marine stratocumulus is used to generate 600 parcel trajectories that follow the mean motions of the cloud. Thermodynamic, dynamic, and radiative cloud properties are stored for each trajectory. An offline trajectory ensemble model (TEM) coupled to a bin microphysical model that includes the influences of radiation on drop growth is driven by the 600-parcel dataset.

In line with previous results, including infrared cooling causes a reduction in the time for collection onset. This collection enhancement increases with drop concentration. Larger concentrations (400 cm−3) show a reduction in collection onset time of as much as 45 min. Including infrared cooling as well as solar heating in the LES and microphysical bin models has a number of effects on the growth of cloud drops. First, shortwave (SW) heating partially offsets cloud-top longwave (LW) cooling, which naturally reduces the influence of LW cooling on drop growth. Second, SW heating dominates over LW cooling at larger drop radii (≳200 μm), which causes moderately sized drops to evaporate. Third, unlike LW cooling, SW heating occurs throughout the cloud deck, which suppresses drop growth. All three of these effects tend to narrow the drop size spectrum. For intermediate drop concentrations (100–200 cm−3), it is shown that SW heating primarily suppresses collection initiation whereas at larger drop concentrations (≳250 cm−3) LW cooling dominates causing enhancements in collection.

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Jerry Y. Harrington and Gwenore F. Pokrifka

Abstract

Measurements show that after facets form on frozen water droplets, those facets grow laterally across the crystal surface leading to an increase in volume and surface area with only a small increase in maximum dimension. This lateral growth of the facets is distinctly different from that predicted by the capacitance model and by the theory of faceted growth. In this paper we develop two approximate theories of lateral growth, one that is empirical and one that uses explicit growth mechanisms. We show that both theories can reproduce the overall features of lateral growth on a frozen, supercooled water droplet. Both theories predict that the area-average deposition coefficient should decrease in time as the particle grows, and this result may help explain the divergence of some prior measurements of the deposition coefficient. The theories may also explain the approximately constant mass growth rates that have recently been found in some measurements. We also show that the empirical theory can reproduce the lateral growth that occurs when a previously sublimated crystal is regrown, as may happen during the recycling of crystals in cold clouds.

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Paul M. Markowski and Jerry Y. Harrington

Abstract

This note reports the preliminary results of an ongoing numerical study designed to investigate what effects, if any, radiative transfer processes can have on the evolution of convective storms. A pair of idealized three-dimensional simulations are conducted to demonstrate the potential dynamical importance of shortwave radiation reductions within the large shadows cast by storms. One of the simulations (the control) is run without surface physics and radiation. In the other simulation, radiative cooling due to cloud shading is emulated by prescribing a cooling rate to the skin temperature at any grid point at which cloud water was present overhead. The imposed skin cooling rate is consistent with past observations. Low-level air temperatures are coupled to the skin cooling in this second simulation by the inclusion of surface sensible heat fluxes using simple bulk aerodynamic drag laws (latent and soil heat fluxes are not included). Significant differences are observed between the two simulated storms, particularly in the evolution of the vertical vorticity field and gust fronts. The storm simulated with emulated cloud shading develops substantially weaker low-level rotation than the storm in the control simulation.

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Christopher M. Hartman and Jerry Y. Harrington

Abstract

The effects of solar heating at a variety of solar zenith angles (Θo) on the vapor depositional growth of cloud drops, and hence the potential for collection enhancement, is investigated. A large eddy simulation (LES) model is used to predict the evolution of marine stratocumulus clouds subject to changes in Θo. During the course of each simulation, LES output is stored for 600 parcel trajectories and is used to drive an offline microphysical model that includes the influence of radiation on drop growth.

Smaller Θo, such as when the sun is overhead, provide strong solar heating, which tends to confine circulations to the cloud layer and leads to long in-cloud residence times for cloud drops. At larger Θo, when solar heating is weak, circulations are stronger and penetrate through the depth of the boundary layer, which causes much shorter in-cloud residence times for cloud drops. Simulations show that this leads to a more rapid collection process in strongly, as compared to weakly solar-heated clouds provided that the liquid water contents of each cloud are similar. When drop vapor growth includes radiative effects, three main results emerge: 1) Solar heating at smaller Θo (0° to 45°) dominates over longwave cooling effects causing a suppression of collection for lower drop concentrations (100 to 200 cm−3). 2) At larger drop concentrations (≳300 cm−3) longwave cooling dominates over solar heating and collection is enhanced. 3) At large Θo (60° to 90°), solar heating is ineffective at modifying the drop size spectrum thus allowing longwave cooling to significantly enhance collection at all drop concentrations above approximately 100 cm−3.

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Anders A. Jensen and Jerry Y. Harrington

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This paper describes and tests a single-particle ice growth model that evolves both ice crystal mass and shape as a result of vapor growth and riming. Columnar collision efficiencies in the model are calculated using a new theoretical method derived from spherical collision efficiencies. The model is able to evolve mass, shape, and fall speed of growing ice across a range of temperatures, and it compares well with wind tunnel data. The onset time of riming and the effects of riming on mass and fall speed between −3° and −16°C are modeled, as compared with wind tunnel data for a liquid water content of 0.4 g m−3. Under these conditions, riming is constrained to the more isometric habits near −10° and −4°C. It is shown that the mass and fall speed of riming dendrites depend on the liquid drop distribution properties, leading to a range of mass–size and fall speed–size relationships. Riming at low liquid water contents is shown to be sensitive to ice crystal habit and liquid drop size. Moreover, very light riming can affect the shape of ice crystals enough to reduce vapor growth and suppress overall mass growth, as compared with those same ice crystals if they were unrimed.

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Jerry Y. Harrington, Kara Sulia, and Hugh Morrison

Abstract

Bulk microphysical schemes use the capacitance model for ice vapor growth in combination with mass–size relationships to determine the evolution of ice water content (IWC) and ice particle maximum dimension in time. These approaches are limited since a single axis length is used, the aspect ratio is usually held constant and mass–size relations have many available coefficients for similar ice types. Fixing the crystal aspect ratio severs the nonlinear link between aspect ratio changes and increased growth rates that occur during crystal growth. A method is presented here for predicting two crystal axes and the crystal aspect ratio in bulk models. Evolution of the ice mass mixing ratio is tied to the evolution of two axis length mixing ratios through the use of a historical axis ratio parameter containing memory of crystal shape. This parameter links the distributions of the two axes, allowing characterization of particle lengths using a single distribution. The method uses four prognostic variables: the mass and number mixing ratios, and two axis length mixing ratios. Development of the method is presented, with testing described in Part II.

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Jonathan L. Petters, Jerry Y. Harrington, and Eugene E. Clothiaux

Abstract

When stratiform-cloud-integrated radiative flux divergence (heating) is dependent on liquid water path (LWP) and droplet concentration Nd, feedbacks between cloud dynamics and this heating can exist. These feedbacks can be particularly strong for low LWP stratiform clouds, in which cloud-integrated longwave cooling is sensitive to LWP and Nd. Large-eddy simulations reveal that these radiative–dynamical feedbacks can substantially modify low LWP stratiform cloud evolution when Nd is perturbed.

At night, more rapid initial evaporation of the cloud layer occurs when Nd is high, leading to more cloud breaks and lower LWP values that both result in less total cloud longwave cooling. Weakened circulations result from this reduced longwave cooling and entrainment drying is able to counteract cloud growth. When Nd is low, the cloud layer is better maintained because cloud longwave cooling is still relatively strong.

During the day, the addition of shortwave warming leads to reduced LWP for all values of Nd and, consequently, further reduced longwave cooling and weakened circulations. For high Nd, these reductions are such that the cloud layer cannot be maintained. For lower Nd, the reductions are smaller and the cloud layer thins but does not dissipate.

These results suggest that low LWP cloud layers are more tenuous when Nd is high and are more prone to dissipating during the day. Comparison with other studies suggests the modeled low LWP cloud response may be sensitive to the initial thermodynamic profile and model configuration.

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