Riming Growth Research Articles

Microphysical mechanisms of wintertime postfrontal precipitation enhancement over the Australian Snowy Mountains

AbstractA heavy orographic precipitation event associated with the postfrontal sector of a midlatitude cyclone over the Australian Snowy Mountains (ASM) was analyzed using field observations and numerical simulations. This event, observed during a 2018 intensive field campaign, was of particular interest as three distinct precipitation episodes were identified within a prolonged postfrontal period. Deep mixed‐phase clouds (MPCs) characterized by cold cloud‐top temperatures (colder than −30°C) and the presence of updrafts extending 3.5–4.5 km above the boundary‐layer height, produced the three enhanced precipitation events over the windward slopes of the ASM. The presence of conditional instabilities and deep updrafts were also found in the sounding and Doppler velocity observations respectively, while the cloud radar observations show the deep MPCs with cloud tops reaching to 6–7 km a.s.l. Orographic convection invigoration was found to be the main mechanism producing the precipitation enhancement over the windward slopes and higher terrain. Using the Weather Research/Forecasting model, we analyzed the rates of microphysical processes to explicitly account for the enhancement of precipitation formation processes in these MPCs. This analysis showed that the precipitation formation processes were further enhanced through depositional and riming growth of ice‐phase hydrometeors during the three precipitation events. Deposition is simulated at higher levels (above the −15°C level) and most likely enabled by deep convective updrafts through the midtroposphere, whereas riming is stronger at lower levels (below −10°C level) due to the persistent production of feeder supercooled liquid water clouds sustained by the orographic lifting.

Impact of Aerosols on Convective System Over the North China Plain: A Numerical Case Study in Autumn

AbstractThe effects of aerosols on deep convective systems are highly uncertain. We used a coupled meteorology and chemistry model to investigate the effects of aerosols on a convective precipitation event that took place in North China from 7 to 10 October 2017. A higher concentration of aerosols can lead to more cloud condensation nuclei, which increases the condensation of water vapor and the formation of cloud droplets. Although the auto‐conversion from cloud droplets to rain can be inhibited due to the diminished effective radius of cloud droplets, more cloud droplets can be accreted by raindrops, resulting in a larger effective radius of raindrops and making them less likely to evaporate. Besides, aerosols can contribute to the vapor deposition of snow, and a larger number of cloud droplets can provide the resources needed for the riming growth of snow in the cloud, which eventually results in more and larger snow particles. Also, the net latent heat released by these microphysical processes can increase the vertical velocity and facilitate the development of the convective system. Thereby, the precipitation in North China during the study period was ultimately enhanced. We analyzed the influence of aerosols on this precipitation event from a microphysical perspective, which might improve our understanding of complex aerosol‐cloud‐precipitation interactions.

Polarimetric Size Sorting Signatures in the Convective Regions of Mesoscale Convective Systems in PECAN: Implications on Kinematics, Thermodynamics, and Precipitation Pathways

An object-based technique was utilized to identify hydrometeor size-sorting signatures at lower levels in the convective regions of 10 mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) field campaign. Composite statistical analysis indicates that the magnitudes of size-sorting signatures (the separation distances between rain diameter maxima and concentration maxima) are nonlinearly correlated to the echo-top height, rain mass beneath the melting level, and precipitation rates at higher percentiles. To explore this correlation, the weather forecasting and research model was used to simulate the 20 June 2015 MCS during PECAN. Statistical analysis of the model outputs indicates more active riming growth and quicker graupel fallout at warmer temperatures near areas with larger separation distances. While updraft intensity above the melting level was also positively correlated to separation distances, this correlation was only statistically significant within certain temperature ranges. Additional analyses reveal that the higher intense precipitation potential near signatures with large separation distances could be attributed to precipitation production from the melted graupel. Finally, spatial correspondence between graupel distribution at the melting level and rain distribution at the lowest model level illustrates the critical role of graupel sedimentation and sorting in creating size-sorting signatures in MCSs during the PECAN field experiment.

Ice Concentration Retrieval in Stratiform Mixed-Phase Clouds Using Cloud Radar Reflectivity Measurements and 1D Ice Growth Model Simulations

Abstract Measurements of ice number concentration in clouds are important but still pose problems. The pattern of ice development in stratiform mixed-phase clouds (SMCs) offers an opportunity to use cloud radar reflectivity (Ze) measurements and other cloud properties to retrieve ice number concentrations. To quantify the strong temperature dependencies of ice crystal habits and growth rates, a one-dimensional (1D) ice growth model has been developed to calculate ice diffusional growth and riming growth along ice particle fallout trajectories in SMCs. The radar reflectivity and fallout velocity profiles of ice crystals calculated from the 1D ice growth model are evaluated with the Atmospheric Radiation Measurements (ARM) Climate Research Facility (ACRF) ground-based high-vertical-resolution radar measurements. A method has been developed to retrieve ice number concentrations in SMCs at a specific cloud-top temperature (CTT) and liquid water path (LWP) by combining Ze measurements and 1D ice growth model simulations. The retrieved ice number concentrations in SMCs are evaluated using integrated airborne in situ and remote sensing measurements and three-dimensional cloud-resolving model simulations with a bin microphysical scheme. The statistical evaluations show that the retrieved ice number concentrations in the SMCs are within an uncertainty of a factor of 2.

High-Resolution Polarimetric Radar Observations of Snow-Generating Cells

AbstractHigh-resolution X-band polarimetric radar data were collected in 19 snowstorms over northern Colorado in early 2013 as part of the Front Range Orographic Storms (FROST) project. In each case, small, vertically erect convective turrets were observed near the echo top. These “generating cells” are similar to those reported in the literature and are characterized by ~1-km horizontal and vertical dimensions, vertical velocities of 1–2 m s−1, and lifetimes of at least 10 min. In some cases, these generating cells are enshrouded by enhanced differential reflectivity ZDR, indicating a “shroud” of pristine crystals enveloping the larger, more isotropic particles. The anticorrelation of radar reflectivity factor at horizontal polarization ZH and ZDR suggests ongoing aggregation or riming of particles in the core of generating cells. For cases in which radiosonde data were collected, potential instability was found within the layer in which generating cells were observed. The persistence of these layers suggests that radiative effects are important, perhaps by some combination of cloud-top cooling and release of latent enthalpy through depositional and riming growth of particles within the cloud. The implications for the ubiquity of generating cells and their role as a mechanism for ice crystal initiation and growth are discussed.

Aerosol Impacts on the Microphysical Growth Processes of Orographic Snowfall

AbstractThe Regional Atmospheric Modeling System was used to simulate four winter snowfall events over the Park Range of Colorado. For each event, three hygroscopic aerosol sensitivity simulations were performed with initial aerosol profiles representing clean, moderately polluted, and highly polluted scenarios. Previous work demonstrates that the addition of aerosols can produce a snowfall spillover effect, during events in which riming growth of snow is prevalent in the presence of supercooled liquid water, that is due to a modified orographic cloud containing more numerous but smaller cloud droplets. This study focuses on the detailed microphysical processes that lead to snow growth in each event and how these processes are modulated by the addition of hygroscopic aerosols. A conceptual model of hydrometeor growth processes is presented, along a vertical orographic transect, that reveals zones of vapor deposition of ice and liquid, riming growth, evaporation, sublimation, and regions in which the Wegener–Bergeron–Findeisen (WBF) snow growth process is active. While the aerosol-induced spillover effect is largely determined by the degree of reduction in ice particle riming, an enhancement in the WBF snow growth process under more polluted conditions largely offsets the loss of rime growth, thus leading to a minimal net change in the regional precipitation.

Investigation of Microphysical Processes Occurring in Organized Convection during NAME

AbstractA major objective of the North American Monsoon Experiment (NAME) was to quantify microphysical processes within convection occurring near the steep topography of northwestern Mexico. A previous study compared examples of isolated convection using polarimetric radar data and noted a dependence on mixed-phase processes via drop freezing and subsequent riming growth along the coastal plain and western slopes, with an even greater role of melting ice in rainfall production over the highest terrain. Despite the higher frequency of these isolated cells compared to organized convective systems, the latter were responsible for 75% of rainfall. Therefore, this study seeks to evaluate the role of mesoscale organization on microphysical processes and describes the evolution of these systems as a function of topography.Similar to isolated convection, both warm-rain and ice-based processes played important roles in producing intense rainfall in organized convection. Although similarities existed between cell types, organized convection was typically deeper and contained greater ice mass, which melted and contributed to the development of outflow boundaries. As convection organized along the slopes, these boundaries spread over the lower terrain, converging with diurnally driven upslope flow, thus allowing for the generation of new convection and propagation toward the coast. Once over lower elevations, additional warm-cloud depth contributed to intense rainfall and allowed for continued ice production. This, along with the development of rear inflow in the trailing stratiform region, led to further development of convective outflow, similar to organized systems in the tropics and midlatitudes.

The Impact of Varying Environmental Conditions on the Spatial and Temporal Patterns of Orographic Precipitation over the Pacific Northwest near Portland, Oregon

Abstract Operational radar data from three winter seasons (2003–06) in Portland, Oregon, in the U.S. Pacific Northwest are used to describe how orographic precipitation varies with cross-barrier wind speed, 0°C level height, and stability over the moderately wide (~50-km half-width) Cascade Mountain Range. Orographic enhancement is specified in terms of location, frequency, and relative intensity of the reflectivity (precipitation field). The typical storm for the region, as defined by the 25th to 75th percentile characteristics, is compared to storms with <25th and >75th percentile characteristics for a given variable. About half of Portland-region storms have a low-level wind direction within a relatively narrow azimuth range. This subset of storms is used to examine the sensitivity of orographic enhancement relative to other environmental variables. Cross-barrier wind speed has a stronger role in determining the magnitude of precipitation frequency than either 0°C level or stability. Cross-barrier wind speed and 0°C level height have separate but comparable roles in determining the frequency of relatively heavier precipitation. The increase in precipitation frequency with stronger cross-barrier wind speed is partially attributed to the higher occurrence of intermittent convective cells intersecting the slope. The area where inferred riming growth occurs over local peaks on the windward slope broadens upslope as the 0°C level height increases. In the Portland region, variations in the squared moist Brunt–Väisälä frequency yield smaller differences in the pattern and intensity of precipitation enhancement than either cross-barrier wind speed or 0°C level height.

Tropical Rain Characteristics and Microphysics in a Three-Dimensional Cloud Model

Abstract The rain characteristics of convective clouds have been investigated numerically. Five different microphysical model settings have been used to study the roles of various mechanisms influencing rain formation: maritime ice (full microphysics, low cloud nuclei), maritime frozen (neglecting ice nuclei, enhancing drop freezing), maritime warm (only warm rain), continental ice (high cloud nuclei), and continental warm. Rain patterns and accumulation, drop growth modes, cell organization, heating rate profiles, and the alignment of rain cells in rainbands all differ greatly with different microphysics. Rainfall amounts were highest with maritime ice. With ice, a single, large rain cell was formed by absorbing small cells from the front and sides of the main cell. Forward in the cell, frozen drop formation dominated. Graupel-based hail fell from the storm center. A unique rain accumulation process produced heavy rain in a sloped updraft. Graupel fell from above, producing a high hail water content near the freezing level by collecting many supercooled drops from merging cells. The apparent heating source peaked at two different levels: one by drop condensation growth at about 3 km, and the other by depositional growth at about 5 km. With maritime ice, a 60-km rain ring appeared and persisted. In the cases of the rainband model, lines of rain cells moved intermittently through the formation of cells at the cloud's leading edge in maritime ice. The results of these investigations, with and without ice, indicates the importance of diffusive and riming growth of ice particles and the associated release of latent heat in the development of convection and rain.

Study of hail density parametrizations

AbstractOne aspect of hailstone growth that has received attention over the last twenty years is the importance of low‐density riming growth in hail. Density is an important parameter in riming growth because it regulates hail size and is also involved in other atmospheric processes, like crystal multiplication, charge transfer, etc. New numerical dynamic and cinematic models take into account the variability of density with growth parameters (such as size of cloud droplets, impact velocity and surface temperature) in order to attain more realistic results. These kinds of models need to have a function that describes the real low‐density riming process.Analysing previous results, it is clear that several different empirical parametrizations are employed to describe density. These parametrizations relate density to growth parameters through the variable X=Vob/Ts, where Vo is the impact velocity of droplets into the target, b is the droplet size, and Ts the hail surface temperature.These empirical expressions give different density values for identical growth conditions. It is important to note that these parametrizations stem from experimental data obtained in very different growth conditions and with different collector geometries.Working with experimental density data, we can show that different parametrizations are equivalent if X is defined in an appropriate way (taking into account correct impact velocity and collector geometry). Different environmental conditions are taken into account by means of Stokes' parameter.Due to this finding, we have aimed at developing a parametrization to be used in cloud models, which satisfactorily represents all experimental results. Copyright © 2002 Royal Meteorological Society

Study of hail density parametrizations

One aspect of hailstone growth that has received attention over the last twenty years is the importance of low-density riming growth in hail. Density is an important parameter in riming growth because it regulates hail size and is also involved in other atmospheric processes, like crystal multiplication, charge transfer, etc. New numerical dynamic and cinematic models take into account the variability of density with growth parameters (such as size of cloud droplets, impact velocity and surface temperature) in order to attain more realistic results. These kinds of models need to have a function that describes the real low-density riming process. Analysing previous results, it is clear that several different empirical parametrizations are employed to describe density. These parametrizations relate density to growth parameters through the variable X=Vob/Ts, where Vo is the impact velocity of droplets into the target, b is the droplet size, and Ts the hail surface temperature. These empirical expressions give different density values for identical growth conditions. It is important to note that these parametrizations stem from experimental data obtained in very different growth conditions and with different collector geometries. Working with experimental density data, we can show that different parametrizations are equivalent if X is defined in an appropriate way (taking into account correct impact velocity and collector geometry). Different environmental conditions are taken into account by means of Stokes' parameter. Due to this finding, we have aimed at developing a parametrization to be used in cloud models, which satisfactorily represents all experimental results. Copyright © 2002 Royal Meteorological Society

The Growth of Atmospheric Ice Crystals: A Summary of Findings in Vertical Supercooled Cloud Tunnel Studies

Measurements of ice crystal growth under free fall in a generation of vertical supercooled cloud tunnels and some static cloud chambers as well as the related theoretical works are summarized. Growth parameters, that is, mass (m), dimensions, apparent density, and fall velocity ( w), show extrema at about 258, 2108, and 2158C where crystals are predominantly column-needle, isometric, and plate-stellardendrite, respectively. Crystal shape enhances with time (t) at about 258 and 2158C, whereas at 2108C the effect is minimal and crystals show strongest tendency to grow into graupel due to the fall velocity maximum discovered early in the series of present studies. At this temperature, switch-over of growth mode toward graupel occurs more quickly as liquid water content (WL) increases. Under a fixed cloud condition, the shape-enhanced crystals hardly grow into graupel and vice versa. The diffusional growth theory, with Maxwellian surface condition and without ventilation, describes well the behaviors of intermediate size crystals for which m } t 3/2 } (2w) 3/2 } (2z) 3/4; z, being the fall distance, is identified. Small crystals grow more slowly due to accommodation coefficient effects and larger ones grow faster due to enhanced ventilation and riming. To include these effects, a generalized growth theory is formulated. A simple theory is developed for graupel/hail growth where m } (WLt) 6 } w6 } (2z) 3, ra being the air density. 23 ra Based on these relationships, the dominance of diffusional growth mechanism for precipitation development in shallow, convectively weak, winter clouds and that of graupel/hail-type riming growth in deep, convectively strong, summer clouds is explained.

日本の日本海側の山岳地域における降雪の形成に貢献する雲粒捕捉成長過程

Heavy snowfall occurs every year in orographic areas of Japan facing the Japan Sea. The riming growth process contributing to the formation of snowfall was studied by radar and microphysical analyses. The observational results can be summarized as follows. Radar analyses showed that snow clouds which formed over the Japan Sea moved from the sea toward the land, and changed from the developing stage to the mature stage and then to the dissipating stage as they moved inland. When the clouds reached an orographic area, they developed again due to topographic updraft. The microphysical measurements showed that rime mass increased with an increase in snowfall intensity, and with an increase in wind speed when the mass flux of ice crystals was subtracted. Besides, it was supposed from the figure of wind speed vs. riming proportion relationship that rime mass increased with an increase in wind speed even if the advection effect of cloud droplets is subtracted. These indicate in the case of a strong wind that falling ice crystals can wash out many cloud droplets in clouds where cloud droplets increased, resulting in a snowfall of high riming proportion and high intensity. Thus, the riming growth process contributes significantly to the formation of orographic snowfall. The observational results also showed that the deposition and aggregation growth processes contributed to the formation of orographic snowfall. It was shown that the liquid water content in snow clouds depends on not only the production rates of clond droplets due to topography-induced updraft, but also the mass flux of ice crystals in clouds. The observational results showed that two cases in which there are only a small mass flux of ice crystals, as compared with the amounts of cloud droplets, have a high potential for artificial seeding experiments.

The relation between prevailing precipitation-particle type and radar echo structure around the Dewa Hills

Snow clouds influenced by topography were observed on 13–19 February 1992, and were analyzed using the data of Doppler radars, surface observations and rawinsondes. The analysis period was divided into two stages (stages I and II) based on the radar echo pattern and the rawinsonde data. The radar echo intensified over the Dewa Hills in both stages. Each stage represented the different type of snow cloud. The cloud type was clearly related to the prevailing precipitation-particle type. The cold outbreak was weak and widespread echoes appeared in stage I. Snowflakes were dominated among the observed precipitation-particles, and weak orographic lifting was expected. It is suggested that deposition growth prevailed over the Dewa Hills in stage I. The cold outbreak was strong and many echo cells were observed in stage II. Graupels dominated among the observed precipitation-particles. The low-level wind favored the strong orographic lifting. It is inferred that riming growth prevailed over the Dewa Hills in stage II. The contribution of graupel to the precipitation intensity was larger when the observation site was closer to the center of an echo cell.

海岸部と内陸部における降雪粒子の雲粒捕捉成長に関する比較

It is of importance to study the regional characteristics of the snowfall-formation mechanism in order to lessen and possibly prevent damage caused by heavy snowfalls. Therefore, snowfalls were observed in areas toward the Japan Sea, Japan, paying attention to the riming growth of snow particles for the purpose of studying the regional characteristics of the snowfall-formation mechanism. The observational results can be summarised as follows. Snowfall, in which the riming process was predominant in the growth of snow particles (snow particles having a mass of rime more than 50% of the total mass), accounted for about 70% of the total snowfalls in coastal areas. On the other hand, the riming process accounted for about 40% of the total snowfalls in inland areas. On the bases of this and other observational results, it was clarified that the riming process plays an important role in the growth of snow particles in coastal areas, whereas in inland areas the role is not so important. It was shown that the contribution of the riming process to the growth of snow particles is controlled by meteorological conditions. Favorable meteorological conditions are found more frequently in coastal areas than in inland areas. These observational facts explain why the riming process is predominant in the growth of snow particles in coastal areas and not in inland areas

An aircraft study of rapid precipitation development and electrification in a growing convective cloud

The rapid initial precipitation growth and initial electrification of a convective cloud, growing as a new cell on the upshear side of a cloud system in Florida, is traced from radar data and aircraft penetrations at the −7°C to −10°C level. This study combines radar, microphysical and electrical measurements so that an examination of the interactions between the cloud dynamics, microphysics and electrification is possible. The first pass (−7°C) was characterized by a strong 23 m/s updraft, all liquid cloud water, no precipitation, and no significant electrification. In the 300 s between the two penetrations, precipitation developed very rapidly from < 15 dBZ to < 45 dBZ, and the vertical component of the electric field increased from below the measurement threshold to − 25 kv/m. The second penetration, which started at − 7°C and ended at − 10°C, was still exclusively updraft, but with lesser peak velocities and a more complex structure; i.e., no downdraft, but with relative minima in the updraft. The microphysics of the second pass displayed a segment of exclusively cloud liquid water (no precipitation size hydrometeors), a small segment of all liquid precipitation size hydrometeors, a small region of mixed hydrometeors and an extensive region of graupel hydrometeors, ranging in size from 100 μm to several mm. High cloud liquid water coexisted with the liquid and graupel hydrometeors in the strong updrafts. The electrification was observed to occur exclusively in the segments of the cloud pass where graupel were observed. Within this graupel region, where the graupel often coexisted with supercooled cloud liquid water, significant electric field occurred only at relative minima in the updraft. These relative velocity minima were also minima in the cloud liquid water content. The observed updraft velocities in these relative minima were close to balance velocities for the observed larger graupel hydrometeors. The strongest updrafts, where the formation and the riming growth of graupel was the greatest (maxima in cloud liquid water content), were not the locations of significant electrification at this flight level.

Evaluation of a Two-Dimensional Kinematic Cloud Model Using Data from a Central Sierra Nevada Orographic Cloud System

Abstract A two-dimensional kinematic cloud model was developed to include snowflake growth by aggregation as well as riming and vapor deposition. The model includes new parameterizations of ice crystals and aggregates of ice crystals. The model was run and evaluated using a dataset gathered by the Sierra Cooperative Pilot Project (SCPP). The model was tested for its sensitivity to two model parameters, a constant relating the mean mass of collected ice crystals to the mean mass of all ice crystals in the cloud, and an ice nucleation rate constant. Varying the collection constant by a factor of 2.5 had only minor effects on the model behavior, but varying the ice nucleation rate by a factor of 50 had significant effects on the relative importance of riming and vapor deposition growth. Two problems that did develop in the model were the result of the model's bulk microphysical assumptions relating graupel and raindrop size to the magnitude of the mixing ratio, so that small concentrations behaved as cloud p...

On the Relative Efficiency of Coalescence and Riming

Abstract Riming growth rates for graupel and frozen raindrops are compared with those of unfrozen drops growing by coalescence. The results indicate that graupel can have a considerable advantage over unfrozen raindrops in regions where the cloud droplets are large (e.g., 20 μm diameter). In this case, the graupel growth rates win also be enhanced by reductions in the graupel bulk densities, at least down to values as low as 0.2 g cm−3 or less. When encountering smaller cloud droplets, however, the apparent advantage of the graupel particles fades, and at times may even be reversed.

Numerical Modeling of Hailstorms and Hailstone Growth. Part II: The Role of Low-Density Riming Growth in Hag Production

The past several years have seen a renewed interest in the importance of low-density riming growth to the development of hailstones. This paper reports on the results of a study that incorporates the physical factors controlling the density of the rime deposit in a two-dimensional, time-dependent numerical cloud model with discretized treatment of the graupel/hail size distribution. Comparisons are made between cases in which the mass-diameter relationship is fixed based on a priori assumed particle densities and cases in which the mass-diameter relationship is allowed to change in accordance with the variable particle density diagnosed from the riming density relationship and past growth history. Compared to the fixed particle density treatment common to earlier work, ice particles of lower density have enhanced surface and cross-sectional area for particles of equal mass, which in turn, increases the effective ventilation experienced by the particles and their capture volumes, thus allowing enhanced diffusional and accretional growth. The lower density particles also experience reduced sedimentation effects due to reduced fallspeeds. This leads to increased residence time in favorable growth environments for both the low density embryos in the region of the embryo curtain and for medium density particles growing to hail in the more active regions of the cloud.

Stochastic riming of plate-like and columnar ice crystals

Riming growth of ice particles is simulated by numerically solving the stochastic collection equations, simultaneously considering coagulation of water droplets. By introducing a special criterion which defines the habit of a riming particle, the influence of this habit on the growth of several kinds of ice particles assumed to be formed during riming was investigated.