Secondary Ice Particle Production Research Articles

Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

Abstract. The seeder–feeder mechanism has been observed to enhance orographic precipitation in previous studies. However, the microphysical processes active in the seeder and feeder region are still being understood. In this paper, we investigate the seeder and feeder region of a mixed-phase cloud passing over the Swiss Alps, focusing on (1) fallstreaks of enhanced radar reflectivity originating from cloud top generating cells (seeder region) and (2) a persistent low-level feeder cloud produced by the boundary layer circulation (feeder region). Observations were obtained from a multi-dimensional set of instruments including ground-based remote sensing instrumentation (Ka-band polarimetric cloud radar, microwave radiometer, wind profiler), in situ instrumentation on a tethered balloon system, and ground-based aerosol and precipitation measurements. The cloud radar observations suggest that ice formation and growth were enhanced within cloud top generating cells, which is consistent with previous observational studies. However, uncertainties exist regarding the dominant ice formation mechanism within these cells. Here we propose different mechanisms that potentially enhance ice nucleation and growth in cloud top generating cells (convective overshooting, radiative cooling, droplet shattering) and attempt to estimate their potential contribution from an ice nucleating particle perspective. Once ice formation and growth within the seeder region exceeded a threshold value, the mixed-phase cloud became fully glaciated. Local flow effects on the lee side of the mountain barrier induced the formation of a persistent low-level feeder cloud over a small-scale topographic feature in the inner-Alpine valley. In situ measurements within the low-level feeder cloud observed the production of secondary ice particles likely due to the Hallett–Mossop process and ice particle fragmentation upon ice–ice collisions. Therefore, secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountaintop observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.

The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch

Abstract. During the winter of 2013 and 2014 measurements of cloud microphysical properties over a 5-week period at the high-alpine site Jungfraujoch, Switzerland, were carried out as part of the Cloud Aerosol Characterisation Experiments (CLACE) and the Ice Nucleation Process Investigation and Quantification project (INUPIAQ). Measurements of aerosol properties at a second, lower site, Schilthorn, Switzerland, were used as input for a primary ice nucleation scheme to predict ice nuclei concentrations at Jungfraujoch. Frequent, rapid transitions in the ice and liquid properties of the clouds at Jungfraujoch were identified that led to large fluctuations in ice mass fractions over temporal scales of seconds to hours. During the measurement period we observed high concentrations of ice particles that exceeded 1000 L−1 at temperatures around −15 °C, verified by multiple instruments. These concentrations could not be explained using the usual primary ice nucleation schemes, which predicted ice nucleus concentrations several orders of magnitude smaller than the peak ice crystal number concentrations. Secondary ice production through the Hallett–Mossop process as a possible explanation was ruled out, as the cloud was rarely within the active temperature range for this process. It is shown that other mechanisms of secondary ice particle production cannot explain the highest ice particle concentrations. We describe four possible mechanisms that could lead to high cloud ice concentrations generated from the snow-covered surfaces surrounding the measurement site. Of these we show that hoar frost crystals generated at the cloud enveloped snow surface could be the most important source of cloud ice concentrations. Blowing snow was also observed to make significant contributions at higher wind speeds when ice crystal concentrations were < 100 L−1.

Cloud Conditions Favoring Secondary Ice Particle Production in Tropical Maritime Convection

Abstract Progress in understanding the formation of ice in lower-tropospheric clouds is slowed by the difficulties in characterizing the many complex interactions that lead to ice initiation and to the dynamic, non-steady-state nature of the clouds. The present study characterizes the conditions where secondary ice particles, specifically identified as needle or thin columnar types, are observed in tropical maritime convection with modest liquid water contents during the Ice in Clouds Experiment-Tropical (ICE-T), based out of St. Croix, U.S. Virgin Islands, and the NASA African Monsoon Multidisciplinary Analyses (NAMMA) in 2006 sampling from Cape Verde, Africa. The properties of the cloud droplet populations relevant to the secondary ice production process and the ice particle populations are characterized as a function of temperature and vertical velocity. These secondary ice particles are observed primarily in regions of low liquid water content and weak vertical velocities. Two situations are examined in detail. First, ice formation is examined by following the tops of a group of ICE-T chimney clouds as they ascend and cool from a temperature of +7° to −8°C, examining the production of the first ice. Then, using the data from a cloud system sampled during NAMMA, the authors elucidate a process that promotes ice multiplication. The intention is that this study will lead both to a better understanding of how secondary ice production proceeds in natural clouds and to more realistic laboratory studies of the processes involved.

In-situ aircraft observations of ice concentrations within clouds over the Antarctic Peninsula and Larsen Ice Shelf

Abstract. In-situ aircraft observations of ice crystal concentrations in Antarctic clouds are presented for the first time. Orographic, layer and wave clouds around the Antarctic Peninsula and Larsen Ice shelf regions were penetrated by the British Antarctic Survey's Twin Otter aircraft, which was equipped with modern cloud physics probes. The clouds studied were mostly in the free troposphere and hence ice crystals blown from the surface are unlikely to have been a major source for the ice phase. The temperature range covered by the experiments was 0 to −21 °C. The clouds were found to contain supercooled liquid water in most regions and at heterogeneous ice formation temperatures ice crystal concentrations (60 s averages) were often less than 0.07 l−1, although values up to 0.22 l−1 were observed. Estimates of observed aerosol concentrations were used as input into the DeMott et al. (2010) ice nuclei (IN) parameterisation. The observed ice crystal number concentrations were generally in broad agreement with the IN predictions, although on the whole the predicted values were higher. Possible reasons for this are discussed and include the lack of IN observations in this region with which to characterise the parameterisation, and/or problems in relating ice concentration measurements to IN concentrations. Other IN parameterisations significantly overestimated the number of ice particles. Generally ice particle concentrations were much lower than found in clouds in middle latitudes for a given temperature. Higher ice crystal concentrations were sometimes observed at temperatures warmer than −9 °C, with values of several per litre reached. These were attributable to secondary ice particle production by the Hallett Mossop process. Even in this temperature range it was observed that there were regions with little or no ice that were dominated by supercooled liquid water. It is likely that in some cases this was due to a lack of seeding ice crystals to act as rimers to initiate secondary ice particle production. This highlights the chaotic and spatially inhomogeneous nature of this process and indicates that the accurate representation of it in global models is likely to represent a challenge. However, the contrast between Hallett Mossop zone ice concentrations and the fairly low concentrations of heterogeneously nucleated ice suggests that the Hallet Mossop process has the potential to be very important in remote, pristine regions such as around the Antarctic coast.

Ice formation and development in aged, wintertime cumulus over the UK: observations and modelling

Abstract. In situ high resolution aircraft measurements of cloud microphysical properties were made in coordination with ground based remote sensing observations of a line of small cumulus clouds, using Radar and Lidar, as part of the Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) project. A narrow but extensive line (~100 km long) of shallow convective clouds over the southern UK was studied. Cloud top temperatures were observed to be higher than −8 °C, but the clouds were seen to consist of supercooled droplets and varying concentrations of ice particles. No ice particles were observed to be falling into the cloud tops from above. Current parameterisations of ice nuclei (IN) numbers predict too few particles will be active as ice nuclei to account for ice particle concentrations at the observed, near cloud top, temperatures (−7.5 °C). The role of mineral dust particles, consistent with concentrations observed near the surface, acting as high temperature IN is considered important in this case. It was found that very high concentrations of ice particles (up to 100 L−1) could be produced by secondary ice particle production providing the observed small amount of primary ice (about 0.01 L−1) was present to initiate it. This emphasises the need to understand primary ice formation in slightly supercooled clouds. It is shown using simple calculations that the Hallett-Mossop process (HM) is the likely source of the secondary ice. Model simulations of the case study were performed with the Aerosol Cloud and Precipitation Interactions Model (ACPIM). These parcel model investigations confirmed the HM process to be a very important mechanism for producing the observed high ice concentrations. A key step in generating the high concentrations was the process of collision and coalescence of rain drops, which once formed fell rapidly through the cloud, collecting ice particles which caused them to freeze and form instant large riming particles. The broadening of the droplet size-distribution by collision-coalescence was, therefore, a vital step in this process as this was required to generate the large number of ice crystals observed in the time available. Simulations were also performed with the WRF (Weather, Research and Forecasting) model. The results showed that while HM does act to increase the mass and number concentration of ice particles in these model simulations it was not found to be critical for the formation of precipitation. However, the WRF simulations produced a cloud top that was too cold and this, combined with the assumption of continual replenishing of ice nuclei removed by ice crystal formation, resulted in too many ice crystals forming by primary nucleation compared to the observations and parcel modelling.

The influence of small aerosol particles on the properties of water and ice clouds

In this paper, results are presented of the influence of small organic- and soot-containing particles on the formation of water and ice clouds. There is strong evidence that these particles have grown from nano particle seeds produced by the combustion of oil products. Two series of field experiments are selected to represent the observations made. The first is the CLoud-Aerosol Characterisation Experiment (CLACE) series of experiments performed at a high Alpine site (Jungfraujoch), where cloud was in contact with the ground and the measuring station. Both water and ice clouds were examined at different times of the year. The second series of experiments is the CLOud Processing of regional Air Pollution advecting over land and sea (CLOPAP) series, where ageing pollution aerosol from UK cities was observed, from an airborne platform, to interact with warm stratocumulus cloud in a cloud-capped atmospheric boundary layer. Combining the results it is shown that aged pollution aerosol consists of an internal mixture of organics, sulfate, nitrate and ammonium, the organic component is dominated by highly oxidized secondary material. The relative contributions and absolute loadings of the components vary with location and season. However, these aerosols act as Cloud Condensation Nuclei (CCN) and much of the organic material, along with the other species, is incorporated into cloud droplets. In ice and mixed phase cloud, it is observed that very sharp transitions (extending over just a few metres) are present between highly glaciated regions and regions consisting of supercooled water. This is a unique finding; however, aircraft observations in cumulus suggest that this kind of structure may be found in these cloud types too. It is suggested that this sharp transition is caused by ice nucleation initiated by oxidised organic aerosol coated with sulfate in more polluted regions of cloud, sometimes enhanced by secondary ice particle production in these regions.

Numerical modelling of mixed-phase frontal clouds observed during the CWVC project

The Met Office's cloud-resolving model was used to model mixed-phase frontal clouds observed as part of the Clouds, Water Vapour and Climate programme between 1999 and 2001. The clouds were studied using the Met Office's C130 aircraft and the Chilbolton radar facility. In the model, precipitation forms in updraughts then falls, giving rise to regions of high radar reflectivity similar to those observed. The vertical motions were driven by conditional instability or shear instability. The sensitivity of the model results to the microphysics was examined by varying the number of primary ice nuclei and switching off the Hallett–Mossop process. It is demonstrated that the ice nucleation processes, both primary nucleation and secondary ice particle production, are key to the precipitation production in the cloud. This occurs both through the detail of cloud microphysics and the release of latent heat of fusion in the cloud. This is very sensitive to the details of the microphysics and can markedly change the cloud dynamics. In both cases it seems that the cloud dynamics can be simulated locally without considering the large-scale forcing. © Royal Meteorological Society, 2005. The contributions of P. R. A. Brown and P. R. Field are Crown copyright.

A modelling study of ice‐spectrum modes in deep frontal clouds

AbstractResults from a one‐dimensional mixed‐layer model with explicit microphysics and a discrete ice‐spectrum were compared to in situ data provided by the Met Office C‐130 aircraft. Bimodal ice‐spectra were observed. The first mode was found at diameters near the lower limit of the instrumentation resolution (25 µm) at a roughly constant concentration. The second mode was found initially at diameters exceeding 200 µm but it shifted to larger sizes and smaller peak concentrations as depths below the cloud top increased.The objectives of the paper are threefold: validation of the model with real cloud data; an analysis of parameters controlling the rate of aggregation, and to show that a secondary ice‐production process is needed to maintain the first mode and produce a second mode which is sensible.A reasonable first mode could only be maintained by producing a constant supply of small ice particles at a rate of at least 10 m−3s−1. Primary nucleation was unable to supply this rate of ice‐particle production at temperatures above −20°C. In the model, it was achieved by invoking a secondary ice‐particle production process, such as the break‐up of evaporating ice crystals. The rate of production required is in broad agreement with laboratory investigations of secondary ice‐particle production during evaporation.The relative importance of the processes of aggregation, deposition and sedimentation to the production of the bimodal spectra is also studied. While it is possible to grow ice particles to large sizes by deposition alone, the number concentration observed in these clouds produces so much competition for water vapour that deposition growth is very limited. The aggregation process is the strongest mechanism for the production of the larger ice‐particles, which then precipitate. Consequently, its strength controls the rate of small ice‐particle production needed, and so is mainly responsible for the second mode. Copyright © Royal Meteorological Society, 2003. P. R. Field's contribution is Crown copyright.

A laboratory study of the effect of velocity on Hallett–Mossop ice crystal multiplication

Laboratory experiments into the effect of ice multiplication in clouds by the Hallett–Mossop (H–M) mechanism have been extended to higher velocities than in earlier studies. The secondary ice particle production rate per milligram of rime accreted is found to be dependent on the velocity of the graupel pellet as it accretes supercooled droplets. In the range 1.5 to 12 m s −1, the maximum secondary ice particle ejection occurs at 6 m s −1 with a rate of one crystal per 70 mg of accreted rime. The relevance of these new results to the particle production mechanism and to behaviour in natural clouds is discussed.

Microphysical Structure and Evolution of a Central Sierra Nevada Orographic Cloud System

This paper describes complex microphysical interactions observed within an orographic cloud system over the central Sierra Nevada of California. Measurements of the physical structure and microphysical characteristics of this cloud system were made during the Sierra Cooperative Pilot Project (SCPP) with a 5-cm scanning radar, an instrumented aircraft, two rawinsonde stations, and a network of precipitation gauges across the mountain. The cloud system extended 130 km upwind of the Sierra Nevada crestline, was 4 km deep, had a base temperature of +5°C, and a top temperature of approximately −15°C. Cloud droplets rising from cloud base grew to sizes >50 μm and first encountered dendritic ice particles descending from cloud top over the middle elevations of the Sierra Nevada. Within this region, rapid removal of the large droplets by riming occurred, and secondary ice-crystal production due to rime splintering was observed. Needles produced by secondary ice-particle production were incorporated into dendritic aggregates falling from cloud top farther toward the mountain crest. The nature of the microphysical interactions within this cloud system and their impact on the distribution, amount, and type of precipitation across the mountain range are discussed. Changes in the distribution of precipitation from the orographic cloud system due to the approach of a short-wave trough are also presented.

Characteristics of Cloud Ice and Precipitation during Wintertime Storms over the Mountains of Northern Colorado

Abstract This article describe cloud ice and precipitation process in 17 wintertime storm systems that occurred over the mountains of northwestern Colorado. Surface, remote sensing and aircraft measurements are used to study cloud and precipitation characteristics. Ice particle concentrations; secondary ice particle production; and ice panicle growth, habits and aggregation are studied in the context of cloud system structure. The region of the cloud near cloud top was found to be a primary source region for ice particles in these clouds. Ice particle concentrations were found to increase over elevated topography and to increase at lower altitudes in deep cloud systems. Approximately 80% of the measured concentrations were within the range of 1-100 L−1, and 50% were less than 20 L−1. The activity of three secondary ice particle production mechanisms was evaluated. Fragmentation was found to be important in cloud systems that contained dendritic crystals. Evidence suggested that the Hallett-Mossop rime spl...

Numerical Simulation of the Effects of Varying Ice Crystal Nucleation Rates and Aggregation Processes on Orographic Snowfall

Abstract The Colorado State University cloud model is applied to the simulation of orogrophic cloud snowfall. A model of ice crystal aggregation processes and primary nucleation and secondary ice particle production of crystals is described. Sensitivity experiments demonstrated that aggregation plays an important role in controlling the fields of cloud liquid water content, ice crystal concentrations, and surface precipitation amounts. The sensitivity experiments also support observations that the air mass is often quite clean in upper levels of stable orographic clouds. Introducing a reduction of available nuclei that can be activated by deposition/sorption processes brought concentrations to within observed values. This study clearly emphasizes the need for a great deal more fundamental research in the physics of aggregation processes and primary and secondary nucleation of ice crystals.

A laboratory study of secondary ice particle production by the fragmentation of rime and vapour-grown ice crystals

Laboratory experiments have been performed to study the strength of rime and vapour-grown ice crystals to investigate the possibility that fragmentation may occur following collisions in cloud. The results show that fragmentation of rime is very unlikely to occur in natural clouds. Vapour-grown crystals are found to be considerably more fragile, with dendrites the weakest crystal habit. In the experiments one or two microscopic fragments were produced. Collisions between dendritic crystals and graupel particles may be able substantially to enhance ice crystal concentrations in some clouds.

A laboratory study of secondary ice particle production by the fragmentation of rime and vapour‐grown ice crystals

AbstractLaboratory experiments have been performed to study the strength of rime and vapour‐grown ice crystals to investigate the possibility that fragmentation may occur following collisions in cloud. The results show that fragmentation of rime is very unlikely to occur in natural clouds. Vapour‐grown crystals are found to be considerably more fragile, with dendrites the weakest crystal habit. In the experiments one or two microscopic fragments were produced. Collisions between dendritic crystals and graupel particles may be able substantially to enhance ice crystal concentrations in some clouds.

Secondary ice particle production during rime growth: The effect of drop size distribution and rimer velocity

AbstractThe rate of production of secondary ice particles during the growth of rime in a supercooled cloud at about ‐5°C is known to depend mainly upon the rate of accretion of large drops (diameter ⩾25 m̈m). New experiments have defined more clearly how ‘splinter’ production is affected by the rate of accretion of small drops (diameter ⩽12m̈m).The relationship between secondary ice production and rimer velocity is explored over a wider velocity range than before. At low velocities, splinter production persists to 0.55 ms−1 and probably lower. There is no evidence of a low‐velocity cut‐off as previously claimed. At high velocities, splinter production persists to velocities >5ms−1. This may have important implications for secondary ice production in hail clouds.

Secondary ice particle production during rime growth: the effect of drop size distribution and rimer velocity

Secondary ice particle production during rime growth: the effect of drop size distribution and rimer velocity

Temperature dependence of secondary ice crystal production during soft hail growth by riming

AbstractSimple experiments confirm that the production of secondary ice particles during the growth of rime depends upon the surface temperature of the riming particle rather than upon the cloud temperature. The required surface temperature of −2.5 to −7.5 °C may be produced over a wide range of cloud temperatures depending on the liquid water content of the cloud and the nature of the riming particle. A particle of high fall velocity in a cloud of high liquid water content may produce secondary ice particles at cloud temperatures considerably below −8 °C.

Temperature dependence of secondary ice crystal production during soft hail growth by riming

Simple experiments confirm that the production of secondary ice particles during the growth of rime depends upon the surface temperature of the riming particle rather than upon the cloud temperature. The required surface temperature of −2.5 to −7.5 °C may be produced over a wide range of cloud temperatures depending on the liquid water content of the cloud and the nature of the riming particle. A particle of high fall velocity in a cloud of high liquid water content may produce secondary ice particles at cloud temperatures considerably below −8 °C.

Freezing modes of riming droplets with application to ice splinter production

AbstractLaboratory studies of riming have shown that, provided the heat loss is fairly symmetrical, the internal freezing pattern of an accreted drop shows two distinct modes in the temperature range −3°C to −9°C and a transition to a third below this temperature. (1) At high temperatures the freezing is dominated by an ice front growing outwards from the rimer with a somewhat enhanced growth rate around the drop edge. (2) At temperatures below about −5°C two ice fronts are evident, one from the substrate, one from the remote edge of the drop. (3) At temperatures of −9°C and below the freezing is essentially symmetrically inwards. Freezing modes (1) and (2) sometimes give rise to protuberances which may be associated with the Hallett‐Mossop ice splinter production. These are suppressed by the addition of ammonia to the drop. The three modes of freezing are accounted for in terms of the pattern of dendritic growth during the first stage of freezing, and it is suggested that this explains the low temperature cut‐off in secondary ice particle production.

The mechanism of splintering during rime growth

We present evidence that the production of secondary ice particles during rime growth on a moving body is greater at low velocities relative to the number of drops accreted (over the range 1.4 to 3 m s−1). This favours the hypothesis that ‘splintering’ is due to the formation, round the periphery of the drop, of an ice shell which later shatters. Shell formation is more likely if drops land lightly and do not spread over the ice substrate.