Ice Splinters Research Articles

Hallett‐Mossop Rime Splintering Dims Cumulus Clouds Over the Southern Ocean: New Insight From Nudged Global Storm‐Resolving Simulations

AbstractIn clouds containing both liquid and ice with temperatures between −3°C and −8°C, liquid droplets collide with large ice crystals, freeze, and shatter, producing a plethora of small ice splinters. This process, known as Hallett‐Mossop rime splintering, and other forms of secondary ice production, can cause clouds to reflect less sunlight and to have shorter lifetimes. We show its impact on Southern Ocean shallow cumuli using a novel suite of five global storm‐resolving simulations, which partition the Earth's atmosphere into 2–4 km wide columns. We evaluate simulated clouds and radiation over the Southern Ocean with aircraft observations from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), and satellite observations from Clouds and the Earth's Radiant Energy System (CERES) and Himawari. Simulations with large concentrations of ice crystals in boundary layer clouds, which agree better with SOCRATES observations, have reduced mixed‐phase cumulus cloud cover and weaker shortwave cloud radiative effects (CREs) that are less biased compared with CERES. Using a pair of simulations differing only in their treatment of Hallett‐Mossop rime splintering, we show that including this process increases ice crystal concentrations in cumulus clouds and weakens shortwave CREs over the Southern Ocean by 10 W m−2. We also demonstrate the key role that global storm‐resolving models can play in detangling the effects of clouds on Earth's climate across scales, making it possible to trace the impact of changes in individual cumulus cloud anvils (10 km2) on the radiative budget of the massive Southern Ocean basin (107 km2).

The Role of Ice Splintering on Microphysics of Deep Convective Clouds Forming Under Different Aerosol Conditions: Simulations Using the Model With Spectral Bin Microphysics

AbstractObservations during the Ice in Clouds Experiment‐Tropical (ICE‐T) field experiment show that the ice particles concentration in a developing deep convective clouds at the level of T = −15 °C reached about 500 L−1, that is, many orders higher than that of ice‐nucleating particle. To simulate microphysics of these clouds, the 2‐D Hebrew University Cloud model (HUCM) with spectral bin microphysics was used in which two main types of ice multiplication mechanisms were included in addition to the Hallet‐Mossop mechanism. In the first ice multiplication mechanism ice splinters form by drop freezing and drop‐ice collisions. Ice multiplication of this type dominates during developing stage of cloud evolution, when liquid water content is significant. At later stage when clouds become nearly glaciated, ice crystals are produced largely by ice splintering during ice‐ice collisions (the second ice multiplication mechanism). Simulations show that droplet size distributions, as well as size distributions of ice particles, agree well with the measurements during ICE‐T. Simulations with different cloud condensation nuclei concentrations show the existence of the “optimum” cloud condensation nuclei concentration (or droplet concentration), at which concentration of ice splinters reaches maximum. In these simulations ice nucleation caused by the direct formation of ice crystals upon ice‐nucleating particles, as well as the Hallett‐Mossop process, has a negligible contribution to the ice crystal concentration.

Comparing model and measured ice crystal concentrations in orographic clouds during the INUPIAQ campaign

Abstract. This paper assesses the reasons for high ice number concentrations observed in orographic clouds by comparing in situ measurements from the Ice NUcleation Process Investigation And Quantification field campaign (INUPIAQ) at Jungfraujoch, Switzerland (3570 m a.s.l.) with the Weather Research and Forecasting model (WRF) simulations over real terrain surrounding Jungfraujoch. During the 2014 winter field campaign, between 20 January and 28 February, the model simulations regularly underpredicted the observed ice number concentration by 103 L−1. Previous literature has proposed several processes for the high ice number concentrations in orographic clouds, including an increased ice nucleating particle (INP) concentration, secondary ice multiplication and the advection of surface ice crystals into orographic clouds. We find that increasing INP concentrations in the model prevents the simulation of the mixed-phase clouds that were witnessed during the INUPIAQ campaign at Jungfraujoch. Additionally, the inclusion of secondary ice production upwind of Jungfraujoch into the WRF simulations cannot consistently produce enough ice splinters to match the observed concentrations. A flux of surface hoar crystals was included in the WRF model, which simulated ice concentrations comparable to the measured ice number concentrations, without depleting the liquid water content (LWC) simulated in the model. Our simulations therefore suggest that high ice concentrations observed in mixed-phase clouds at Jungfraujoch are caused by a flux of surface hoar crystals into the orographic clouds.

Observations of the Origin and Distribution of Ice in Cold, Warm, and Occluded Frontal Systems during the DIAMET Campaign

Abstract Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L−1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L−1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m−3, but lower in the cold front case.

Modeling Study of Ice Formation in Warm-Based Precipitating Shallow Cumulus Clouds

Abstract Observations of large concentrations of ice particles in the dissipating stage of warm-based precipitating shallow cumulus clouds point to the limitations of scientists’ understanding of the physics of such clouds and the possible role of cloud dynamics. The most commonly accepted mechanisms of ice splinter production in the riming process have limitations to properly explain the rapid production of ice bursts. A more detailed description of the temporal and spatial evolution of hydrometeors and their interaction with cloud condensation nuclei and ice nuclei is needed to understand this phenomenon. A cloud model with bin-resolved microphysics can describe the time-dependent evolution of liquid droplets and ice particles and provide insights into how the physics and dynamics and their interaction may result in ice initiation and ice multiplication. The authors developed a 1.5-dimensional nonhydrostatic convective cloud and aerosol interaction model with spectral (bin) microphysics. The number and mass concentrations of aerosols, including ice nuclei and cloud condensation nuclei, were explicitly followed. Since both in situ observations of bioaerosols and laboratory experiments pointed to efficient nucleation capabilities at relative warm temperatures, it was assumed that ice-nucleating bioaerosols are involved in primary ice particle formation in condensation and immersion modes. Results show that bioaerosols can be the source of primary ice pellets, which in turn lead to high ice concentrations.

Numerical Simulation of Effects of Cloud Top Temperatures and Generating Cells on Secondary Ice Production in Stratiform Clouds with a Detailed Microphysical Model

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.

Clouds in Tropical Cyclones

Abstract Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

Clouds in Tropical Cyclones

Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

On the onset of the ice phase in boundary layer Arctic clouds

Airborne measurements in slightly supercooled Arctic boundary layer stratocumulus have been carried out in Spitsbergen on 29 May during the ASTAR 2004 campaign. Cloud measurements have been performed in both warm and cold sectors of a cold front passing the observation area. The results show a north–south gradient in freezing properties and thus evidence of significant differences in the cloud microstructure. Ahead of the front line, in the warm sector (cloud top temperature at −4°C), no ice particles were detected. The cloud formed in clean air conditions (aerosol concentration of 300 cm−3) with subsequent large effective diameter (20–26 μm) and low concentration (50 cm−3) of cloud droplets. Therefore, the collision‐coalescence process was effective, favoring the drizzle formation with concentration up to 300 L−1 (D > 50 μm). In the cold sector behind the front, with a lower cloud top temperature (−6°C), ice crystals were observed in the entire cloud layer, and no droplets larger than about 50 μm (drizzle) were detected. The observations confirm high ice particle concentrations (up to 50 L−1) even with rather warm cloud top (−6°C) compared to previous studies in Arctic clouds. The shattering of isolated drops during freezing and the ice splinter production during riming appear to be the most likely processes to explain the observations of high ice concentration in the cold sector. Analysis of back trajectories did not reveal significant differences in the origin of the air masses in the warm and cold sectors that might have contributed to the differentiation of aerosol composition and thus cloud properties. A cloud top temperature colder than −4°C appears to be required for the onset of the ice phase in this slightly supercooled stratiform cloud.

Microstructures and precipitation development in cumulus and small cumulonimbus clouds over the warm pool of the tropical Pacific Ocean

AbstractIn situ airborne measurements obtained in convective clouds in the vicinity of the Marshall Islands on 15 days in July and August 1999 are used to determine the microphysical structures and precipitation‐producing mechanisms in these clouds. The liquid water contents of the clouds were generally well below adiabatic values, even in newly risen turrets. This is attributed to the entrainment of ambient air and to the very efficient removal of cloud water by the collision and coalescence of drops. The formation of raindrops began when the concentration of droplets with radii <15 µm exceeded ∼3 cm−3 or, equivalently, when the effective cloud drop radius reached ∼ 13 µm. Clouds rained when their depth exceeded ∼1.5 km. Due to rainout, the effective radius of cloud droplets began decreasing ∼2–4 km above cloud base.Extremely high concentrations of ice particles (often >500 litre−1) formed very rapidly at temperatures between −4 and −10 °C. High‐resolution imagery of these particles, which were primarily sheaths, needles, frozen drops and irregular ice fragments, indicates that the high concentrations of ice were initiated by the freezing of individual drops, some of which fragmented upon freezing, accompanied by ice splinter production during riming. However, for ice particles < 100 µm in maximum dimension, frozen drops and ice fragments were much more numerous than columnar ice crystals, the latter being indicative of ice splinter production. Narrow (10–50 m wide) streamers of precipitation containing large (<3 mm diameter) solid and liquid particles were often encountered in growing clouds. It is proposed that the particles in these streamers grew rapidly by riming in water‐rich zones and, under appropriate conditions, the ice particles produced by both drop fragmentation and riming resulted in exceptionally high localized concentrations of ice. Convective clouds consisting entirely of liquid water produced rain rates that were similar to those from deep convective clouds containing ice.Since the concentration of drizzle and raindrops within a few kilometres of cloud base was highly correlated with cloud depth, and therefore cloud top temperature, the concentration of raindrops in cumulus and cumulo‐nimbus clouds over the warm pool of the tropical Pacific Ocean can be inferred from satellite measurements of cloud top temperatures. Copyright © 2005 Royal Meteorological Society.

The influence of aerosol concentrations on the glaciation and precipitation of a cumulus cloud

AbstractThe response of the glaciation and precipitation of a multi‐thermal cumulus cloud to changes in the aerosol concentration has been assessed in a series of sensitivity tests with the UMIST Explicit Microphysics Model (EMM). A simulation of this cloud from the Met Office cloud‐resolving model (CRM) has been utilized in these tests. This cumulus cloud was observed by aircraft during the initial stage of its growth over New Mexico on 10 August 1987. The growth of the simulated cloud is divided into two parts: a shallow phase followed by a deep phase. Maximum values of the cloud depth in these two phases were 5 and 9 km, respectively.In the EMM simulations including only the shallow phase, the precipitation efficiency was found to decrease substantially with increasing atmospheric concentrations of cloud condensation nuclei (CCN). Also, the graupel mixing ratio and total ice concentration were found to decrease as normalized CCN concentrations were increased above typical continental values. These changes are explicable in terms of: (1) the Hallett–Mossop (H–M) process at −3 to −8°C and the freezing of supercooled raindrops in collisions with ice splinters dominating the glaciation; and (2) the warm‐rain process being more significant for the overall precipitation production than the ice process in these particular simulations. The almost complete suppression of precipitation by extreme CCN concentrations corresponding to a forest‐fire plume in the EMM simulation is consistent with the analysis by Rosenfeld of satellite images of Indonesian cumuli engulfed by smoke from biomass burning.A clear tendency for ice crystals to be smaller and more numerous in the anvil was found with increasing CCN concentrations beyond typical continental values in long‐term simulations that included the deep phase. The sensitivity of the precipitation rate to the normalized CCN concentration was found to be relatively low in these deep cases. Copyright © 2002 Royal Meteorological Society.

Ice particles in stratiform clouds in the Arctic and possible mechanisms for the production of high ice concentrations

The presence of ice particles in clouds affects precipitation processes, the radiative properties of the clouds, and the derivation of cloud properties from remote sensing measurements. High ice particle concentrations occur often in slightly to moderately supercooled clouds in the Arctic. This paper combines data collected in a common type of ice‐producing arctic cloud (stratocumulus) with calculations based on laboratory experiments to elucidate mechanisms that might be responsible for the ice. Ice splinters produced during riming could account for the relatively high concentrations of ice particles in clouds that encompass temperatures between −2.5°C and −8°C. However, it has generally been assumed that ice splinters grow into pristine ice crystal habits, whereas detailed measurements in an arctic stratocumulus cloud showed that only 32% of the ice particles were pristine crystals (needles, sheaths, short columns, and plates) and 10% were broken pieces of needles or sheaths. Thirty‐seven percent of the ice particles were not identifiable crystal types, 20% were frozen drops, and 1% were aggregates and graupel. The large numbers of unidentifiable ice particles could have originated from the fragmentation of delicate ice crystals and the shattering of some drops during freezing in free fall. These two mechanisms may also play a role in the production of relatively high ice particle concentrations in moderately supercooled arctic clouds that lie outside of the temperature zone where ice splinter production by riming occurs.

The glaciation of a cumulus cloud over New Mexico

The Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi-thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud. The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett-Mossop (H-M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud-base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H-M splinter production and auto-conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm-rain process in the EMM is fundamental to the production of graupel, H-M splinters and precipitation. Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H-M process, with ice production being dominated by the freezing of supercooled raindrops in the H-M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.

The glaciation of a cumulus cloud over New Mexico

AbstractThe Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi‐thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud.The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett‐Mossop (H‐M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud‐base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H‐M splinter production and auto‐conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm‐rain process in the EMM is fundamental to the production of graupel, H‐M splinters and precipitation.Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H‐M process, with ice production being dominated by the freezing of supercooled raindrops in the H‐M band, and the immediate and continuous production of ice splinters as supercooled droplets freeze onto them.

The production of high ice‐crystal concentrations in stratiform clouds

AbstractThe appearance of ice crystals in high concentrations, of ∼ 500 1−1 in the −3 to −8°C zone of stratiform clouds, recently reported by Bower et al., is explained by the shedding of splinters by riming ice crystals at rates observed in the laboratory earlier by Hallett and Mossop. The appearance of a second maximum at −15°C, with small crystals of diameter less than 125 μm in concentrations of ∼ 1000 1−1, is attributed to the transport of small ice splinters produced in the Hallet‐Mossop zone that grow at small ice supersaturations in weak updraughts between the −8°C and −12°C levels, and thereafter at water saturation in convective cells containing liquid water and updraughts of 1 m s−1 between the −12°C and −15°C levels.

A parametrization of the ice water content observed in frontal and convective clouds

The properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global-climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied. Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud-top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett–Mossop process which operates at temperatures around −6°C, is the dominant mechanism operating. The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds. These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett-Mossop splinter-production temperature range.

A parametrization of the ice water content observed in frontal and convective clouds

AbstractThe properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global‐climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied.Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud‐top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett–Mossop process which operates at temperatures around −6°C, is the dominant mechanism operating.The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds.These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett‐Mossop splinter‐production temperature range.

Ice particle concentrations and precipitation development in small polar maritime cumuliform clouds

AbstractIncreases in the concentrations of ice particles, as well as precipitation development, can proceed very rapidly in even quite small maritime cumuliform clouds. In the upper portions of these clouds, ice particle concentrations can increase from <1 per litre to >100 per litre in ∼10 minutes, even when cloud tops are warmer than −12°C. Just prior to the onset of the high ice particle concentrations in the upper regions of these clouds, a few liquid and frozen drizzle drops appear; these are followed almost immediately by high concentrations of largely vapour‐grown ice crystals. Subsequently, the ice particle concentrations gradually decline as the ice particles grow, aggregate and fall out. Crystal habits and sizes indicate that nearly all of the ice crystals encountered in the upper regions of the clouds form close to those regions.Prior to the formation of high ice particle concentrations, the clouds generally contained, near cloud top, droplets >30μm diameter in concentrations >1cm−3, and often drizzle drops (100–400μm diameter) in concentrations >1 per litre. For maritime and continental cumuliform clouds with widths D ≥ 3km, the breadth of the droplet spectrum near the cloud top is a better predictor of the maximum ice particle concentrations that will develop in this region than is cloud‐top temperature.Exceptions to the general picture described above are the very narrow (D<3km), ‘chimney‐type’, maritime cumuliform clouds with tops that quickly subside after reaching their maximum altitude. For cloud‐top temperatures between −7 and −13°C these clouds produce fewer ice particles (∼1–20 per litre), even though droplets with diameters >30μm are present in concentrations >1cm−3 and drizzle drops are often present in concentrations >1 per litre.Several proposed mechanisms for the formation of high ice particle concentrations in clouds are discussed in the light of these field observations. In a detailed case‐study, ice particle concentrations produced by the ejection of ice splinters during riming were calculated to be about an order of magnitude less than the ice particle concentrations measured in the upper regions of the wider maritime clouds. The freezing of evaporating drops by contact nucleation could be responsible for the frozen drops that precede the high concentrations of vapour‐grown ice crystals. The latter could be produced in localized regions of unexpectedly high supersaturation that may be produced when drops begin to grow by collisions. This explanation is consistent with several of our findings concerning the conditions under which high concentrations of vapour‐grown crystals appear almost spontaneously in maritime clouds. Similarities between these natural crystals, and the conditions under which they appear, and those produced artificially by dry‐ice seeding and by aircraft flying through supercooled clouds are discussed.

Microphysical Properties of Glaciating Cumulus Clouds: Comparison of Measurements With A Numerical Simulation

AbstractObservational data on the time evolution of the concentrations of drops and ice particles in glaciating cumuli exhibiting an ice particle enhancement mechanism are compared with results of a mathematical simulation model. the model deals with kinetic equations for each type of cloud particle. Sensitivity tests concerning parameters related to the rime‐splinter process show that observations and numerical simulation results agree well when a rate corresponding to 140 to 250 droplets with diameter greater than 20‐25 μm producing one ice splinter is applied. Moreover, in estimating from model results the time interval over which the ice particle concentration increases by a factor of 104, a reasonable span of 50 min is obtained.

The effect of rimer surface temperature on ice splinter production by the Hallett-Mossop process

The effect of rimer surface temperature on ice splinter production by the Hallett-Mossop process