High Concentrations Of Ice Particles Research Articles

Extremely High Concentrations of Ice Particles in East Asian Dust‐Infused Baroclinic Storm (DIBS) Cirrus Shield: Dominant Role of Dust Ice Nucleation Effect

AbstractActing as efficient ice‐nucleating particles, dust particles can affect cirrus cloud properties through heterogeneous nucleation. A dust‐infused baroclinic storm (DIBS) over East Asia in May 2017 was examined using satellite observations and the Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) to study the dust effects on cirrus clouds. Satellite observations showed that the ice number concentration (Nice) was extremely high, exceeding 1–10 cm−3 at the cirrus shield of the DIBS, and the ice effective radius (rei) was about 10–30 μm. An aerosol‐aware heterogeneous ice nucleation parameterization (Ullrich et al., 2017, https://doi.org/10.1175/jas-d-16-0074.1) was implemented into WRF‐Chem to explore the formation mechanism of such a high Nice. The simulated Nice from Ullrich17 was 1–2 orders of magnitude higher than that from the default scheme (Cooper, 1986, https://doi.org/10.1007/978-1-935704-17-1_4), and the rei was 2–3 times smaller. The results from Ullrich17 were more consistent with satellite observations, which can be attributed to the enhanced deposition nucleation from the high dust concentrations. Our results demonstrated that the extremely high Nice could be fully explained by the heterogeneous nucleation of dust particles in DIBS. A conceptual framework is proposed to explain the dust‐cloud interactions in DIBS. This study confirms the significant dust effects on cirrus clouds in DIBS and highlights the importance of using aerosol‐aware ice nucleation parameterizations in modeling cirrus clouds when the dust is present.

Multi-thermals and high concentrations of secondary ice: a modelling study of convective clouds during the Ice in Clouds Experiment – Dust (ICE-D) campaign

Abstract. This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the Facility for Airborne Atmospheric Measurements (FAAM) aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 ∘C. Sensitivity studies were performed using existing parameterization schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these increased the secondary ice particle concentration but not by a sufficient amount. Simulations that involved only dust particles as ice-nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice-nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement of the freezing efficiency or relaxation of the onset temperature produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water in the Hallett–Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. It is possible of course that several mechanisms, some of them only recently discovered, may be responsible for producing the ice particles in clouds. This study highlights the fact that the dynamics of the clouds likely play an important role in producing high concentrations of secondary ice particles in clouds.

Potential use of the GLM for nowcasting and data assimilation

Based on the relationship between lightning and thunderstorm microphysics, this paper aims to determine the averaged vertical profiles of polarimetric variables for different classes of lightning density according to the GLM grid and then evaluate the potential use of these profiles for data assimilation in models with high spatial and temporal resolutions. Polarimetric variables from an X-band radar located in Campinas-SP and data from the Brazilian Network were used to detect the microphysics properties and atmospheric discharges of clouds (GLM proxy). The main differences between the lightning density class-averaged profiles for the four variables of ZH, ZDR, KDP and ρHV were observed in the region above the melting layer. For the most intense lightning classes, the signatures associated with high concentrations of ice particles at high altitudes, the presence of supercooled drops above the freezing level and the occurrence of large and more oblate raindrops were observed. To analyze the possible use of reflectivity profiles as a way to indirectly assimilate GLM information into forecast models, two case studies were conducted using the Weather Research and Forecasting model. The analyses and forecasts obtained with the assimilation of radar data (reflectivity factor and Doppler winds) and with the indirect assimilation of GLM lightning density rates through mean reflectivity profiles were evaluated against a control run assimilating no data. Overall, the two assimilation experiments offered substantial improvements over the control run in terms of short-term forecasts of reflectivity patterns and storm motion. These encouraging results supports the ability of the GLM data to positively contribute to nowcasting and forecasting of convective-scale systems, especially over the vast regions of the South American continent currently suffering from limited and, even, an utter lack of observations.

Analysis and Automated Detection of Ice Crystal Icing Conditions Using Geostationary Satellite Datasets and In Situ Ice Water Content Measurements

<div class="section abstract"><div class="htmlview paragraph">Recent studies have found that high mass concentrations of ice particles in regions of deep convective storms can adversely impact aircraft engine and air probe (e.g. pitot tube and air temperature) performance. Radar reflectivity in these regions suggests that they are safe for aircraft penetration, yet high ice water content (HIWC) is still encountered. The aviation weather community seeks additional remote sensing methods for delineating where ice particle (or crystal) icing conditions are likely to occur, including products derived from geostationary (GEO) satellite imagery that is now available in near-real time at increasingly high spatio-temporal detail from the global GEO satellite constellation. A recent study using a large sample of co-located GEO satellite and in-situ isokinetic evaporator probe (IKP-2) total water content (TWC) datasets found that optically thick clouds with tops near to or above the tropopause in close proximity (≤ 40 km) to convective updrafts were most likely to contain high TWC (TWC ≥ 1 g m<sup>-3</sup>). These parameters are detected using automated algorithms and combined to generate a HIWC probability (PHIWC) product at the NASA Langley Research Center (LaRC). Seven NASA DC-8 aircraft flights were conducted in August 2018 over the Gulf of Mexico and the tropical Pacific Ocean during the HIWC Radar II field campaign. The convection sampled during four flights was observed by GOES-16 at 1- or 5-minute intervals, providing the first opportunity to analyze product performance from this new satellite. This paper will (1) present initial comparisons between GOES-16 and IKP-2 datasets during HIWC Radar II, (2) demonstrate GOES-16 products for select periods when high TWC was encountered with an emphasis on three flights with 1-minute imagery, (3) compare GOES observations and derived products from the HIWC Radar I and II campaigns.</div></div>

Factors controlling secondary ice production in cumulus clouds

Aircraft measurements of two cumulus clouds were made during the Ice and Precipitation Initiation in Cumulus campaign over the British Isles. The 18 May 2006 cloud had high concentrations of ice particles and conditions were conducive for the Hallett–Mossop (HM) process of secondary ice production, but the 13 July 2005 cloud had low concentrations. A bin‐resolved cloud model was used to investigate several factors that are known to control the HM process using the observations of the two clouds. For the 2006 cloud, the model results show that the fast production of graupel by directly freezing of supercooled raindrops through collisional collection with ice particles was crucial to the activation of the HM process. Switching off raindrop freezing led to much delayed and suppressed formation of graupel particles, and hence a negligible HM process. Sensitivity studies were performed on the concentration of primary ice particles required to kick‐start the HM process. It was found that a concentration of the first ice as low as 0.01 L−1 could be sufficient, as long as there was a large enough concentration of cloud droplets (small and large) available when a significant number of graupel particles developed in the HM temperature zone. For the modelled 2005 cloud, the HM process did not operate effectively mainly because of the low concentration of supercooled raindrops and hence graupel. The HM process was also hindered by the relatively greater number of aerosols, and higher temperatures at cloud base and top.

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.

Observed microphysical structure of nimbostratus in northeast cold vortex over China

Airborne measurements were collected during a stepwise ascent within a nimbostratus cloud associated with a northeast cold vortex in Jilin Province over China on 20 June 2005 to study cloud structure and ice particle spectra. The microphysical structure of the nimbostratus was elucidated by King liquid water probe and Particle Measuring Systems (PMS) probes aboard the research aircraft. The PMS 2D images provide detailed information on crystal habits. A thick layer of supercooled cloud is observed and Hallett–Mossop ice multiplication process is used to explain very high ice particle concentrations in the temperature region between −3°C and −6°C. From near cloud top to melting layer, ice crystals shape in the form of columns, needles, aggregations and plates. In addition, significant horizontal variability was evident on the scale of few hundred meters. Ice particle spectra in this cloud were adequately described by exponential relationships. Relationship between the intercept (N0) and slope (λ) parameters of an exponential size distribution was well characterized by a power law.

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.

Orographic Influences on an Oahu Flood

Abstract On 2 April 2006, Oahu’s Ko‘olau Mountain Range endured more than 6 h of heavy rain with accompanying flash flooding along its northeast-facing slopes. The storm responsible for the event left a pattern of precipitation characteristic of orographic anchoring of convection with extreme rainfall gradients along the slopes and maxima along the crest of the mountain range. In fact, this was the third flash-flood event to impact the Ko‘olau Mountains in just over 1 month, with each event occurring under conditions of moist southeasterly flow at low levels and moderate conditional instability. Under these conditions persistent convection and localized heavy rainfall often occur over the Ko‘olau Mountain Range. Mesoscale analyses of the thunderstorm complex responsible for the 2 April 2006 heavy rain event and the results of a high-resolution numerical simulation employing the Weather Research and Forecasting (WRF) model are described in this study. Key features of the convection that contributed to the longevity of the event include repeat formation of convective cells along the eastern side of the central Ko‘olaus, minimal horizontal cloud motion, and strong updrafts that sloped toward the northwest in the lower levels. The westerly shear of the low-level flow determined the pattern of accumulated precipitation by aligning the slope of the convective updrafts nearly parallel to the southeast-to-northwest-orientated Ko‘olau Mountain Range. The microphysical structure of the convection was complex, with the vertical advection of hydrometeors originating below the freezing level facilitating high concentrations of ice particles and an environment conducive to charge separation and lightning.

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.

Microphysical Structure of a Nimbostratus Over Jilin Province Provided by Airborne Observations

Airborne measurements were collected during a stepwise ascent within a nimbostratus cloud associated with a cold vortex depression over the Jilin Province on 21 June 2005 to study cloud structure and ice particle spectra. The melting layer of the nimbostratus was clearly defined in the radar images. The microphysical structure of the nimbostratus was elucidated by a King liquid water probe and Particle Measuring Systems (PMS) probes aboard the research aircraft. The PMS 2-D images provided detailed information of ice crystal transformations. A thick layer of supercooled cloud was observed, and the high ice particle concentrations at temperatures ranging from 3°C to 6°C were consistent with Hallett-Mossop ice multiplication. The shape of ice crystals from near the cloud top to the melting layer were in the form of columns, needles, aggregations, and plates. In addition, significant horizontal variability was evident on the scale of few hundred meters. Particle spectra in this cloud were adequately described by exponential relationships. Relationship between the intercept (N 0 ) and slope (λ) parameters of an exponential size distribution was well characterized by a power law.

Ice Microphysics Observations in Hurricane Humberto: Comparison with Non-Hurricane-Generated Ice Cloud Layers

Abstract Measurements are presented that were acquired from the National Aeronautics and Space Administration (NASA) DC-8 aircraft during an intensive 3-day study of Tropical Storm/Hurricane Humberto on 22, 23, and 24 September 2001. Particle size distributions, particle image information, vertical velocities, and single- and dual-wavelength Doppler radar observations were obtained during repeated sampling of the eyewall and outer eye regions. Eyewall sampling temperatures ranged from −22° to −57°C and peak updraft velocities from 4 to 15 m s−1. High concentrations of small ice particles, in the order 50 cm−3 and above, were observed within and around the updrafts. Aggregates, some larger than 7 mm, dominated the larger sizes. The slope of the fitted exponential size distributions λ was distinctly different close to the eye than outside of that region. Even at low temperatures, λ was characteristic of warm temperature growth (λ < 30 cm−1) close to the eye and characteristic of low temperature growth outside of it as well (λ > 100 cm−1). The two modes found for λ are shown to be consistent with observations from nonhurricane ice cloud layers formed through deep convection, but differ markedly from ice cloud layers generated in situ. It is shown that the median, mass-weighted, terminal velocities derived for the Humberto data and from the other datasets are primarily a function of λ. Microphysical measurements and dual wavelength radar observations are used together to infer and interpret particle growth processes. Rain in the lower portions of the eyewall extended up to the 6- or 7-km level. In the outer eye regions, aggregation progressed downward from between 8.5 and 11.9 km to the melting layer, with some graupel noted in rainbands. Homogeneous ice nucleation is implicated in the high concentrations of small ice particles observed in the vicinity of the updrafts.

Homogeneous Ice Nucleation in Subtropical and Tropical Convection and Its Influence on Cirrus Anvil Microphysics

Abstract This study uses a unique set of microphysical measurements obtained in a vigorous, convective updraft core at temperatures between −33° and −36°C, together with a microphysical model, to investigate the role of homogeneous ice nucleation in deep tropical convection and how it influences the microphysical properties of the associated cirrus anvils. The core and anvil formed along a sea-breeze front during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL–FACE). The updraft core contained two distinct regions as traversed horizontally: the upwind portion of the core contained droplets of diameter 10–20 μm in concentrations of around 100 cm−3 with updraft speeds of 5–10 m s−1; the downwind portion of the core was glaciated with high concentrations of small ice particles and stronger updrafts of 10–20 m s−1. Throughout the core, rimed particles up to 0.6-cm diameter were observed. The anvil contained high concentrations of both small particles and large aggregates. Thermodynamic analysis suggests that the air sampled in the updraft core was mixed with air from higher altitudes that descended along the upwind edge of the cloud in an evaporatively driven downdraft, introducing free-tropospheric cloud condensation nuclei into the updraft below the aircraft sampling height. Farther downwind in the glaciated portion of the core, the entrained air contained high concentrations of ice particles that inhibit droplet formation and homogeneous nucleation. Calculations of droplet and ice particle growth and homogeneous ice nucleation are used to investigate the influence of large ice particles lofted in updrafts from lower levels in this and previously studied tropical ice clouds on the homogeneous nucleation process. The preexisting large ice particles act to suppress homogeneous nucleation through competition via diffusional and accretional growth, mainly when the updrafts are < 5 m s−1. In deep convective updrafts > 5–10 m s−1, the anvil is the depository for the small, radiatively important ice particles (homogeneously nucleated) and the large ice particles from below (heterogeneously or secondarily produced, or recycled).

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.

Simulations of the glaciation of a frontal mixed‐phase cloud with the Explicit Microphysics Model

AbstractSimulations with the Explicit Microphysics Model (EMM) of a case of lightly precipitating, glaciated stratiform cloud are presented. This frontal cloud was observed by the UK Met Office C‐130 aircraft and the dual‐polarization radar at Chilbolton in southern England.The Hallett–Mossop (H‐M) process was found to cause extremely high number concentrations of crystal columns of up to almost 1000 l−1 in the H‐M region (−3 to −8°C) of the updraught in the EMM control simulation. Similarly high number concentrations of ice particles were seen in, and in the vicinity of, ascending thermals in the aircraft observations. Such concentrations are orders of magnitude higher than the ice nucleus (IN) concentration. Moreover, the dendritic growth of highly planar particles of primary ice produces a peak in differential reflectivity of almost 4 dB just below the cloud top (−15°C). Primary ice particles are generally larger than H‐M splinters and mostly determine the simulated radar properties of the model cloud.Tests with the EMM revealed significant sensitivities of the average ice number concentration, the cloud‐ and ice‐water paths, and the surface precipitation rate, to atmospheric concentrations of IN and cloud condensation nuclei for this frontal‐cloud case. The layer of supercooled cloud‐water located just below cloud top in the control is depleted by evaporation when the IN concentration is augmented. © Royal Meteorological Society, 2003. P. R. Field's contribution is Crown copyright

An overview of microphysical properties of Arctic clouds observed in May and July 1998 during FIRE ACE

Microphysical data were collected by the NCAR C‐130 research aircraft during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment Arctic Cloud Experiment (FIRE ACE). Boundary layer clouds 100 to 400 m thick were observed on 11 of the 16 missions. The all‐water clouds varied from being adiabatic and homogeneous with monomodal drop spectra to subadiabatic and inhomogeneous with bimodal drop spectra and drizzle. The subadiabatic clouds were observed to be actively mixing near cloud top. The adiabatic clouds provided a test of the performance of the liquid water content (LWC) probes but only in low LWC conditions. A mixed‐phase boundary layer cloud displayed striking variability in the hydrometeor fields on a horizontal scale of 10 km and a vertical scale of 100 m. Cloud Particle Imager (CPI) data showed separate regions with small supercooled cloud drops, supercooled drizzle (at −25°C) and graupel particles. A deep stratus cloud with its base at 2 km (+2°C) and top at 6 km (−25°C) contained drizzle near cloud top and (lower in the cloud) very high (2500 to 4000 L−1) concentrations of ice particles in conditions that did not meet all the Hallett‐Mossop criteria. CPI data showed that an Arctic cirrus cloud was composed of very high (∼100,000 L−1) concentrations of small ice particles interspersed with single, large (mostly bullet rosette) crystals. The data showed that the cirrus cloud was inhomogeneous on scales down to tens of meters. The average ice particle concentrations measured in the cirrus by the FSSP and CPI probes were several hundred to a few thousand per liter, much higher than commonly found in the literature.

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.

Characterizing the Supercooled Large Droplet Environment with Corresponding Turboprop Aircraft Response

This continuing study evaluated turboprop aircraft performance response to various environmental conditions. These conditions included clear air, warm rain, ice only, mixed phase, and supercooled drops encountered during 19 separate e ights. Supercooled droplets consisting of cloud, drizzle, and rain sizes were the main focus of this study. Aircraft response was quantie ed by rates of change in aircraft rateof-climb capability, lift and drag coefe cients and, lift over drag ratio. The aircraft performance parameters were compared to environmental hydrometeor parameters, such as 80% volumetric diameter (80VD)*liquid water content (LWC), quantifying the environmental conditions. Results show that encounters with supercooled drizzle drops, or ZL, resulted in maximum rates of performance degradation. These high rates of degradation forced the pilot to take evasive action within 5 min of entering these hazardous conditions. The Wyoming King Air experienced substantial increases in drag and stall speed, substantial decreases in climb capability, and signie cant lift degradations while encountering ZL. Encounters with supercooled cloud and rain-sized drops resulted in minor to low rates of performance degradation, whereas encounters with supercooled drops in low ice particle concentrations resulted in only minor rates of degradation. In addition, aircraft response to high ice particle concentrations and low liquid water, following a ZL encounter, resulted in rapid performance recovery, possibly because of erosion of ice on the airframe. Aggregates of needles and hexagonal plates led to the highest rates of recovery, whereas cold, clear air resulted in the lowest rates of recovery. Furthermore, e ight analyses facilitated quantifying ZL horizontal extents and encounter frequencies. For example, ZL measured by a one-dimensional cloud probe during atmospheric research e ights exceeded 0.05 g /m 3 twice per 10 e ight hours over a horizontal length of 29 km. The results presented herein show a strong relationship between aircraft response and environmental parameters utilizing the largest drops in the hydrometeor distribution (80VD*LWC and ZL LWC). The results suggest that the most severe icing is actually caused by drizzle-sized drops as opposed to freezing rain. In addition, the results suggest that activating the de-icing boots (with typical chordwise coverage) after a ZL encounter may have little effect on aircraft performance recovery.

Microstructures of low and middle‐level clouds over the Beaufort Sea

AbstractAirborne measurements in low and middle‐level clouds over the Beaufort Sea in April 1992 and June 1995 show that these clouds often have low droplet concentrations (<100 cm3) and relatively large effective droplet radii. The highest average droplet concentrations overall were measured in altocumulus clouds that formed in airflows from the south that passed either over the North American continent or were from Asia. Droplet concentrations in low clouds tended to be higher in April than in June. The low clouds in June occasionally contained drops as large as 35 μm diameter; in these clouds the collision‐coalescence process was active and produced regions of extensive drizzle. Cloud‐top droplet concentrations were significantly correlated with aerosols beneath their bases, but appeared to be relatively unaffected by aerosols above their tops. Anthropogenic sources around Deadhorse, Alaska, increased local cloud droplet concentrations.Ice particle concentrations were generally low in April, but high ice particle concentrations were encountered in June when cloud‐top temperatures were considerably higher. On two days in June, tens per litre of columnar and needle ice crystals were measured in stratocumulus with top temperatures between −4 and −9 °C. Ice particle concentrations were poorly correlated with temperature (r = 0.39) but, for the two data sets as a whole, the concentrations of ice particles tended to increase with increasing temperature from −30 to −4.5 °C. Ice particle concentrations correlated better with the size of the largest droplets (r = 0.61).The most common mixed‐phased cloud structure encountered was a cloud topped by liquid water that precipitated ice. Liquid‐water topped clouds were observed down to temperatures of −31 °C. They are likely common in the Arctic, and may play an important role in the radiation balance of the region.Temperature lapse rates in the clouds were generally complex, reflecting either layering, due to differential advection, or radiational effects. In these cases, vertical profiles of liquid‐water content and effective cloud droplet radius did not vary systematically with height above cloud base, as they do in well‐mixed clouds. However, when the temperature in a cloud decreased with height at or very near the pseudo‐adiabatic lapse rate, the cloud liquid‐water content (and various measures of the broadness of the cloud droplet size distribution) generally increased monotonically with height until very close to cloud top.For clouds consisting entirely of droplets, or droplets and ice crystals, cloud coverage increased by about 10% when the definition of a cloud was changed from 10 to 5 droplets per cubic centimetre. For clouds containing ice particles, cloud coverage and/or cloud depth increased by about 40% when the definition of a cloud was changed from 1 to 0.1 ice particles per litre.The significance of these observations with respect to the effects of clouds on the radiation budget of the Arctic, and the potential for modification of arctic clouds by pollution, are discussed.

Cloud Particle Measurements in Thunderstorm Anvils and Possible Weather Threat to Aviation

Since 1990, there have been at least 10 known incidents where jet aircraft have experienced loss of thrust in one or more turbofan engines while maneuvering in the anvil region near the central core of a thunderstorm. The exact cause of the uncommanded thrust reduction, commonly called engine rollback, is still under investigation. It appears that the rollback incidents may be associated with ingestion of high mass concentrations of ice particles, snow, and possibly small concentrations of supercooled liquid water in the anvil region. The characteristics of cloud particles in thunderstorm anvils have not been extensively studied. Results from analysis of aircraft observations in the anvils of midlatitude and tropical thunderstorms are discussed. Aircraft and limited radar observations show that most anvils associated with small, garden-variety thunderstorms contain low ( < ∼ 0.4 g m -3 ) mass concentrations of ice particles. In larger, more intense midlatitude storms, anvils may contain ice water contents from 1 to 3 g m -3 . The mean of the maximum particle dimension in the anvil region of the more intense storms showed a strong modal size of about 2 mm. The particles themselves appear to be ice crystals and aggregates of ice crystals, i.e., snowflakes