Potential Ice Nuclei Research Articles

Characteristics of clouds, precipitation, and latent heat in midlatitude frontal system mixed with dust storm from GPM satellite observations and WRF simulations

A heavy dust storm originating in Mongolia and Inner Mongolia traveled to Northeast China and met a midlatitude frontal system on May 3, 2017. The potential ice nuclei (IN) effects of mineral dust aerosols on the vertical structure of clouds, precipitation, and latent heat (LH) were studied using Global Precipitation Mission (GPM) satellite observations and Weather Research and Forecasting (WRF) model simulations. The WRF simulations correctly captured the main features of the system, and the surface rain rate distribution was positively correlated with data retrieved from the GPM Microwave Imager. Moreover, the correlation coefficient increased from 0.31 to 0.54 with increasing moving average window size. The WRF-simulated rainfall vertical profiles are generally comparable to the GPM Dual-Frequency Precipitation Radar (DPR) observations, particularly in low layers. The joint probability distribution functions of the rain rate at different altitudes from the WRF simulation and GPM observations show high positive correlation coefficients of ~0.80, indicating that the assumptions regarding the raindrop size distribution in the WRF model and DPR retrieval were consistent. Atmospheric circulation analysis and aerosol optical depth observations from the Himawari-8 satellite indicated that the dust storm entered only a narrow strip of the northwest edge of the frontal precipitation system. The WRF simulations showed that in carefully selected areas of heavy dust, dust can enhance the heterogeneous ice nucleation process and increase the cloud ice, snowfall, high-altitude precipitation rate, and LH rate in the upper layers. This effect is significant at temperatures of −15 °C to −38 °C and requires dust number concentrations exceeding 10<sup>6</sup> m<sup>−</sup><sup>3</sup>. It is important to accurately classify the dusty region in this type of case study. In the selected vertical cross section, the WRF-simulated and DPR-retrieved LH have comparable vertical shapes and amplitudes. Both results reflect the structure of the tilted frontal surface, with positive LH above it and negative LH below it. The simulated area-averaged LH profiles show positive heating in the entire column, which is a convective-dominated region, and this feature is not significantly affected by dust. DPR-based LH profiles show stratiform-dominated or convective-dominated shapes, depending on the DPR retrieval product.

Assessment of a Microphysical Ensemble Used to Investigate the OWLeS IOP4 Lake-Effect Storm

AbstractMicrophysical processes within mixed-phase convective clouds can have cascading impacts on cloud properties and resultant precipitation. This paper investigates the role of microphysics in the lake-effect storm (LES) observed during intensive observing period 4 of the Ontario Winter Lake-effect Systems field campaign. A microphysical ensemble is composed of 24 simulations that differ in the microphysics scheme used (e.g., Weather Research and Forecasting Model microphysics options or a choice of two bulk adaptive habit models) along with changes in the representation of aerosol and potential ice nuclei concentrations, ice nucleation parameterizations, rain and ice fall speeds, spectral indices, ice habit assumptions, and the number of moments used for modeling ice-phase hydrometeors in each adaptive habit model. Each of these changes to microphysics resulted in varied precipitation types at the surface; 15 members forecast a mixture of snow, ice, and graupel, 7 members forecast only snow and ice, and the remaining 2 members forecast a combination of snow, ice, graupel, and rain. Observations from an optical disdrometer positioned to the south of the LES core indicate that 92% of the observed particles were snow and ice, 5% were graupel, and 3% were rain and drizzle. Analysis of observations spanning more than a point location, such as polarimetric radar observations and aircraft measurements of liquid water content, provides insight into cloud composition and processes leading to the differences at the surface. Ensemble spread is controlled by hydrometeor type differences spurred by processes or parameters (e.g., ice fall speed) that affect graupel mass.

Ship-based measurements of ice nuclei concentrations over the Arctic, Atlantic, Pacific and Southern oceans

Abstract. Ambient concentrations of ice-forming particles measured during ship expeditions are collected and summarised with the aim of determining the spatial distribution and variability in ice nuclei in oceanic regions. The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica. In comparison to continental observations, ship-based measurements of ambient ice nuclei show 1 to 2 orders of magnitude lower mean concentrations. To quantify the geographical variability in oceanic areas, the concentration range of potential ice nuclei in different climate zones is analysed by meridionally dividing the expedition tracks into tropical, temperate and polar climate zones. We find that concentrations of ice nuclei in these meridional zones follow temperature spectra with similar slopes but vary in absolute concentration. Typically, the frequency with which specific concentrations of ice nuclei are observed at a certain temperature follows a log-normal distribution. A consequence of the log-normal distribution is that the mean concentration is higher than the most frequently measured concentration. Finally, the potential contribution of ship exhaust to the measured ice nuclei concentration on board research vessels is analysed as function of temperature. We find a sharp onset of the influence at approximately −36 ∘C but none at warmer temperatures that could bias ship-based measurements.

Redistribution of ice nuclei between cloud and rain droplets: Parameterization and application to deep convective clouds

AbstractIn model studies of aerosol‐dependent immersion freezing in clouds, a common assumption is that each ice nucleating aerosol particle corresponds to exactly one cloud droplet. In contrast, the immersion freezing of larger drops—“rain”—is usually represented by a liquid volume‐dependent approach, making the parameterizations of rain freezing independent of specific aerosol types and concentrations. This may lead to inconsistencies when aerosol effects on clouds and precipitation shall be investigated, since raindrops consist of the cloud droplets—and corresponding aerosol particles—that have been involved in drop‐drop‐collisions. Here we introduce an extension to a two‐moment microphysical scheme in order to account explicitly for particle accumulation in raindrops by tracking the rates of selfcollection, autoconversion, and accretion. This provides a direct link between ice nuclei and the primary formation of large precipitating ice particles. A new parameterization scheme of drop freezing is presented to consider multiple ice nuclei within one drop and effective drop cooling rates. In our test cases of deep convective clouds, we find that at altitudes which are most relevant for immersion freezing, the majority of potential ice nuclei have been converted from cloud droplets into raindrops. Compared to the standard treatment of freezing in our model, the less efficient mineral dust‐based freezing results in higher rainwater contents in the convective core, affecting both rain and hail precipitation. The aerosol‐dependent treatment of rain freezing can reverse the signs of simulated precipitation sensitivities to ice nuclei perturbations.

Simulations of Ice Nucleation by Kaolinite (001) with Rigid and Flexible Surfaces.

Nucleation of ice by airborne particles is a process vital to weather and climate, yet our understanding of the mechanisms underlying this process is limited. Kaolinite is a clay that is a significant component of airborne particles and is an effective ice nucleus. Despite receiving considerable attention, the microscopic mechanism(s) by which kaolinite nucleates ice is not known. We report molecular dynamics simulations of heterogeneous ice nucleation by kaolinite (001) surfaces. Both the Al-surface and the Si-surface nucleate ice. For the Al-surface, reorientation of the surface hydroxyl groups is essential for ice nucleation. This flexibility allows the Al-surface to adopt a structure which is compatible with hexagonal ice, Ih, at the atomic level. On the rigid Si-surface, ice nucleates via an unusual structure that consists of an ordered arrangement of hexagonal and cubic ice layers, joined at their basal planes where the interfacial energy cost is low. This ice structure provides a good match to the atomistic structure of the Si-surface. This example is important and may have far-reaching implications because it demonstrates that potential ice nuclei need not be good atomic-level matches to particular planes of ice Ih or cubic ice, Ic. It suggests that surfaces can act as effective ice nuclei by matching one of the much larger set of planes that can be constructed by regular arrangements of hexagonal and cubic ice.

Morphological characteristics of bioaerosols from contrasting locations in southern tropical India – A case study

Bioaerosols, which are ubiquitous in the earth's atmosphere, are poorly characterized in terms of their physical and chemical properties. Improved knowledge of their physical and chemical properties is essential to have a better understanding of their dispersion and long-range transport in the atmosphere and at the same time to assess their role as potential Ice Nuclei (IN). In the present work, possibly for the first time we report the morphological characteristics of bioaerosols from marine urban and high altitude continental regions in Southern India. The samples were collected using polycarbonate filter paper and analyzed using Scanning Electron Microscope (SEM) coupled with Energy-dispersive Spectra Detector (EDX/EDS). The observed bioaerosols exhibited great variability in their morphological features over this region of the world. At these contrasting environments, we found that fungal spores constituted the major fraction of the total observed bioaerosols. Pollen grains, plant and insect fragments, and lot of other non-identified bio-particles were also observed constituting the remaining fraction. Further, the classification of fungal spores exhibited strong variability over this region. For example, fungal spores of both Ascomycota and Basidiomycota class were seen in abundance in marine environment, while Ascomycota especially Cladosporium were seen in abundance in high altitude continental environment. Our findings also suggest that increase in diversity of bioaerosol particles at marine site appeared to coincide with precipitation. It appears that vast diversity in the morphological features of bioaerosols exists over this region, which should further be studied using advanced online techniques for better quantification under contrasting environments. However, the diversity observed in morphological characteristics of bioaerosols at these two contrasting locations is limited and restricted to these two sites and season of the year, and should therefore not be generalized over a large spatio-temporal scale; emphasizing the need for more similar studies covering larger geographical area.

Large‐eddy simulation of three mixed‐phase cloud events during ISDAC: Conditions for persistent heterogeneous ice formation

AbstractA Classical‐Nucleation‐Theory‐based parameterization for heterogenous ice nucleation, including explicit dependencies of the nucleation rates on the number concentration, size, and composition of the ambient aerosol population, is implemented in a cloud‐scale, large‐eddy simulation model and evaluated against Arctic mixed‐phase cloud events observed during Indirect and Semi‐Direct Aerosol Campaign (ISDAC). An important feature of the parameterization is that the ice nucleation efficiency of each considered aerosol type is described using a contact angle distribution which evolves with time so that the model accounts for the inhibition of ice nucleation as the most efficient ice‐forming particles are nucleated and scavenged. The model gives a reasonable representation of first‐order (ice water paths) and second‐order (ice crystal size distributions) ice microphysical properties. The production of new ice crystals in the upper part of the cloud, essential to guarantee sustained mixed‐phase conditions, is found to be controlled mostly by the competition between radiative cooling (resulting in more aerosol particles becoming efficient ice nuclei as the temperature decreases), cloud‐top entrainment (entraining fresh particles into the cloud), and nucleation scavenging of the ice+forming aerosol particles. The relative contribution of each process is mostly determined by the cloud‐top temperature and the entrainment rates. Accounting for the evolution of the contact angle probability density function with time seems to be essential to capture the persistence of in‐cloud ice production without having to, for example, increase the free tropospheric aerosol concentration. Although limited to only three cases and despite important limitations of the parameterization (e.g., the present version only considers dust and black carbon as potential ice nuclei), the results suggest that modeling the time evolution of the ice nuclei population ability to form ice is required to accurately model Arctic mixed‐phase cloud processes.

Effects of chemical aging on the ice nucleation activity of soot and polycyclic aromatic hydrocarbon aerosols.

The role of soot particles as ice nuclei (IN) in heterogeneous freezing processes in the atmosphere remains uncertain. Determination of the freezing efficiency of soot is complicated by the changing properties of soot particles undergoing atmospheric aging processes. In this study, the heterogeneous freezing temperatures of droplets in contact with fresh and oxidized soot particles were determined using an optical microscope apparatus equipped with a sealed cooling stage and a CCD video camera. Experiments were also conducted using fresh and oxidized polycyclic aromatic hydrocarbons (PAHs), including anthracene, pyrene, and phenanthrene, as potential ice nuclei. Chemical changes at the surface of the aerosols caused by exposure to ozone were characterized using Fourier transform infrared spectroscopy with horizontal attenuated total reflectance (FTIR-HATR). In addition, Brunauer-Emmett-Teller (BET) measurements were used to determine the specific surface areas of the soot particles. Mean freezing temperatures on fresh particles ranged from -19 to -24 °C, depending on the IN composition and size. In all cases, exposure to ozone facilitated ice nucleation at warmer temperatures, by 2-3 °C, on average. In addition, nucleation rate coefficients for a single temperature and IN type increased by as much as 4 orders of magnitude because of oxidation. Furthermore, a fraction of the oxidized soot particles froze at temperatures above -10 °C. A modified version of classical nucleation theory that accounts for a range of contact angles on nucleation sites within an IN population was used to derive the probability of freezing as a function of temperature for each type of IN. In summary, our results suggest that atmospheric oxidation processes may both extend the range over which particles can act as ice nuclei to warmer temperatures and increase heterogeneous nucleation rates on soot and pollutant aerosols.

Different contact angle distributions for heterogeneous ice nucleation in the Community Atmospheric Model version 5

Abstract. In order to investigate the impact of different treatments for the contact angle (α) in heterogeneous ice nucleating properties of natural dust and black carbon (BC) particles, we implement the classical-nucleation-theory-based parameterization of heterogeneous ice nucleation (Hoose et al., 2010) in the Community Atmospheric Model version 5 (CAM5) and then improve it by replacing the original single-contact-angle model with the probability-density-function-of-α (α-PDF) model to better represent the ice nucleation behavior of natural dust found in observations. We refit the classical nucleation theory (CNT) to constrain the uncertain parameters (i.e., onset α and activation energy in the single-α model; mean contact angle and standard deviation in the α-PDF model) using recent observation data sets for Saharan natural dust and BC (soot). We investigate the impact of the time dependence of droplet freezing on mixed-phase clouds and climate in CAM5 as well as the roles of natural dust and soot in different nucleation mechanisms. Our results show that, when compared with observations, the potential ice nuclei (IN) calculated by the α-PDF model show better agreement than those calculated by the single-α model at warm temperatures (T; T > −20 °C). More ice crystals can form at low altitudes (with warm temperatures) simulated by the α-PDF model than compared to the single-α model in CAM5. All of these can be attributed to different ice nucleation efficiencies among aerosol particles, with some particles having smaller contact angles (higher efficiencies) in the α-PDF model. In the sensitivity tests with the α-PDF model, we find that the change in mean contact angle has a larger impact on the active fraction at a given temperature than a change in standard deviation, even though the change in standard deviation can lead to a change in freezing behavior. Both the single-α and the α-PDF model indicate that the immersion freezing of natural dust plays a more important role in the heterogeneous nucleation than that of soot in mixed-phase clouds. The new parameterizations implemented in CAM5 induce more significant aerosol indirect effects than the default parameterization.

Aircraft observations of elevated pollution layers near the foothills of the Himalayas during CAIPEEX‐2009

AbstractPre‐monsoon aerosols in the northern part of India may play an important role in the advancement of the monsoon. This study investigates the properties of aerosols and their spatial and vertical distribution near the foothills of the Himalayas using data from an instrumented aircraft during the Cloud Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX), 2009. CAIPEEX was conducted over five days during May 2009 in the northern part of India near the foothills of the Himalayas. On all the flight days thick haze was observed, with elevated aerosol layers up to 4 km with varying concentrations. The sources are identified as being from local anthropogenic activities such as biomass burning, as inferred from MODIS fire maps and dust from local as well as from long‐range transport, as suggested by trajectory analysis. The aerosol size distributions depict the increases in both fine and coarse mode aerosols in polluted layers. This indicates that aerosols over this region are well mixed and the vertical distribution is a mixture of both biomass burning and dust aerosols at different altitudes, as also observed by CALIPSO and inferred from Mie calculations. Clouds observed above the elevated aerosol layers showed higher droplet concentrations (200–1400 cm−3) with small effective radii (3.5 to < 6 µm). Ice phase observed above 6 km at temperatures lower than −14°C might be due to the presence of dust aerosol acting as potential ice nuclei. Copyright © 2012 Royal Meteorological Society

Internally mixed sulfate and organic particles as potential ice nuclei in the tropical tropopause region

Cirrus clouds are ubiquitous in the tropical tropopause region and play a major role in the Earth's climate. Any changes to cirrus abundance due to natural or anthropogenic influences must be considered to evaluate future climate change. The detailed impact of cirrus clouds on climate depends on ice particle number, size, morphology, and composition. These properties depend in turn on the nucleation mechanism of the ice particles. Although it is often assumed that ice nucleates via a homogeneous mechanism, recent work points to the possibility that heterogeneous ice nucleation is important in the tropical tropopause region. However, there are very few studies of depositional ice nucleation on the complex types of particles likely to be found in this region of the atmosphere. Here, we use a unique method to probe depositional ice nucleation on internally mixed ammonium sulfate/palmitic acid particles, namely optical microscopy coupled with Raman microscopy. The deliquescence and efflorescence phase transitions of the mixed particles were first studied to gain insight into whether the particles are likely to be liquid or solid in the tropical tropopause region. The ice nucleating ability of the particles was then measured under typical upper tropospheric conditions. It was found that coating the particles with insoluble palmitic acid had little effect on the deliquescence, efflorescence, or ice nucleating ability of ammonium sulfate. Additional experiments involving Raman mapping provide new insights into how the composition and morphology of mixed particles impact their ability to nucleate ice.

The ice nucleating ability of pollen:: Part II. Laboratory studies in immersion and contact freezing modes

Laboratory tests were conducted of the ice nucleating ability of four kinds of pollen in the immersion and the contact freezing modes. The diameters of the selected pollen were between 25 and 70 μm. The experiments were carried out at the Mainz vertical wind tunnel with freely suspended supercooled droplets at temperatures down to −28 °C. The immersion freezing experiments were conducted with drops of radii between 250 and 375 μm formed from distilled water with a defined amount of pollen added. The drops were freely floated in the wind tunnel while being supercooled. For the contact freezing experiments, a short burst of pollen was allowed to collide with freely suspended, supercooled pure water drops of 360-μm radius. The results showed that particle-free water drops in particle-free air in the wind tunnel did not freeze at temperatures above −28 °C while water drops containing pollen froze at temperatures as high as −9 °C, and water drops colliding with pollen froze at temperatures −5 °C and lower. Combined with earlier results about the ice nucleating ability of some bacteria, marine plankton, and leaf litters, the present results confirm the importance of biological aerosol particles as potential ice nuclei at relatively warm temperatures.

潜在的氷晶核の測定結果について

自然大気中に沃素のVaperを加えた時活性化する核,すなわち潜在的氷晶核は,-8°Cの様な高い温度でも有効であることが観測された.これらの核の濃度の温度スペクトルを自然氷晶核の場合と比較すると,作用温度の下降とともに核濃度は指数関数的に増加する点ではよい一致を示したが,そのカーブは自然氷晶核に比べて急であった. 潜在的氷晶核の大きさは,電子顕微鏡を用いて調べた結果,個体粒子とすれば10分の数ミクロン又はそれ以下であることがわかった.Mixing typeのcold chamberを用いての方法と, millipore filterを用いたfilter法と,二つの方法についてみると,filter法は比較観測には便利であるが,-12°C以下の温度では潜在的氷晶核の測定には誤差を考慮しなくてはならないと思われる.また二つの測定法による核数を比較するとfi1ter法の方がmixing chamberによる場合の値より一桁少かった.濃度変化の傾向をみるために,東京と軽井沢で観測を行った.その結果,東京でも軽井沢でも高い日も低い日もあり,また日による変化は非常にはげしく,時間的変化も激しいことがわかった.

The Production of Potential Ice Nuclei by Gasoline Engines

Potential ice nuclei, particles which become ice nuclei when properly treated, are produced in automotive exhaust. They are activated by iodine vapor and are believed to be lead particles originating from the tetraethyl lead mixed with the fuel. The sensitivity of nucleation measurements provides an excellent method of counting submicron lead particles and monitoring the automotive component of air pollution. Measurements reported here show that the production of potential ice nuclei by a gasoline engine is at least 2 × 107 per gram Pb at −10C, 1 × 1010 per gram Pb at −15C and 1 × 1012 per gram Pb at −20C. Some simple calculations are presented, showing that large enough numbers of potential ice nuclei are produced by the routine burning of gasoline to be useful in cloud and weather modification research. In particular, a DC-6 aircraft burning 6 gal min−1 of gasoline should produce at least 4 × 1011 nuclei sec−1 at −20C. It is demonstrated that ethylene diiodide can be added to the gasoline supply of an automobile and that the automobile will then act as an inexpensive source of large numbers of ice nuclei.