Ice Nuclei Concentrations Research Articles

Variations of ice nuclei concentration induced by rain and snowfall within a local forested site in Japan

Abstract Biological ice nuclei (IN) such as certain species of bacteria and fungi are believed to have impacts on ice nucleation in mixed-phase clouds at temperatures warmer than −15 °C. Recent studies have indicated that rain is closely related to increases of biological IN in the near-surface atmosphere. However, variations of IN concentrations during rain and snowfall have not been compared. In the present study, field measurements of atmospheric IN were carried out under fine, cloudy, rain and snow at a local forested site in Japan. IN concentrations at −7 °C in spring were dramatically increased by rain, and concentrations associated with rain (0.86–2.2 m−3) were greater than 2.6 times higher than the mean concentration during fine weather (0.33 m−3). In winter, concentrations associated with rain (1.6 to >5.7 m−3) were also higher than those under cloudy sky (1.1 m−3), but increases were not observed during snowfall (0.21–0.4 m−3). Detectable IN concentrations associated with rain considerably decreased after heat treatment at 90 °C, indicating that IN increased during rain were likely biological substances such as heat-sensitive ice nucleation active proteins. Consequently, different types of precipitation may have varying effects on IN concentration associated with biological substances.

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.

New particle-dependent parameterizations of heterogeneous freezing processes: sensitivity studies of convective clouds with an air parcel model

Abstract. Based on the outcome of laboratory results, new particle-dependent parameterizations of heterogeneous freezing were derived and used to improve and extend a two-dimensional spectral microphysics scheme. They include (1) a particle-type-dependent parameterization of immersion freezing using the numbers of active sites per mass, (2) a particle-type and size-resolved parameterization of contact freezing, and (3) a particle-type-dependent description of deposition freezing. The modified microphysical scheme was embedded in an adiabatic air parcel model with entrainment. Sensitivity studies were performed to simulate convective situations and to investigate the impact of ice nuclei concentrations and types on ice formation. As a central diagnostic parameter, the ice water fraction (IWF) was selected, which is the relation of the ice water content to the total amount of water in the condensed form. The following parameters were varied: initial aerosol particle number size distributions, types of ice nucleating particles, final temperature, and the fractions of potential ice nucleating particles. Single and coupled freezing processes were investigated. The results show that immersion freezing seems to be the most efficient process. Contact freezing is constrained by the collision kernel between supercooled drops and potential ice nucleating particles. The importance of deposition freezing lies in secondary ice formation; i.e., small ice particles produced by deposition nucleation trigger the freezing of supercooled drops by collisions. Thus, a broader ice particle spectrum is generated than that by immersion and contact freezing. During coupled immersion–contact and contact–deposition freezing no competition was observed, and both processes contribute to cloud ice formation but do not impede each other. As already suggested in the literature, mineral dust particles seem to be the most important ice nucleating particles. Biological particles are probably not involved in significant ice formation. The sensitive parameters affecting cloud properties are temperature, aerosol particle composition and concentration, and particle size distribution.

In-situ measurements of cloud-precipitation microphysics in the East Asian monsoon region since 1960

A large number of in-situ measurements of cloud-precipitation microphysical properties have been made since 1960, including measurements of particle size distribution, particle concentration, and liquid water content of clouds and rain. These measurements have contributed to considerable progress in understanding microphysical processes in clouds and precipitation and significant improvements in parameterizations of cloud microphysics in numerical models. This work reviews key findings regarding cloud-precipitation microphysics over China. The total number concentrations of various particles vary significantly, with certain characteristic spatial scales. The size distributions of cloud droplets in stratiform clouds can generally be fit with gamma distributions, but the fit parameters cover a wide range. Raindrop size distributions (RSDs) associated with stratiform clouds can be fit with either exponential or gamma distributions, while RSDs associated with convective or mixed stratiform-cumuliform clouds are best fit with gamma distributions. Concentrations of ice nuclei (IN) over China are higher than those observed over other regions, and increase exponentially as temperature decreases. The particle size distributions of ice crystals, snow crystals, and hailstones sampled at a variety of locations can be reliably approximated by using exponential distributions, while aerosol particle size distributions are best described as the sum of a modified gamma distribution and a Junge power-law distribution. These results are helpful for evaluating and improving the fidelity of physical processes and hydrometeor fields simulated by microphysical parameterizations. The comprehensive summary and analysis of previous work presented here also provide useful guidelines for the design of future observational programs.

Estimated desert-dust ice nuclei profiles from polarization lidar: methodology and case studies

Abstract. A lidar method is presented that permits the estimation of height profiles of ice nuclei concentrations (INC) in desert dust layers. The polarization lidar technique is applied to separate dust and non-dust backscatter and extinction coefficients. The desert dust extinction coefficients σd are then converted to aerosol particle number concentrations APC280 which consider particles with radius > 280 nm only. By using profiles of APC280 and ambient temperature T along the laser beam, the profile of INC can be estimated within a factor of 3 by means of APC-T-INC parameterizations from the literature. The observed close relationship between σd at 500 nm and APC280 is of key importance for a successful INC retrieval. We studied this link by means of AERONET (Aerosol Robotic Network) sun/sky photometer observations at Morocco, Cabo Verde, Barbados, and Cyprus during desert dust outbreaks. The new INC retrieval method is applied to lidar observations of dust layers with the spaceborne lidar CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) during two overpasses over the EARLINET (European Aerosol Research Lidar Network) lidar site of the Cyprus University of Technology (CUT), Limassol (34.7° N, 33° E), Cyprus. The good agreement between the CALIOP and CUT lidar retrievals of σd, APC280, and INC profiles corroborates the potential of CALIOP to provide 3-D global desert dust APC280 and INC data sets.

Implementing marine organic aerosols into the GEOS-Chem model

Abstract. Marine-sourced organic aerosols (MOAs) have been shown to play an important role in tropospheric chemistry by impacting surface mass, cloud condensation nuclei, and ice nuclei concentrations over remote marine and coastal regions. In this work, an online marine primary organic aerosol emission parameterization, designed to be used for both global and regional models, was implemented into the GEOS-Chem (Global Earth Observing System Chemistry) model. The implemented emission scheme improved the large underprediction of organic aerosol concentrations in clean marine regions (normalized mean bias decreases from −79% when using the default settings to −12% when marine organic aerosols are added). Model predictions were also in good agreement (correlation coefficient of 0.62 and normalized mean bias of −36%) with hourly surface concentrations of MOAs observed during the summertime at an inland site near Paris, France. Our study shows that MOAs have weaker coastal-to-inland concentration gradients than sea-salt aerosols, leading to several inland European cities having >10% of their surface submicron organic aerosol mass concentration with a marine source. The addition of MOA tracers to GEOS-Chem enabled us to identify the regions with large contributions of freshly emitted or aged aerosol having distinct physicochemical properties, potentially indicating optimal locations for future field studies.

Persistent after-effects of heavy rain on concentrations of ice nuclei and rainfall suggest a biological cause

Abstract. Rainfall is one of the most important aspects of climate, but the extent to which atmospheric ice nuclei (IN) influence its formation, quantity, frequency, and location is not clear. Microorganisms and other biological particles are released following rainfall and have been shown to serve as efficient IN, in turn impacting cloud and precipitation formation. Here we investigated potential long-term effects of IN on rainfall frequency and quantity. Differences in IN concentrations and rainfall after and before days of large rainfall accumulation (i.e., key days) were calculated for measurements made over the past century in southeastern and southwestern Australia. Cumulative differences in IN concentrations and daily rainfall quantity and frequency as a function of days from a key day demonstrated statistically significant increasing logarithmic trends (R2 > 0.97). Based on observations that cumulative effects of rainfall persisted for about 20 days, we calculated cumulative differences for the entire sequence of key days at each site to create a historical record of how the differences changed with time. Comparison of pre-1960 and post-1960 sequences most commonly showed smaller rainfall totals in the post-1960 sequences, particularly in regions downwind from coal-fired power stations. This led us to explore the hypothesis that the increased leaf surface populations of IN-active bacteria due to rain led to a sustained but slowly diminishing increase in atmospheric concentrations of IN that could potentially initiate or augment rainfall. This hypothesis is supported by previous research showing that leaf surface populations of the ice-nucleating bacterium Pseudomonas syringae increased by orders of magnitude after heavy rain and that microorganisms become airborne during and after rain in a forest ecosystem. At the sites studied in this work, aerosols that could have initiated rain from sources unrelated to previous rainfall events (such as power stations) would automatically have reduced the influences on rainfall of those whose concentrations were related to previous rain, thereby leading to inhibition of feedback. The analytical methods described here provide means to map and delimit regions where rainfall feedback mediated by microorganisms is suspected to occur or has occurred historically, thereby providing rational means to establish experimental set-ups for verification.

Response of Natural Ice Nuclei to Deposited Silver Iodide

Aim: To demonstrate that deposition of silver iodide particles on the ground creates secondary ice nuclei and that these interfere with analyses of experiments designed to increase rainfall. Methods: Measurements of ice nucleus concentrations following the cessation of deposition of silver iodide during cloud seeding show that persistent secondary nuclei had been generated. The information is used to predict the effects on analyses of cloud seeding experiments. Effects on rainfall in large areas surrounding cloud seeding operations are then examined. Results: The predictions have explained effects such as a decrease in seeding effects with increasing application of silver iodide or with the duration of its application. Widespread downwind increases in rainfall appear to have accompanied all cloud seeding experiments. The explanation offered is that silver iodide increases the airborne concentration of ice nucleating bacteria or their properties. The secondary nuclei appear to be more efficient in stimulating rainfall than the silver iodide nuclei. Conclusions: Attempts to increase precipitation from clouds by seeding with silver iodide ice nuclei has been motivated by water shortages in many parts of the world. This paper has indicated that the frequent apparent slight or negligible success of such operations may have been due to the influence of the silver on ice nuclei of microbiological origin. An understanding of that influence is urgently required. Much larger and more widespread gains in rainfall than have been found in the past might then be obtained. Original Research Article

The characteristics of atmospheric ice nuclei measured at the top of Huangshan (the Yellow Mountains) in Southeast China using a newly built static vacuum water vapor diffusion chamber

A newly built static vacuum water vapor diffusion chamber was built to measure the concentration of ice nuclei (INs) at the top of Huangshan (the Yellow Mountains) in Southeast China. The experiments were conducted under temperatures between −15°C and −23°C and supersaturations with respect to ice between 4% and 25%. The results show that the average IN concentration was in the range of 0.27 to 7.02L−1, when the temperature was varied from −15°C to −23°C. The changes in IN concentrations with time were correlated with the change of number concentration of the aerosol particles of 0.5–20μm in diameter. The square correlation coefficients (R2) between IN and coarse aerosol particles (0.5–20μm in diameter) were all higher than 0.60, much higher than that (0.10) between IN and smaller particles (0.01–0.5μm). The concentration of ice nuclei at 14:00 LST was significantly higher than that at 08:00 LST, which is correlated with the diurnal variation of the concentration of aerosol particles. A parametric equation was developed based on measurements to represent the variations of IN concentration with temperature and supersaturation.

Characterisation of bioaerosol emissions from a Colorado pine forest: results from the BEACHON-RoMBAS experiment

Abstract. The behaviour of primary biological aerosols (PBAs) at an elevated, un-polluted North American forest site was studied using an ultra violet-light induced fluorescence (UV-LIF) measurement technique in conjunction with hierarchical agglomerative cluster analysis (HA-CA). Contemporaneous UV-LIF measurements were made with two wide-band integrated bioaerosol spectrometers, WIBS-3 and WIBS-4, which sampled close to the forest floor and via a continuous vertical profiling system, respectively. Additionally, meteorological parameters were recorded at various heights throughout the forest and used to estimate PBAP (Primary Biological Aerosol Particle) fluxes. HA-CA using data from the two, physically separated WIBS instruments independently yielded very similar cluster solutions. All fluorescent clusters displayed a diurnal minimum at midday at the forest floor with maximum concentration occurring at night. Additionally, the number concentration of each fluorescent cluster was enhanced, to different degrees, during wet periods. A cluster that displayed the greatest enhancement and highest concentration during sustained wet periods appears consistent with behaviour reported for fungal spores. A cluster that appears to be behaviourally consistent with bacteria dominated during dry periods. Fluorescent particle concentrations were found to be greater within the forest canopy than at the forest floor, indicating that the canopy was the main source of these particles rather than the minimal surface vegetation, which appeared to contribute little to overall PBA concentrations at this site. Fluorescent particle concentration was positively correlated with relative humidity (RH), and parameterisations of the aerosol response during dry and wet periods are reported. The aforementioned fungal spore-like cluster displayed a strong positive response to increasing RH. The bacteria-like cluster responded more strongly to direct rain-fall events than other PBA types. Peak concentrations of this cluster are shown to be linearly correlated to the log of peak rainfall rates. Parallel studies by Huffman et al. (2013) and Prenni et al. (2013) showed that the fluorescent particle concentrations correlated linearly with ice nuclei (IN) concentrations at this site during rain events. We discuss this result in conjunction with our cluster analysis to appraise the candidate IN.

Aerosol effects on cirrus through ice nucleation in the Community Atmosphere Model CAM5 with a statistical cirrus scheme

AbstractA statistical cirrus scheme that tracks ice saturation ratio in the clear‐sky and cloudy portion of a grid box separately has been implemented into the Community Atmosphere Model CAM5 to provide a consistent treatment of ice nucleation and cloud formation. Simulated ice supersaturation and ice crystal number concentrations strongly depend on the number concentrations of heterogeneous ice nuclei (IN), subgrid temperature formulas, and the number concentration of sulfate particles participating in homogeneous freezing, while simulated ice water content is insensitive to these perturbations. Allowing 1–10% of dust particles to serve as heterogeneous IN is found to produce ice supersaturation in better agreement with observations. Introducing a subgrid temperature perturbation based on long‐term aircraft observations produces a better hemispheric contrast in ice supersaturation compared to observations. Heterogeneous IN from dust particles alter the net radiative fluxes at the top of atmosphere (TOA) (−0.24 to −1.59 W m−2) with a significant clear‐sky longwave component (0.01 to −0.55 W m−2). Different cirrus treatments significantly perturb the net TOA anthropogenic aerosol forcing from −1.21 W m−2 to −1.54 W m−2, with a standard deviation of 0.10 W m−2. Aerosol effects on cirrus exert an even larger impact on the atmospheric component of the radiative fluxes (2 or 3 times the changes in the TOA radiative fluxes) and therefore through the fast atmosphere response on the hydrological cycle. This points to the urgent need to quantify aerosol effects on cirrus through ice nucleation and how these further affect the hydrological cycle.

High ice nucleation activity located in blueberry stem bark is linked to primary freeze initiation and adaptive freezing behaviour of the bark.

Controlled ice nucleation is an important mechanism in cold-hardy plant tissues for avoiding excessive supercooling of the protoplasm, for inducing extracellular freezing and/or for accommodating ice crystals in specific tissues. To understand its nature, it is necessary to characterize the ice nucleation activity (INA), defined as the ability of a tissue to induce heterogeneous ice nucleation. Few studies have addressed the precise localization of INA in wintering plant tissues in respect of its function. For this purpose, we recently revised a test tube INA assay and examined INA in various tissues of over 600 species. Extremely high levels of INA (-1 to -4 °C) in two wintering blueberry cultivars of contrasting freezing tolerance were found. Their INA was much greater than in other cold-hardy species and was found to be evenly distributed along the stems of the current year's growth. Concentrations of active ice nuclei in the stem were estimated from quantitative analyses. Stem INA was localized mainly in the bark while the xylem and pith had much lower INA. Bark INA was located mostly in the cell wall fraction (cell walls and intercellular structural components). Intracellular fractions had much less INA. Some cultivar differences were identified. The results corresponded closely with the intrinsic freezing behaviour (extracellular freezing) of the bark, icicle accumulation in the bark and initial ice nucleation in the stem under dry surface conditions. Stem INA was resistant to various antimicrobial treatments. These properties and specific localization imply that high INA in blueberry stems is of intrinsic origin and contributes to the spontaneous initiation of freezing in extracellular spaces of the bark by acting as a subfreezing temperature sensor.

Influence of heterogeneous freezing on the microphysical and radiative properties of orographic cirrus clouds

Abstract. The influence of heterogeneous freezing on the microphysical and optical properties of orographic cirrus clouds has been simulated with the large eddy simulation model EULAG. Idealised simulations with different concentrations of ice nuclei (IN) in a dynamically dominated regime with high vertical velocities have been performed. Furthermore the temperature at cloud formation as well as the critical supersaturation for initiation of heterogenous freezing have been varied. The shortwave, longwave and net cloud forcing has been calculated under the assumption that the clouds form between 06:00 and 12:00 local time (LT) or between 12:00 and 18:00 LT. In general it can be seen that the onset of homogeneous freezing is shifted in time depending on the IN concentration, as part of the available water vapour is depleted before the critical threshold for homogeneous freezing is reached. Although the high vertical velocities in an orographic gravity wave lead to a strong adiabatic cooling followed by high ice supersaturations, even a small number concentration of IN of the order of 5 L−1 is able to strongly decrease the simulated ice crystal number burden (ICNB), ice water path (IWP) and optical depth of the cloud. In general, the ICNB, IWP and optical depth strongly decrease when the IN concentrations are increased from 0 to 50 L−1. The absolute values of the shortwave, longwave and net cloud forcing are also reduced with increasing IN concentrations. A cloud will produce a net warming or cooling depending on the IN concentration, the temperature and the time of day when the cloud forms. The clouds that form between 06:00 and 12:00 LT are mainly cooling, whereas the clouds with the same microphysical properties can lead to a warming when they form between 12:00 and 18:00 LT. In order to predict the radiative forcing of cirrus clouds it is therefore necessary to take the correct dynamical and thermodynamical processes as well as the possible existence and freezing threshold of heterogeneous IN into account, not only for low vertical velocities but also for dynamically dominated regimes like orographic cirrus.

Observation of a freezing drizzle episode: A case study

On 5 February 2012 an episode of freezing precipitation took place in the Guadarrama Mountains, at the center of the Iberian Peninsula. This precipitation affected high elevations, where temperatures remained below freezing because of snow cover that had accumulated from snowfall during the previous days. The case study was recorded by surface synoptic observations (SYNOP) at Navacerrada Pass meteorological observatory (belonging to the National Weather Service of Spain). To study winter cloud systems during the TEcoAgua project, a multichannel ground-based microwave radiometer (MMWR), Micro Rain Radar (MRR-2), and isothermal cloud chamber were installed in the study area, thus permitting the monitoring of the freezing precipitation event.Analysis using Meteosat Second Generation (MSG) satellite data and observations permitted the determination of factors that triggered the freezing precipitation event. Freezing drizzle was interspersed with the passage of a warm and cold front. During frontal passage, mid-level clouds inhibited the generation of freezing drizzle, with snowfall recorded in the study area. However, during the period between the two fronts, an absence of mid-level clouds permitted low-level orographic clouds to persist upwind of the mountain system, producing freezing drizzle at the surface. The decisive factors for the generation of freezing drizzle were high humidity at low levels, weak mesoscale updrafts caused by the topography, stability at mid levels, cloud-top temperatures warmer than −15°C, and low concentrations of ice nuclei.

Effects of environmental conditions on inducing charge structures of thunderstorms over Eastern India

Abstract It is well known that environmental conditions like convective instability, aerosol loading, and availability of moisture content affect the polarity of charge structures of thunderstorms. The electrical characteristics of thunderstorms observed during the pre-monsoon season of year 2009, over Eastern India were studied to identify the effects of different environmental conditions on charge structures of thunderstorms occurring over this region. Electric field and Maxwell current data suggest that at least one of these thunderstorms had an inverted charge structure. Doppler RADAR, radiosonde, and Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) data have been used to compare the microphysical and dynamical characteristics of these thunderstorms. The thermo dynamical structure observed by radiosonde during the day on which an inverted polarity thunderstorm was observed showed very high CAPE in the mixed-phase region compared to other thunderstorm days. Furthermore, the AOD peaked 1 day before this thunderstorm. The back trajectories of winds also suggest that the aerosols might have been transported from a desert region on that day. It has been proposed that the large ice nuclei concentration can produce dominant positive charge in the lower portion of the mixed-phase region by maintaining ice saturation.

Contribution of pollen to atmospheric ice nuclei concentrations

Abstract. Recent studies have suggested that the ice-nucleating ability of some types of pollen is derived from non-proteinaceous macromolecules. These macromolecules may become dispersed by the rupturing of the pollen grain during wetting and drying cycles in the atmosphere. If true, this mechanism might prove to be a significant source of ice nuclei (IN) concentrations when pollen is present. Here we test this hypothesis by measuring ambient IN concentrations from the beginning to the end of the 2013 pollen season in Raleigh, North Carolina, USA. Air samples were collected using a swirling aerosol collector twice per week and the solutions were analysed for ice nuclei activity using a droplet freezing assay. Rainwater samples were collected at times when pollen grain number concentrations were near their maximum value and analysed with the drop-freezing assay to compare the potentially enhanced IN concentrations measured near the ground with IN concentrations found aloft. Ambient ice nuclei spectra, defined as the number of ice nuclei per volume of air as a function of temperature, are inferred from the aerosol collector solutions. No general trend was observed between ambient pollen grain counts and observed IN concentrations, suggesting that ice nuclei multiplication via pollen grain rupturing and subsequent release of macromolecules was not prevalent for the pollen types and meteorological conditions typically encountered in the southeastern US. A serendipitously sampled collection after a downpour provided evidence for a rain-induced IN burst with an observed IN concentration of approximately 30 per litre, a 30-fold increase over background concentrations at −20 °C. The onset temperature of freezing for these particles was approximately −12 °C, suggesting that the ice-nucleating particles were biological in origin.

The characteristics of atmospheric ice nuclei measured at different altitudes in the Huangshan Mountains in Southeast China

The concentration of ice nuclei (IN) and the relationship with aerosol particles were measured and analyzed using three 5-L mixing cloud chambers and a static diffusion cloud chamber at three altitudes in the Huangshan Mountains in Southeast China from May to September 2011. The results showed that the mean total number concentration of IN on the highest peak of the Huangshan Mountains at an activation temperature (T a) of −20°C was 16.6 L−1. When the supersaturation with respect to water (S w) and with respect to ice (S i) were set to 5%, the average number concentrations of IN measured at an activation temperature of −20°C by the static diffusion cloud chamber were 0.89 and 0.105 L−1, respectively. A comparison of the concentrations of IN at three different altitudes showed that the concentration of IN at the foot of the mountains was higher than at the peak. A further calculation of the correlation between IN and the concentrations of aerosol particles of different size ranges showed that the IN concentration was well correlated with the concentration of aerosol particles in the size range of 1.2–20 μm. It was also found that the IN concentration varied with meteorological conditions, such as wind speed, with higher IN concentrations often observed on days with strong wind. An analysis of the backward trajectories of air masses showed that low IN concentrations were often related to air masses travelling along southwest pathways, while higher IN concentrations were usually related to those transported along northeast pathways.

Weather Features Associated with Aircraft Icing Conditions: A Case Study

In the context of aviation weather hazards, the study of aircraft icing is very important because of several accidents attributed to it over recent decades. On February 1, 2012, an unusual meteorological situation caused severe icing of a C-212-200, an aircraft used during winter 2011-2012 to study winter cloud systems in the Guadarrama Mountains of the central Iberian Peninsula. Observations in this case were from a MP-3000A microwave radiometric profiler, which acquired atmospheric temperature and humidity profiles continuously every 2.5 minutes. A Cloud Aerosol and Precipitation Spectrometer (CAPS) was also used to study cloud hydrometeors. Finally, ice nuclei concentration was measured in an isothermal cloud chamber, with the goal of calculating concentrations in the study area. Synoptic and mesoscale meteorological conditions were analysed using the Weather Research and Forecasting (WRF) model. It was demonstrated that topography influenced generation of a mesolow and gravity waves on the lee side of the orographic barrier, in the region where the aircraft experienced icing. Other factors such as moisture, wind direction, temperature, atmospheric stability, and wind shear were decisive in the appearance of icing. This study indicates that icing conditions may arise locally, even when the synoptic situation does not indicate any risk.

Laboratory studies of immersion and deposition mode ice nucleation of ozone aged mineral dust particles

Abstract. Ice nucleation in the atmosphere is central to the understanding the microphysical properties of mixed-phase and cirrus clouds. Ambient conditions such as temperature (T) and relative humidity (RH), as well as aerosol properties such as chemical composition and mixing state play an important role in predicting ice formation in the troposphere. Previous field studies have reported the absence of sulfate and organic compounds on mineral dust ice crystal residuals sampled at mountain top stations or aircraft based measurements despite the long-range transport mineral dust is subjected to. We present laboratory studies of ice nucleation for immersion and deposition mode on ozone aged mineral dust particles for 233 < T < 263 K. Heterogeneous ice nucleation of untreated kaolinite (Ka) and Arizona Test Dust (ATD) particles is compared to corresponding aged particles that are subjected to ozone concentrations of 0.4–4.3 ppmv in a stainless steel aerosol tank. The portable ice nucleation counter (PINC) and immersion chamber combined with the Zurich ice nucleation chamber (IMCA-ZINC) are used to conduct deposition and immersion mode measurements, respectively. Ice active fractions as well as ice active surface site densities (ns) are reported and observed to increase as a function of decreasing temperature. We present first results that demonstrate enhancement of the ice nucleation ability of aged mineral dust particles in both the deposition and immersion mode due to ageing. We also present the first results to show a suppression of heterogeneous ice nucleation activity without the condensation of a coating of (in)organic material. In immersion mode, low ozone exposed Ka particles showed enhanced ice activity requiring a median freezing temperature of 1.5 K warmer than that of untreated Ka, whereas high ozone exposed ATD particles showed suppressed ice nucleation requiring a median freezing temperature of 3 K colder than that of untreated ATD. In deposition mode, low exposure Ka had ice active fractions of an order of magnitude higher than untreated Ka, whereas high ozone exposed ATD had ice active fractions up to a factor of 4 lower than untreated ATD. From our results, we derive and present parameterizations in terms of ns(T) that can be used in models to predict ice nuclei concentrations based on available aerosol surface area.

High concentrations of biological aerosol particles and ice nuclei during and after rain

Abstract. Bioaerosols are relevant for public health and may play an important role in the climate system, but their atmospheric abundance, properties, and sources are not well understood. Here we show that the concentration of airborne biological particles in a North American forest ecosystem increases significantly during rain and that bioparticles are closely correlated with atmospheric ice nuclei (IN). The greatest increase of bioparticles and IN occurred in the size range of 2–6 μm, which is characteristic for bacterial aggregates and fungal spores. By DNA analysis we found high diversities of airborne bacteria and fungi, including groups containing human and plant pathogens (mildew, smut and rust fungi, molds, Enterobacteriaceae, Pseudomonadaceae). In addition to detecting known bacterial and fungal IN (Pseudomonas sp., Fusarium sporotrichioides), we discovered two species of IN-active fungi that were not previously known as biological ice nucleators (Isaria farinosa and Acremonium implicatum). Our findings suggest that atmospheric bioaerosols, IN, and rainfall are more tightly coupled than previously assumed.