Up in the clouds

Nucleation and Atmospheric Aerosols, 2007

Quarterly Journal of the Royal Meteorological Society, 2011

† The contributions of P. R. A. Brown and R. Cotton were prepared in the course of their employment at the Met Office, UK, and are published with the permission of the Controller of HMSO and the Queen's Printer for Scotland. A combination of modelling studies and ground-based and aircraft measurements is used to examine the development of ice particles in convective clouds observed over the Black Forest mountains during the Convective and Orographically-induced Precipitation Study (COPS). High concentrations of relatively small ice particles were observed in the weaker northern cell that developed on convergence lines over the mountains in the much-studied 15 July 2007 case. The conditions in the cloud were not conducive for the Hallett-Mossop process. Instead, the explanation for such high concentrations of ice is likely associated with the type of ice nuclei ingested into the cloud. Biological nuclei, oxidised organic aerosol particles in the polluted air vented from the Murg valley into the cloud base, and desert dust are all possible candidates. A model sensitivity test with biological nuclei produced similar concentrations of ice particles to the observations. In contrast, the high concentration of ice particles measured in clouds that advected over the Black Forest mountains on 11 July 2007 were likely due to the Hallett-Mossop process. The deep convective cell at the southern end of the COPS domain on 15 July 2007 developed in less polluted air than the shallower northern cloud. A model sensitivity test with lower aerosol loading produced a more vigorous cloud with a higher top, more precipitation, and greater reflectivity, more similar to the radar observations. The results suggest that aerosol particles vented out of the valleys could have a significant impact on orographically induced precipitation. Copyright

1.BACKGROUND This paper focuses on wintertime orographic clouds not associated with major synoptic dis- turbances, but formed by strong winds across major barriers. Such clouds constitute a major source of precipitation in many regions of the world and are frequent targets of cloud seed- ing activities aimed at increasing the mountain snowpack. For these reasons such clouds have been extensively observed and mod- elled. However, these systems are no excep- tion to the general difficulty of predicting the concentrations of ice particles that form in them at different temperatures. The most fre- quently made assumption is that ice particle concentrations follow ice nucleus concentra- tions, which in turn are assumed to be an ex- ponential function of temperature. This as- sumption is justified by the absence of pro- cesses of secondary ice generation and the re- latively simple dynamics of these clouds. Yet, it is known that the observed ice concentrations vary over large ranges and a...

Atmospheric Chemistry and Physics, 2017

In situ single particle analysis of ice particle residuals (IPRs) and out-of-cloud aerosol particles was conducted by means of laser ablation mass spectrometry during the intensive INUIT-JFJ/CLACE campaign at the high alpine research station Jungfraujoch (3580 m a.s.l.) in January-February 2013. During the 4-week campaign more than 70 000 out-of-cloud aerosol particles and 595 IPRs were analyzed covering a particle size diameter range from 100 nm to 3 µm. The IPRs were sampled during 273 h while the station was covered by mixed-phase clouds at ambient temperatures between −27 and −6 • C. The identification of particle types is based on laboratory studies of different types of biological, mineral and anthropogenic aerosol particles. The outcome of these laboratory studies was characteristic marker peaks for each investigated particle type. These marker peaks were applied to the field data. In the sampled IPRs we identified a larger number fraction of primary aerosol particles, like soil dust (13 ± 5 %) and minerals (11 ± 5 %), in comparison to out-of-cloud aerosol particles (2.4 ± 0.4 and 0.4 ± 0.1 %, respectively). Additionally, anthropogenic aerosol particles, such as particles from industrial emissions and lead-containing particles, were found to be more abundant in the IPRs than in the out-of-cloud aerosol. In the out-of-cloud aerosol we identified a large fraction of aged particles (31 ± 5 %), including organic material and secondary inorganics, whereas this particle type was much less abundant (2.7 ± 1.3 %) in the IPRs. In a selected subset of the data where a direct comparison between out-ofcloud aerosol particles and IPRs in air masses with similar origin was possible, a pronounced enhancement of biological particles was found in the IPRs.

2018

Ice particle residuals (IPRs) and the total aerosol particle population were sampled in parallel during mixed phase cloud events at the high altitude research station Jungfraujoch in January/February 2017. Particles were sampled on boron substrates by use of multi MINI cascade impactors operated behind an ice selective counterflow impactor (Ice-CVI) for IPRs and a heated total inlet for the total aerosol particles. Total aerosol samples were collected with a dilution setup to match the much longer sampling time behind the Ice-CVI. About 4000 particles from ten Ice-CVI samples (from seven days of cloud events at temperatures between-10 °C and-18°C) were analysed and classified with operator controlled scanning electron microscopy. Contamination particles (identified by their chemical composition) most likely originating from abrasion in the Ice-CVI and collection of secondary ice, were excluded from the further analysis. Approximately 3000 total aerosol particles from five days in clouds were also analysed. Enrichment and depletion of the different particle groups (within the IPR fraction relative to total aerosol reservoir) are presented as odds ratio relative to alumosilicate (particles only consisting of Al, Si and O), which was chosen as reference due to the large enrichment of this group relative to total aerosol and the relatively high number concentration of this group in both total aerosol and the IPR samples. Complex secondary particles and soot are the major particle groups in the total aerosol samples, but are not found in the IPR fraction and are hence strongly depleted. C-rich particles (most likely organic particles) showed a smaller enrichment compared to alumosilicates by a factor of ~20. The particle groups with similar enrichment as alumosilicate are silica, Fe-alumosilicates, Ca-rich, Ca-sulphates, sea salt and metal/ metal oxide. Other-alumosilicates-consisting of variable amounts of Na, K, Ca, Si, Al, O, Ti and Fe-are somewhat more (factor ~2) and Pb-rich more (factor ~8) enriched than alumosilicates. None of the sampled IPR groups showed a temperature or size dependence in respect to ice activity, which might be due to the limited sampling temperature interval and the similar size of the particles. Footprint plots and wind roses could explain the different total aerosol composition in one sample (carbonaceous particle emission from the urban/industrial area of Po Valley), but this did not affect the IPR composition. Taken into account the relative abundance of the particle groups in total aerosol and the ice nucleation ability, we found that silica, alumosilicates and other-alumosilicates were the most important ice particle residuals at Jungfraujoch during the mixed phase cloud events in winter 2017.

Tellus B, 2015

The state of a slightly supercooled ephemeral cloud can be changed by the presence of a few particles capable of catalysing freezing, and potentially result in precipitation. We investigated the atmospheric abundance of particles active as ice nuclei at (88C (IN (8) over the course of a year at the high-alpine station Jungfraujoch (3580 m.a.s.l., Switzerland) through the use of immersion freezing assays of particles collected on quartz microfibre filters. In addition, we determined IN (8 on a hill in the planetary boundary layer 95 km northwest of Jungfraujoch and in the dust laden Saharan Air Layer reaching Tenerife. Results indicate a strong seasonality of IN (8 at Jungfraujoch. Values were largest during summer (between 1 and 10 m (3) and about two orders of magnitude smaller during winter. Sahara dust events had a negligible influence on IN (8 at Jungfraujoch. Seasonality in the boundary layer was not observed in the upper, but in the lower bound of IN (8 values. Values B1m (3 were only found on cold winter days, when IN (8 were more likely to have already been activated and deposited than on warmer days. A good correlation between IN (8 and maximum daily temperature at Jungfraujoch (R 2 00.54) suggests IN (8 abundance at Jungfraujoch may be limited most of the year by microphysical processing related to IN activation in approaching air masses.

Atmospheric Chemistry and Physics Discussions, 2015

During the winter of 2013 and 2014 measurements of cloud microphysical properties over a five 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) Measure-5 ments 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 10 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 Hallet-Mossop process as a possible explana-15 tion 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 4 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 20 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 .

This paper presents results from the "INUIT-JFJ/CLACE 2013" field campaign at the high alpine research station Jungfraujoch in January/February 2013. The chemical composition of ice particle residuals (IPR) in a size diameter range of 200-900 nm was measured in orographic, convective and non-convective clouds with a single particle mass spectrometer (ALABAMA) under ambient conditions characterized by temperatures between −28 and −4 • C and wind speed from 0.1 to 21 km h −1. Additionally, background aerosol particles in cloud free air were investigated. The IPR were sampled from mixed-phase clouds with two inlets which selectively extract small ice crystals in-cloud, namely the Counterflow Virtual Impactor (Ice-CVI) and the Ice Selective Inlet ACPD

Annales Geophysicae, 2000

When the University of Bonn lidar on the Esrange (68°N, 21°E), Sweden, was switched on in the evening of July 18, 1998, a geometrically and optically thin cloud layer was present near 14 km altitude or 400 K potential temperature, where it persisted for two hours. The tropopause altitude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmospheric temperatures near 230 K were approximately 40 K too high to support ice particles at stratospheric water vapour pressures of a few ppmv. The isentropic back trajectory on 400 K showed the air parcels to have stayed clear of active major rocket launch sites. The air parcels at 400 K had travelled from the Aleutians across Canada and the Atlantic Ocean arriving above central Europe and then turned northward to pass over above the lidar station. Parcels at levels at AE25 K from 400 K had come from the pole and joined the 400 K trajectory path above eastern Canada. Apparently the cloud existed in a ®lament of air with an origin dierent from those ®laments both above and below. Possibly the 400 K level air parcels had carried soot particles from forest wild ®res in northern Canada or volcanic ash from the eruption of the Korovin Volcano in the Aleutian Islands.

Nature

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