Quantifying the effect of riming on snowfall using ground-based observations

Ground-based observations of ice particle size distribution and ensemble mean density are used to quantify the effect of riming on snowfall. The rime mass fraction is derived from these measurements by following the approach that is used in a single ice-phase category microphysical scheme proposed for the use in numerical weather prediction models. One of the characteristics of the proposed scheme is that the prefactor of a power law relation that links mass and size of ice particles is determined by the rime mass fraction, while the exponent does not change. To derive the rime mass fraction, a mass-dimensional relation representative of unrimed snow is also determined. To check the validity of the proposed retrieval method, the derived rime mass fraction is converted to the effective liquid water path that is compared to microwave radiometer observations. Since dual-polarization radar observations are often used to detect riming, the impact of riming on dual-polarization radar variables is studied for differential reflectivity measurements. It is shown that the relation between rime mass fraction and differential reflectivity is ambiguous, other factors such as change in median volume diameter need also be considered. Given the current interest on sensitivity of precipitation to aerosol pollution, which could inhibit riming, the importance of riming for surface snow accumulation is investigated. It is found that riming is responsible for 5% to 40% of snowfall mass. The study is based on data collected at the University of Helsinki field station in Hyytiala during U.S. Department of Energy Biogenic Aerosols Effects on Clouds and Climate (BAECC) field campaign and the winter 2014/2015. In total 22 winter storms were analyzed, and detailed analysis of two events is presented to illustrate the study.Peer reviewe

Ice nucleation abilities of soot particles determined with the Horizontal Ice Nucleation Chamber

Ice nucleation by different types of soot particles is systematically investigated over the temperature range from 218 to 253&thinsp;K relevant for both mixed-phase (MPCs) and cirrus clouds. Soot types were selected to represent a range of physicochemical properties associated with combustion particles. Their ice nucleation ability was determined as a function of particle size using relative humidity (RH) scans in the Horizontal Ice Nucleation Chamber (HINC). We complement our ice nucleation results by a suite of particle characterization measurements, including determination of particle surface area, fractal dimension, temperature-dependent mass loss (ML), water vapor sorption and inferred porosity measurements. Independent of particle size, all soot types reveal absence of ice nucleation below and at water saturation in the MPC regime (T &gt; 235&thinsp;K). In the cirrus regime (T ≤ 235&thinsp;K), soot types show different freezing behavior depending on particle size and soot type, but the freezing is closely linked to the soot particle properties. Specifically, our results suggest that if soot aggregates contain mesopores (pore diameters of 2–50&thinsp;nm) and have sufficiently low water–soot contact angles, they show ice nucleation activity and can contribute to ice formation in the cirrus regime at RH well below homogeneous freezing of solution droplets. We attribute the observed ice nucleation to a pore condensation and freezing (PCF) mechanism. Nevertheless, soot particles without cavities of the right size and/or too-high contact angles nucleate ice only at or well above the RH required for homogeneous freezing conditions of solution droplets. Thus, our results imply that soot particles able to nucleate ice via PCF could impact the microphysical properties of ice clouds.</p

Observed relations between snowfall microphysics and triple-frequency radar measurements

2015JD023156Recently published studies of triple-frequency radar observations of snowfall have demonstrated that naturally occurring snowflakes exhibit scattering signatures that are in some cases consistent with spheroidal particle models and in others can only be explained by complex aggregates. Until recently, no in situ observations have been available to investigate links between microphysical snowfall properties and their scattering properties. In this study, we investigate for the first time relations between collocated ground-based triple-frequency observations with in situ measurements of snowfall at the ground. The three analyzed snowfall cases obtained during a recent field campaign in Finland cover light to moderate snowfall rates with transitions from heavily rimed snow to open-structured, low-density snowflakes. The observed triple-frequency signatures agree well with the previously published findings from airborne radar observations. A rich spatiotemporal structure of triple-frequency observations throughout the cloud is observed during the three cases, which often seems to be related to riming and aggregation zones within the cloud. The comparison of triple-frequency signatures from the lowest altitudes with the ground-based in situ measurements reveals that in the presence of large (>5 mm) snow aggregates, a bending away in the triple-frequency space from the curve of classical spheroid scattering models is always observed. Rimed particles appear along an almost horizontal line in the triple-frequency space, which was not observed before. Overall, the three case studies indicate a close connection of triple-frequency signatures and snow particle structure, bulk snowfall density, and characteristic size of the particle size distribution.Peer reviewe

Making a 16-electron bromo (or iodo) complex of ruthenium(II) and a C—F bond in one pot

The 16e– bromo or iodo complexes [RuX(dppp)2]+ (dppp = 1,3-bis(diphenylphosphino)propane, X = Br (1c), I (1d)) and [RuX(dppe)2]+ (dppe = 1,2-bis(diphenylphosphino)ethane, X = Br (2c), I (2d)) have been prepared exploiting the reaction of the fluoro complexes [RuF(dppp)2]+ (1a) and [Tl(µ-F)2Ru(dppe)2]+ (3) with activated alkyl bromides or iodides. The X-ray structures of 1c, 1d, 2c, and 2d suggest that the distortion of the Y-shaped trigonal-bipyramidal structure of [MX(P∩P)2]+ is possibly related to the formation of intramolecular hydrogen bonds between the halide ligand and the ortho-hydrogen atoms of the neighbouring phenyl rings. The five-coordinate species 1c, 1d, 2c, and 2d react with H2 to form the dihydrogen complexes [RuX(η2-H2)(P∩P)2]+. The reaction of the dppp derivatives 1c and 1d with H2 (P = 1 atm, 1 atm = 101.322 kPa) is an equilibrium. Quantitative formation of [RuBr(η2-H2)(dppp)2] (4c) is obtained under H2 pressure (100 bar, 1 bar = 100 kPa), whereas the iodo analogue is not stable under analogous conditions. The less crowded dppe derivatives 2c and 2d react quantitatively with H2 under ambient pressure. The iodo and bromo derivatives [RuX(η2-H2)(P∩P)2]+ contain elongated dihydrogen ligands, as indicated by their transverse relaxation times T1 (min). The present data suggest that Cl, Br, and I have similar donor properties in these dihydrogen complexes, and that the different chemical behaviour in the Cl, Br, I series is mainly a result of steric effects.Key words: 16e– complexes, ruthenium, fluoro complexes, bromo complexes, iodo complexes, dihydrogen complexes. </jats:p