The formation of ice particles in the Earth's atmosphere strongly affects the properties of clouds and their impact on climate. Despite the importance of ice formation in determining the properties of clouds, the Intergovernmental Panel on Climate Change (IPCC, 2007) was unable to assess the impact of atmospheric ice formation in their most recent report because our basic knowledge is insufficient. Part of the problem is the paucity of quantitative information on the ability of various atmospheric aerosol species to initiate ice formation. Here we review and assess the existing quantitative knowledge of ice nucleation by particles immersed within supercooled water droplets. We introduce aerosol species which have been identified in the past as potentially important ice nuclei and address their ice-nucleating ability when immersed in a supercooled droplet. We focus on mineral dusts, biological species (pollen, bacteria, fungal spores and plankton), carbonaceous combustion products and volcanic ash. In order to make a quantitative comparison we first introduce several ways of describing ice nucleation and then summarise the existing information according to the time-independent (singular) approximation. Using this approximation in combination with typical atmospheric loadings, we estimate the importance of ice nucleation by different aerosol types. According to these estimates we find that ice nucleation below about −15 °C is dominated by soot and mineral dusts. Above this temperature the only materials known to nucleate ice are biological, with quantitative data for other materials absent from the literature. We conclude with a summary of the challenges our community faces.

/pdf/ice-nucleation-by-particles-immersed-in-supercooled-cloud-4c88zmbtg2.pdf

Ice nucleation by particles immersed in supercooled cloud droplets

The 39-day long eruption at the summit of Eyjafjallajokull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011 kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km3) transported by 3–10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010 kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010 kg (2%) are considered to have been transported longer than 600–700 km with <108 kg (<0.02%) reaching mainland Europe.

/pdf/ash-generation-and-distribution-from-the-april-may-2010-2nud1l5m62.pdf

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland

. The April–May, 2010 volcanic eruptions of Eyjafjallajokull, Iceland caused significant economic and social disruption in Europe whilst state of the art measurements and ash dispersion forecasts were heavily criticized by the aviation industry. Here we demonstrate for the first time that large improvements can be made in quantitative predictions of the fate of volcanic ash emissions, by using an inversion scheme that couples a priori source information and the output of a Lagrangian dispersion model with satellite data to estimate the volcanic ash source strength as a function of altitude and time. From the inversion, we obtain a total fine ash emission of the eruption of 8.3 ± 4.2 Tg for particles in the size range of 2.8–28 μm diameter. We evaluate the results of our model results with a posteriori ash emissions using independent ground-based, airborne and space-borne measurements both in case studies and statistically. Subsequently, we estimate the area over Europe affected by volcanic ash above certain concentration thresholds relevant for the aviation industry. We find that during three episodes in April and May, volcanic ash concentrations at some altitude in the atmosphere exceeded the limits for the "Normal" flying zone in up to 14 % (6–16 %), 2 % (1–3 %) and 7 % (4–11 %), respectively, of the European area. For a limit of 2 mg m−3 only two episodes with fractions of 1.5 % (0.2–2.8 %) and 0.9 % (0.1–1.6 %) occurred, while the current "No-Fly" zone criterion of 4 mg m−3 was rarely exceeded. Our results have important ramifications for determining air space closures and for real-time quantitative estimations of ash concentrations. Furthermore, the general nature of our method yields better constraints on the distribution and fate of volcanic ash in the Earth system.

/pdf/determination-of-time-and-height-resolved-volcanic-ash-n9xc7uu1ci.pdf

Determination of time- and height-resolved volcanic ash emissions and their use for quantitative ash dispersion modeling: the 2010 Eyjafjallajökull eruption

. The Cloud-Aerosol Lidar with Orthogonal Polarization
(CALIOP) version 4.10 (V4) level 2 aerosol data products, released in
November 2016, include substantial improvements to the aerosol subtyping and
lidar ratio selection algorithms. These improvements are described along
with resulting changes in aerosol optical depth (AOD). The most fundamental
change in the V4 level 2 aerosol products is a new algorithm to identify aerosol
subtypes in the stratosphere. Four aerosol subtypes are introduced for
stratospheric aerosols: polar stratospheric aerosol (PSA), volcanic ash,
sulfate/other, and smoke. The tropospheric aerosol subtyping algorithm was
also improved by adding the following enhancements: (1) all aerosol subtypes
are now allowed over polar regions, whereas the version 3 (V3) algorithm
allowed only clean continental and polluted continental aerosols; (2) a new
“dusty marine” aerosol subtype is introduced, representing mixtures of
dust and marine aerosols near the ocean surface; and (3) the “polluted
continental” and “smoke” subtypes have been renamed “polluted
continental/smoke” and “elevated smoke”, respectively. V4 also revises
the lidar ratios for clean marine, dust, clean continental, and elevated
smoke subtypes. As a consequence of the V4 updates, the mean 532 nm AOD
retrieved by CALIOP has increased by 0.044 (0.036) or 52 % (40 %) for
nighttime (daytime). Lidar ratio revisions are the most influential factor
for AOD changes from V3 to V4, especially for cloud-free skies. Preliminary
validation studies show that the AOD discrepancies between CALIOP and
AERONET–MODIS (ocean) are reduced in V4 compared to V3.

/pdf/the-calipso-version-4-automated-aerosol-classification-and-4u5tjok927.pdf

The CALIPSO Version 4 Automated Aerosol Classification and Lidar Ratio Selection Algorithm

Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.

/pdf/stratospheric-aerosol-observations-processes-and-impact-on-2ujdtgxydt.pdf

Stratospheric Aerosol--Observations, Processes, and Impact on Climate

. Airborne lidar and in-situ measurements of aerosols and trace gases were performed in volcanic ash plumes over Europe between Southern Germany and Iceland with the Falcon aircraft during the eruption period of the Eyjafjalla volcano between 19 April and 18 May 2010. Flight planning and measurement analyses were supported by a refined Meteosat ash product and trajectory model analysis. The volcanic ash plume was observed with lidar directly over the volcano and up to a distance of 2700 km downwind, and up to 120 h plume ages. Aged ash layers were between a few 100 m to 3 km deep, occurred between 1 and 7 km altitude, and were typically 100 to 300 km wide. Particles collected by impactors had diameters up to 20 μm diameter, with size and age dependent composition. Ash mass concentrations were derived from optical particle spectrometers for a particle density of 2.6 g cm−3 and various values of the refractive index (RI, real part: 1.59; 3 values for the imaginary part: 0, 0.004 and 0.008). The mass concentrations, effective diameters and related optical properties were compared with ground-based lidar observations. Theoretical considerations of particle sedimentation constrain the particle diameters to those obtained for the lower RI values. The ash mass concentration results have an uncertainty of a factor of two. The maximum ash mass concentration encountered during the 17 flights with 34 ash plume penetrations was below 1 mg m−3. The Falcon flew in ash clouds up to about 0.8 mg m−3 for a few minutes and in an ash cloud with approximately 0.2 mg m−3 mean-concentration for about one hour without engine damage. The ash plumes were rather dry and correlated with considerable CO and SO2 increases and O3 decreases. To first order, ash concentration and SO2 mixing ratio in the plumes decreased by a factor of two within less than a day. In fresh plumes, the SO2 and CO concentration increases were correlated with the ash mass concentration. The ash plumes were often visible slantwise as faint dark layers, even for concentrations below 0.1 mg m−3. The large abundance of volatile Aitken mode particles suggests previous nucleation of sulfuric acid droplets. The effective diameters range between 0.2 and 3 μm with considerable surface and volume contributions from the Aitken and coarse mode aerosol, respectively. The distal ash mass flux on 2 May was of the order of 500 (240–1600) kg s−1. The volcano induced about 10 (2.5–50) Tg of distal ash mass and about 3 (0.6–23) Tg of SO2 during the whole eruption period. The results of the Falcon flights were used to support the responsible agencies in their decisions concerning air traffic in the presence of volcanic ash.

/pdf/airborne-observations-of-the-eyjafjalla-volcano-ash-cloud-3dz485lnbg.pdf

Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010

The Greenland tip jet is a narrow, low-level jet periodically developing in the lee of Cape Farewell. It has recently been identified as an important mesoscale meteorological element driving the thermohaline circulation in the Atlantic Ocean. Due to the heat loss by the enhanced turbulent transport over the sea the sporadically occurring "corner wind" initiates the deep sea convection in the Irminger Sea (Pickart et al. 2003). This area is considered to be an additional source of deep sea water (so called Labrador Sea Water) and the subject is strongly discussed among oceanographers (Dickson 2003). Doyle and Shapiro (1999) investigated the Greenland tip jet by means of real case and idealized model simulations. They explored the flow response to large-scale topography which is particularly difficult to predict (Ólafsson, 1998, Schär 2002). Flow over and around Greenland is affected by the large and very steep elevation change between the ocean and the central plateau of about 3000 m, the roughness of the jagged mountains around the coastline, and the predominant katabatic flows from the plateau down the coast. Greenland may influence the atmospheric flow several thousand kilometers downstream (Scorer 1988). Vertically, the flow past Greenland induces disturbances up to stratospheric altitudes (e. g. Leutbecher and Volkert 2000). Most of the recent papers study the influence of Greenland’s topography on the impinging westerly flow and the lee cyclogenesis by numerical simulations (e.g. Kristjánsson and McInnes, 1999 Schwierz and Davis, 2003, Petersen et al. 2003). Observations of the flow field in the lee of Greenland are rare. Here, we present airborne Doppler wind lidar and dropsonde observations of a tip jet event on 24 November 2003. The observations took place during the Atlantic THORPEX Regional Campaign (ATReC), which was conducted from October to December 2003 under the lead of EUCOS with participation of NOAA, NCAR, UCAR, NASA, NRL, DLR, and Environment Canada. During that time the DLR research aircraft Falcon was based in Keflavik, Iceland. The goal of the DLR participation was to deliver targeted dropsonde and wind lidar observations. Instruments

R. Simmet

Papers

Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010

Wind Lidar Observations in the Lee of Greenland