Spatial structure and dispersion of the 16 / 17 April 2010 volcanic ash cloud over Germany

S. Emeis, W. Junkermann, K. Schäfer, R. Forkel, P. Suppan, H. Flentje, S. Gilge, W. Fricke, M. Wiegner, V. Freudenthaler, S. Groß, L. Ries, F. Meinhardt, C. Münkel, and F. Obleitner Institute for Meteorology and Climate Research (IMK-IFU), Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany German Weather Service, Offenbach and Hohenpeißenberg, Germany Meteorological Institute, Ludwig-Maximilians University, Munich, Germany Federal Environmental Agency, Germany Vaisala GmbH, Hamburg, Germany Institute for Meteorology and Geophysics, University of Innsbruck, Austria


Introduction
The emission of geogenic material and smoke and their dispersion in the atmosphere has influenced human societies at all times.Except from the luckily rare occasions of meteorite impacts (see, e.g., Pollack et al., 1983), the most prominent types of these events are the advection of material from wind erosion such as desert dust (Shao, 2008), from large fires (Damoah et al., 2004), and from volcanic ash plumes (Woods et al., 1995) over populated areas.All these types of events can lead to reduced incoming shortwave radiation, reduced visibility, and -in extreme cases -even to adverse health impacts and degradation of technical functions such as jet aircraft turbines.In Europe desert dust advection from the Sahara happens by and then on the front side of approaching troughs from the West (Ansmann et al., 2003).These events, which are sometimes visible for the human eye as colourless haze in higher atmospheric layers, rarely affect the normal life of biota.Aviation over Europe is usually not affected by Saharan dust, although Simpson et al. (2003) do not rule out impacts of Asian dust on aviation.The advection of volcanic ash clouds on the other hand has several times led to remarkable effects on air traffic due to the lower melting point of ejected material as compared to desert dust (Casadevall, 1992), and due to the sharp-edged nature of the emitted particles (see, e.g., Tupper et al., 2006 for a list of some studies on past events).In order to avoid damages and failures to aircraft in operation, a global network of nine Volcanic Ash Advisory Centres (VAAC) has been established in the 1990s (ICAO, 2000).The dispersion of volcanic ash clouds over Northern and Central Europe is currently computed by the Lagrangian model NAME (Numerical Atmospheric-dispersion Modelling Environment) and the simulation results are interpreted and issued by the London VAAC (Witham et al., 2007).
A prominent event of a tropospheric advection of volcanic ash to Europe was the eruption of the Icelandic volcano Laki in Iceland which commenced on 8 June, 1783 and lasted until 8 February, 1784.This was the most violent, extensive and prolonged volcanic episode which has occurred in the northern hemisphere during the modern era (Grattan and Brayshay, 1995).The volcano generated SO 2 at a rate of 1•7 million tonnes per day during the first 6 weeks of the eruption.In addition, huge amounts of hydrofluoric and hydrochloric acid were produced during this period.The resulting "dry fog" was present nearly constantly during late June, July, and August of 1783 in Britain, Scandinavia, France, Belgium, The Netherlands, Germany, and Italy, where it shrouded the sun and moon, reduced visibility, affected human health, and withered vegetation.Air-pollution concentrations during this 18th century event were at least as great as those recorded during modern urban air-pollution episodes, and these conditions probably persisted or recurred throughout Europe during the summer of 1783 (Durand and Grattan, 2001).Introduction

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Full Recently, on 14 April, 2010 the Icelandic volcano Eyjafjallaj ökull erupted fiercely and its ash cloud was advected by northwesterly winds nearly directly towards Central Europe.The ash cloud was not observable by the naked eye, but nevertheless it had massive impact on the European air traffic.Aircraft were grounded in larger parts of Europe for more than five days between 15 and 21 April, 2010.The traffic bans for different parts of Europe were based mainly on the forecasted ash cloud dispersion from the London VAAC dispersion model.Thus, the assessment and forecasting of the spatial structure and the dispersion of such volcanic clouds has become a major issue in public, economics and aviation as well as in discussions at scientific conferences.This paper will cover the detection and analysis of the spatial structure and dispersion of the volcanic ash cloud mainly by optical ground-based remote sensing, some in-situ air quality measurements in Southern Germany close to the Alps, and a Eulerian dispersion model simulation with MCCM (Grell et al., 2000).The analysis will concentrate on the propagation of the first southward-moving ash cloud which arrived over Germany on 16 and 17 April, 2010.It will report on its detection by a ground-based remote sensing network.It will address the identification of the volcanic origin of the detected aerosol cloud.The influence of the Alps at the southern frontier of Germany on the ash dispersion will be shortly investigated.Finally, the remote sensing network data will be used to verify a dispersion simulation with the Eulerian model.We will not discuss subsequent regional or global dimming effects or any other climate impact issue, because Eyjafjallaj ökull only ejected material into the troposphere.
A related paper by Sch äfer et al. ( 2010) will analyse the interaction between the ash cloud and the atmospheric boundary layer and focus on air quality and health issues.Introduction

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Full 2 Data, Weather situation, and instruments

2010 Eyjafjallaj ökull eruptions
The Eyjafjallaj ökull is a 1666 m high ice-covered volcano near the southern tip of Iceland at 63  1.This information serves to characterize the strength and height of the eruption since quantitative emission data is not available.The material from the very first eruption day on 14 April, 2010, which was the fiercest one, will be in the focus of this paper.

Weather situation
The transport of volcanic ash clouds from volcanoes in Iceland towards Central Europe depends on the height of the eruption cloud and the prevailing wind patterns over Western, Northern, and Central Europe.The eruption height of Eyjafjallaj ökull was between 3 km minimum and 9.5 km maximum (Petersen, 2010, see also Table 1).This led to an injection of material into the middle and partly also upper troposphere but not into the stratosphere.The mean emission height roughly coincided with the height of the 500 hPa layer of the troposphere.Therefore, Fig. 1 left shows the flow patterns in this 500 hPa layer at 48 h intervals from the period with air traffic bans.It shows a rather stationary weather situation with high pressure over the Atlantic and the British Isles, a weak and decaying trough over Central and Southwestern Europe and anticyclonic activity to the North.From 19 April onwards, a trough over Scandinavia was forming.Remarkable is the persisting low wind speed situation over Central Europe, which led to the phenomenon that the ash cloud which had been advected towards Central Europe so rapidly between 14 and Introduction

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Full The right-hand column in Fig. 1 shows the predictions of the London VAAC based on simulations with the Lagrangian dispersion model NAME.It simulated a transport of volcanic ash towards Scandinavia on the first day after the eruption.48 h later the core of the ash cloud had rapidly moved southward and covered the whole of Central Europe.Partly a transport of ash back towards the Atlantic due to the anticyclonic (clockwise) circulation over the British Isles is visible as well.The following days the cloud was more or less stagnant over Central Europeand started to disappear on 21 April, so that the flight ban could be cancelled on this day.

Measurement instruments
Instrumentation, which delivered the data for the present assessment includes optical surface-based remote sensing devices all over Germany and a few ground-based insitu instruments in Southern Germany and onboard an ultralight aircraft.

Optical remote sensing
The ceilometer network of the German Weather Service (DWD), which now consists of 36 Jenoptik CHM15K instruments operating at 1064 nm allows for an areal observation of aerosol backscatter over Germany (Flentje et al., 2010a, b).These biaxial ceilometers provide vertical profiles of particle and molecular back-scattering in an atmospheric column from about 600m above ground level up to 15 km with a vertical resolution of 15 m at a 5-7 kHz repetition rate.The wavelength of 1064 nm provides relatively large contrast to molecular scattering, thus highlighting aerosol structures, but the scattering efficiency drops sharply for particles with radii well below 1 µm, which limits the accuracy of a single profile.See Flentje et al. (2010b) for additional data from a VAISALA LD-40 ceilometer, operated at the Schneefernerhaus, and further references.
Quantitative assessment of optical properties of the volcanic ash layer is derived from lidar measurements (Meteorological Institute of the Ludwig-Maximilians University

2003
).In the present paper data from the multi-wavelength lidar system MULIS (e.g., Freudenthaler et al., 2009)  Innsbruck (and at the airport Vienna Schwechat), which have been analysed by the Institute of Meteorology and Geophysics of the University of Innsbruck, Austria (IMG-IBK).This ceilometer is a one-lens ceilometer using infrared light of 910 nm.It offers a height resolution of 10 m from about 30 m above ground to a maximum range of 7500 m.A comparison on the optical configuration of mono-axial and biaxial ceilometers can be found in Emeis (2010).Introduction

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In-situ instruments
The ultralight aircraft of IMK-IFU flew in the area of Augsburg in the late afternoon of 19 April, 2010.This research aircraft is based on a weight shift ultralight (Junkermann, 2001) and is equipped with a suite of instruments for the measurement of aerosol and radiation properties, ozone and meteorological parameters.Instrumentation suitable for the detection of the volcanic ash cloud consists of an optical particle counter GRIMM 1.108 with 15 size bins between 300 nm and 20 µm, an open path nephelometer for the measurement of the extinction, and a seven-wavelength aethalometer, MAGEE AE42, for the spectral characterisation of the aerosol.The flight was performed about 10 km northwest of the airport of Jesenwang ( 48• 10.46 N, 11 • 10.50 E, between Munich and Augsburg) with the intention to be comparable to the ceilometer and LIDAR measurements in the vicinity.Measurement of aerosol properties are performed at Schauinsland station in 1200 m a.s.l.run by the German Environmental Agency (UBA).Schauinsland is a summit site in the southern Black Forest in the southwestern corner of Germany.SO 2 is measured with a TE43CTL Thermo Scientific, PM 10 with a Thermo Fisher FH62IR.Particle size spectrum is obtained from a scanning mobility particle sizer (SMPS) with range from 10-800 nm (differential mobility analyser from IFT, Leipzig , with condensation particle counter CPC 3772).The size spectrum sample has been differentiated by an alternating treatment with and without thermo-denuder at 300 • C. The black carbon content of aerosol was measured with a MAAP (Multi Angle Absorption Spectrometer MLU 5012).Aerosol measurements at the UBA station at Schneefernerhaus close to the Zugspitze in an altitude of 2650 m a.s.l. are carried out with a SMPS (model TSI 3080 with condensation particle counter 3010 CPC) for number concentrations of particle size distribution from 10-800 nm.Continuous quality assurance of measured number concentrations for the size distributions has been done with parallel measurements of TSI 3772 butanol and TSI 3785 water CPC.Chemical analysis is made for SO 2 Introduction

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Full with a Thermo Scientific TE43i TLE (operated by DWD) and for black carbon with a MAAP (MLU Carusso, Model 5012).Forward and backward scattering coefficients are obtained with a three-wavelength integrating nephelometer TSI 3563.Additionally, at both stations, PM 10 daily samples with Digitel HiVol sampler have been collected.For mass determination 22 cm filters were equilibrated, premeasured, transferred to the measuring site, and after sampling brought back to the central lab, and after a renewed equilibration the second measurement for determination of mass difference was performed.
For Innsbruck, half hourly data of the concentration of SO 2 und PM 10 were provided by the Umweltbundesamt GmbH, Austria.

Dispersion model MCCM
The online coupled meteorology atmospheric chemistry model MCCM (Grell et al., 2000) is a meso-scale flow simulation and weather forecast model based on the 5thgeneration Penn State/NCAR Mesoscale Model (MM5, Grell et al., 1994) frequently used for investigations of air quality during episodes, real time weather and air quality forecasts, as well as for the investigation of climate impact on regional air quality.It includes a choice of three gas phase chemistry modules (Haas et al., 2010).MCCM has been operated here with the RADM gas phase chemistry (Stockwell et al., 1990).Aerosol processes are described with the modal aerosol module MADE/SORGAM (Schell et al., 2001) which distinguishes three modal size distributions.The description of the Aitken mode and the accumulation mode processes includes the inorganic as well as organic compounds and considers interactions with the gas phase.For the coarse mode sedimentation is taken into account but no interaction with the gas phase is considered.For the simulations of the volcanic ash the ash was attributed entirely to the coarse mode and handled as mineral dust in MADE/SORGAM.
The simulations starting on 14 April 00:00 UTC were set up for the whole of Europe with two different horizontal resolutions, 45 km (114×92 grid points) and 25 km, respec-Introduction

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Full tively (199×169 grid points).The atmosphere between the surface and the 50 hPa level is resolved in 33 layers, with a vertical resolution that deceases with height.The thickness of the lowest layer is 30 m near the ground and in the free troposphere the layers are between 450 m and 700 m thick.The emission of ash from the volcano is considered to occur within a vertical column of variable height.Top of the emission plume is modulated according to the published plume height measured by the weather radar operated by the Icelandic Met Office (Petersen, 2010, see also Table 1).The emission source strength of airborne material during the first 3 days of the eruption was assumed to be 5% of the published amount of tephra (140×10 6 m 3 ).Depending on the observed plume height this corresponds to a mineral aerosol emission of 10 000 t/h to 50 000 t/h.Furthermore, a SO 2 source of 1000 t/h to 5000 t/h was arbitrarily assumed.

Proof of the volcanic origin of the cloud
Before we present the results from the remote sensing network and the numerical simulations, we have to verify the volcanic origin of the observed aerosol cloud.This is necessary, since ceilometer observations record pure backscatter intensity, which does not allow for a distinction between more spherical particles such as small water droplets in haze and clouds and more sharp-edged particles such as volcanic ash particles.Three different approaches to validate the volcanic origin of the aerosols detected by the ceilometers are pursued here: 1. from the depolarisation ratio obtained with special lidars, 2. from back trajectories from GME analyses of DWD, 3. from simultaneous increase of SO 2 and particle concentrations at mountain observatories on arrival of the cloud.Introduction

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Investigation with depolarisation lidars
The Raman lidar MULIS of MIM at Maisach west of Munich can analyse the linear depolarisation ratio.Figure 2 shows results from measurements with this lidar on the very early morning of 17 April, 2010.The left frame, displaying the backscatter coefficient, indicates the existence of two layers with increased aerosol content: a lower layer below about 2 km a.g.l., which is probably a residual layer from the boundary layer evolution of the preceding day and an elevated layer above around 3 km a.g.l.
The right-hand frame, displaying the depolarisation ratio, reveals a remarkable difference between these two layers.While the backscatter from the lower layer exhibits nearly no depolarisation, the upper layer signal is considerably depolarised.This quite high depolarisation ratio of up to nearly 0.4 is a strong evidence for the presence of non-spherical particles such as they are expected from a volcanic eruption.

Back trajectories
The origin of an air mass may be analysed from the measured wind field and from back trajectories.Analyses of vertical wind from ECMWF (European Centre for Medium-Range Weather Forecasts, Reading, UK, Fig. 3) and DWD global model back trajectories (Fig. 4) indicate that the aerosol cloud which arrived at Hohenpeißenberg observatory of the German Weather Service southwest of Munich was advected as an initially higher elevated tilted layer and simultaneously subsided by about 1-2 km per day while crossing Germany on 17 April, 2010.The meteorological analysis (Fig. 3) shows a vertical velocity of 0.3 Pa/s (0.1 Pa/s) corresponding to a subsidence rate of about 2000 m and 700 m per day at 500 and 700 hPa respectively.The DWD GME trajectory (Fig. 4) indicates a similar descent rate from about 4000 m to 3200 m (each m a.s.l.) during the 24 h before arrival at Hohenpeißenberg.Also the back trajectories indicate that this was about 1000 m per day (roughly 0.01 m/s).Simultaneously, Fig. 4 shows that the ash travelled about 1000 km in 24 h horizontally before arriving at Hohenpeißenberg.The thick curve in Fig. 4 (labelled "High", arriving at about Introduction

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Full 3000 m a.s.l. at Hohenpeißenberg) indicates that the air mass in about 2.5 km height carrying the ash over the Hohenpeißenberg on 17 April, 19:00 CET very probably had passed Iceland three days ago.Thus a volcanic origin of the ash load in this air mass is very likely.

Comparison to surface measurements
Observations at mountain tops were the only means (except aircraft measurements with the Dimona of Metair in Switzerland on 17 April, see www.metair.ch),which allowed for an in-situ proof of the volcanic nature of the first advected dust particles detected by the ceilometers before the ash material was included into the atmospheric boundary layer (see Sch äfer et al., 2010).The flights of the Falcon of DLR, Germany (Schumann et al., 2010), and the ultralight aircraft data addressed in Section 5.2 did not take place before 19 April.
Figure 5 shows that about at 10:30 on 17 April a large amount of very fine particles around 10 nm reached the Zugspitze.A similar event is observed at Schauinsland two hours later.The delay at Schauinsland is probably due to the much lower height (1200 m compared to the 2670 m of the Global Atmospheric Watch (GAW) station close to the peak of the Zugspitze).Measurements with thermo denuders showed that these particles evaporate at 300 • C. Additionally, MAAP measurements indicated that these fine particles are not black.Further analyses with a water CPC and a butanol CPC yield hints for a high water solubility of these particles.All these facts prove the arriving particles to be most likely small liquid droplets which formed from sulphur dioxide and water vapour during the travel from Iceland to Europe.This finding is supported by sulphur dioxide time series which show a similar structure like the aerosol time series.

Remote sensing observations
Remote sensing with ceilometers gives time-height cross-sections of the optical backscatter intensity.A typical result from such an instrument is shown in Fig. 6 which is explained in a bit more detail here.It shows one day of range-corrected but otherwise uncalibrated optical backscatter intensity observed at Augsburg, Southern Germany on 17 April, 2010.The most striking feature is the slanted filament-like structure of a shallow ash layer of several hundreds of metres thickness visible during the first half of the day descending from about 3000 above ground (i.e.roughly 3500 m a.s.l.) to about 1500 m above ground at noon.This seems to show an apparent sinking of the ash layer of about 3000 m per day.But referring to Sect.4.2 the sinking motion is only in the order of 1000 m per day.Therefore the rest (2000 m per day) of the apparent sinking must be attributed to a slanted orientation of this ash layer that is advected over the ceilometer site.This means that the leading edge of the ash layer over Augsburg was at about 3500 m a.s.l.while that part of the ash layer 500 km upstream, which was to arrive at Augsburg about 12 h later, was at 2500 m a.s.l.This gives an inclination of the shallow ash layer in the lower troposphere over Germany of about 1:500.The brownish-red structures at 1500 to 2000 m a.g.l. in the very left of Fig. 6 are normal water clouds.Also the structures underneath these clouds are not related to the volcanic ash.Those clouds obstruct the view of the instrument to the layers above the clouds.Therefore, the detection of the shallow ash layer above is interrupted for the periods in which lower-level clouds appeared.
The development of the boundary layer is also visible from CL31 soundings as the one displayed in Fig. 6.The increasing depth of this layer can be followed from the lighter blue area adjacent to the surface which reaches its maximum depth in the later afternoon with more than 1000 m.The detection of the boundary layer is due to the enhanced aerosol content in this layer originating from surface sources.depolarisation.Therefore, this near-surface aerosol is definitely no volcanic ash. Figure 6 also shows that the ash cloud was not mixed into the boundary layer at Augsburg that afternoon, because a very narrow dark line remains faintly visible between the backscatter due to the ash aloft and the backscatter due to the boundary-layer aerosol.
The arrival of the volcanic ash layer over Germany on 15/16 April was documented by all ceilometers of the DWD ceilonet and other ceilometers and lidars.Figure 7 shows measurements in Southern Germany at Augsburg, Weihenstephan, Hohenpeißenberg (DWD-Ceilonet, CHM15k), at Munich (LMU, CHM15kx), and the LMU-lidars at Maisach with a slightly different colour code.Shown are the range corrected ceilometer signals (1064 nm) as time-height cross-sections from 15:00 UTC (16 April) till 24:00 UTC ( 17April).The vertical axis is height above sea level from 0 to 8 km.The fourth frame in Fig. 7 shows time-height cross-sections as derived from MULIS at Maisach.The layer could be clearly observed from 17:00 UTC in an altitude of more than 7 km.In the following time the measurements were interrupted for two hours and partly influenced by low clouds, nevertheless, the temporal evolution of the ash layer could clearly be demonstrated.At midnight, the layer was detected at 4 km height; at 12:00 UTC on 17 April it was between 2 and 2.5 km and still clearly separated from the planetary boundary layer.The maximum signal over Maisach was observed between 05:00 and 09:00 UTC.After 15:00 UTC mixing with the boundary layer began at some places.Note, that volcanic ash was present throughout the day in heights up to 7 km.
As MULIS is a sophisticated aerosol lidar with e.g.pulse energies of roughly four orders of magnitude larger compared to the ceilometers, these data can serve as reference.
The arrival of the ash layer at about 17:00 UTC in Augsburg in an altitude of 6-7 km is hardly visible due to obscuration of low level clouds, however, after 20:30 UTC, the layer is clearly visible.The height gradually decreases from 5 km to 2.2 km within 15 h.This observation is supported by the observations from the nearby CL31 ceilometer of IMK-IFU (Fig. 6, see also there for a distinction between sinking motion and the advection of a slanted layer explaining the observed decrease in height of the ash layer).The comparison of the upper frame of Fig. 7  information from these two different types of ceilometer.While the CHM15k give a somewhat clearer image for the free troposphere, the mono-axial CL31 also covers the development of the boundary layer during this day.This boundary-layer development is not discernable from the CHM15k data from Augsburg.
In Weihenstephan, the visibility of the ash layer was reduced due to a higher amount of low level clouds.The first observations were around 23:00 UTC on 16 April, from then the layer was visible with interruptions until 10:30 UTC, when it became undistinguishable from the boundary layer.In Munich, the volcanic ash could be observed since 18:00 UTC of 16 April with only short interruptions due to low level clouds.At 18:00 UTC the ash layer was visible between 6 and 7 km.At 15:00 UTC (17 April) the layer got mixed with the planetary boundary layer in a height of 2.2 km.After 17:00 UTC, the aerosol of the volcanic eruption and the boundary layer could not be distinguished from the ceilometer data.The corresponding ceilometer data from Hohenpeißenberg are shown in the third frame of Fig. 7.The general trend of the development of the ash layer is similar to the one at Augsburg and Weihenstephan, however, the layer was visible from 19:00 UTC in about 6.5 km (a.s.l.), i.e. somewhat later than at the other stations, due to the more southern position of this site.The faint ash layers above the main layer could not be observed by any of the ceilometers.
The ash cloud rapidly lost its identity when entering into the mountainous region of the Alps. Figure 8 only shows a weak signal which may be interpreted as optical backscatter from the ash cloud at about 3200 m a.s.l. between 07:00 and 11:00 CET.It can be speculated that the strongly increased vertical exchange over the Alps (see, e.g., Furger et al., 2000or Grell et al., 2000) leads to a rapid dilution of the ash cloud so that the ash concentration decreases and the sharp contours of the cloud decay.Synoptic weather analysis reveals that this development is also asscociated to a meso-scale low pressure system to the south of the Alps inducing a change from northeasterly to southerly wind directions and associated topographic subsidence at the northern fringes of the Alps.This interpretation is supported by consideration of regional radiosoundings (Innsbruck and Munich) which are characterized by a large Introduction

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Probing the boundary layer with an ultralight aircraft
The ultralight aircraft took off at the airport of Jesenwang, 48 • 10.46 N, 11 • 07.50 E at 14:40 CET for a vertical profile up to 4000 m a.s.l. and returned to the ground at 16:30 CET on 19 April, 2010.The flight was performed about 10 km northwest of this airport with the intention to be comparable to the ceilometer and LIDAR measurements in the vicinity.Unfortunately, for logistic reasons, it was not possible to fly two days earlier.But luckily, the flight documented the front of another flush of volcanic material over Southern Germany which is worth to be compared with the numerical simulations of this event (see below).Therefore, a short analysis of this aircraft data is included here.
Due to the slow true airspeed of about 25 m/s the flight pattern allows for focussing on the vertical distribution of the aerosols.The flight enabled a comparison between pre-volcanic aerosol and the volcanic ash plume, as it took place just during the arrival of a new flush of volcanic ash.In an altitude of 3200 m a.s.l. a layer of 2/8 stratocumulus clouds and a temperature inversion was detected.Above this level clear skies prevailed.At a maximum altitude of 3650 m a.s.l. the aircraft stayed for several minutes followed by a slow descent back to ground.Between the two profiles a clear exchange of the air masses was observed as shown in Fig. 9.
Most of the measured parameters changed significantly between ascent and descent.The profiles of the dew point and extinction coefficient (Fig. 9b/c reaching the free troposphere, later in the descending profile down to an elevation of about 500 m above ground.Data on particle size distributions are available only up to 2000 m a.s.l. in the ascent and below 1800 m a.s.l. in the descent.The numbers of large particles and the total suspended particle mass derived from the size distribution did not change significantly as it would be expected from the change in the extinction coefficient (Fig. 10) This is an indication that the optical properties of the two air masses are different.Figure 10a-c shows the related optical absorption measured with the seven-wavelength aethalometer.
In Fig. 10 again, the red trace indicates the ascent, the green one the descent.Most striking within this data is the behaviour of the absorption at 370 to 420 nm and in the visible range at 590 nm.While in the ultraviolet the absorption decreased remarkably, the absorption in the visible increased.No significant difference was observed at the 880 nm wavelength, typically used for black carbon measurement.Figure 10c gives the average absorption equivalent to black carbon mass in the seven channels.The aged air mass measured before arrival of the volcanic plume shows a bimodal structure with absorption in the UV and the near infrared with a minimum in the visible.This is typical for an aged air mass with some contribution of organic matter.The replacing air mass, expected to be of volcanic origin had a quite different spectral fingerprint.The smooth spectra with slight increase in the absorption from the UV to the infrared is typical for a more homogeneous aerosol mixture like it is observed in layers of Saharan dust.It also has been seen previously during research flights in Mexico during the MILAGRO campaign (Grutter et al., 2008) during a passage a few hundred m below the sulphur dioxide plume of the volcano Popocatepetl.Summarizing these results indicates that from a particle-size point of view a volcanic ash plume would be difficult to identify.Combination of particle size, optical properties, and selected trace gases is a more promising approach for in situ measurements.However, without aerosol chemistry measurements a more robust identification of a volcanic plume would require at least the additional measurement of sulphur dioxide.Introduction

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MCCM model results
The main focus of the MCCM simulation presented below is on the dispersion of the ash cloud and not on the absolute concentrations of particles as the emission source strength of the volcano is based on rough estimates only.
Figure 11 shows ash concentrations from simulations with MCCM at a height of approximately 3 km above the ground.Although the simulated distribution of the ash concentration shows a slightly more detailed structure for a horizontal resolution of 25 km, the main features of the ash cloud can already be recognized for a grid size of 45 km.Due to the uncertainty of the amount of air borne ash emitted by the volcano the concentrations given in Fig. 11 should not be considered as absolute values.
A time-height cross-section from the MCCM results for 17 April, 2010 is shown in Fig. 12.It can be compared to the ceilometer observations displayed in the lowest frame of Fig. 7. Except for a small bias towards a too early arrival, the time of the arrival of the bulk mass of the ash cloud in Southern Germany is in quite good agreement with the ceilometers measurements for both horizontal resolutions.However, the simulated ash cloud is much thicker than observed.This seems to be a feature that is also found for simulations of the ash cloud with other Eulerian models (e.g.Elbern, 2010).For the simulations shown here this may mostly be attributed to the comparatively coarse vertical resolution that has been chosen in order to keep the numerical effort within reasonable limits.In order to resolve structures with a vertical extension of only some hundred meters, a much better vertical resolution and a much higher numerical effort would be required.Another reason for the too large vertical extent of the simulated ash cloud might be the simple representation of the eruption plume.
The overall slanted shape of the ash cloud layer 16 and 17 on April is well depicted although, probably due to coarse vertical resolution of the model, the shallowness of the cloud is not reproduced.The temporal course of the ash clouds' bulk mass in Fig. 12 on the other hand agrees quite well with the ceilometers measurements, so that it may be assumed that the overall horizontal advection of the cloud in the model Introduction

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Full is not spoiled by too strong vertical diffusion.Horizontal advection is addressed further in the next Section.Figure 12 also shows a second increase in ash concentrations on the afternoon of 19 April.This second event fits quite well to the ultralight aircraft observation described in Sect.5.2 above, although the exact arrival time of this second plume is slightly different between observation and MCCM simulation.The ultralight aircraft observed the arrival of the second plume around 16:00 h CET while the MCCM simulation sees it near the surface around 18:00 h CET.But given the long way the ash cloud has been advected during the five days before all the way from Iceland, this seems to be a rather good agreement.
Figure 13 compares the arrival times at 3 km a.s.l. of the first ash cloud over Europe on 16 and 17 April, 2010 from DWD ceilonet observations and numerical modelling with MCCM.The height of 3 km was chosen for this comparison in order to reduce the error in the ceilometers measurements resulting from the presence of clouds.Only the arrival time of this first and intense flush of ash was clearly deducible from the DWD ceilonet.Later flushes were still visible but due to clouds and missing sharp ash fronts no exact arrival time could be inferred from the ceilonet instruments.The comparison in Fig. 13 shows general agreement between observations and numerical simulation.Smaller deviations are partly due to the presence of clouds which obstructed the ceilometers' view on the leading edge of the ash clouds at some locations.
In Southeastern Germany a slightly too fast movement of the simulated ash cloud can be observed, but on the other hand the lag in the west is reproduced properly.Further, the ash transport across the northern Alps is somewhat too fast in the model, probably due to a lack of decelerating orographic impact at the lower levels in combination with a vertically too diffused representation of the ash layer (compare Figs. 7 and 12).Introduction

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Conclusions
The eruption of volcanoes itself is still unpredictable.But once the ash cloud has been emitted, ground-based observations and numerical predictions of the dispersion of the cloud are possible.There is a fundamental need for reliable predictions for air traffic security reasons as well as for air quality aspects.
This study has shown that the first ash cloud arriving over Germany on 16/17 April, 2010 was a quite shallow polluted layer which was only several hundreds of metres deep and which was oriented slantwise in the troposphere.In this layer the aerosol concentration was so large that this layer could easily be followed by simple groundbased optical remote sensing instruments such as ceilometers.Quantification how large a concentration must be at least in order to be detected by ceilometers could not be derived from the available data due to the absence of continuous in-situ concentration data.The observation of later flushes of the ash cloud with lesser concentrations was disturbed partly by cloudy weather as well.Generally, ceilometers only give meaningful results during clear sky conditions in the lower and middle troposphere.
It is important to note with respect of volcanic ash detection that ceilometer information needs back-up by additional measurements for the identification of the volcanic origin of detected aerosol clouds.Ceilometers only give range-corrected backscatter information.Depolarisation measurements seem to be a good means for this, but also back trajectories might be helpful.Also the apparent sinking of the ash cloud with time in the time-height sections derived from ceilometer observations needs additional interpretation.The slantwise oriented ash cloud was subject to large-scale sinking motion during its advection over Germany.Therefore, the rapid apparent sinking of the ash cloud signal in single ceilometer time-height sections is due to two reasons: the large-scale sinking and the advection of an inclined layer which is at lower altitudes at its rear end.
Eulerian numerical models are a good means to predict the dispersion of the ash clouds.The comparison presented here has shown the principal ability of such a model Figures to perform this task.The progression of the leading edge of the first flush of the ash was simulated quite well.Even after about 120 h of travelling of parts of the ash layer the numerical simulation resulted in a difference in arrival times of a new flush of ash of only about 2 h, which is an error of less than two percent.Simultaneously, this study has shown that the evaluation of dispersion models is possible with a ground-based optical remote sensing network of ceilometers.The case presented here is presumably the first example of a comparison between a numerical model result and data from a ceilometer network.
Mountainous terrain seems to have a considerable influence on such ash cloud dispersion.Due to the enhanced and modified vertical motions over such terrain vertical dilution of the ash cloud is much stronger over mountainous terrain.Therefore lower tropospheric aerosol clouds might easily lose their identity when they have to pass larger mountain chains.Maybe, this is also one of the reasons why there appeared to be a difference between the simulated and observed progression of the leading edge of the ash layer close to the Alps.This issue needs further consideration in future.
The above results indicate that for air traffic security the combination of a modern dispersion model together with a well-designed ceilometer network which is supported by special profiling measurements (depolarisation, spectral aerosol properties) may be a good means to predict the dispersion of thicker volcanic ash clouds.Whether this is sufficient to avoid aircraft hazards has to investigated in more detail later when threshold values for hazardous ash concentrations are available from engineering sciences.Introduction

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Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | in Munich (MIM) performed in the framework of EARLINET (see, e.g., B ösenberg et al., Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | For further details of the interaction of the ash cloud with surface air quality see Sch äfer et alDiscussion Paper | Discussion Paper | Discussion Paper | 5 Analysis of the temporal and spatial structure of the cloud Fig. 2 has demonstrated that the near-surface aerosol in this lower layer does not show any 26129 Discussion Paper | Discussion Paper | Discussion Paper | and Fig. 6 give an impression of the different Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |spread and near adiabatic temperature gradients during the latter period.Moreover, enhanced SO 2 and PM 10 concentrations occurred at several locations in the area of Innsbruck during 17 April which was particularly pronounced in the early afternoon (Amt der Tiroler Landesregierung, 2010; Umweltbundesamt Wien, 2010).Such surface concentration increases were not yet present in the foreland of the Alps on this day.These findings indicate topographically forced vertical mixing within the Alpine region in contrast to the situation in the Alpine foreland.
) indicate the arrival of a new air mass which was first seen in the ascending profile shortly before Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
36 W. It had already some minor eruption earlier in 2010.A major outbreak occurred on 14 April, 2010.This activity lasted until 22 May, 2010.Since 23 May only water vapour was emitted from the volcano.Some details on the first days of this eruption are listed in Table • 38 N 19 •