Measuring Volcanic Plume and Ash Properties from Space

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Measuring volcanic plume and ash properties from space

R. G. GRAINGER1*, D. M. PETERS1, G. E. THOMAS1, A. J. A. SMITH1, R. SIDDANS2, E. CARBONI1 & A. DUDHIA1 1Sub-Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK 2Science and Technology Facilities Council Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK *Corresponding author (e-mail: [email protected])

Abstract: The remote sensing of volcanic ash plumes from space can provide a warning of an aviation hazard and knowledge on eruption processes and radiative effects. In this paper new algorithms are presented to provide volcanic plume properties from measurements by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), the Advanced Along Track Scanning Radiometer (AATSR) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). A challenge of remote sensing is to provide near-real-time methods to identify, and so warn of, the presence of volcanic ash. To achieve this, a singular vector decomposition method has been developed for the MIPAS instrument on board the Environmental Satellite. This method was applied to observations of the ash clouds from the eruptions of Nabro and the Puyehue–Cordoń Caulle in 2011 and led to a sensitive volcanic signal flag which was capable of tracking changes in the volcanic signal spectra as the plume evolved. A second challenge for remote sensing is to identify the ash plume height. This is a critical parameter for the initialization of algorithms that numerically model the evolution and transport of a volcanic plume. As MIPAS is a limb sounder, the identification of ash also provides an estimate of height provided the plume is above about 6 km. This is comple- mented by a new algorithm, Stereo Ash Plume Height Retrieval Algorithm, that identifies plume height using the parallax between images provided by Along Track Scanning Radiometer-type instruments. The algorithm was tested on an image taken at 14:01 GMT on 6 June 2011 of the Puyehue–Cordoń Caulle eruption plume and gave a height of 11.9 + 1.4 km, which agreed with the value derived from the location of the plume shadow (12.7 + 1.8 km). This plume height was similar to the height observed by MIPAS (12 + 1.5 km) at 02:56 GMT on 6 June. The quantitative use of satellite imagery and the full exploitation of high-resolution spectral measurements of ash depends upon knowing the optical properties of the observed ash. Laboratory measurements of ash from the 1993 eruption of Mt Aso, Japan have been used to determine the refractive indices from 1 to 20 mm. These preliminary measurements have spectral features similar to ash values that have been used to date, albeit with slightly different positions and strengths of the absorption bands. The refractive indices have been used to retrieve ash properties (plume height, optical depth and ash effective radius) from AATSR and SEVIRI instruments using two versions of Oxford-RAL Retrieval of Aerosol and Cloud (ORAC) algorithm. For AATSR a new ash cloud type was used in ORAC for the analysis of the plume from the 2011 Eyjafjallajo¨kull eruption. For the first c. 500 km of the plume ORAC gave values for plume height of 2.5–6.5 km, optical depth 1–2.5 and effective radius 3–7 mm, which are in agreement with other observations. A weakness of the algorithm occurs when underlying cloud invalidates the assumption of a single cloud layer. This is rectified in a modified version of ORAC applied to SEVIRI measurements. In this case an extra model of a cloud underlying the ash plume was included in the range of applied models. In cases where the plume overlay cloud, this new model worked well, showing good agreement with correlative Cloud–Aerosol Lidar with Orthogonal Polarization observations.

Volcanic plumes formed by explosive eruptions are smaller solid particles (radii ,2 mm) referred to mixtures of gas, quenched and fragmented silicate as volcanic ash (Schmid 1981). The effects of air- material (tephra) and other aerosol particles borne ash include: derived from both the magmatic emissions and background air (e.g. Mather et al. 2003; Durant † Aviation: volcanic ash is a hazard to aviation et al. 2010; Ilyinskaya et al. 2010; Oppenheimer (Casadevall 1994). Before March 2010 the et al. 2010). The particles created during a volcanic Civil Aviation Authority did not permit civil air- event are classiﬁed according to size with the craft to ﬂy in the presence of volcanic ash.

From:Pyle, D. M., Mather,T.A.&Biggs, J. (eds) Remote Sensing of Volcanoes and Volcanic Processes: Integrating Observation and Modelling. Geological Society, London, Special Publications, 380, http://dx.doi.org/10.1144/SP380.7 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

Following the 2010 Eyjafjallajo¨kull eruption this Volcanic cloud properties zero-tolerance approach was changed to permit flights within ash concentrations less than The characterization of a volcanic cloud can be 2 × 1023 gcm23 (CAA 2010). broken into two scales: † Climate: injection of volcanic ash into the strato- † plume macrophysical properties, principally sphere and troposphere influences the Earth’s plume morphology, which is usually simply radiation balance by interacting with both solar described by plume altitude and thickness; and thermal radiation as a function of the ash’s † plume microphysical properties, which charac- optical properties (Solomon et al. 2007). In the terize the ash properties through quantities such troposphere, volcanic aerosols indirectly modify as ash shape, size and composition. climate by acting as cloud condensation nuclei. This happens over the range of volcanic activ- Plume height ity from quiescent degassing (Yuan et al. 2011) to large volcanic eruption plumes. For Plinian eruptions plume height is determined † Human health: high levels of respirable ash (par- by the rate and intensity of the magma discharge, ticle radius ,5 mm) in the air are not yet known the density of erupted material and the temperature to result in serious injury or disease from inhala- difference of the plume with the surrounding air tion (Horwell & Baxter 2006). However, acute (Sparks 1986). The eruption column typically over- respiratory symptoms are commonly reported shoots the height of neutral buoyancy and relaxes by people during and after ash falls (Blong back to form a layer of material whose thickness 1984). is typically a few kilometres for a tropospheric eruption. Sparks et al. (1986) observed that cloud Heavy ash fall may result in the collapse of roofs thickness is driven by the size of this overshoot; under the weight of ash and this can be deadly for the ratio of umbrella cloud thickness to the maxi- people within buildings. The deposition of volcanic mum height of momentum driven ascent is typically ash can increase trace metal (iron) concentrations 0.25–0.3. While atmospheric diffusion and sedi- in the local environment, especially following mentation will tend to thicken the plume over time, explosive eruptions (Martin et al. 2009). If depo- the variation of wind with height can shear the sition occurs in the ocean, this can increase the pro- plume into thin layers. For example lidar observa- ductivity of phytoplankton in areas with limited tions of plumes from the 2010 Eyjafjallajo¨kull erup- nutrients (Jones & Gislason 2008; Gabrielli et al. tion gave layer thicknesses less than 1 km (Sicard 2008). et al. 2011). Although models can calculate the dis- Changes in style of activity are common dur- persion of ash (Witham et al. 2007), their accuracy ing eruptions, with explosive eruptions being par- depends critically on source parameters, in parti- ticularly favoured by high (magmatic) gas content cular the eruption column height (Stohl et al. 2011). and high melt viscosity (andesitic to rhyolitic mag- mas), or by the presence of external water (e.g. Ash particle size and shape Sheridan & Wohletz 1983; Scandone et al. 2007). Component and morphological analyses of the Figure 1 shows an SEM image of volcanic ash col- erupted ash and comparison of these data with lected from the 1993 eruption of Mt Aso, Japan. The those from other monitoring techniques demon- ash particles cover a wide ranges of sizes and strate a clear relationship between ash features and display shapes from the near-spherical to highly styles of explosive activity (e.g. Heiken & Wohletz complex angular shapes. In general, volcanic ash 1985; Martin et al. 2008; Rust & Cashman 2011; particle size distributions tend to be multimodal Taddeucci et al. 2004). with the size, shape and composition being a func- Remotely sensed ash properties could potentially tion of the processes of formation. The majority of be used particles range from nanometres to millimetres in size (Heiken & Wohletz 1985). The volcanic ash † to warn of an aviation hazard; particles within a plume can include: † to act as the basis of a quantitative estimate of † volcanic glass formed from fragments of the – ash fall out, molten part of magma that cooled and solidified – volcanic perturbation of the radiative field, and during eruption; – climate perturbations; † minerals or crystals that grow within the magma † to investigate the interactions between ash while it is below the Earth’s surface; and the biosphere (e.g. cloud seeding, ocean † rock fragments from the walls of the magma fertilization); conduit; † to quantify volcanic process involved with ash † particles formed from the condensation of generation and plume evolution. volcanic gases. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Preliminary measurements of ash optical properties An irony of remote sensing is that often properties of the target need to be known to make full use of the remote measurement. In the case of gaseous species this is the molecular spectroscopy of the target gas. For aerosol or cloud, a priori information is needed on the complex refractive index, and to a lesser extent on the size and shape of the particles. For volcanic ash, a difficulty is that these assumed properties are likely to change during an eruption. The fewer a priori assumptions that are made about the target, the lower is the chance of making spurious deductions caused by variations in a quantity assumed constant. Existing measurements of volcanic ash refractive indices, as listed in Table 1, are extremely limited. There are a few more published refractive indices of volcanic material (e.g. basalt, Pollack et al. 1973; Egan et al. 1975; andesite, Pollack et al. Fig. 1. SEM micrograph of sieved (radius ,22.5 mm) ash from the 1993 eruptions of Mt Aso, Japan. This 1973; Egan et al. 1975; pumice, Volz 1973; obsid- sample was collected from a bomb-shelter where 1–2 m ian, Pollack et al. 1973; granite, Toon et al. 1977); of ash had accumulated. these should in principle show similar spectral signatures to the ash. In order to exploit new instruments such as high-resolution spectrometers, refer- Hence the composition, grain-size and mor- ence refractive indices are needed for a range of phology of volcanic ash particles contain important ashes and their mineral and rock components. information about the processes of magma ascent and fragmentation in volcanic eruptions (e.g. Transmission measurements Heiken & Wohletz 1985; Rust & Cashman 2011). Further process alter the particle properties such as Reported here are preliminary refractive indices † adsorption of species into the particle surface; derived from transmission spectra of resuspended † fragmentation by the explosive expansion of volcanic ash. Figure 3 outlines the basic configura- volcanic gases; tion of the experiments undertaken. A sample of † coagulation of particles; ash from the 1993 eruption of Mt Aso was col- † condensation of gases (e.g. water or sulphuric lected from a bomb shelter, where 1–2 m of ash acid) to coat the particle; and had accumulated. Data on the composition of the † sedimentation. ash will be addressed in future studies. For context, the composition of the 1989 Aso eruption The size and shape of ash particles are also critical scoria is reported by Ono et al. (1995) as SiO2 ¼ in influencing the transport of ash through the atmo- 54.71 wt%, Na2O ¼ 3.01 wt%, K2O ¼ 2.00 wt%. sphere. In the absence of coagulation and vertical The ash sample was sieved to ,22.5 mm and this winds, the process that determines the maximum fraction was resuspended and introduced into an life-time of a particle is sedimentation. Figure 2 aerosol test cell. The aerosol cell has optical win- shows the calculated fallout time for spherical par- dows fitted, allowing the aerosol transmission to ticles as a function of particle size and initial be measured via a Fourier transform spectrometer height. It is suggested that the non-sphericity of (FTS). The aerosol size distribution was determined ash particles further slows their fallout (Riley using techniques insensitive to particle refractive et al. 2003). Given this caveat it is apparent that index and the aerosol vented into a fume cupboard. particles whose radii are greater than about 15 mm All of the measurements were undertaken at the are removed from the plume within the first day Molecular Spectroscopy Facility at the Rutherford following the eruption. Very fine ash (radii less than Appleton Laboratory. The aerosol cell used had an c.1mm) is predicted to exist in the atmosphere for optical path-length of 26 cm. Spectral intensity over a year, but this is contradicted by observations measurements were made using a Bruker IFS-66 that suggest very fine ash falls out faster than fluid FTS. Measurements of the detected spectrum were dynamics predicts (Rose & Durant 2009). Further obtained with and without the aerosol to calculate work is required to understand this anomaly. the transmission spectrum, T(l). A correction was Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

1 day 1 week

1 year

20 Altitude (km) 10

1 hour

1 year 1 day 1 week

0 0.1 1.0 10.0 100.0 Particle radius (μm)

Fig. 2. Fallout time calculated for spherical particles (density of 2.4 g cm23) as a function of particle radius and injection height based on the US Standard Atmosphere 1976 (National Oceanic and Atmospheric Administration et al. 1976). made to the transmission spectrum to remove water coefﬁcient to be calculated from and carbon-dioxide gas absorption lines; this was achieved via a separate retrieval of these gas 1 ext ext 2 species’ concentrations. b = Q (r, m(l), l)pr n(r) dr, (2) 0

Data analysis and results where Qext is the extinction efﬁciency, r is the particle radius, m the complex refractive index The refractive indices were determined from the and n(r)dr is the number of particles with radii transmission spectrum. Aerosol cell transmission between r and r + dr. relates to the physical properties of the aerosol via The complex refractive index wavelength Bouguer’s Law: dependence m(l) is represented by a damped harmonic oscillator model to reduce the number of − ext( ) T(l)=e b l x, (1) model parameters to less than the number of measured spectral points. Optimal estimation is where T(l) is the transmission, bext the volume then used to derive the band model parameters extinction coefﬁcient at wavelength l, and x the (and hence refractive index) and aerosol size distri- path-length though the test cell. Assuming a parti- bution. The method has been described in detail by cle scattering model (Mie theory or T-matrix to Thomas et al. (2005). deal with non-spherical particles) and knowing The derived refractive index of Aso ash is shown the particle size distribution allows the extinction in Figure 4. The refractive index is dominated by

Table 1. Existing measurements of volcanic ash spectral complex refractive indices (m ¼ n + ik)

Ash sample Refractive index Spectral Reference component range (mm)

Mt Spur n and k 0.34, 0.36, 0.38 Krotkov et al. (1999) Mt St Helens k only 0.3–0.7 Patterson (1981) Mayon k only 1–16 Patterson (1994) El Chicho´n k only 0.3–0.7 Patterson et al. (1983) Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Fig. 3. Simplified diagram of experimental configuration. a broad absorption band situated at about 9.5 mm wavelength owing to absorption lines from atmos- which can be associated with the stretching vibra- pheric and volcanic gases, as well as broad-scale tion of Si–O. The much weaker and narrower features principally owing to particulate absorption band at about 3 mm is probably from O–H stretch. or emission. While the gas lines have provided These are preliminary results; data at wavelengths important insights into volcanic processes (Burton shorter than 1 m (not plotted) are a forward model et al. 2001, 2003; Oppenheimer et al. 2006; extrapolation and should be used with caution. Edmonds et al. 2003; Sawyer et al. 2008), the ash features have not been analysed to the same extent. In the infrared, ash is detectable by its dis- Identifying volcanic ash tinctive emission spectra. This is shown in A volcanic plume perturbs the Earth’s radiation field Figure 5, where the similar amounts of material of by scattering and absorbing radiation and by emit- the same size are shown to have distinct emission ting radiation in the infrared. In the shortwave the spectra. The spectra are presented in terms of particle size and refractive index mean that a volca- optical depth t defined for a plume of thickness L as nic plume resembles a cloud from which it may only be distinguishable by the cloud’s morphology. L The infrared (IR) transmission or emission spectra t = bext dz, (3) of volcanic plumes show a rapid variation with 0

3.0 Basalt Andesite 2.5 Obsidian Volcanic pumice 2.0 Aso ash 1.5 Real part 1.0 0.5 2 4 6 8 10 12 14 Wavelength (μm)

10.000

1.000

0.100

0.010 Imaginary part 0.001 2 4 6 8 10 12 14 Wavelength (μm)

Fig. 4. Real and imaginary refractive index components determined from an Aso ash sample. The refractive indices of basalt, andesite, obsidian (Pollack et al. 1973) and volcanic pumice (Volz 1973) are also shown. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

Wavelength (μm) 1 2 3 4 5 6 7 10 15 20

0.4 AATSR SEVIRI MIPAS

0.3 −− Aso ash −− Sulphuric acid −− Water ice −− Water 0.2 Optical depth

0.1

0.0 30000 3000 1400 1200 1000 800 600 Wavenumber (cm−1)

2 23 Fig. 5. Simulated optical thicknessfor a 100 m-thick plumeconsistingof a log–normaldistribution(N0 ¼ 2 × 10 cm , rm ¼ 1 mm, S ¼ 1.7) of spherical particles. The plume is assumed to consist of Aso ash (using the preliminary refractive indices), sulphuric acid (refractive indices from Luo et al. 1996), water ice (refractive indices from Warren & Brandt 2008) and water (refractive indices from Hale & Querry 1973). The black lines at the top of the plot indicate the AATSR and SEVIRI channels and the MIPAS spectral range.

while the ash size distribution is characterized by the T12 at these two wavelengths rather than a radiance. effective radius re deﬁned by The magnitude of the brightness temperature differ- 1 ence has been shown to be a function of the ash size r3 n(r) dr (Prata 1989a; Wen & Rose 1994). 0 While a powerful and useful tool, there are cases re = 1 . (4) 2 ( ) where the technique is not robust (Prata et al. 2001). r n r dr They include: 0 Volcanic aerosol is assumed to conform to a (1) strong temperature inversions near the sur- log–normal distribution described by face, which can result in a negative T11 –T12 in clear sky conditions; N 1 1 (ln r − ln r )2 n(r)=√0 exp − m , (5) (2) dry soil, for example deserts or wind blown 2p ln(S) r 2 ln2(S) dust, which is also often characterized by a negative T11 –T12, for the same reasons as vol- where N0 is the total particle number density, rm is canic ash; the median of the size distribution and S is a par- (3) cloud tops that overshoot the tropopause, ameter than controls the spread of the size distri- which can also show a negative T11 –T12 bution. For a log–normal distribution rm and re owing to the inversion of the temperature gra- are related by dient in the stratosphere; 5 (4) very thick volcanic ash, ash mixed with ice r = r exp ln2 S . e m 2 (quite a common occurrence, since volcanoes generally emit large quantities of water Figure 5 shows that, between about 10 and vapour), or ash which lies below an airmass 12 mm, the optical thickness of ash and sulphuric with a high water vapour content; acid decreases whereas water or ice increases. Mea- (5) instrument noise or collocation errors between surement channels centred near 11 and 12 mm are the 11 and 12 mm channels. common on meteorological imaging instruments and have been widely exploited to study volcanic ash Use of a 3.7 mm channel and information from clouds. Examples are listed in Table 2. These instru- shortwave channels can alleviate this problem ments generally report brightness temperatures T11, (Ellrod et al. 2003) as their dependence on the Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Table 2. Eruption quantiﬁed using the brightness temperature difference

Instrument Eruption re (mm) Reference

ATSR-2 Mt Ruapehu, 1996 3–5.1* Prata & Grant (2001) AVHRR Mt Galunggung, July 1982 Prata (1989a, b) AVHRR El Chicho´n, 1982 4–8 Rose et al. (2000) AVHRR Cerro Hudson, 1991 6–9 Rose et al. (2000) AVHRR Mt Spurr, August 1992 2.4–3.15 Wen & Rose (1994) AVHRR Mt Spurr, August 1992 0.5–6 Yu et al. (2002) AVHRR Mt Spurr, August 1992 Krotkov et al. (1999) AVHRR Mt Etna, November 2006 Spinetti et al. (2007) AVHRR Mt Etna, October 2002 Filizzola et al. (2007) GOES Montserrat, December 1997 0.6–6 Yu et al. (2002) MODIS Mt Etna, November 2006 1.6–3.3 Spinetti et al. (2007); Corradini et al. (2008)

*Particle radii estimated from the reported mean radii by multiplying by 1.5; this is based on assuming a log–normal distribution with a spread of 1.5. properties of volcanic ash differs from the 8–12 mm in the ash composition. This is reﬂected in the var- region. iability of infrared spectra measured from satellites The ash signature depends on the composition for different volcanoes and eruptions (Clarisse et al. and size distribution of ash particles. Figure 6 2010; Gangale et al. 2010). shows an example of different optical depths com- puted using the same size-distribution (with an Other volcanic products effective radius of 2 mm) and different refractive indices. The published infrared spectral refractive In addition to ash volcanic eruption can release indices of volcano-related ash and rock show a large quantities of gas, the principle ones being large variability, presumably because of changes H2O, CO2, CO, SO2,H2S and HCl. Of these SO2

Wavelength (μm) 1 2 3 4 5 6 7 10 15 20

0.4 AATSR SEVIRI MIPAS −− Basalt 0.3 −− Andesite −− Obsidian −− Volcanic pumice −− Aso ash 0.2 Optical depth

0.1

0.0 30000 3000 1400 1200 1000 800 600 Wavenumber (cm−1)

Fig. 6. Optical depth as a function of wavenumber for a 100 m-thick ash plume consisting of a log–normal distribution 2 23 (N0 ¼ 2 × 10 cm , rm ¼ 1 mm, S ¼ 1.7) of spherical particles of different composition. The chosen size distribution and plume thickness gives a column loading of 3.7 g m22. Refractive index data is from Pollack et al. (1973) for basaltic glass, andesite and obsidian, and from Volz (1973) for pumice. The Aso values used the refractive indices given earlier. The black lines at the top of the plot indicate the AATSR and SEVIRI channels and the MIPAS spectral range. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL. has been observed from space (e.g. Read et al. 1993; path-length over which the mirrors moved was Walker et al. 2012) and its evolution tracked remo- decreased, reducing the resolution of the spectra to tely. While there are several observations of ash and 0.0625 cm21. The reduced resolution nominal scan SO2 being collocated, this is not always true, the pattern consists of 27 observations of the Earth’s most famous example being the El Chichoń 1982 limb with 1.5 km spacing in the lower atmosphere. eruption when the ash and SO2 cloud were observed A complete scanning profile, then, takes about to be spatially separated (Seftor et al. 1997). Other 65 s, resulting in over 1000 scans taken daily. examples include the Hekla 2000 eruption (Rose There is nominally 330 km horizontally between et al. 2003) and the Eyjafjallajo¨kull eruption of the first (highest) and last (lowest) sweeps in a 2010 (Thomas & Prata 2011). The reason for this scan with along-track horizontal spacing between is that the SO2 and ash had different altitudes of scans of 410 km. The field-of-view is about 3 km neutral buoyancy within the eruption column and vertically and 30 km horizontally. as a result were separated by wind shear. Hence, Although MIPAS has been used extensively to while SO2 may indicate the presence of ash, it determine atmospheric trace gas properties (e.g. should not be used as a proxy for ash. Burgess et al. 2006; Payne et al. 2007) and to observe clouds (e.g. Spang et al. 2005; Hurley et al. 2011), to date there have been no reports of Remote sensing of volcanic plume MIPAS observations of volcanic ash. and ash properties There are three distinct time regimes for remote Recognition of volcanic ash using singular sensing of volcanic ash: vector decomposition of MIPAS spectra † the identification of volcanic ash for hazard The Oxford MIPAS retrieval of clouds (Hurley avoidance – in this case a fast (real time) algo- et al. 2011) uses the continuum radiance from a rithm is needed that recognizes ash with a low narrow band of wavenumber ranges between 930 (less than once per year) false detection rate; and 965 cm21 to retrieve a cloud effective fraction † the characterization of volcanic plume properties defined as the fraction of the field-of-view that is (e.g. altitude, erupted mass) in near real time for covered by optically thick cloud. This is used to – plume model initialization and validation, find the highest MIPAS scan altitude at which – and to help quantify the immediate impact of cloud is present. The cloud top height, cloud extinc- ash on the biosphere; tion and cloud top temperature are retrieved using † a full and detailed characterization of volcanic the cloud effective fraction and the continuum ash properties to help understand volcanic pro- radiances from the scan above, at and below the cesses (both in the plume and potentially field-of-view at which the cloud top is found. within the volcano itself). However this approach does not determine cloud type. Extinction, cloud height and cloud top tem- More than 20 different satellite instruments have perature are retrieved, but any significant particulate been used to measure volcanogenic gases and/ plume can also show up in the retrievals, since the or particles. Earlier work is well summarized in process identifies cases where the field-of-view is Oppenheimer (1998) and Francis & Rothery (2000) not clear. The retrieval of volcanic plumes is not a while Thomas & Watson (2010) provide infor- problem per se. These do have legitimate height, mation on more recent missions. Here we concen- extinction and temperature, but it is useful to trate on three instruments that have been relatively know when the cloud is composed of volcanic ash, underexploited for volcanological studies. as opposed to water or ice. Plumes can be flagged roughly by inspecting MIPAS the ratio of radiances from different areas of the MIPAS continuum. For ash it was found that an The Michelson Interferometer for Passive Atmos- appropriate flag was: pheric Sounding (MIPAS) is a limb viewing Four- ier transform spectrometer measuring from 685 R(800−830 cm−1) ( ) to 2410 cm21 at a spectral resolution of 0.025 , 1.3, 6 R(935−960 cm−1) cm21. MIPAS was launched in March 2002 on the European Space Agency’s Environmental Satel- where R is the mean continuum radiance between lite (ENVISAT). The ENVISAT spacecraft is in an the lower and upper bounds of the wavenumbers 800 km Sun-synchronous polar orbit with a nomi- indicated in the subscripts. The flag correctly ident- nal repeat period of 35 days. In March 2004, ified many areas where the volcanic output from MIPAS began experiencing irregular motion of the Puyehue and Nabro was observable in MIPAS scanning mirrors. To rectify this problem, the measurements. This flag is particularly good at Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES picking out strong signals, but in areas with less shown in the bottom panel, has almost no concen- clear ash loadings, the method will not work. tration before the eruption, but sporadically shows Using singular vector decomposition (SVD), it is up post-eruption. Arrows show where the original possible to identify plumes more definitely, picking volcanic flag has been triggered. The new method up weaker signals, and signals which also include identifies more scans that include ash than found regular clouds. A similar approach to that used by using the continuum flag. Hurley et al. (2009) is adopted (although in that case, singular vectors were obtained for a set of AATSR calculated radiances, not measured atmospheric spectra). First, before a volcano has erupted, sev- The Advanced Along Track Scanning Radiometer eral days of MIPAS data at the volcano’s latitude (AATSR) is the current operational instrument in and a range of nominal altitudes are taken as a a series of nadir viewing radiometers (ATSR-1, ‘clean atmosphere training set’ for atmosphere not ATSR-2 and AATSR, commonly referenced as containing a volcanic plume. Singular vectors, vi, ATSR). AATSR, like MIPAS, is part of the pay- are fitted using the method given by Press et al. load on ENVISAT. A feature of the ATSR instru- (1992). These vectors represent the orthogonal ments is that they provide two views of the same components of the given spectra, so that any of the scene within c. 90 s of each other, the first centred training set spectral measurements, R, can be exac- on a viewing zenith angle of 558, the second at tly reproduced by a weighted sum of the singular nadir. The parallax between the two views allows vectors, obtained through a least squares fit: for the determination of the altitude of features by purely geometric means. For AATSR each = ( ) R livi, 7 view is made in seven spectral channels centred at i 0.56, 0.66, 0.86, 1.6, 3.7, 11 and 12 mm and reported where li is the weight of the ith singular vector. as Sun-normalized reflectances in the shortwave Since the training set is expected to contain clear (i.e. R0.56, R0.66, R0.86, R1.6) and brightness tempera- sky and cloudy sky data, it can be used to fit tures in the infrared (i.e. T3.7, T11, T12). As a polar measurements after the eruption where volcanic orbiting instrument, it provides a single day-time signals are not present. There are as many singular and single night-time overpass at approximately vectors as original measurements used in the train- 10:30 a.m./p.m. local time. The instrument swath ing set, but the principal modes of variation are found width of 512 km (with a 1 km pixel size) means only in the first few terms. It is therefore appropriate that it takes the instrument three days to provide to truncate the sum given in Equation 7 after a few near-global coverage. Thus, for a given eruption, terms. In this case, the first nine singular vectors AATSR will only be able to provide a measure- were found to contain 90% of the variability bet- ment of the proximal plume once every three days. ween different measured spectra in the training set. Previous volcanological work using the ATSR For post-volcanic measurements, the flag defined series has included the thermal monitoring of vol- by Equation 6 is used to provide cases where there canic hotspots (e.g. Wooster & Rothery 1997). Prata is a strong volcanic signal. The original singular & Grant (2001) determined ash properties from the vectors are fitted to these new measurements, and 11 and 12 mm brightness temperatures in addi- the residual is that part of the spectrum that cannot tion to plume height determined from the paral- be explained by our prior vectors for clean sky lax formed between the ATSR forward and nadir (and is therefore assumed to contain a volcanic sig- views. Here, the AATSR instrument is used to pro- nature). From several days of data, these residuals duce volcanic cloud products from two independent are collected, and used to obtain a new set of singu- methods: lar vectors that contain the principal orthogonal (1) The stereo matching approach is used to signals for volcanic plumes. The volcanic singular produce an estimate of geometric ash cloud vector is specific to the fitted ensemble as it contains top height. information about aerosol composition that may (2) The brightness temperatures, in conjunction vary during an eruption and between different with knowledge of the vertical temperature volcanic events. structure of the atmosphere, are used to deter- Figure 7 shows an example of fitting the calcu- mine ash cloud height using the shortwave lated singular vectors to real data. The values of li channels to constrain the optical depth and for the first two ‘clean’ singular vectors and the microphysical properties of the cloud. first volcanic singular vector are shown. Note the much greater magnitude of the first two panels, Identifying ash in AATSR imagery which capture the strongest modes of variability for the clean signal, the diurnal cycle and the pres- The identification of ash in imager data has tra- ence of cloud. The first volcanic singular vector, ditionally relied upon the brightness temperature Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

SVDclear #0 )

λ Pre-volcano measurements Post-volcano measurements 6×104

4×104

2×104

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SVDvolc #0 )

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Fig. 7. Fit coefficients, li, for the first two clear singular vectors, and the first volcanic singular vector for the Nabro eruption at MIPAS tangent measurement heights of 15 + 1.5 km. The x-axis gives an arbitrary measurement number (which is increasing in time). Data taken after the eruption are shown in orange. The diurnal cycle can be clearly seen in the top two vectors (which capture 70% of variability). Arrows mark points where the volcanic plume was seen, using the test given in Equation 6. While the arrows mark the strongest signals, the SVD method is able to distinguish weaker volcanic signatures. difference between 11 and 12 mm channels. In day- Lean (2009) provides a test specifically for AATSR light scenes, the shortwave channels can also in daylight conditions whereby a pixel which meets provide additional constraints – ash will typically all four of the following conditions: have a low albedo compared with cloud and is also more absorbing in the green than the red. T11 − T12 , −0.1 These features have been combined to produce an T11 − T3.7 , −20 AATSR volcanic ash flag, which uses thresholds (9) on the following quantities: R0.67 , 0.1 NDVI , 0.1 † T11 –T12; † T11 –T3.7; is deemed to be ash. For night-time scenes, the solar † R0.67; reflectance proportion of the 3.7 mm signal drops to † The Normalized Difference Vegetation Index zero, as do the shortwave channels, and the test (NDVI), defined as becomes: − − − T11 T12 , 0.1 = R0.55 R0.67 ( ) (10) NDVI . 8 T − T , 0 R0.55 + R0.67 11 3.7 Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Using these tests, AATSR data can be rapidly in a clearly defined plume, the correlation method checked for evidence of volcanic ash. As these is sufficient for evaluating our approach. thresholds are instrument- and calibration-dependent, further research would be needed for their SEVIRI use on other ATSR instruments. The Spinning Enhanced Visible and Infrared Imager (SEVIRI) is a scanning radiometer on-board the Stereo Ash Plume Height Retrieval operational Meteosat Second Generation weather Algorithm satellite, which is in a geostationary orbit nominally located on the Greenwich meridian. SEVIRI pro- The Stereo Ash Plume Height Retrieval Algorithm vides image data of the Earth in 11 narrow-band (SAPHRA) consists of two stages: channels centred at 0.635, 0.81, 1.64, 3.92, 6.25, 7.35, 8.70, 9.66, 10.8, 12.0 and 13.4 mm. A key (1) the detection of possible volcanic ash plumes, feature of this instrument is its continuous imaging based on the test specified in Equation 9 with a baseline repeat cycle of 15 min. The during the day and Equation 10 at night; imaging sampling distance is 3 km at the sub- (2) the estimation of the height of detected satellite point. SEVIRI has been used to identify plumes using stereo-matching of the forward volcanic hotspots (Wooster et al. 2000) as wells as and nadir brightness temperature difference to study lava flows at erupting volcanoes (Hirn images. et al. 2009). Ash and SO2 have also been measured Stereo matching is a well-established tool for the using the five channels centred at 6.25, 7.35, 8.70, determination of cloud and plume heights, from 10.8 and 12.0 (Prata & Kerkmann 2007). More both ATSR instruments (Muller et al. 2007; Prata recently Stohl et al. (2011) used SEVIRI estimates & Grant 2001) and NASA’s Multi-angle Imaging of ash mass loading to infer the ash emission from Spectro-Radiometer on board the Terra satellite the Eyjafjallajo¨kull 2010 eruption as a function of (Moroney et al. 2002). The method employed here time and altitude. is the simplest approach to the problem of matching features in the two views: ORAC retrieval of ash plume height and physical properties (1) T11 –T12 is calculated for the entire scene in which ash has been detected, in both views. The Oxford-RAL Retrieval of Aerosol and Cloud (2) The correlation between the two brightness (ORAC) algorithm is an optimal estimation retrie- temperature difference images is calculated val scheme designed to provide estimates of aero- × using a sliding window of n n pixels, sol optical depth and effective radius, cloud top where n is c.3. pressure, height and temperature, cloud particle (3) The images are offset by one pixel in the effective radius, cloud optical depth and cloud type along-track direction, and step 2 is repeated. (generally liquid water or ice) from multi-spectral (4) Step 3 is repeated 20 times, creating corre- imagery. The scheme has been used to produce a lation images of the two brightness tempera- cloud product from the full ATSR-2 and AATSR ture difference fields for offsets of 0–20 record up to 2010 (Poulsen et al. 2011; Sayer pixels. et al. 2011). The ORAC cloud scheme has been (5) The offset for maximum correlation is deter- modified to determine ash cloud properties by mined for each pixel, and converted to a using a radiative forward model based on ash geometric altitude. particles described by a log–normal distribution (the log–normal spread is fixed at 1.77 but the The use of T –T ensures that the ash plume has a 11 12 effective radius is allowed to vary in the range strong contrast to the rest of the image, and gives a 0.01–10 mm) and the Aso refractive indices large change in correlation as the two views are described in the section ‘Ash particle size and aligned. Under the naive assumption that the uncer- shape’. This principally involves creating lookup tainty in the retrieved height is entirely due to the tables describing the reflectance and transmission alignment, a one-pixel error implies an altitude of the ash plume for a range of solar and satellite uncertainty of approximately 1.4 km. Other geo- geometries. metric cloud/plume height retrievals often use Assuming the plume is homogeneous, the inte- more complex pattern matching algorithms to gral in Equation 3 can be completed, so the plume produce a height map, such as the Multi-point thickness, L, and optical depth t are related by Matchers M2, M3 (Muller et al. 2002) and M4 (Muller et al. 2007), which could be investigated for use here. However, as we are interested solely t = bextL (11) Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

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The ash loading m (usually in g m23) can be cal- imagery; ash clouds cannot be well fitted using culated from either water or ice cloud properties and vice versa. 1 Here we will demonstrate an ability to deter- = 4 3 ( ) × t mine ash properties when a plume is located above m r pr n r dr ext 0 3 b a cloud. This is particularly useful as there were mass per unit volume L many instances of ash overlying cloud during 1 the Eyjafjallajo¨kull, 2010 and Grimsvo¨tn, 2011 3 4prert r n(r) dr eruptions. = 0 , (12) In summary, quantifying a volcanic cloud 3bext requires knowledge of ash loading, composition, morphology and size, as well as the physical dimen- where r is the density of the ash (Wen & Rose sions and location of the ash plume. In addition, 1994). If the particle size-distribution is assumed tephra from explosive volcanic eruptions holds to be log–normal the column loading becomes information about magma dynamics in the critical zone where fragmentation occurs and eruption = 4 rt 3 2 ( ) m ext pre exp (2 ln S), 13 style is decided. In this work new algorithms are 3 s presented that determine plume and ash properties where s ext is the average extinction cross-section based on preliminary laboratory measurements of per particle given by ash optical properties. bext sext = . (14) N0 Case study I: the 2011 Puyehue–Cordoń Caulle eruption CAA restrictions on aircraft flights are in terms of the density of ash rather than the column The Puyehue–Cordoń Caulle volcanic complex amount. There are three approaches to estimate (Singer et al. 2008) forms part of the Andes and is this quantity from the mass loading: located in Puyehue National Park, Chile. The erup- (1) Use a correlative measurement of the plume tion began in June and continued through the thickness, L, for example from lidar, then esti- remaining months of 2011. A summary of the first mate the mass density through two months of activity is given below. From 27 April, seismic activity was detected. m r = . (15) The frequency of earthquakes increased on 2 June L and was followed, on 4 June, by an explosion (2) Assume a typical volume extinction coeffi- from Puyehue–Cordoń Caulle that produced a cient to convert a plume’s optical depth into plume of ash and gas, rising to an altitude of physical depth. 12.2 km above sea-level (a.s.l.) as noted by Obser- (3) Assume that the plume thickness is the same vatorio Volcanologico de los Andes del Sur as its altitude. This then provides a minimum (OVDAS) (Chile) (2011) and Servicio Nacional de possible mass density under the assumption Geologia y Mineria scientists (Servicio Nacional that the plume is homogeneous. de Geologia y Mineria 2011a, b, c, d ). Initially, plumes drifted south at 5 km a.s.l. and drifted west It is worth noting that the simultaneous retrieval of and east at an altitude of about 10 km. all state parameters provided by the optimal esti- From 4 June and on the following days the mation method ensures that a physically consistent ash plumes were generally transported in the SE to and numerically optimal estimate of the state is pro- NE sector. The Buenos Aires VAAC reported that duced. It is clear that, although thermal IR channels on 4 June ash plumes rose to altitudes of 10.7– mostly provide information on the cloud top pres- 13.7 km and drifted 870 km ESE (Global Volcan- sure, height and temperature, the transmission and ism Program 2011a). On 5 June, the ash plume emission values of the cloud will depend on the was estimated to be between 10.7 and 12.2 km optical depth and effective radius, which are mostly located 1700 km ESE of the volcano over the Atlan- determined from the shortwave channels. Conver- tic Ocean. On 6 June, a new part of the plume was sely, the above- and below-cloud transmissions in located 170 km ENE of the mountain, while the the shortwave will be weakly (if we assume the previous part of the plume continued to be trans- channels used are atmospheric window channels) ported ESE over the ocean. On the 7 June the ash dependent on the cloud height. Owing to the large plume was reported at heights of 5.5–9.8 km (Glo- differences in the optical properties of ash, water bal Volcanism Program 2011a). Volcanic activ- droplets and ice crystals, ORAC has been proved to ity continued over the following days, leading to provide an effective way of detecting ash in satellite the cancellation of several flights in Argentina, Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Uruguay and Brazil. By 12 June, the ash plume had eruption as viewed by AATSR on the 6 of June. reached Australia and New Zealand. On 13 June, a Figure 8 shows an image generated from red, MODIS image showed a large volcanic ash plume green and blue channels of the MEdium Resolution over Argentina, which was reported to lie between Imaging Spectrometer (MERIS), which is on board 4 and 8 km (NASA Earth Observatory 2011). the Envisat platform with AATSR. Superimposed From 22 June to 5 July, the eruption continued on the MERIS image is a visible – near-infrared with less activity and lower plume heights in the false colour image from AATSR. The primary ash range 2–4 km (Global Volcanism Program 2011a). plume is very clear, and its altitude can be estimated On 7–8 July an increase in the plume height from the shadow it casts on the surface as led to the cancellation of flights in Argentina and 12.7 + 1.8 km (the solar elevation at the time the Uruguay (France-Presse 2011). During the follow- satellite over-passed the plume was about 208). A ing few days the plume altitude decreased and was less distinct, lower-altitude ash cloud can also be reported to be 3 km (9 July), 1–2 km (17 July) made out to the south of the main plume, particu- and 5 km (18 July) above the crater. larly in the AATSR image. Between 25 July and 1 August, the eruption con- Figure 9a shows the results of applying the tinued with plumes rising 2–5 km above the crater. ash flag to both views of the AATSR instrument. Eruptive processes continued through the remaining The ash plume is successfully detected in both months of 2011 (Global Volcanism Program 2011a). views of the instrument. The greater sensitivity of the forward view, owing to its greater atmos- Using AATSR to Identify the 2011 Puyehue– pheric path-length, results in a larger number of Cordoń Caulle ash cloud height ash pixels, including some possible false detections along the southern edge of the image, but the two SAPHRA has been used to identify the height of the views generally produce very consistent results. ash plume from the 2011 Puyehue–Cordoń Caulle The parallax between the two views is also very

Fig. 8. A combined AATSR-MERIS false colour image of the Puyehue–Cordo´n Caulle eruption plume taken at 14:01 GMT on 6 June 2011. The central section of the image shows the AATSR nadir view, with the MERIS image providing context. The volcano itself is located in the left-hand section of the MERIS image, and the coast of Argentina is visible on the right. North is approximately towards the top of the image. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

Fig. 9. (a) The pixels determined to be ash from the AATSR image shown in Figure 8. Red denotes pixels detected in the nadir view only, green in the forward view only and yellow in both. The parallax between the two views is clear in the main northern plume, but, as one would expect, it is much less evident in the southern (much lower) plume. (b) The stereo-retrieved ash height from the ash plume shown in Figure 8.

clear in this image, particularly along the northern † As the two views of ATSR are obtained at very edge of the main plume. different viewing geometries, they will see The results of the stereo matching technique, different parts of the plume and any surrounding using a 3 × 3 pixel sliding window, are given in clouds, as well as shadows cast by both the Figure 9b. Heights retrieved in the main plume are plume and clouds. typically around 12 km, while the lower ash cloud † The native pixel size in the forward view of to the south has produced a height of approximately ATSR is approximately twice the size of that 5 km. There is also evidence that there is some in the nadir view. Although Level 1b data from decrease in the main plume height from the west the instrument has both views interpolated onto to the east of the image. a common grid, this resolution difference pro- Figure 10 shows a histogram of the heights from duces noticeable differences in the clarity of Figure 9b, as well as two Gaussians, which have features in the two views. been fitted to the two peaks apparent in the figure. † There may be some evolution in the scene over Under the assumption that the heights of both dis- the 90 s gap between the forward and nadir tinct plumes in the scene are constant throughout scans of the instrument; however, for most the image, the plume heights can be characterized scenes this will be insignificant on the 1 km as 11.9 + 1.4 km for the main plume and 5.3 + spatial scale of an instrument pixel. 2.5 km for the cloud to the south. † Stereo matching will be most successful for The variability in the retrieved height of the images with strong features that lie perpendicu- lower ash plume is greater than might be expected lar to the satellite track, as they will provide from alignment error alone. The stereo matching clear markers of parallax. Thus, for the greater method has several limitations related both to the part of an ATSR orbit, plumes which extend in measurements provided by the instrument and to an east–west direction will be better retrieved. the assumption that the two images of the plume † ATSR has a collocation error of typically one will match when the parallax is removed. These pixel (although often more) between the for- uncertainties can be summarized as follows: ward and nadir views. Thus, features at sea-level Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

will often not coincide in the two images. This The geographical coverage of the plume can be error is probably the most serious of those inferred by looking for areas where the least listed here, as it can result in a consistent bias squares fit of the singular vectors to a MIPAS spec- in the retrieved height of up to a few kilometres. trum requires a significant contribution from the However, it is possible to correct for: in the first volcanic singular vector, that is, lvolc,1.In results presented here, surface features have these areas, the ‘clean’ singular vectors could not been used to align the two views by eye before satisfactorily represent the scene. Care must be applying the height retrieval algorithm. There used in interpreting these areas as no effort has is also an on-going effort to produce a global cor- been made to account for unobserved portions of rection for the entire ATSR-2 and AATSR time the plume. These occur through MIPAS’s limited series, which should solve this problem. spatial sampling, by the plume being obscured by water or ice cloud, or by the plume occurring All of these limitations can contribute to an uncer- below MIPAS’s minimum sampling altitude. tainty which is significantly greater than the c. Plotting the area of plume coverage over the next +1.4 km error that results from the 1 km resolution few months, it is seen that, starting from a small of AATSR, especially for diffuse ash clouds. localized area around the site of the volcano, the However, for a well-defined, favourably aligned plume spreads out, reaching a peak in coverage plume, such as the main plume in this case study, around 30 days after the eruption, before rapidly the variability in the retrieved plume height shows dropping off. Figure 12a shows the Southern very good agreement with the expected error. Hemisphere the day after the eruption has begun. A signal can be seen at the emission location of Using MIPAS to identify the 2011 Puyehue– the volcano (marked with a triangle), as well as a Cordoń Caulle ash cloud further plume to the east, which is from a later overpass of the satellite. Figure 12b shows the much Figure 11 shows the first volcanic singular vector increased coverage, but weaker signal strength of calculated using flagged spectra that were thought the volcanic signature a month later. to contain the plume from the Puyehue eruption, Over the course of the next two months, similar viewed at an altitude of 12 km. The positive slope plots (not shown) show the coverage of the plumes through the centre of the A-band is a well-known increasing while travelling around the globe from feature to be expected, since it agrees with the west to east. Figure 13 shows fractional coverage empirical flag that was used to mark volcanic pro- between 608S and 208S for this time. Singular files. The SVD can be re-calculated at later times vectors calculated at the start of the eruption were after the eruption in order to see the evolution of just as effective at observing the plume vectors cal- the plume’s optical properties. culated a month later, suggesting that the plume

10000

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Fig. 10. Histogram of the retrieved plume height shown in Figure 9b. Gaussians representing the mean height and uncertainty of both plumes are plotted. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

SVDvolc #0 (Puyehue, 12.0km)

Band A Band AB Band B 0.04

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Fig. 11. The principle volcanic singular vector for the Puyehue eruption, calculated using data from the week after the eruptions, when the empirical flag defined in Equation 6 is positive. The three shortest MIPAS wavenumber bands, A, AB and B, were fitted, but excluding the ozone region of AB band (1020–1070 cm21), and the beginning of the A band (685–765 cm21), where carbon dioxide and ozone absorption is extremely strong. Calculations were restricted to the latitude range 20–608S. did not significantly change in composition over the periods of the eruption were not observed as they course of its lifetime. The reported altitudes of the did not reach high enough altitudes. ash plumes during June 2011 are generally in agreement with the MIPAS flagged locations of the plume. For example the peak plume altitude on 6 Case study II: the 2011 Nabro eruption June was the 12 + 1.5 km range, which is in agreement with both the reported observations and the Nabro is a stratovolcano 2218 m high, located in the AATSR stereo height retrieval. Plumes from later Afar depression in Eritrea, close to the border with

Fig. 12. The spatial variation of the 1st volcanic singular vector’s inﬂuence following the Puyehue eruption. Plots are from 6 June 2011 (upper) and 13 July 2011 (lower) between 608S and 208S at 12 km. The location of the volcano is marked with a triangle. Data have been gridded and averaged to account for irregular sampling of the satellite. This irregular sampling is also responsible for the discontinuities in the plume in the upper ﬁgure where bands of clear sky are probably due to overpasses before the plume had extended across the South Atlantic Ocean. As time evolved the plume was seen to spread out spatially, become weaker in signal strength, and move from west to east, circling the world several times. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

12.0km Puyehue

20% SVD calculated between following dates. Jun 6 - Jun 10 Jun 21 - Jun 25 Jul 6 - Jul 8 15% Jul 21 - Jul 25 Aug 5 - Aug 9

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Fig. 13. The spatial coverage of the Puyehue volcanic plume at 12 km for the geographical region shown in Figure 12. Coverage is calculated as the ratio of flagged profiles (where lvolc,1 is significant) to all available profiles. The different coloured lines indicate volcanic singular vectors calculated during different measurement periods.

Ethiopia (Wiart & Oppenheimer 2005). There are July (at an altitude less than 5.5 km) and on 17 no ground measurements for this volcano and it is July (Global Volcanism Program 2011b). an example of the importance of volcanic monitoring using satellite remote sensing. The seismic activity started in the evening of the 12 June 2011 Using MIPAS to identify the 2011 Nabro with a magnitude 5.1 earthquake and continued for ash cloud several hours (USGS Earthquake Hazards Program 2011). The first satellite image with a discernible Since different eruptions produce varied particles, volcanic plume is the SEVIRI false colour image the singular vector signatures have large differ- of 21:00 UTC of 12 June (EUMETSAT 2011). In ences. Within the AB and B bands, Figure 14 subsequent imagery the plume drifts to the NW so shows that the Nabro signature appears to contain that by the evening of 13 June the plume is over the SO2 n1 and n2 absorption bands, which are Egypt. On 14 June an ash cloud was reported over centred at 1152 and 1362 cm21 respectively (this southern Israel (The Jerusalem Post 2011), which is not clearly visible in the Puyehue vector). This is consistent with satellite observations of SO2. Vol- result can give us confidence that the technique is canic plumes were reported at altitudes of 9.1– working. Preliminary retrievals of IASI data show 13.7 km on 13 June, 6.1–10.7 km on 14 June and that SO2 emissions from Nabro are around an order 6.1–7.9 km during 15–20 June (Global Volcanism of magnitude greater than those from Puyehue. Program 2011b). Figure 15 shows a change in the spatial distri- On 15–16 June satellite measurements of SO2 by bution of the plume over the course of the next six the Infrared Atmospheric Sounding Interferometer weeks. Starting from a localized zone, the area (IASI) show a plume located over central Asia. taken up fills much of the Northern Hemisphere, The IASI measurements show two distinct filaments but, as one would expect, weakens in signal of SO2 over Asia (one at 10 km altitude and one at strength. 15 km). The SO2 plume reached the east coast of Figure 16 shows the spatial coverage of the China on 18 June. During 22–26 June large plume as a function of the volcanic singular vec- amounts of sulphur dioxide continued to be detected tor calculated on different dates. The singular by satellite sensors. The Toulouse VAAC reported vector found at the start of the eruption (but fitted that during 26–27 June plumes rose to altitudes up through the entire time period) reaches a maxi- to 6.1 km. The satellite images show that the SO2 mum about 40 days after the start of the eruption signal gradually decreased until 28 June. The last before decaying. The singular vector found at the ash plumes from this eruption were reported on 16 end of the eruption (also fitted through the entire Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL.

SVDvolc #0 (Nabro, 15.0km)

0.08 Band A Band AB Band B

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Fig. 14. The principle singular volcanic singular vector for the Nabro eruption, calculated using data from the week after the eruptions, when the empirical flag defined in Equation 6 is positive. As with Figure 11, the three shortest MIPAS wavenumber bands, A, AB and B, were fitted, excluding the ozone region of AB band, and the beginning of the A band. Calculations were restricted to the latitude range 20–708N. The SO2 n1 and n2 absorption bands can be seen at 1152 and 1362 cm21.

time period) steadily increases with time. This is precipitating and the SO2 converting into sulphate consistent with the ash and SO2 (represented by aerosol. The ﬁnal singular vector is consistent with the initial singular vector) dispersing, the ash the sulphate aerosol concentrations increasing

Fig. 15. As Figure 12, but for the Nabro eruption. Signiﬁcant volcanic signals between 308N and 708Nat15+ 1.5 km are shown. Nabro is located at 138N and is shown by the triangle. The upper ﬁgure, showing the day after the eruption has the plume emerging. A month later (lower), the volcanic signature can be seen all over the Northern Hemisphere, more evenly but more weakly distributed. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

15.0km Nabro 30% SVD calculated between following dates. Jun 14 - Jun 17 Jun 29 - Jul 2 25% Jul 15 - Jul 18 Jul 29 - Aug 2 Aug 13 - Aug 16 20%

15%

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Fig. 16. As Figure 13, but for a nominal altitude of 15 km and the Nabro geographical region shown in Figure 15. following the oxidation of sulphur-containing vol- et al. 2011; Stohl et al. 2011) and the wind con- canic gases into sulphuric acid. Further work is ditions transported the ash plume in a SE direc- needed to turn this qualitative interpretation of the tion, towards Europe. MIPAS observation into quantitative estimates of aerosol concentration, composition and conversion rates. Phase II: 18 April to 4 May From the evening of 18 April there was a magmatic Case study III: the April–May 2010 eruption phase; water and ice from the glacier were not inside the vent. The intensity of the eruption was Eyjafjallajo¨kull eruption one order of magnitude lower than phase I, and there The Eyjafjallajo¨kull is a stratovolcano, located close was a reduction in the amount of ash injected into to Iceland’s southern coast (Sigmundsson et al. the atmosphere. The altitude of the eruption 2010). As reported by Global Volcanism Program column was between 2 and 5 km (Zehner 2012; (2011c), after an initial eruptive phase (from 20 Stohl et al. 2011). March 2010) of lava flow but no significant ash and SO2 emission, an explosive eruptive phase of the Eyjafjallajo¨kull volcano began on the 14 April Phase III: 5–24 May 2010. This was anticipated by a series of earth- Between 3 and 5 May an increase in seismic activity quakes in the night between 13 and 14 April. Fol- was reported followed by a more intense explosive lowing Zehner (2012), the explosive part of this phase of the eruption. Ash production increased eruption can be divided into three phases. and the eruption column altitude was reported between 4 and 10 km (Stohl et al. 2011). In this Phase I: 14–18 April period ash plumes were transported over Europe and the Atlantic ocean. This was a phreatomagmatic eruption phase, and ice The volcanic plumes from the eruption of Eyjaf- and water from the ice cup above the caldera were jallajo¨kull starting in April 2010 resulted in the directly in contact with the fresh magma in the cancellation of 100 000 flights over Europe, affect- vent. This produced a faster cooling of the ejected ing roughly 10 million passengers (European Com- magma and a large amount of ash injected into the mission 2010). The airline industry lost an estimated atmosphere, as well as steams plumes. During this £153 million per day during the 15–21 April 2010 period the injection altitude of the plume was esti- period when European airspace was closed (Maz- mated as between 2 and 10 km height (Marzano zocchi et al. 2010). Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

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ORAC–AATSR retrieval of the 2010 effective radius and the retrieval cost, for those Eyjafjallajo¨kull ash plume pixels picked as ash. Retrieval cost is a measure of the quality of the data. High-quality retrievals The Eyjafjallajo¨kull eruption plume is highlighted have a cost of about one. For the Eyjafjallajo¨kull in the AATSR-based false colour image shown in eruption, a major issue is the ubiquity of thin Figure 17a. The plume is clearly visible and can layers of ash over a thick liquid water cloud (often be seen to extend over cloud-free land and ocean, under a pronounced temperature inversion). These as well as over low-lying cloud as it bends from locations are identified by a high cost in the single an east–west alignment to a southerly direction. layer retrieval. There are also several relatively small convective The retrieved cloud top height for the ash clouds that appear to coincide with the ash plume plume in this scene ranges from approximately in this region. The ORAC retrieval scheme has 6.5 km close to the volcano, down to approximately been applied to this image, using volcanic ash 2.5 km near the southern edge of the image. The optical properties in addition to the standard water areas of the ash plume associated with the convec- and ice cloud types. Figure 17b shows the best tive clouds apparent in the false colour image fitting cloud type for each retrieved pixel in this show a much higher altitude than the surrounding scene for which plume and often have been selected as ice cloud † a priori surface reflectance data were available rather than ash. This seems a reasonable result, (the MODIS surface BRDF product was not but is an example of where the assumptions of available for much of Iceland, unfortunately the retrieval have broken down. Examination of including the region under the ash plume); the retrieval cost shows that the regions where the † the retrieval converged, producing an optical false colour image shows that both ash and water depth of greater than 0.2; cloud are present have greatly elevated cost, indi- † the cloud top height was greater than zero. cating that the retrieval did not produce a good fit to the measurements and is probably inaccurate – No cloud mask was applied to the scene before in this case the retrieved height is probably an the retrieval was run, so the latter two tests are intermediate level between an ash layer and the needed to remove clear-sky pixels. height of underlying water cloud. In these cases The ash plume is readily apparent, but it is also the retrieved optical depth and effective radius clear that ash has produced the best fit in regions also show artefacts associated with the presence where we would not expect any ash to be present. of cloud. Ignoring those retrievals whose cost is This is a product of the under-constrained nature greater than 2 suggests that, within the first c. of the retrieval problem – in some cases where the 500 km of the plume, its height was in the range forward model is not a good representation of the 2.5–6.5 km, the optical depth was 1–2.5 and the real world (multi-layer cloud, cloud edges, mixed effective radius was 3–7 mm. The retrieved effec- phase cloud), it is to be expected that ash might tive radius is typical of those reported for other provide the best fit to the measurements by chance eruptions given in Table 2. alone. With this proviso in mind, Figure 18 shows An interesting feature of the plume, which is the retrieved optical depth, cloud top height, the apparent in its east–west aligned portion, is the

Fig. 17. (a) A false colour image (designed to highlight volcanic ash as green-brown) of the Eyjafjallajo¨kull eruption plume constructed from images acquired by AATSR at 12.13 GMT on 6 May 2010. (b) Best ﬁtting cloud type for the scene. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES

Fig. 18. The retrieved (a) optical depth, (b) cloud-top height, (c) effective radius and (d) retrieval cost for those pixels identified as ash. strong gradient in effective radius and cloud top Extended ORAC–SEVIRI retrieval of the 2010 height along its southern edge, which is associated Eyjafjallajo¨kull ash plume overlying clouds with a distinct change in the appearance of the plume in the false colour image. The retrieval of By modifying the look-up tables the ORAC cloud larger particles at a lower altitude along this edge scheme is also applicable to measurements by suggests that there may have been a vertical wind MSG SEVIRI. As with the AATSR work, ORAC shear acting on the plume, as larger particles was first applied in the standard manner, fitting the would be expected to be found lower in the plume optical depth, effective radius and altitude of a owing to their higher settling velocity, resulting in single-layer cloud model, considered to be infinite- subtly different trajectories for particles of differ- simally geometrically thin in the vertical. The ent sizes. However, it is also possible that this scheme was applied three times, assuming optical feature is an artefact caused by the breakdown of the plane-parallel approximation near the edge of the plume, so no firm conclusion can be drawn -18 -12 from these results alone. 15.0 Figure 19 shows the ash mass density derived 12.5

from the ORAC estimates of optical depth and 66 effective radius using Equation 13. The grey 66 ) values are obvious cloud contamination and have 2 10.0 2 values in the range of about 50–80 g m . The total 7.5 mass in the scene is 0.83 Tg if the cloud contami- 63 63 nation is included and 0.24 Tg, if all the pixels Ash mass (g/m 5.0 with an ash density of greater than 20 g m2 are ignored. Given the plume covers about 12 h of 2.5 emissions, this represents an emission rate of × 7 21 0.0 1.8 10 gs . This value is very similar to the 6 -18 -12 May 2010 emission value of 2.0 × 107 gs21 reported by Stohl et al. (2011). Fig. 19. Ash mass density for the 6 May 2010. Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

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Fig. 20. Example ORAC results for a SEVIRI scene at 14:00 UT on 7 May 2010, compared with co-incident CALIOP lidar observations. The top left-hand panel shows the CALIOP-measured lidar back-scatter with the ORAC retrieved cloud-top height superimposed on top. As indicated in the legend, triangles show results from the single layer scheme and open squares show the height (of the upper layer) from the two-layer scheme. Error bars are shown in both cases, although those from the single layer scheme are too small to be seen on this scale. Colours indicate the particle type which best matches observations. The bottom left-hand panel shows the retrieved optical depth. In the bottom-right is a false colour image from SEVIRI based on the 8.7, 11 and 12 mm channels, in which ash appears as bright red/orange or pink. The CALIOP track is shown as a black line, with the section shown in the left-hand panels highlighted in green. properties of (a) liquid cloud, (b) ice cloud and (c) examining the retrieval cost, since in most cases volcanic ash in turn. Measurements in all SEVIRI the multi-layer scene cannot be well represented channels were used except the 3.92 and 9.66 mm by any of the single-layer models fitted. channels. The cloud type present in each scene An alternative approach is to run a two-layer was identified by selecting the retrieval with the scheme when the retrieval cost exceeds some pre- most consistent match to observations in all chan- determined level. The two-layer scheme includes nels (as measured by the optimal estimation cost an extended version of the ORAC forward model function), again in a similar fashion to the AATSR (Siddans et al. 2010), which represents a second retrievals. cloud layer, also in terms of its optical depth, effec- While this approach was successful in simple tive radius and height. The retrieval state vector is scenes, as was noted in the AATSR analysis, the extended to include these three additional param- ash plume often occurred over low-lying (optically eters; however, it is not generally feasible to thick) liquid cloud and in these circumstances the obtain reliable information on all three quantities retrieved height was significantly lower than that for both cloud layers. Here the problem is con- observed by the CALIOP (Cloud–Aerosol Lidar strained by assuming the lower layer of liquid with Orthogonal Polarization) lidar. This is an cloud to be optically thick and have effective issue that also affects the standard cloud retrieval, radius of 10 mm. Only the height of the lower when thin ice cloud is present over thicker liquid layer cloud is retrieved, together with the height, cloud: in these circumstances the scheme will fit radius and optical depth of the upper layer. Sensi- an effective height that lies between the uppermost tivity to the actual optical depth and effective cloud top and the lowest cloud base, generally at radius of the lower layer is avoided by only using an altitude which corresponds to approximately thermal infrared channels in the two-layer scheme one optical depth from the top (Siddans et al. (as demonstrated for water cloud in Watts et al. 2010; Poulsen et al. 2011). Usually multi-layer 2011). This retrieval relies on the fact that SEVIRI cloud (both ice over liquid and ash over liquid) has thermal infrared channels that are selectively can be recognized, and flagged as unreliable, by sensitive to cloud at different altitudes because of Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

VOLCANIC PLUME AND ASH PROPERTIES the varying levels of trace-gas absorption in each space than was previously available. Weaker sig- channel. In particular, water vapour absorption in nals, and signals that have been partially masked the 6.3 and 7.3 mm channels and CO2 absorption by other effects, are more easily observed. Perio- at 13 mm give information on height complemen- dic recalculation of the volcanic singular vectors tary to that from the window channels at 8.7, 11 through the course of an event suggested a change and 12 mm. Furthermore, the spectral dependencies in aerosol composition as the plume aged for the of cloud/ash optical properties provide distinct Puyehue–Cordoń Caulle but not for the Nabro information on cloud optical depth and, to a lesser plume. This was consistent with the first eruption extent, particle size. being sulphur-rich, leading to a change in cloud This scheme is applied twice: once assuming the composition from ash to sulphate. upper layer to be ice and once assuming it to be ash. The identification of ash plume height is critical The result of these two retrievals that gives the best for the initialization of algorithms that numerically fit (lowest cost) to the observations is selected as model the evolution and transport of a volcanic being the most appropriate model for the upper plume. The MIPAS results suggest an altitude of layer. about 15 km for the initial 2011 Puyehue–Cordoń This approach was trialled on SEVIRI image Caulle plume, which was higher than the 12.7 km of the Eyjafjallajo¨kull plume observed at 14:00 on height estimated from the plume shadow. The 7 May 2010. Figure 20 shows the SEVIRI retrieved latter value is in better agreement with a new algor- height compared with CALIOP lidar backscatter ithm, SAPHRA, that gives a main plume height profile (at 0.532 mm). CALIOP shows an ash 11.9 + 1.4 km using the parallax between images plume from about 6 to 9 km in altitude, mainly provided by AATSR. between 478 and 498N, although possibly extend- The SAPHRA approach lends itself to near-real- ing further north at a slightly lower altitude, with time application, as it is inherently simple and does amounts close to the noise level. CALIOP also not rely on any information beyond the ATSR shows an optically thick layer of liquid cloud at an radiances themselves. It could be envisaged that altitude of 1 km across the whole section. The the algorithm could form the basis for an opera- single layer scheme identifies the main part of this tional product which would detect and report the ash plume consistently, but estimates low altitude height of volcanic ash plumes detected by AATSR and high optical depth. The two-layer scheme has (and later SLSTR) on a routine basis. Despite the been run for scenes in which the retrieval cost func- lack of daily global coverage, this information tion is more than twice the expected value based could very valuable in helping to constrain the dis- on the modelled instrument noise and estimated persion models used by the VAAC centres to issue forward model accuracy. For these scenes, the two- volcanic hazard warnings. layer scheme returns a more correct estimate of the The quantitative use of satellite imagery and ash plume height and also detects the presence of the full exploitation of high-resolution spectral ash between 49.58 and 508N (although here the measurements of ash depends upon knowing the height remains apparently underestimated). While optical properties of the observed ash. Laboratory it is not possible to directly validate the retrieved measurements of ash from the 1993 eruption of Mt optical depths, it is clear that the single layer Aso, Japan have been used to determine the refrac- scheme retrieves a value approximating the total tive indices from 1 to 20 mm. The refractive indices optical depth of the ash (or ice) and underlying have been used to retrieve ash properties from the liquid cloud, while the two-layer result gives a AATSR and SEVIRI instruments using two ver- more plausible estimate for the thin ash layer. sions of the ORAC algorithm. For AATSR, a new cloud type in ORAC was used in the analysis of the plume from the 2011 Conclusions Eyjafjallajo¨kull eruption, giving retrieved values of plume height 2.5–6.5 km, optical depth 1–2.5 In this paper new algorithms have been presented and effective radius 3–7 mm, which were in agree- that provide volcanic plume properties from mea- ment with other observations. A weakness of the surements by the MIPAS, AATSR and SEVIRI. A algorithm occurred when underlying cloud invali- singular vector decomposition method, developed dated the assumption of a single cloud layer. In for the MIPAS, has been applied to observations these cases the retrieval errors are underestimated of ash clouds from the eruptions of Nabro and because the approach neglects the inherent uncer- Puyehue–Cordoń Caulle in mid 2011. The geo- tainty caused by assuming the scene to be contain graphical locations of the clouds based on MIPAS only a single cloud layer. This was rectified in a were in agreement with observations obtained by modified version of ORAC applied to SEVIRI. In other methods. The SVD method provides a far this case an extra model of a cloud underlying the more powerful tool for flagging volcanic ash from ash plume was included in the range of applied Downloaded from http://sp.lyellcollection.org/ at Oxford University on July 9, 2013

R. G. GRAINGER ET AL. models. Because the two-layer scheme explicitly to thank A. Hurst for providing the Aso ash sample and takes into account the multi-layer nature of the gratefully acknowledge the NASA Langley Atmospheric scene, it reports much larger errors, but these are Science Data Centre for CALIOP data. The authors also more consistent with the observed discrepancies thank F. Prata of NILU for identifying CALIOP orbits ¨ with CALIOP. In cases where the plume overlay that sampled the 2010 Eyjafjallajokull plume. cloud this new model worked well, showing good agreement with correlative CALIOP observations. It should be noted that the ORAC retrieval, as References it currently stands, has several weaknesses which Blong, R. 1984. Volcanic Hazards: a Sourcebook limit its applicability to volcanic ash detection and on the Effects of Eruptions. Academic Press, characterization, many of which it shares with Orlando, FL. other algorithms: Burgess, A., Grainger,R.G.&Dudhia, A. 2006. Zonal mean atmospheric distribution of sulphur hexafluoride † ORAC assumes plane-parallel cloud in its for- (SF6). Geophysical Research Letters, 33, L07809, ward model. Volcanic ash plumes are often not http://dx.doi.org/10.1029/2005GL025410 plane-parallel (as they are typically a narrow Burton, M., Oppenheimer, C., Horrocks,L.& plume). This can result in significant biases in Francis, P. 2001. Diurnal changes in volcanic plume the retrieved parameters, including height. chemistry observed by lunar and solar occultation † As with all ash detection schemes, in cases where spectroscopy. Geophysical Research Letters, 28, ash is mixing with large quantities of either ice 843–846. Burton Allard Mure Oppenheimer or liquid water, there will come a point where , M., , P., ,F.& ,C. 2003. FTIR remote sensing of fractional magma the ash cloud becomes indistinguishable from degassing at Mount Etna, Sicily. In: Oppenheimer, a normal water or ice cloud. Furthermore, even C., Pyle,D.M.&Barclay, J. (eds) Volcanic Degas- if the cloud is determined to be ash, the presence sing. Geological Society, London, Special Publi- of water will alter its radiative properties and cations, 213, 281–293. produce a bias in the retrieval. CAA 2010. Volcanic ash, a briefing from the Civil † For optically thin ash, an accurate knowledge of Aviation Authority, https://www.caa.co.uk/docs/ the surface properties (particularly its reflec- 2011/VolcanicAshBriefing.pdf (accessed December tance) becomes important and the Lambertian 2011). Casadevall surface reflectance assumption can become pro- , T. J. (ed.) 1994. Volcanic ash and aviation safety. Proceedings of the 1st International Sym- blematic. This can be improved through the use a posium on Volcanic Ash and Aviation Safety. USGS forward model that includes a model of surface Bulletin 2047, US Geological Survey, Washington, bidirectional reflectance distribution function. DC. Such a model is being introduced for ORAC Clarisse, L., Hurtmans, D., Prata, A. J., Karagulian, cloud retrievals, but the accuracy of the retrieval F., Clerbaux, C., De Maziere,M.&Coheur, P.-F. is still dependant on prior knowledge of the 2010. Retrieving radius, concentration, optical depth, surface reflectance. and mass of different types of aerosols from high- † The retrieval of multi-layer cloud systems is still resolution infrared nadir spectra. Applied Optics, 49, experimental and there remain obstacles to its 3713–3722. Corradini, S., Spinetti,C.et al. 2008. Mt. Etna widespread application, not least the reliable tropospheric ash retrieval and sensitivity analysis detection of multi-layer scenes. using moderate resolution imaging spectroradio- † The need for knowledge of the atmospheric tem- meter measurements. 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