Modelling concentrations of volcanic ash encountered by aircraft in past eruptions

Atmospheric Environment 48 (2012) 219e229

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Modelling concentrations of volcanic ash encountered by aircraft in past eruptions Claire Witham*, Helen Webster, Matthew Hort, Andrew Jones, David Thomson Met Office, FitzRoy Road, Exeter EX1 3PB, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 February 2011 Received in revised form 24 June 2011 Accepted 27 June 2011

Prolonged disruption to aviation during the AprileMay 2010 eruption of Eyjafjallajökull, Iceland resulted in pressure to predict volcanic ash plume concentrations for the purpose of allowing aircraft to fly in regions with low ash contamination. Over the past few decades there have been a number of incidents where aircraft have encountered volcanic ash resulting in damage to the aircraft and loss of power to engines. Understanding the volcanic ash concentrations that these aircraft have encountered provides important input to determining a safe concentration limit. Aircraft encounters with six volcanic eruption plumes have been studied and ash concentrations predicted using the atmospheric dispersion model NAME. The eruptions considered are Galunggung 1982, Soputan 1985, Redoubt 1989, Pinatubo 1991, Hekla 2000 and Manam 2006. Uncertainties in the eruption source details (start time, stop time and eruption height) and in the aircraft encounter location and flight path are found to be major limitations in some cases. Errors in the driving meteorological data (which is often coarse in resolution for historic studies) and the lack of eruption plume dynamics (e.g. umbrella cloud representation) results in further uncertainties in the predicted ash concentrations. In most of the case studies, the dispersion modelling shows the presence of ash at the aircraft encounter location. Maximum ash concentrations in the vicinity of the aircraft are predicted to be at least 4000 mg m
3 although confidence in the estimated concentrations is low and uncertainties of orders of magnitude are shown to be possible. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: Volcanic ash Dispersion modelling Aircraft encounters Ash concentration

1. Introduction The AprileMay 2010 volcanic eruption of Eyjafjallajökull in Iceland highlighted the disruption that can occur to aviation when a volcano emits large quantities of ash into the atmosphere near to and within busy airspace. At the start of the eruption, accepted guidance on volcanic ash was that it should be completely avoided by aircraft (ICAO, 2002). This guidance was used as the basis for the International Civil Aviation Organization’s volcanic ash contingency plan for the European region (ICAO, 2009). As the event continued and much of international aviation to and from Europe was brought to a standstill, pressure mounted from the airlines to allow aircraft to fly in regions of low ash contamination. By the end of the incident, aircraft were allowed to fly within regions of low ash concentration in the predicted ash plume as long as the airlines then conducted enhanced checks and maintenance schedules. The decision to allow aircraft to fly in predicted low levels of volcanic ash contamination was taken by the UK Civil Aviation Authority (CAA) after discussions with aircraft engine manufactures,

* Corresponding author. Tel.: þ44 1392 885017; fax: þ44 1392 885681. E-mail address: claire.witham@metoffice.gov.uk (C. Witham).

European transport ministers, EUROCONTROL and input from various scientific experts including those from the Met Office, UK. Volcanic ash concentrations were estimated within three zones: the low contamination zone, where peak ash concentrations were predicted to be between 200 and 2000 mg m
3, the medium contamination zone, with predicted peak ash concentrations between 2000 and 4000 mg m
3, and the high contamination zone, with predicted peak ash concentrations greater than 4000 mg m
3. The high contamination zone was designated as dangerous and therefore not cleared for flights by the regulators. The lower limit of the low contamination zone, namely 200 mg m
3, corresponds, with various modelling assumptions, to the “visual ash” concentration threshold used previously by the London Volcanic Ash Advisory Centre (VAAC) (Leadbetter and Hort, 2011; Witham et al., 2007) to determine the extent of the volcanic ash plume. Over the past 30 years there have been a number of incidents in which aircraft have encountered volcanic ash plumes, resulting in damage to the aircraft and, in the most serious cases, loss of power to all engines (Guffanti et al., 2010). Fortunately, to date, there has been no loss of life, but these historic cases demonstrate the danger of volcanic ash to aviation and the necessity of being able to predict the location of the ash plume. Reconstructing the ash concentrations which aircraft are predicted to have flown through in historic

1352-2310/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.06.073

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cases is an important step in understanding the possible impacts and determining a safe concentration limit. There are, however, a number of uncertainties which hinder the process of modelling ash concentrations accurately and confidently. This paper discusses some of these modelling challenges using examples of six past volcanic eruptions between 1982 and 2006 associated with aircraft ash encounters. Atmospheric dispersion modelling of the volcanic eruptions is used to predict ash concentrations encountered by the aircraft based on the modelling procedures used by the London VAAC during the 2010 Eyjafjallajökull eruption. 2. Methods Six volcanic eruptions associated with aircraft encounters have been studied (Table 1). These include the well known all-engine flame-out incidents that occurred following the 1982 Galunggung and 1989 Mount Redoubt eruptions. The atmospheric dispersion model NAME (Numerical Atmospheric-dispersion Modelling Environment, Jones et al. (2007)) has been used to model the transport and spread of the ash plume in each of the case studies. NAME is one of the many models that are currently used for volcanic ash dispersion modelling (Langmann et al., 2012). NAME is the UK Met Office’s atmospheric dispersion model and the operational model of the London VAAC. It was initially developed following the Chernobyl accident but now has a wide range of applications including emergency response work to predict the transport and spread of airborne diseases and other hazardous material (e.g. chemical, biological and nuclear), volcanic ash modelling, dust modelling, air quality forecasting, and determination of pollutant source strengths and locations (e.g. Derwent et al., 2007; Leadbetter and Hort, 2011; Redington et al., 2009; Webster et al., 2006). NAME has been compared against observations and other atmospheric dispersion models in a number of validation studies (Galmarini et al., 2004; Gloster et al., 2010; Potempski et al., 2008; Ryall and Maryon, 1998; Witham et al., 2007) and is under a continual development program to support additional functionalities and to improve the scientific representation of atmospheric processes. Version 6.0 has been used in this study. NAME is a Lagrangian particle model in which large numbers of model ‘particles’ are released and tracked through the model atmosphere. Each model ‘particle’ represents a certain mass of the released material and the transport and dispersion of this material is governed by input meteorological data. For volcanic ash modelling, a uniform mass release profile from the volcano summit to the observed/estimated plume top is used. A particle size distribution based on measurements of volcanic ash from Hobbs et al. (1991) is used (Table 2). It is assumed that volcanic ash particles with diameters larger than 100 mm fall out near to the source and are of minimal atmospheric interest so therefore are not included.

Table 2 The particle size distribution for volcanic ash used by NAME. Diameter (mm)

Cumulative mass fraction

0.1 0.3 1.0 3.0 10.0 30.0 100.0

0 0.001 0.006 0.056 0.256 0.956 1.0

Sedimentation of volcanic ash particles due to gravitational settling is included within NAME, with the sedimentation rate a function of the particle density and size. The mass of airborne material is reduced over time due to wet and dry deposition processes. Prior to the 2010 Eyjafjallajökull eruption, NAME’s modelling of volcanic ash was conducted in terms of nominal release rates and VAFTAD visual ash thresholds (as described in Witham et al. (2007)). During the Eyjafjallajökull eruption, a new approach was followed to model real volcanic ash concentrations using estimated mass release rates. This allows multiple eruptive phases, including periods of rest by the volcano, to be modelled in a continuous simulation. Complex eruptive episodes can be represented, provided detailed information on the timing and eruption height is known. A uniform mass release rate profile from the volcano summit to the observed/estimated plume top is still used, but the true volcanic ash release rate is estimated by:

M ¼ 88:2H1=0:225

(1)
1

where M is the mass release rate in kg s and H is the height of the plume top above the volcano summit in km. Eq. (1) was determined by calibrating the VAFTAD table using Mastin et al. (2009) and fitting a continuous curve to the VAFTAD data (Leadbetter and Hort, 2011; Webster et al., submitted for publication). The estimated mass release rate determined using Eq. (1) compares well with the equation given by Mastin et al. (2009), which is obtained from plume height and mass eruption rate information from a number of well studied volcanic eruptions. Between 90% and 99.9% of this total emitted mass is thought to fall out close to source due to gravitational settling of large tephra grain sizes, aggregation of smaller grain sizes and organised downdrafts. Under the procedure used here, hourly average volcanic ash concentrations are generated at a horizontal output resolution of 0.1  0.1 over three vertical layers: FL000-200, FL200-350 and FL350-550, where FL are flight levels which are approximately equivalent to 100’s of feet. To account for unresolved peak concentrations within these thick vertical layers, a conversion factor (a peak-to-mean ratio) is used to convert the NAME predicted mean volcanic ash concentration into peak concentrations. In

Table 1 The volcanic eruptions considered. For further information on the encounters see Guffanti et al. (2010). Volcano

Eruption date

Effects of aircraft encounters

NWP met data

Galunggung, Indonesia

June 1982

ERA-40

Soputan, Indonesia

May 1985

Redoubt, Alaska

December 1989

Pinatubo, Philippines

June 1991

Hekla, Iceland Manam, Papua New Guinea

February 2000 July 2006

747 in-flight loss of power to all 4 engines, heavy abrasion to leading edges and windows 747 in-flight observations of St Elmo’s fire and dust in cabin. Subsequently found to have damage to all 4 engines 747 in-flight loss of power to all 4 engines. Damage to aircraft estimated at $80 million At least 20 encounters causing damage to engines, windows, probes, lights and aircraft edges. In-flight failure of engine(s) in two encounters. DC-8 damage to all 4 engines. Gulfstream II twin-engine failure

ERA-40

ERA-Interim ERA-Interim

ERA-Interim, MetUM 60 km MetUM 40 km

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reality, this conversion factor and the factor accounting for near-source fall out have been found to roughly cancel out (Webster et al., submitted for publication) and this is assumed here. However, departures from this are a significant source of uncertainty in the results. The horizontal resolution and time averaging used here are higher than those often used for emergency response when model run time is key. This higher resolution gives better representation for cases where the aircraft encounter occurred near to the source or the eruption duration was short-lived, situations which are common in the six case studies. For volcanic ash modelling, NAME is usually driven by numerical weather prediction (NWP) data from the Met Office Unified Model (MetUM) (Davies et al., 2005). Over the years, the resolution of the global MetUM has increased and accuracy has improved, due to advances in both science and computing. Currently the operational global MetUM has a horizontal resolution of approximately 25 km. For historic events, however, the availability of meteorological data and the lower resolution of the NWP models at these times, limits the modelling. For the most recent of the historic cases, namely Manam, the Met Office’s global MetUM analysis data is available at a horizontal resolution of approximately 40 km (Table 1). For the earlier historic cases, ERA-Interim and ERA-40 NWP re-analysis meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF, http://www.ecmwf.int/research/era/ do/get/index) have been used with horizontal resolutions of approximately 85 km and 125 km, respectively. All of the meteorological data used have a temporal resolution of 3-h. For the Hekla 2000 eruption both MetUM and ERA-Interim data are used to demonstrate the influence of NWP uncertainty. Further uncertainties in the eruption source parameters (eruption start and stop time and height), the aircraft encounter location and the particle size distribution all contribute to uncertainties in the modelling. The lack of representation of eruption dynamics, including for example umbrella cloud formation and explicit particle aggregation, also leads to possible errors in predictions. Quantifying and understanding these errors and uncertainties is necessary to aid decision-making.

221

giving a maximum height of approximately 16 km in this region. An eruption height of 14 km has been assumed, but simulations have also been conducted for eruption heights of 12 km and 16 km. The maximum predicted ash concentration encountered by the aircraft is highly dependent on where the aircraft is assumed to have been and through which part of the plume its flight path would have taken it as well as the specifics of the eruption. The height of the aircraft seems to be well established at an altitude of w11.5 km (flight level FL370) at the time of the encounter, but there is considerable discrepancy about its horizontal location. Reports place the aircraft at different distances from named locations (e.g. 130 miles south-east of Jakarta (pers. comm. Rolls Royce); 150 km west-south-west of Galunggung (Smithsonian Institution, 1982); 330 km south of Jakarta (Johnson and Casadevall, 1994); 230 km south of Jakarta (Hanstrum and Watson, 1983)). Guffanti et al. (2010) have looked at the reported locations for this incident and note that the map of the flight route in Tootell (1985) is incorrect. Here we use the location derived from the aircraft’s black box (pers. comm. Rolls Royce) of approximately 107.5 E, 8.0 S and assume that it was flying in a south-easterly direction on its route from Kuala Lumpur to Perth. Fig. 1 shows the NAME simulated plume at the time of the encounter for FL350-FL550 for an eruption up to 14 km. This demonstrates that the aircraft is likely to have flown across the centre line of the plume on its flight path. The predicted ash concentration at the reported aircraft encounter location is w4000 mg m
3; whilst the predicted maximum ash concentration that the aircraft may have flown through, based on the largest predicted concentrations in the nearby vicinity, is w180,000 mg m
3. For the lower eruption height of 12 km, the predicted ash concentration at the reported aircraft encounter location is w900 mg m
3; whilst the predicted maximum ash concentration is w45,000 mg m
3. For an eruption height of 16 km, the predicted ash concentration at the aircraft encounter location is approximately 8000 mg m
3 with maximum nearby concentrations up to 320,000 mg m
3. Hence it can be seen that the uncertainty in the eruption height results in significant uncertainty in the predicted ash concentrations. The large uncertainties in the

3. Results 3.1. Galunggung The encounter of a British Airways 747 aircraft with the ash plume from Galunggung, Indonesia on 24 June 1982 (Johnson and Casadevall, 1994) is perhaps the best known aircraft-ash encounter. It led to the in-flight failure of all four engines and abrasion of the windshield and wing surfaces, and was a major milestone in the recognition of the threat to aviation from volcanic ash. However detailed information on the eruption on 24 June 1982 has been difficult to find. Residents reported seeing ash emissions from 12:00 UTC (19:00 local time (LT)) (Smithsonian Institution, 1982), but no plume is evident on satellite data at 12:00 UTC. The volcano had started erupting by the time the next satellite image was available at 18:00 UTC (Hanstrum and Watson, 1983). The encounter time has been reported as 13:44 UTC (Diamond, 1986) and 15:00 UTC (Hanstrum and Watson, 1983), but the latter is believed to be in error and has been discounted. In light of these timing uncertainties, the start time of the eruption has been taken as 12:00 UTC and the end time of the eruption has been taken to be after the encounter. Estimates of the ash cloud height from satellite data (Hanstrum and Watson, 1983; Katili and Sudradjat, 1984; Sawada, 1987) suggest that it was in excess of 12 km and most likely around 14 km (all plume heights are given as above sea level). Bluth et al. (1994) reported that the sulphur dioxide cloud was below the tropopause,

6

Jakarta

7

Galunggung

Encounter

8

9

106

107

108

109

110

Maximum value = 1.8e+06 ug/m3

1.0e+01

1.0e+02

1.0e+03

1.0e+04

1.0e+05

1.0e+06

1.0e+07

Fig. 1. NAME simulated hourly mean ash plume (mg m
3) between FL350 and FL550 from 13:00 to 14:00 UTC on 24/06/1982 for the eruption at Galunggung assuming an eruption height of 14 km.

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other key eruption source parameters, namely eruption start time and eruption end time, mean that the estimated concentrations are highly uncertain. 3.2. Soputan The eruption of Soputan, Indonesia in 1985 led to the encounter of a Qantas Airways 747 aircraft with an ash cloud, which resulted in all four engines and more than a dozen windows needing to be replaced (Tupper and Wunderman, 2009). The eruption occurred at 10:15 UTC (18:15 LT) on 19 May 1985 and ended at 17:30 UTC (01:30 LT on 20 May 1985) (Smithsonian Institution, 1985). Ground based observations suggest that the eruption column rose to 4 km altitude, but satellite observations place the eruption height at 12 km (Tupper and Wunderman, 2009). This discrepancy is due partly to the limitations of ground based observations in situations of cloud cover. Based on the satellite evidence, an eruption height of 12 km has been assumed. Similar to the Galunggung case study, there is uncertainty in the aircraft’s encounter location. In this case, it appears that misquoting latitude and longitude coordinates in arc minutes as decimal coordinates has propagated through the literature. The encounter coordinates are reported by Guffanti et al. (2010) and SEAN (1985) as 0.5 N, 124.54 E in decimal coordinates, but the coordinates from the original crew report are 00 500 N, 124 540 E at 16:58 UTC (flight engineer) and 00 500 N, 124 590 E at 17:00 UTC (captain) (pers. comm. Rolls Royce). This gives a positional error of 0.33 latitude and 0.36 longitude, which is w55 km at this latitude, between the literature and crew coordinates. Here, the flight engineer coordinates have been taken as correct and used in the modelling of this event. The encounter occurred at approximately 17:00 UTC and lasted for 7e8 min, during which an orange glow and sparks were observed around the nose of the aircraft and the windshield, and a haze filled the cabin (Smithsonian Institution, 1985). For the NAME analysis, only ECMWF ERA-40 data at approximately 125 km horizontal resolution are available. Fig. 2(a) shows that the NAME predicted ash plume places the centre of the ash cloud just to the west of the aircraft’s reported encounter location. No ash is predicted at the location for the time of the encounter, but it is believed that there may be errors in the coarse NWP wind data used. Comparison of the vertical integral (total column) of the modelled plume (Fig. 2(b)) with satellite observations of the ash cloud from an overpass of the TOMS instrument at 04:00 UTC the next day (Fig. 2(c)) shows that the actual plume was located further south-eastwards than was modelled. This suggests that the axis of the plume is likely to have been more towards the encounter location. The aircraft therefore probably flew through a more concentrated part of the eruption cloud than was modelled for the encounter location. Altering the NWP dataset to “correct” for this discrepancy in predicted winds is not possible. Therefore in an attempt to account for this uncertainty, the estimated ash concentration has been taken from the highest concentration part of the plume at the same distance downwind from the volcano as the aircraft was from the summit. This gives an estimated maximum ash concentration of 200,000 mg m
3. 3.3. Redoubt The eruption of Mount Redoubt, Alaska on 14 and 15 December 1989 consisted of four major eruptive phases. The largest eruption on the 15th occurred shortly before a KLM 747 flew to the east of the volcano on route from Amsterdam to Anchorage. The aircraft encountered the ash cloud at 20:47 UTC (11:47 LT) and subsequently lost power to all four engines (Casadevall, 1994). The crew eventually managed to restart all engines and descended to land at

Anchorage. Damage to the aircraft was estimated to be $80 million (Hobbs et al., 1991). The four eruptions have been modelled in a continuous NAME simulation using the source parameters outlined in Table 3. Fig. 3 shows the flight path of the aircraft taken from Casadevall (1994). At Loc1 the aircraft turned towards the south east and started to descend from FL390 to avoid the ash cloud based on reports from other aircraft in the vicinity. At Loc2 the aircraft encountered the ash plume at FL250. Engine power was increased to climb out of the cloud, but after just a minute and a half at FL279 all four engines stalled at Loc3. This sequence of events fits well with the NAME simulated plume, which shows that Loc2 is within the ash cloud and, assuming a direct route from Loc2 to Loc3, the maximum concentration for this section of the plume is at Loc3. The NAME predicted ash concentration at Loc2 is w4000 mg m
3 and at Loc3 is w55,000 mg m
3. Allowing for some uncertainty in the route, the maximum concentration in the vicinity of Loc2 and Loc3 is w70,000 mg m
3. 3.4. Hekla The eruption of Hekla, Iceland on the 26 February 2000 has been linked to engine damage of a NASA DC-8 research aircraft that was flying to the north of Iceland on 28 February 2000 (Grindle and Burcham, 2003). The eruption started at 18:20 UTC on 26 February and the most vigorous phase ended at w20:00 UTC (Lacasse et al., 2004; Smithsonian Institution, 2000). During this time the eruption column extended up to 12 km altitude (Lacasse et al., 2004). The simulations show that the ash plume initially travelled north-eastwards over Iceland and then became elongated in the east-west direction as it continued to travel north-east over the North Atlantic. This resulted in a long filament of dilute ash remaining around 75 e77 N for the next two days. This position agrees with satellite detections of the volcanic plume (primarily for sulphur dioxide and ice) in Rose et al. (2003) and Watson et al. (2004). During the early morning of 28 February, the NASA aircraft was on direct route from Edwards Air Force Base, California to Kiruna, Sweden when it encountered the volcanic plume (Grindle and Burcham, 2002). The encounter lasted between approximately 05:08 UTC and 05:18 UTC. At this time the flight was in total darkness, but research instruments on the aircraft recorded the presence of aerosols and gases that were indicative of a volcanic plume (Rose et al., 2006). Information on the exact location of the encounter is not consistent between different reports in the literature (e.g. Pieri et al. (2002) versus Rose et al. (2006)), but based on the in-flight data available from the SOLVE project website (http:// espoarchive.nasa.gov/archive/arcs/solve/data/dc8) it appears to have been within the region 75.7 N to 76.5 N and
9.2 E to
4.7 E. Differences between the quoted altitude of the aircraft also exist, with FL370, 11.3 km (which is wFL370) and 10.4 km (wFL340) all given. This difference is due to whether the quoted altitude was originally recorded as a pressure altitude (FL370) or a GPS altitude in metres (10.4 km). Fig. 4 shows the results from the two NAME simulations produced using different driving NWP data. Both of the simulations show that the ash cloud was present in the encounter zone at the time reported, although the ERA-Interim driven data fit better with the encounter coordinates. Ash was also present at lower levels, but was slightly further south due to weaker winds at lower altitudes. To reach Kiruna, the aircraft would have transected the plume. The highest predicted ash concentrations in the encounter region are w800 mg m
3 and w4000 mg m
3 for the MetUM and ERA-Interim NWP respectively. The maximum ash concentrations on the flight path are w5000 mg m
3 and w4000 mg m
3 respectively. These

C. Witham et al. / Atmospheric Environment 48 (2012) 219e229

223

Fig. 2. (a) NAME simulated hourly mean ash plume between FL350 and FL550 from 16:00 to 17:00 UTC on 19/05/1985 for the eruption of Soputan. The black line shows the approximate route of the aircraft’s flight path from NW to SE between two way points. (b) NAME simulated total column mean ash plume from 03:00 to 04:00 UTC on 20/05/1985. (c) The aerosol index at w04:00 UTC (12:00 LT) on 20/05/1985 detected by the TOMS instrument on the NIMBUS 7 polar orbiting satellite (TOMS Volcanic Emissions Group, UMBC/ NASA GSFC (http://toms.umbc.edu/)).

values corroborate evidence from the onboard research instruments that the aircraft flew through diffuse volcanic ash on this flight (Hunton et al., 2005). This particular case is complicated by the fact that the aircraft subsequently carried out a number of survey flights from Kiruna, Sweden that may have also sampled the dilute volcanic plume. The damage to the engines that was identified on the aircraft’s return to California may have been due to accumulated effects of multiple

flights through a dilute ash cloud, but evidence in Grindle and Burcham (2002) suggests that the encounter on 28 February may have been the primary cause. 3.5. Manam Manam is a small volcanic island off the north coast of Papua New Guinea. On 17 July 2006, following many months of sporadic

224

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ash was not visible on satellite imagery making verification of the eruption difficult for the Darwin VAAC (Tupper et al., 2007). Possible ash transport scenarios have been determined using movements of upper level cirrus in satellite data by Tupper et al. (2007). These suggest that a transport time of 3 h is needed for a high altitude plume from Manam to reach the aircraft’s location. NAME modelling of a 12 km eruption height (Fig. 5) suggests that a transport time of 3.5e4 h is necessary for a plume to reach the encounter location, which agrees well. NAME modelling of a plume for the observed eruption height of 4.6 km shows that ash from such a low-level eruption would have been transported to the north-west along the coastline and not towards the encounter location. This north-west transport matches the sulphur dioxide signal detected by the OMI satellite instrument, but cannot explain the aircraft encounter. However, lofting of ash by convection has been recognised as a mechanism for vertical ash transport in the tropics (e.g. Tupper et al., 2006) and could have contributed in this case. Analysis of the NWP meteorology does not suggest any large-scale convective activity in the region at this time, but it is unlikely that this coarse data would represent smallscale convection. To properly investigate possible ash concentrations, a plausible eruption scenario needs to be established. Here, an eruption period of 01:30 UTC to 02:00 UTC on 17 July 2006 has been used. Fig. 5 shows that an eruption with a maximum height of 12 km places the ash cloud within the aircraft’s survey zone at the time of the encounter. The aircraft was flying NW-SE transects so could have crossed the plume multiple times before the engines cut-out, although the engine failure mechanism suggests a sudden engine flame-out, as no power reduction was noticed beforehand (pers. comm. Rolls Royce).

Table 3 Eruption source parameters used for the December 1989 eruptions of Mount Redoubt. Data are derived from the Smithsonian Global Volcanism Program weekly reports (Smithsonian Institution, 1989). Eruption number

Start time (UTC)

Stop time (UTC)

Plume top height (km asl)

1 2 3 4

19:13 10:40 12:38 19:17

20:00 11:40 13:38 20:00

10.5 6.5 6.5 12.0

14/12/1989 15/12/1989 15/12/1989 15/12/1989

14/12/1989 15/12/1989 15/12/1989 15/12/1989

eruptive activity, a continuous ash eruption was observed from Manam and reported by a ground observer to reach 3 km above sea level. At a similar time, a pilot report stated that an ash cloud from Manam was reaching altitudes of w4.6 km, but its exact extent (lateral and vertical) was obscured by cloud cover above this height. At 05:18 UTC (15:18 LT) a Gulfstream II aircraft, engaged in survey work approximately 270 km to the south west of Manam experienced a twin-engine flame-out over the Papua New Guinea highlands at an altitude of FL390 (w12 km). Both engines were restarted following descent to FL240 (7.3 km) and the aircraft returned to its base airport on reduced power. Engine examination pointed to volcanic ash as the only plausible candidate for the engine failure (for full details on this incident see Tupper et al. (2007)). To explain the incident, Tupper et al. (2007) proposed that ash from Manam must have been erupted to 12 km or lofted up to this height at some time prior to the aircraft encounter. There is no evidence from observations (ground-based or satellite) of an eruption of this magnitude, and high-level cloud cover meant that

Loc1

63

Loc2 Loc3 Loc4 Loc5

62

Anchorage 61

Redoubt 60

59

58

57 154

153

152

151

150

149

148

147

146

Maximum value = 1.1e+05 ug/m3

1.0e+01

1.0e+02

1.0e+03

1.0e+04

1.0e+05

1.0e+06

Fig. 3. NAME simulated hourly mean ash plume between FL200 and FL350 from 20:00 to 21:00 UTC on 15/12/1989 for the eruptions at Redoubt. The lines mark the aircraft’s flight path as it encountered the plume, changed course to recover and then continued on to Anchorage to land (location numbers refer to the those in Casadevall (1994)).

C. Witham et al. / Atmospheric Environment 48 (2012) 219e229

a

225

b 80

80

75

75

70

70

65

65

60

60

55

55 20

15

10

5

0

5

20

10

15

10

1.0e+02

1.0e+03

1.0e+04

0

5

10

Maximum value = 6.4e+03 ug/m3

Maximum value = 4.8e+03 ug/m3

1.0e+01

5

1.0e+05

1.0e+01

1.0e+06

1.0e+02

1.0e+03

1.0e+04

1.0e+05

1.0e+06

Fig. 4. NAME simulated hourly mean ash plume between FL350 and FL550 from 05:00 to 06:00 UTC on 28/02/2000 for the eruption of Hekla. The black rectangle outlines the region of the encounter location and the black line shows the aircraft’s flight path. (a) Using MetUM 60 km NWP, (b) using ERA-Interim re-analysis NWP.

mass is lofted to a layer of a similar depth, then concentrations would be on the order of 5e10% of these values. Due to the large uncertainties in the eruption source details, the modelled ash concentrations are clearly highly speculative.

The predicted ash concentration at the encounter location for a 12 km eruption is w40 mg m
3, but the maximum estimated concentration within the survey zone at this time is w19,000 mg m
3. If a smaller eruption is considered of say 5e6 km height, where all of the ash

3

4

Manam

5

6

Encounter

7

8

9 141

142

143

144

145

146

147

148

149

Port Moresby

Maximum value = 1.9e+04 ug/m3

1.0e+01

1.0e+02

1.0e+03

1.0e+04

1.0e+05

1.0e+06

Fig. 5. NAME simulated hourly mean ash plume between FL350 and FL550 from 05:00 to 06:00 UTC on 17/07/2006 for a 12 km eruption at Manam between 01:30 and 02:00 UTC. The black lines denote the region where the survey aircraft was carrying out traverses before the encounter.

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3.6. Pinatubo The June 1991 eruption of Pinatubo consisted of around 17 explosive eruptions over three days before the main climactic eruption phase started on the 15 June (Hoblitt et al., 1996; Holasek et al., 1996; Wolfe and Hoblitt, 1996). This climactic phase lasted approximately 21 h during which the eruption height decreased from around 40 kme13 km. Between 12 and 18 June sixteen damaging in-flight encounters between jet aircraft and the Pinatubo ash clouds were reported (see Casadevall et al. (1996) for full details), many at distances of up to 2000 km from the volcano. The most serious encounter occurred on 17 June when a 747 lost power to two engines whilst in flight. As a result of the encounters, ten engines needed to be replaced, including all four engines on one aircraft. To model the Pinatubo ash cloud, changes in the eruption height throughout all of the eruptive phases were used in the source parameters (a total of 26 source configurations within one simulation). This required assumptions about the duration and eruption height of each phase, as well as a degree of simplification of the activity. For example, for the climactic phase of the eruption a mean eruption height for each 3-h period was used, rather than attempting to represent higher frequency changes. This was only possible due to the well-documented range of observations that provide details of the eruption phases (Hoblitt et al., 1996; Holasek et al., 1996; Wolfe and Hoblitt, 1996). Details on the full source parameter sequence used are available in Webster et al. (2011). The first aircraft encounter occurred during the initial phases of the eruption. At 04:20 UTC on 12 June 1991 a 747, flying at FL370 approximately 170 km from Pinatubo, reported a 3 min encounter

with volcanic ash at 14 000 N and 119 300 E (incident 91-01 in Casadevall et al. (1996)). All four engines were subsequently replaced. Fig. 6 shows that the encounter location was on the edge of the predicted plume at this time and ash concentrations at this location are estimated to be w400 mg m
3. The aircraft was flying from Bangkok to Manila (approximately west to east) so it is possible that it flew through a higher concentration part of the plume. Estimated maximum concentrations in the centre of the plume are 2,000,000 mg m
3. In the following days, the climactic phase of the eruption led to the formation of a large umbrella cloud with a top height of w34 km at its centre (Koyaguchi and Tokuno, 1993). This is an intrusive gravity current formed by the encounter of upward and downward moving material in the eruption column around the level of neutral buoyancy. This cloud spread out over an area of more than 1000 km in diameter in less than 11 h (Holasek et al., 1996) and its large radial velocity (up to 20 m s
1) resulted in significant upwind spread of the ash plume. NAME is not currently designed to model the spread of an umbrella cloud as a gravity current so a simplified representation of the umbrella cloud was included in the simulations using a continuous ellipsoid source of diameter 550 km (corresponding to satellite observations at 08:41 UTC on 15 June in Holasek et al. (1996)) between altitudes of 20 km and 38.5 km. This source was introduced on the morning of the climactic eruption phase and it was assumed that its subsequent transport was by mean and turbulent modelled winds only (and subject to sedimentation), with radial velocities of the umbrella cloud negligible. This is a much simplified representation of a complex feature, but it was hoped that it would account, in some way, for the umbrella cloud within the modelling.

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Fig. 6. NAME simulated hourly mean ash plume between FL350 and FL550 from 04:00 to 05:00 UTC on 12/06/1991 for the eruptions at Pinatubo.

C. Witham et al. / Atmospheric Environment 48 (2012) 219e229

Examination of the simulated ash plume shows that the inclusion of the umbrella cloud source within NAME spreads the ash cloud over a more appropriate area (as shown in satellite observations (Holasek et al., 1996)) than simulations without this source (Fig. 7). However, this simplified approach does not accurately represent the subsequent spread of the umbrella cloud. The simulated plume is subject to much less lateral spread than was observed and is predominantly transported westwards by the mean forecast wind. In reality, the umbrella cloud spread out over a wider area. This suggests that the radial velocities cannot be neglected even once the umbrella cloud has reached a horizontal extent of 550 km. The inability of NAME to accurately represent the spread of the umbrella cloud hinders prediction of the concentrations that may have been encountered by aircraft during this period and at some reported encounter locations no ash is predicted. No estimates of possible encountered ash concentrations are given here since the errors involved are believed to be substantial. The umbrella cloud is not the only source of uncertainty in the model simulations, as satellite imagery (Holasek et al., 1996) shows the ash cloud from the 15 June 1991 eruptions was partly transported towards the south-east, which is not represented. At this time, Typhoon Yunya passed within 75 km of Mount Pinatubo bringing strong storms and heavy rains to the region (Oswalt et al., 1996). This small-scale typhoon is not well captured by the 85 km resolution ERA-Interim NWP data available for this period, which is thought to explain some of the discrepancy between the modelled and observed ash plume. This NWP uncertainty further hinders accurate prediction of ash concentrations for this event.

dispersion model hinders accurate simulation of both the formation of the umbrella cloud and its spread. Table 4 summarises the results of the NAME predictions for the six case studies and the major limitations experienced during the modelling process. In most of the case studies investigated here, the modelling suggests that maximum concentrations in the nearby vicinity of the aircraft encounter locations were at least 4000 mg m
3 (equating to the “high contamination zone” introduced during the 2010 Eyjafjallajökull eruption) and in most cases significantly higher. This work has demonstrated that the uncertainties involved can be large and hence confidence in the predicted ash concentrations is low. Some attempt has been made to quantify these uncertainties and the Galunggung (eruption height), Soputan (aircraft location and NWP) and Hekla (NWP) examples demonstrate that variations in predicted ash concentrations of orders of magnitude are possible. Reducing these uncertainties presents a number of challenges for recreating past encounters: 1. Obtaining accurate information on the eruption source parameters is essential for modelling. Parameters required as a minimum include start and end times of all eruption phases and the associated eruption heights. 2. Precise information on the aircraft’s location at the time of the encounter needs to be available, ideally from the black box. The latitude, longitude, and height of the aircraft at the start of the encounter are vital pieces of information. Even small errors in these, such as those caused by misinterpretation of coordinates in arc minutes, can place the aircraft outside of the modelled ash plume. Knowledge of the flight path is also very useful for interpreting which part of the plume the aircraft may have flown though and hence the ash concentrations that may have been encountered before or after the reported encounter location. The flight path can also provide information on whether the effects were due to a sudden encounter with the ash cloud or were due to cumulative exposure. This could be an important factor for “lower” concentration encounters.

4. Discussion In most of the cases examined NAME predicts volcanic ash where the aircraft encounters occurred, which gives confidence that the model is able to predict the dispersion of ash clouds from the eruptions considered. The main exception is Pinatubo, where the lack of representation of volcano plume dynamics in the

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Fig. 7. NAME simulated hourly mean ash plume between FL350 and FL550 from 14:00 to 15:00 UTC on 15/06/1991 for the eruptions at Pinatubo (a) without an umbrella cloud representation and (b) including a simple representation of the umbrella cloud.

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Table 4 Estimates of the concentrations of volcanic ash encountered by aircraft and the major limitations in producing these predictions. Values for Pinatubo are not given due to the large uncertainties involved in these simulations. Eruption

Concentration at encounter location (mg m
3)

Maximum concentration in vicinity (mg m
3)

Major limitations

Galunggung (14 km height) Soputan

4000

180,000

Large uncertainties in height and timing of eruption episodes.

0

200,000

Uncertainties in NWP. Uncertain eruption height. Concentration strongly dependent on aircraft’s proximity to volcano and its flight path.

55,000 800 and 4000 using different NWP data 40

70,000 5000 and 4000 using different NWP data 19,000

N/A

N/A

Redoubt Hekla Manam (12 km height) Pinatubo

3. Reconstructions of past eruptions are restricted to coarser resolution historical or re-analysis NWP meteorological data than is available for today’s operational forecasts. This data generally has larger uncertainties associated with it than current NWP and may mean that modelled ash clouds do not match other sources of information e.g. satellite data. Having accurate eruption source parameters is important for volcano modelling in general and especially for encounters that occur close to the volcano and within only a few hours of the eruption start, as is the case in several of the case-studies considered here. Knowledge of the eruption height is important, as even minor changes in modelled plume height can cause large changes in predicted concentrations if ash is emitted at altitudes where winds are in different directions. The Manam encounter demonstrates the problems associated with predicting concentrations when the reported eruption height does not fit the encounter altitude. For eruptions in poorly monitored locations or those occurring at night, constraining the eruption details can be very difficult. Even satellite data cannot be relied upon if there is cloud cover. Information on the start of the eruption is essential, but it is useful to have enough detail to model multiple eruption episodes in one simulation. This was vital for the Mount Redoubt and Pinatubo cases, as it enabled the evolution of the ash cloud and which eruption phases were responsible for the encounters to be understood. The impact of errors within the NWP data on modelled concentrations is illustrated well by the Hekla case. Although the two NWP datasets produce similar predictions for the plume latitude, differences in the positions of the northerly and westerly edges led to large differences in predicted ash concentrations. The ash concentration in the encounter location is significantly higher with the ERA-Interim NWP because the centre of the ash plume corresponds with the encounter location. The maximum estimated concentrations are more similar, but still suggest an uncertainty of 25%. Whilst this is significant, the differences arising from other uncertainties could be considerably larger. The difference in scale between the Pinatubo eruption and the other eruptions highlights one of the current limitations of conventional dispersion models for modelling volcanic eruptions, in that they do not represent gravity currents associated with umbrella clouds. Accurate modelling of the lateral and upwind spread of the umbrella cloud from large volcanic eruptions is consequently not possible in these models at present. Refining dispersion models so that they include better representations of these volcanic dynamics is desirable. Considerable effort has been expended within this study in tracking down data and reconciling inconsistencies to reduce the uncertainties as much as possible. However, many sources of

Uncertainties in NWP. Discrepancy in reported encounter location and influence of subsequent flights. Unknown eruption start and end times and height. Lack of small-scale convection in NWP. Lack of volcano umbrella cloud dynamics in the dispersion model. Uncertainties in NWP. Lack of information on aircraft flight paths.

uncertainty still exist for these historical case studies meaning that great care should be taken when attributing aircraft damage or observed phenomena to the presented concentrations. 5. Conclusions This work has examined the concentrations of ash that may have been encountered during known aircraft encounters with volcanic ash clouds. In most cases the dispersion model predicts ash in these locations, but the uncertainties involved prevent confident prediction of ash concentrations. Many of the causes of uncertainty are outside of the modeller’s control, e.g. eruption source parameters and flight information. The lack of such data prevents truly comprehensive reconstruction of past events and indeed potentially presents significant problems for analysis of future events. Some reductions in modelling errors are possible through improvements to the representation of volcanic ash and the dynamics of eruption clouds in dispersion models and this is particularly important for large eruptions on the scale of Pinatubo 1991. Simple analyses of some of the uncertainties have demonstrated that changes in predicted ash concentrations of orders of magnitude are possible. Consequently, this work does not provide sufficient information on which to derive unsafe concentration thresholds for aviation. To increase confidence in the use of predicted concentrations, further effort is needed to reduce errors and understand and quantify the uncertainties involved. Acknowledgements The authors are grateful to David Hyde and Danny Goodall at Rolls Royce who supplied information on the encounter locations; Andrew Tupper for information and discussion on the Manam case study; Fred Prata for useful discussion on some of the case studies and Steve Sparks for comments on a draft of this paper. References Bluth, G.J.S., Casadevall, T.J., Schnetzler, C.C., Doiron, S.D., Walter, L.S., Krueger, A.J., Badruddin, M., 1994. Evaluation of sulphur dioxide emissions from explosive volcanism: the 1982e1983 eruptions of Galunggung, Java, Indonesia. Journal of Volcanology and Geothermal Research 63, 243e256. Casadevall, T.J., 1994. The 1989e1990 eruption of Redoubt volcano, Alaska: impacts on aircraft operations. Journal of Volcanology and Geothermal Research 62, 301e316. Casadevall, T.J., Delos Reyes, P.J., Schneider, D.J., 1996. The 1991 Pinatubo eruptions and their effects on aircraft operations. In: Newhall, C.G., Punongbayan, R.S. (Eds.), Fire and Mud e Eruptions and Lahars of Mount Pinatubo, Philippines. University of Washington Press, Seattle and Philippine Institution of Volcanology and Seismology, Quezon City, pp. 1071e1085. Davies, T., Cullen, M.J.P., Malcolm, A.J., Mawson, M.H., Staniforth, A., White, A.A., Wood, N., 2005. A new dynamical core for the Met Office’s global and regional

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