IVATF/1-IP/18
22/7/10
INTERNATIONAL VOLCANIC ASH TASK FORCE (IVATF)
FIRST MEETING
Montréal, 27 to 30 July 2010
Agenda Item 3: Results of the EUR/NAT VATF Meeting (Plenary)
3.1: Evaluation of the Eyjafjallajokul eruption and lesson learned
SUMMARY OF UK’S OBSERVATIONAL NETWORK USED TO MONITOR THE
EYJAFJALLAJOKULL ASH PLUME AND VERIFY THE NAME DISPERSION MODEL
PREDICTIONS
(Presented by United Kingdom)
SUMMARY
This paper summarises the diverse range of observational technologies used
by VAAC London (Met Office in the UK) to monitor the movement and
dispersal of the volcanic ash cloud from the Eyjafjallajokull volcano in April
and May 2010. It also summarises the attempts to use these observations to
validate and verify the predictions of the NAME dispersion model.
1.
INTRODUCTION
1.1.
Following the explosive eruption of the Eyjafjallajokull volcano in Iceland on 14 April 2010
the Met Office in the UK put a lot of effort into validating the predictions of the NAME dispersion model
VAAC London used to predict the advection and dispersion of the resulting ash plume. The regulator in
the UK was asking London VAAC not only to give guidance on where the ash cloud was but also to
predict the actual ash mass concentration levels so that they could use effectively engine manufacturer
advise on safe and unsafe levels of ash that aircraft could fly through.
1.2.
Operational and research based observing capabilities in the UK were exploited to monitor
volcanic ash. There are a number of observation types which form useful indicators of the presence of
aerosol, including volcanic ash, but there remains considerable difficulty in distinguishing between
aerosol and water cloud, and no completely objective means of distinguishing between volcanic dust and
other natural or anthropogenic aerosol.
1.3.
This paper summarises the observing capability that was utilised in the UK and briefly shows
some of the verification we were able to do of the model predictions.
(9 pages)
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2.
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DISCUSSION
2.1
The following paragraphs provide a short summary of the diverse range of observational
technologies used during the volcanic eruption.
2.2
Operational satellites
2.2.1
High resolution satellite remote sensing measurements are available from low earth
orbiting satellites (orbital altitude around 850km) but only provide snapshots once a day if the sensor is
using visible channels (the sun must be up), or twice per day if they are using infra-red channels (no
dependence on sunlight). Geostationary satellites provide data at lower spatial resolution (they orbit at a
considerably higher altitude above the Earth ~36,000km) but they are able to provide data typically at 15
minute intervals, and therefore enable volcanic plumes to be detected and, to some extent, their progress
tracked.
2.2.2
Infra-red retrieval algorithms detect the difference in the so-called brightness
temperatures (BT) derived from radiation emitted at around 10.8μm and that emitted at around 12μm. If
this BT difference is negative then the signal is very likely originating from volcanic ash, provided the ash
particles are relatively small (around 10-12 μm). However, the presence of clouds (especially ice clouds)
and the natural dispersal of the ash particles quickly attenuate the signal and consequently the signature of
the ash plume diminishes quite quickly in space and time. Presenting the BT differences using a palette of
colours enables the plume to be distinguished by the naked eye near to its source but also results in
significant ambiguity in the picture as the plume moves away from the source.
2.2.3
Visible retrieval algorithms typically measure the reflectance in the 0.3-1.7μm range of
the electromagnetic spectrum. Over dark ocean surfaces, for example, volcanic ash will be observable as a
slightly brighter plume, while over underlying cloud, the volcanic ash will be observed as a slightly
darker plume. Once again, the careful application of colour can help the human eye detect the location of
the plume but, in a similar way to the infra-red based imagery, there are deficiencies with this method at
distances away from the site of the eruption. The main limitation of satellite retrievals is that they are
interfered with by underlying or overlying cloud; they have limited capabilities to observe at low levels in
the atmosphere (due in part to the effect of the overlying atmospheric column) and most clearly
distinguish the high density ash near the core of the plume.
2.2.4
It was noted during the incident that on occasions there were thin streams of volcanic ash
seen on satellite imagery that were subsequently measured by the DLR Falcon to have volcanic ash
concentrations of less than 30 micrograms per cubic metre which is relatively low compared to the ash
mass concentration thresholds now being used in Europe. This suggests that it is not possible to
distinguish between safe and unsafe levels of volcanic ash by the use of satellite data alone.
2.2.5
Although much progress was made during the incident on the interpretation of satellite
data to help identify areas of volcanic ash, the full extent and spatial distribution of volcanic ash is not
fully discernible from satellite data alone.
2.3
Research aircraft
2.3.1
In-situ aircraft measurements are capable of providing high quality validation data for the
models. Instruments have been developed that measure particle sizes, particle shapes, and gas phase
measurements such as sulphur dioxide. Filter measurements may be made for subsequent analysis which
can reveal the size, shape, chemical composition and the fusion point of the material. The main problems
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with in-situ measurements are that the research aircraft that are used in probing volcanic plumes are
subject to exactly the same problems as any other aircraft; abrasion problems, engine damage and
ultimately engine failure cannot be ruled out, so there is an understandable reluctance to probe very dense
ash plumes.
2.3.2
The Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 aircraft was
unavailable at the time the eruption started as it was scheduled for essential maintenance and had been
stripped down. Instruments were transferred from it to the Natural Environment Research Council
(NERC) Dornier 228 aircraft and this was used for flights on 15, 16, 18, 19, 20 and 21 April to provide
in-situ observations of volcanic ash. Met Office staff worked through the night of 18/19 April loading
equipment at Exeter and driving to Cranfield to enable it to be fitted to the FAAM, which subsequently
flew 11 sorties between 20 April and 18 May. It made a valuable contribution to the in-situ detection and
remote sensing of the volcanic ash plume.
2.3.3
The BAe 146 is owned by BAe Systems and operated by Directflight Limited under
contract to support the UK atmospheric science community. The aircraft’s role is to monitor atmospheric
composition and structure, and it is a unique facility within the UK. The BAe 146 has the same
vulnerability to volcanic ash as any other jet-engine aircraft, but the particular nature of the jet engines
means that if it did suffer damage, the jet engines would need to be rebuilt as they have only one spare
engine available. Directflight and BAe systems requested an insurance waiver from UK Government to
fly the aircraft in a civil contingencies role during the volcanic ash incident. The purpose of the waiver
was to protect Directflight and BAe Systems from any costs or loss of income that might arise from
damage to the aircraft whilst flying in volcanic ash. Negotiations over the insurance waiver were
complex and protracted, and UK Government would only provide a waiver for a limited period of about 1
week at a time. This meant that negotiations had to be repeated at intervals during the incident, with
considerable distraction from operational and scientific activities.
2.3.4
Command and control of the airborne resources was initially confused because there was
no agreed plan to operate the Dornier in a civil contingencies role. In addition, the civil contingencies
plan that was in place prior to this incident specifically states that the BAe 146 will not fly in volcanic
ash. Thus command and control, and rules of engagement, had to be agreed as the situation evolved.
2.4
Surface Based Observing
2.4.1
The most effective surface based measurement system for detecting the presence of
volcanic plumes is lidar (Light Detection And Ranging which is an optical remote sensing technology
that measures properties of scattered light to find range and/or other information of a distant target) which
emits pulses of laser light and detects the backscattered signal. However, lidar will detect everything in
the atmosphere, including low and high level cloud as well as volcanic ash. Additionally, lidar signals
cannot penetrate through thick clouds so low level clouds obscure detection of aerosol plumes higher up
in the atmosphere. Semi-quantitative measurements of aerosol are only possible if the lidar is used
together with a sun-photometer during daylight. Notwithstanding these issues, considerable effort within
the Met Office and elsewhere was required to obtain data from research lidars. Instruments at 5 sites
operated by NERC and at one Met Office site were monitored as part of the volcanic ash observing
network.
2.4.2
Low powered ceilometers, operationally used by the Met Office to observe the cloud
base, work on the same principle but lack the power and the detailed detection elements of lidar systems.
They are however capable of providing limited information on the altitude of thick aerosol layers in the
absence of clouds.
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2.4.3
At about 125 observing sites in the UK, transmissometers measure atmospheric visibility,
and in conditions of low humidity, these instruments can detect aerosol.
2.4.4
Radiation monitors are present at 3 sites in the UK and atmospheric aerosol can be
detected through a change in levels of radiation observed by these instruments.
2.4.5
Trained observers are present for all or part of the day at about 30 sites in the UK and
during daylight they can observe atmospheric aerosol if cloud amounts allow. The observers are also able
to detect the deposition of volcanic ash on surfaces.
2.4.6
Aerosol radiosondes (an instrument attached to a buoyant balloon) can observe the
presence of aerosols in the atmosphere and give an indication of particle size. Only 3 of these
radiosondes were available to the Met Office at the time of the incident, though more were quickly
ordered. The aerosol radiosondes can only be launched at sites where skilled staff are present and
facilities exist to track and receive information from the instruments.
2.4.7
The Met Office operates a lightning detection system known as the Arrival Time
Difference (ATD) system. ATD can detect most cloud-to-ground lightning strikes over Europe and the
North Atlantic. During some volcanic eruptions, the volcanic ash plume can create flashes of lightning
which are detected by ATD. Although ATD does not provide a method of identifying the volcanic ash
plume away from the volcano, it is helpful in giving guidance on the intensity and continuing nature of an
eruption.
2.5
Intercomparisons
2.5.1
The comparison of NAME model predictions with observations of volcanic ash from
various sources has been examined for a number of occasions. Satellite observations are found to provide
a useful verification of the central core of the ash plume in regions where the observations are not heavily
contaminated by cloud. In the cases analysed the location of this central core in the forecast corresponds
very well with the satellite image. Figure 1 and 2 shows examples of where the NAME model prediction
has been compared with processed satellite imagery.
2.5.2
Forecasts were also compared with a number of lidar observations from around the UK.
The observations often show a strong horizontal layering with the majority of the ash concentrated in
relatively shallow layers, particularly, when the ash cloud is several days old. In general, the forecasts are
unable to resolve this layering, primarily because the vertical distribution of material from the source at
the volcano is unknown, but also because of limitations in the vertical resolution of the models.
However, comparisons of NAME predictions in vertical layers have been made with lidar observations
and this does show reasonable agreement between predicted ash mass concentrations and the peak values
of mass concentrations measured in the layers. Figure 2 shows a comparison between the predicted
movement of the ash cloud over Wales and the excellent agreement both in terms of timing and ash mass
concentration as measured by a lidar in Aberystwyth.
2.5.3
Some of the best quality observations of ash come from research flights carried out by the
FAAM BAe 146 aircraft and the NERC Dornier aircraft, and using research balloon instruments. These
provided the only quantitative measurements of ash concentration in the atmosphere and the most
definitive test that aerosol observed is of volcanic origin. For more details of the quantitative comparisons
of the NAME predictions and the aircraft measurements contact VAAC London.
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2.5.4
Model winds used to drive the dispersion model have also been verified. This showed
that the Met Office model provides some of the best wind forecasts available from any global model in
the world.
2.5.5
Overall, the cases analysed indicate that ash is observed in significant quantities in areas
consistent with the forecasts, giving us a good level of confidence in the model predictions.
2.5.6
The Met Office during the volcanic eruption also gained a great deal of confidence in the
model’s prediction through comparisons with other VAAC’s dispersion model output. At different times
during the event VAAC London was able to compare NAME predictions with output from:
a) MOCAGE (Meteo-France);
b) MLDP0 (Environment Canada);
c) SNAP (Met.no, Norway);
d) SILAM (FMI, Finland);
e) MACC (ECMWF); and
f) HYSPLIT (NOAA, USA).
2.5.7
Figure 3 shows a typical comparison between 3 of these models.
2.5.8
Satellite, airborne, and ground based quantitative, scientific observations of volcanic ash
are fundamentally important for VAACs to be able to provide reliable and accurate guidance on the future
movement and dispersion of ash clouds, particularly if more detailed predictions are required of ash mass
concentrations. Unfortunately, these sorts of observations are relatively scarce over Europe and the North
Atlantic. If the volcanic activity in Iceland is moving into a more active phase then not only do we need
more accurate estimates of the source strength of the volcano but we also need a more flexible and
abundant monitoring network in Europe.
3.
3.1
ACTION BY THE IVATF
The IVATF is invited to note the contents of this paper.
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Comparison with NAME
1200 UTC 10/05/2010
Edge of ‘visible’ plume on satellite retrievals
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Figure 1 – Comparison of NAME model output and processed satellite imagery for 12z on 10th May 2010. The image
on the left is the processed satellite image. The top right hand picture is the NAME model prediction for the same
time overlaid with a purple boundary indicating the extent of the ash visible on the satellite picture. The bottom right
graphic shows the NAME prediction using the red, grey and black thresholds now used in Europe.
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Satellite imagery supports the model, but difficult to
interpret because of the presence of cloud
Ash
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Results from Aberystwyth lidar
Results from Aberystwyth lidar
10:44UTC
10:02UTC
Increasing
Not much
here
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Results from Aberystwyth lidar
© Crown copyright Met Office PRESENTATION DELIVERED WITH EXPERT COMMENTARY. PLEASE CHECK AGAINST DELIVERY
Results from Aberystwyth lidar
11:00UTC
11:48UTC
AOD=∫ke * conc * dz
Increasing
Increasing
AOD=0.35
dz = 3000m
ke = 0.6m2g-1
Conc
© Crown copyright Met Office PRESENTATION DELIVERED WITH EXPERT COMMENTARY. PLEASE CHECK AGAINST DELIVERY
= 190gm-3
= 70-600gm-3
© Crown copyright Met Office PRESENTATION DELIVERED WITH EXPERT COMMENTARY. PLEASE CHECK AGAINST DELIVERY
Figure 2 – The graphic at the previous page shows the processed satellite picture for 12z on 4 May.
Overlayed on it and agreeing very well with it is the forward edge of the predicted position of the plume
from the NAME model which is shown in full in the bottom right hand corner of the top graphic. The white
dot on the satellite picture shows the position of the Aberystwyth Lidar. The bottom 4 graphs shows the
movement of the edge of the ash cloud as it moves over the top of Aberystwyth either side of 12z between
1km and 4km above the surface.
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Figure 3 – Comparison of dispersion model results for 12z on 10 May 2010 from the NAME model (top),
MOCAGE model (middle) and MLDP0 model (bottom)
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