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| Titel |
Big grains go far: understanding the discrepancy between tephrochronology and satellite infrared measurements of volcanic ash |
| VerfasserIn |
J. A. Stevenson, S. C. Millington, F. M. Beckett, G. T. Swindles, T. Thordarson  |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 5 ; Nr. 8, no. 5 (2015-05-19), S.2069-2091 |
| Datensatznummer |
250116362
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| Publikation (Nr.) |
 copernicus.org/amt-8-2069-2015.pdf |
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| Zusammenfassung |
| There is a large discrepancy between the size of volcanic ash particles
measured on the ground at least 500 km from their source volcano (known as
cryptotephra) and those reported by satellite remote sensing (effective
radius of 0.5–9 μm; 95% of particles < 17 μm diameter). Here
we present new results from the fields of tephrochronology (a dating
technique based on volcanic ash layers), dispersion modelling and satellite
remote sensing in an attempt to understand why. A literature review and
measurements of prehistoric and recent eruptions were used to characterise
the size range of cryptotephra grains. Icelandic cryptotephra deposited in
NW Europe has lognormal particle size distributions (PSDs) with median
lengths of 20–70 μm (geometric standard deviation: 1.40–1.66; 95th
percentile length: 42–126 μm). Grain-size range estimates from the
literature are similar. We modelled the settling of volcanic ash using
measured fall velocities of ash particles, a release height typical of
moderate Icelandic eruptions (10 km), and a wind speed typical for NW Europe
(10 m s−1), to show that an ash cloud can transport particles up to
80 μm diameter up to 850 km in 24 h. Thus, even moderately sized
Icelandic eruptions can be expected to deposit cryptotephra on mainland
Europe. Using simulated satellite infrared data for dispersion-model-derived
ash clouds, we demonstrate a systematic bias towards small grain sizes in
retrievals of volcanic ash clouds that contain large proportions of
cryptotephra-sized grains. As the median radius of the simulated PSD
increases, fewer ash-containing pixels are correctly identified. Where
retrievals are made of simulated clouds with mass median radii larger than
~ 10 μm, the mean retrieved reff plateaus at around 9 μm.
Assuming Mie scattering by dense spheres when interpreting satellite infrared
brightness temperature difference (BTD) data puts an upper limit on retrieved
particle sizes. If larger, irregularly shaped ash grains can also produce a
BTD effect, this will result in further underestimation of grain size, e.g. in
coarse ash clouds close to a volcano. |
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