1

Volcanic ash and SO2 retrievals from MODIS and SEVIRI:

overview and links to SHIVA project

S. Corradini and L. Merucci

1

INGV

26 July 2013 – Oxford – SHIVA Meeting

2

• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

3

• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

Ash retrieval in the TIR spectral range

The cloud discrimination is based on Brightness Temperature Difference algorithm [Prata et al.,

GRL,1989] (+ water vapor correction [Prata and Grant, CSIRO, 2001; Corradini et al., JARS, 2008])

BTD = Tb(11m) - Tb(12m)

BTD < 0 volcanic ash

BTD > 0 meteo clouds

The ash retrievals are based on computing the simulated inverted arches curves “BTD-

Tb(11m)” varying the AOD () and the particles effective radius (re) [Wen and Rose, JGR, 1994]

re

M(n,m)

5 Ash

Mas

s

Effe

ctive

Rad

ius

AO

D

SO2 retrieval in the TIR spectral range

Ash effect on SO2 retrieval During an eruption generally ash and gases are emitted simultaneously The plume ash particles reduce the top of atmosphere radiance in the entire TIR spectral range, including the channels used for the SO2 retrieval The net effect is a significant SO2 overestimation[Corradini et al., AMT, 2008; Kearney and Watson, JGR, 2008]

n

iiS

imn

mnemnjsM

imnS

imnc w

R

rcRRmn

s1

2

),,(

),(),(,),,(),,(2 ),,(),(

SO2 retrieval

MODIS TIR response functions

Procedure [Realmuto et al., JGR, 1994; Teggi et al., JGR, 1999]

n

iiS

imn

jsM

imnS

imnc w

R

cRRmn

s1

2

),,(

,),,(),,(2 )(),(

Sensor radiance Simulated radiance

7.3 m

8.6 m

for UTLS plumes

for LT plumes

SEVIRI - 12 August 2011

MODIS – 24 November 2006

After Correction

Total mass = 5947 t

Before correction

Total mass = 18728 t

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• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

TOA Radiance computation

Radiative Transfer Model

The only particles detectable in the TIR spectral range

),,( ,, esM rcR

8 values from 0.7 to 10 m, constant step in a log scale

21 values from 0 to 10 g/m2, step 0.5 g/m2

9 values from 0 to 5,constant step in a log scale

Ash Optical Properties

Satellite geometryVolc. cloud

geometry

P, T, W

Spectral surface emissivity and temperature

Each RTM input parameter has an uncertainty that lead to ash and SO2

retrieval errors

W Ts hp tp opt. prop RTM Input parameters

(20%) (2K) (3%) (0.5 km) (50%) (type) RTM Parameter Uncertainty

50

40

30

20

10

% Retrieval

Errors Ash mass[Corradini et al., 2009]

SO2 (8.7m)

[Corradini et al., 2010

Pugnaghi et al., 2013]

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• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

1) The plume can be regarded as a region characterised by a dip in the radiance. VPR removes the plume from the image by linearly interpolating the radiances in the region surrounding the detected volcanic plume, obtaining the radiances that would have been measured by the sensor if the plume was absent

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Volcanic Plume Removal (VPR) procedure for the simultaneous retrievals of ash and SO2 [Pugnaghi et al., 2013]

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2) The new image and the original data allow computation of plume transmittance in the TIR-MODIS bands 29, 31, and 32 (8.6, 11.0 and 12.0 μm) by applying a simplified model consisting of a uniform plume at a fixed altitude and temperature

pupp LLL ,0

3) To correct the uncertainties of the simplified model considered, the transmittances are then refined with a polynomial relationship obtained by means of MODTRAN simulations adapted for the geographical region, ash type, and atmospheric profiles

32,31

32,31AODash

p e

4) From the transmittance of the channels centered around 11 and 12 m:

the AOD31,32 depends linearly on the plume AOD550 (with null offset), but with a slope which is a function of the particle size

229,29,29,

SOp

ashpp

csSOp e 2

29,

)()1(, pppu TBL where

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The volcanic cloud altitude and temperature are the only input

parameters required to run the procedure

Because the effect of the atmosphere and surface is extracted

directly by the image (i.e. an ‘ideal‘ correction is realized), the

ash and SO2 errors due to the uncertainty of the mentioned

parameters are drastically reduced (Pugnaghi et al., in preparation)

The estimated overall ash and SO2 mass retrieval errors considering W, Ts and uncertainties of 20%, 2K and 3%

respectively, is less than 20%

Comparison between VPR and LUT retrievalsThe SO2 and ash masses and fluxes retrieved from the VPR procedure have been compared with the results obtained by applying the established LUT retrieval approach in 2 case studies.

VPR

LUT

2006, December 3rd, at 12:10 UTC, MODIS-Aqualow SO2 plume altitude 3.75 km

2011, October 23rd at 21:30 UTC, MODIS-Terraash and SO2 plume altitude 5.5 km

Total Mass

[t]

3.12.2006

SO2

23.10.2011

SO2

23.10.2011

Ash

VPR 1040 3721 2679

LUT 1268 3379 3383

Mean Flux [t/d]

(v=5 m/s) (v=12 m/s) (v=12 m/s)

VPR 5139 34593 19690

LUT 6099 34682 25050

Flux

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• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

Mie Code (spherical approximation)

Size Distribution

(log-normalMean=1.7,

SD=0.2)

Ash Optical PropertiesSing. Scatt. Albedo Ext. Coeff

Asym. Param.Abs. Coeff.

[Tirelli, 2006]

Ash Refractive Index

SO2 retrieval (7.3, 8.7 m) Ash retrieval (11, 12 m)

ARIA Database

MODISTerra

Eyja eruptionMay 10, 2010

22:45 UTC

Plumeaxis

Plumetransects

Flux computation:Ft = mt * ws

Andesite Obsidian Pumice MinDust Eyja

[courtesy from Taddeucci J. and Misiti V., INGV-Rome]

phase I: 01 April 2010phase IIa: 16-21 April 2010phase IIb: 11 May 2010

[Borisova et al., JGR, 2012]

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Ash characterization for the Eyja 2010 eruption

O

M

P

E

A Andesite-Pollack(1973)

Obsidian-Pollack(1973)

Pumice-Volz(1973)

Mineral Dust-Balkanski(2007)

Eyja-Peters(2013)

A

O

O

A

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Linking the IR transmittance to size and type of volcanic ash particles [Scollo et al., JGR submitted]

Use of the infrared spectroscopy to investigate the spectral signature of volcanic ash particles.As instrument we used a Bruker Equinox-55 FTIR spectrometer in the range 7000-600 cm-1 (1.43-16.67 μm) to analyse the infrared transmittance of ash particles on KBr windows

• Presence of the Christiansen effect (high transmission at a given wavelength in the infrared region)

• Decrease of optical depth with decrease of particle radius ( =-ln(T))

• Defining a and b as the distance in optical depth between the minimum and maximum optical depth values with respect to the continuum, the ratio a/b can be compared with the size of the volcanic ash particles.

~1250 cm-1

R (m)

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• Considering the same refractiv index, the peack of the Christiansen effect remain quite constant for different particles effective radii

• It vary, varying the refractive index

• From the syntetic simulations, the Christiansen peaks are placed at ~1250 and ~ 1359 cm-1 for basaltic glass and obsidian

• Andesite is excluded

• The basaltic glass refractive index gives the best approximation to the laboratory measurements

Basaltic Glass

R(m)

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• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

Etna 2011 activity:12 events, from 12 January to 15 November, characterized by ash, SO2 and condensed water vapour emissions

Condensed Water Vapour effect on ash and SO2 retrievals

It completely cover the ash signal. These volcanic clouds are indistinguishable from meteorological clouds.

13 January 2011, Catania

BTD < - 0.2BTD > 1.5

M(n,m)

re

WV retrieval and correction and SO2 amount[Corradini et al., in preparation]

MODIS10/04/11

12:30UTC

Before Correction

After Correction

n

iiS

imn

mnemnjsM

imnS

imnc w

R

rcRRmn

s1

2

),,(

),(),(,),,(),,(2 ),,(),(

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• Ash and SO2 retrieval from multispectral data in the TIR spectral range

• Error Assessment– Atmospheric and Surface Parameters– Ash Optical properties

• Water/Ice plume retrievals and SO2 correction

• Conclusions and Contribute to the SHIVA project

Overview

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Our approach for ash and SO2 retrievals is based on a massive use of radiative transfer models

The uncertainties on water vapour profile, surface temperature and surface emissivity is reduced by using the VPR approach

[WP3 – Retrieval comparison with other satellite instruments]

The bigger ash and SO2 retrieval errors derive from the uncertainty on ash refractive index

[WP1 – Retrieval error sensitivity considering the uncertainty on size distribution, components, etc.]

A procedure, that uses hyperspectral TIR measurements, has been proposed to investigate the volcanic ash composition

[WP2 – Ground transmittance analysis]

The volcanic cloud water/ice particles are retrieved and the SO2 abundance corrected [WP3 – Comparison with same retrieval made by IASI]

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Thanks for the attention

MODIS Aqua,

13 May 2010, 13: UTC55

MODIS Aqua,

13 May 2010, 13: UTC55

SO

2 M

ass

(t/k

m2)

[IASI: Carboni et al., ACP, 2012

AIRS: Thomas and Prata, ACP, 2011]

Ash = Andesite

Retrieval errors on atmospheric and surface parameters uncertainties for the VPR

procedure