1 Volcanic ash and SO 2 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 SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 correction Conclusions and Contribute to the SHIVA project Overview

3 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 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 = T b (11  m) - T b (12  m) BTD < 0 volcanic ash BTD > 0 meteo clouds The ash retrievals are based on computing the simulated inverted arches curves “BTD- T b (11  m)” varying the AOD (  ) and the particles effective radius (r e ) [ Wen and Rose, JGR, 1994 ] rere  M (n,m)

5 Ash Mass Effective Radius AOD

SO 2 retrieval in the TIR spectral range Ash effect on SO 2 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 SO 2 retrieval  The net effect is a significant SO 2 overestimation [Corradini et al., AMT, 2008; Kearney and Watson, JGR, 2008] SO 2 retrieval MODIS TIR response functions  Procedure [Realmuto et al., JGR, 1994; Teggi et al., JGR, 1999] 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 = t

8 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 correction Conclusions and Contribute to the SHIVA project Overview

TOA Radiance computation Radiative Transfer Model The only particles detectable in the TIR spectral range 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 geometry Volc. cloud geometry P, T, W Spectral surface emissivity and temperature

Each RTM input parameter has an uncertainty that lead to ash and SO 2 retrieval errors W Ts  hp tp opt. prop RTM Input parameters (20%) (2K) (3%) (0.5 km) (50%) (type) RTM Parameter Uncertainty % Retrieval Errors Ash mass [Corradini et al., 2009] SO 2 (8.7  m) [Corradini et al., 2010 Pugnaghi et al., 2013]

11 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice retrievals and SO 2 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 12 Volcanic Plume Removal (VPR) procedure for the simultaneous retrievals of ash and SO 2 [Pugnaghi et al., 2013]

13 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 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 4) From the transmittance of the channels centered around 11 and 12  m: the AOD 31,32 depends linearly on the plume AOD550 (with null offset), but with a slope which is a function of the particle size where

14 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 SO 2 errors due to the uncertainty of the mentioned parameters are drastically reduced (Pugnaghi et al., in preparation) The estimated overall ash and SO 2 mass retrieval errors considering W, Ts and  uncertainties of 20%, 2K and 3% respectively, is less than 20%

Comparison between VPR and LUT retrievals The SO 2 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 3 rd, at 12:10 UTC, MODIS-Aqua low SO 2 plume altitude 3.75 km 2011, October 23 rd at 21:30 UTC, MODIS-Terra ash and SO 2 plume altitude 5.5 km Total Mass [t] SO SO Ash VPR LUT Mean Flux [t/d] (v=5 m/s)(v=12 m/s) VPR LUT Flux

16 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 correction Conclusions and Contribute to the SHIVA project Overview

Mie Code (spherical approximation) Size Distribution (log-normal Mean=1.7, SD=0.2) Ash Optical Properties Sing. Scatt. AlbedoExt. Coeff Asym. Param. Abs. Coeff. [Tirelli, 2006]

Ash Refractive Index SO 2 retrieval (7.3, 8.7  m ) Ash retrieval (11, 12  m ) ARIA Database

MODIS Terra Eyja eruption May 10, :45 UTC Plume axis Plume transects Flux computation: F t = m t * w s Andesite Obsidian Pumice MinDust Eyja

[courtesy from Taddeucci J. and Misiti V., INGV-Rome] phase I: 01 April 2010 phase IIa: April 2010 phase IIb: 11 May 2010 [Borisova et al., JGR, 2012] 20 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

21 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 cm-1 ( 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)

22 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)

23 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 correction Conclusions and Contribute to the SHIVA project Overview

Etna 2011 activity: 12 events, from 12 January to 15 November, characterized by ash, SO 2 and condensed water vapour emissions Condensed Water Vapour effect on ash and SO 2 retrievals It completely cover the ash signal. These volcanic clouds are indistinguishable from meteorological clouds. 13 January 2011, Catania

BTD < BTD > 1.5 M (n,m) rere  WV retrieval and correction and SO 2 amount [Corradini et al., in preparation] MODIS 10/04/11 12:30UTC

Before Correction After Correction

27 Ash and SO 2 retrieval from multispectral data in the TIR spectral range Error Assessment – Atmospheric and Surface Parameters – Ash Optical properties Water/Ice plume retrievals and SO 2 correction Conclusions and Contribute to the SHIVA project Overview

28 Our approach for ash and SO 2 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 SO 2 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 SO 2 abundance corrected [WP3 – Comparison with same retrieval made by IASI]

29 Thanks for the attention

MODIS Aqua, 13 May 2010, 13: UTC55 MODIS Aqua, 13 May 2010, 13: UTC55 SO 2 Mass (t/km 2 ) [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