105 research outputs found
Retrieval of SO2 from thermal infrared satellite measurements: correction procedures for the effects of volcanic ash
The simultaneous presence of SO2 and ash in
a volcanic plume can lead to a significant error in the SO2
column abundance retrieval when multispectral Thermal InfraRed
(TIR) data are used. The ash particles within the
plume with effective radii from 1 to 10μm reduce the Top Of
Atmosphere (TOA) radiance in the entire TIR spectral range,
including the channels used for SO2 retrieval. The net effect
is a significant SO2 overestimation.
In this work the interference of ash is discussed and two
correction procedures for satellite SO2 volcanic plume retrieval
in the TIR spectral range are developed to achieve an
higher computational speed and a better accuracy.
The ash correction can be applied when the sensor spectral
range includes the 7.3 and/or 8.7μm SO2 absorption bands,
and the split window bands centered around 11 and 12μm
required for ash retrieval. This allows the possibility of simultaneous
estimation of both volcanic SO2 and ash in the
same data set. The proposed ash correction procedures have
been applied to the Moderate Resolution Imaging Spectroradiometer
(MODIS) and the Spin Enhanced Visible and Infrared
Imager (SEVIRI) measurements. Data collected during
the 24 November 2006 Mt. Etna eruption have been used
to illustrate the technique. The SO2 and ash estimation is carried
out by using a best weighted least squares fit method and
the Brightness Temperature Difference (BTD) procedures,
respectively. The simulated TOA radiance Look-Up Table
(LUT) needed for the SO2 column abundance and the ash
retrievals have been computed using the MODTRAN 4 Radiative
Transfer Model.
The results show the importance of the ash correction on
SO2 retrievals at 8.7μm, where the corrected SO2 column abundance values are less than 50% of the uncorrected values.
The ash correction on SO2 retrieval at 7.3μm is much
less important and only significant for low SO2 column abundances.
Results also show that the simplified and faster
correction procedure underestimates the ash correction compared
with the more time consuming but more accurate correction
procedure. Such underestimation is greater for instruments
having better ground pixel resolution, i.e. greater for
MODIS than for SEVIRI
Wavelength influence in sub-pixel temperature retrieval using the dual-band technique
The thermal model proposed by Crisp and Baloga (1990) for active lava flows considers thermal flux as a function
of the fractional area of two thermally distinct radiant surfaces. In this model, the larger surface area corresponds
to the cooler crust of the flow and the other, much smaller to fractures in the crust. These cracks temperature
is much higher than the crust one and approaches the temperature of the molten or plastic interior flow.
The dual-band method needs two distinct SWIR (short wave infrared) bands to formulate a two equations system
from the simultaneous solution of the Planck equation in each band. The system solutions consist in the crust
temperature and the fractional area of the hot component. The dual band technique originally builds on data acquired
by sensors (such as Landsat TM) with two SWIR bands only. The use of hyperspectral imaging spectrometers
allows us to test the dual-band technique using different wavelengths in the SWIR range of the spectrum.
DAIS 7915 is equipped with 40 bands into the range 1.54-2.49 nm which represent potential input in dual band
calculation. This study aims to compare results derived by inserting assorted couples of wavelengths into the
equation system. The analysis of these data provides useful information on dual-band technique accuracy
A new simplified approach for simultaneous retrieval of SO2 and ash content of tropospheric volcanic clouds: an application to the Mt Etna volcano
A new procedure is presented for simultaneous estimation of SO2 and ash abundance in a volcanic plume, using
thermal infrared (TIR) MODIS data. Plume altitude and temperature are the only two input parameters required to run
the procedure, while surface emissivity, temperature, atmospheric profiles, ash optical properties, and radiative transfer
models are not necessary to perform the atmospheric corrections. The procedure gives the most reliable results when the surface under the plume is uniform, for example above the ocean, but still produces fairly good estimates in more challenging and not easily modelled conditions, such as above
land or meteorological cloud layers. The developed approach was tested on the Etna volcano. By linearly interpolating the radiances surrounding a detected volcanic plume, the volcanic plume removal (VPR)
procedure described here computes the radiances that would have been measured by the sensor in the absence of a plume, and reconstructs a new image without plume. 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. 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.
Bands 31 and 32 are SO2 transparent and, from their transmittances, the effective ash particle radius (Re), and aerosol optical depth at 550 nm (AOD550) are computed. A simple relation between the ash transmittances of bands 31 and 29
is demonstrated and used for SO2 columnar content (cs) estimation. Comparing the results of the VPR procedure with
MODTRAN simulations for more than 200 000 different cases, the frequency distribution of the differences shows the following: the Re error is less than ±0.5 μm in more than
60% of cases; the AOD550 error is less than ±0.125 in 80% of cases; the cs error is less than ±0.5 gm−2 in more than 60% of considered cases. The VPR procedure was applied in two case studies of recent eruptions occurring at the Mt Etna volcano, Italy, and successfully compared with the results obtained from the established SO2 and ash assessments based on look-up tables (LUTs). Assessment of the sensitivity to
the plume altitude uncertainty is also made.
The VPR procedure is simple, extremely fast, and can be adapted to other ash types and different volcanoes
Aerosol optical thickness of Mt. Etna volcanic plume retrieved by means of the Airborne Multispectral Imaging Spectrometer (MIVIS)
Within the framework of the European MVRRS project (Mitigation of Volcanic Risk by Remote Sensing
Techniques), in June 1997 an airborne campaign was organised on Mt. Etna to study different characteristics of
the volcanic plume emitted by the summit craters in quiescent conditions. Digital images were collected with
the Airborne Multispectral Imaging Spectrometer (MIVIS), together with ground-based measurements. MIVIS
images were used to calculate the aerosol optical thickness of the volcanic plume. For this purpose, an inversion
algorithm was developed based on radiative transfer equations and applied to the upwelling radiance data measured
by the sensor. This article presents the preliminary results from this inversion method. One image was selected
following the criteria of concomitant atmospheric ground-based measurements necessary to model the atmosphere,
plume centrality in the scene to analyse the largest plume area and cloudless conditions. The selected image was
calibrated in radiance and geometrically corrected. The 6S (Second Simulation of the Satellite Signal in the Solar
Spectrum) radiative transfer model was used to invert the radiative transfer equation and derive the aerosol optical
thickness. The inversion procedure takes into account both the spectral albedo of the surface under the plume and
the topographic effects on the refl ected radiance, due to the surface orientation and elevation. The result of the
inversion procedure is the spatial distribution of the plume optical depth. An average value of 0.1 in the wavelength
range 454-474 nm was found for the selected measurement day
A new simplified approach for simultaneous retrieval of SO2 and ash content of tropospheric volcanic clouds: an application to the Mt Etna volcano
A new procedure is presented for simultaneous estimation
of SO2 and ash abundance in a volcanic plume, using
thermal infrared (TIR) MODIS data. Plume altitude and
temperature are the only two input parameters required to run
the procedure, while surface emissivity, temperature, atmospheric
profiles, ash optical properties, and radiative transfer
models are not necessary to perform the atmospheric corrections.
The procedure gives the most reliable results when the
surface under the plume is uniform, for example above the
ocean, but still produces fairly good estimates in more challenging
and not easily modelled conditions, such as above
land or meteorological cloud layers. The developed approach
was tested on the Etna volcano.
By linearly interpolating the radiances surrounding a detected
volcanic plume, the volcanic plume removal (VPR)
procedure described here computes the radiances that would
have been measured by the sensor in the absence of a plume,
and reconstructs a new image without plume. 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 \u3bcm) by applying a simplified model consisting of a
uniform plume at a fixed altitude and temperature. 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.
Bands 31 and 32 are SO2 transparent and, from their transmittances,
the effective ash particle radius (Re), and aerosol
optical depth at 550 nm (AOD550) are computed. A simple
relation between the ash transmittances of bands 31 and 29
is demonstrated and used for SO2 columnar content (cs) estimation.
Comparing the results of the VPR procedure with
MODTRAN simulations for more than 200 000 different
cases, the frequency distribution of the differences shows the
following: the Re error is less than \ub10.5 \u3bcm in more than
60% of cases; the AOD550 error is less than \ub10.125 in 80%
of cases; the cs error is less than \ub10.5 gm 122 in more than
60% of considered cases. The VPR procedure was applied in
two case studies of recent eruptions occurring at the Mt Etna
volcano, Italy, and successfully compared with the results obtained
from the established SO2 and ash assessments based
on look-up tables (LUTs). Assessment of the sensitivity to
the plume altitude uncertainty is also made.
The VPR procedure is simple, extremely fast, and can be
adapted to other ash types and different volcanoes
Spectral emissivity and temperature maps of the Solfatara crater from DAIS hyperspectral images
Quantitative maps of surface temperature and spectral emissivity have been retrieved on the Solfatara crater at Pozzuoli (Naples) from remote sensing hyperspectral data. The present study relies on thermal infrared images collected on July 27, 1997 by the DAIS hyperspectral sensor owned by the German aerospace center (DLR). The Emissivity Spectrum Normalization method was used to make temperature and emissivity estimates. Raw data were previously transformed in radiance and corrected for the atmospheric contributions using the MODTRAN radiative transfer code and the sensor response functions. During the DAIS flight a radiosonde was launched to collect the atmospheric profiles of pressure, temperature and humidity used as input to the code. Retrieved temperature values are in good agreement with temperature measurements performed in situ during the campaign. The spectral emissivity map was used to classify the image in different geo-mineralogical units with the Spectral Angle Mapper method. Areas of geologic interest were previously selected using a mask obtained from an NDVI image calculated with two channels of the visible (red) and the near infrared respectively
Spectral emissivity and temperature maps of the Solfatara crater from DAIS hyperspectral images
Quantitative maps of surface temperature and spectral emissivity have been retrieved on the
Solfatara crater at Pozzuoli (Naples) from remote sensing hyperspectral data. The present study
relies on thermal infrared images collected on July 27, 1997 by the DAIS hyperspectral sensor,
owned by the German aerospace center (DLR). The Emissivity Spectrum Normalization method
was used to make temperature and emissivity estimates. Raw data were previously transformed in
radiance and corrected for the atmospheric contributes using the MODTRAN radiative transfer code
and the sensor response functions. During the DAIS flight a radiosonde was launched to collect the
atmospheric profiles of pressure, temperature and humidity used as input to the code. Retrieved
temperature values are in good agreement with temperature measures performed in situ during the
campaign. The spectral emissivity map was used to classify the image in different geomineralogical
units with the Spectral Angle Mapper method. Areas of geologic interest were
previously selected using a mask obtained from an NDVI image calculated with two channels of the
visible (red) and the near infrared respectively
Insights into rapid explosive volcanic processes from ground- and space-based intraday SO2 flux measurements
Observations of volcanic degassing yield insights into the sub-surface magmatic processes which control volcanic activity during both quiescent and eruptive phases. By combining information on volcanic gas flux with constraints on original dissolved volatile contents the volume of degassing magma can be determined. Comparisons between the volume of degassing magma, erupted volume, and ground deformation allow mass balance calculations to be performed, providing insight into the shallow volcanic plumbing system dynamics. For these reasons there is great interest in improving the quality and frequency of volcanic gas flux measurements.
Ultraviolet and infrared remote sensing techniques allow determination of SO2 column amounts within volcanic plumes. By calculating SO2 column amounts in a profile orthogonal to the plume-wind direction and multiplying the integrated SO2 cross section by the wind speed the SO2 emission rate can be retrieved. There are currently three main approaches for determining volcanic SO2 fluxes; (i) ground-based mini-DOAS systems, (ii) ground-based SO2 imaging cameras, and (iii) satellite-based infrared and ultraviolet imaging.
Here we examine SO2 fluxes obtained by the Flux Automatic Measurement (FLAME) network of scanning mini-DOAS instruments installed at Mt. Etna and by the MODIS instrument aboard the NASA EOS satellite AQUA during the 2006 eruption of Mt. Etna, Sicily, Italy. Mt. Etna produced a highly variable eruptive activity from the South-East crater, characterised by explosive sequences, which could be either ash-rich or ash-poor, lava effusion, partial flank collapse and periods of quiescence. We examined intraday variations in SO2 flux measured with FLAME and MODIS during both ash-rich and ash-poor explosive phases. In general, good agreement was found between the datasets. Of particular interest was the successful recalculation and validation of temporal variations in SO2 flux recorded in a single image from MODIS. By examining the temporal evolution of gas emissions our results provide insight in the eruption mechanism driving the explosive activity
MODIS volcanic ash retrievals vs FALL3D transport model: a quantitative comparison.
Satellite retrievals and transport models represents the key tools to monitor the volcanic clouds evolution. Because of the harming effects of fine ash particles on aircrafts, the real-time tracking and forecasting of volcanic clouds is key for aviation safety. Together with the security reasons also the economical consequences of a disruption of airports must be taken into account. The airport closures due to the recent Icelandic Eyjafjöll eruption caused millions of passengers to be stranded not only in Europe, but across the world. IATA (the International Air Transport Association) estimates that the worldwide airline industry has lost a total of about 2.5 billion of Euro during the disruption.
Both security and economical issues require reliable and robust ash cloud retrievals and trajectory forecasting. The intercomparison between remote sensing and modeling is required to assure precise and reliable volcanic ash products.
In this work we perform a quantitative comparison between Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of volcanic ash cloud mass and Aerosol Optical Depth (AOD) with the FALL3D ash dispersal model.
MODIS, aboard the NASA-Terra and NASA-Aqua polar satellites, is a multispectral instrument with 36 spectral bands operating in the VIS-TIR spectral range and spatial resolution varying between 250 and 1000 m at nadir. The MODIS channels centered around 11 and 12 micron have been used for the ash retrievals through the Brightness Temperature Difference algorithm and MODTRAN simulations. FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of volcanic particles that outputs, among other variables, cloud column mass and AOD. Three MODIS images collected the October 28, 29 and 30 on Mt. Etna volcano during the 2002 eruption have been considered as test cases.
The results show a general good agreement between the retrieved and the modeled volcanic clouds in the first 300 km from the vents. Even if the modeled volcanic cloud area is systematically wider than the retrieved area, the ash total mass is comparable and varies between 35 and 60 kt and between 20 and 42 kt for FALL3D and MODIS respectively. The mean AOD values are in good agreement and approximately equal to 0.8.
When the whole volcanic clouds are considered the ash areas and the total ash masses, computed by FALL3D model are significantly greater than the same parameters retrieved from the MODIS data, while the mean AOD values remain in a very good agreement and equal to about 0.6.
The volcanic cloud direction in its distal part is not coincident for the 29 and 30 October 2002 images due to the difference between the real and the modeled local wind fields. Finally the MODIS maps show regions of high mass and AOD due to volcanic puffs not modeled by FALL3D
Volcanic ash detection and retrievals using MODIS data by means of neural networks
Volcanic ash clouds detection and retrieval represent a key issue for aviation safety due to the harming effects on aircraft. A lesson learned from the recent Eyjafjallajokull eruption is the need to obtain accurate and reliable retrievals on a real time basis. <br><br> In this work we have developed a fast and accurate Neural Network (NN) approach to detect and retrieve volcanic ash cloud properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) data in the Thermal InfraRed (TIR) spectral range. Some measurements collected during the 2001, 2002 and 2006 Mt. Etna volcano eruptions have been considered as test cases. <br><br> The ash detection and retrievals obtained from the Brightness Temperature Difference (BTD) algorithm are used as training for the NN procedure that consists in two separate steps: ash detection and ash mass retrieval. The ash detection is reduced to a classification problem by identifying two classes: "ashy" and "non-ashy" pixels in the MODIS images. Then the ash mass is estimated by means of the NN, replicating the BTD-based model performances. A segmentation procedure has also been tested to remove the false ash pixels detection induced by the presence of high meteorological clouds. The segmentation procedure shows a clear advantage in terms of classification accuracy: the main drawback is the loss of information on ash clouds distal part. <br><br> The results obtained are very encouraging; indeed the ash detection accuracy is greater than 90%, while a mean RMSE equal to 0.365 t km<sup>−2</sup> has been obtained for the ash mass retrieval. Moreover, the NN quickness in results delivering makes the procedure extremely attractive in all the cases when the rapid response time of the system is a mandatory requirement
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