The ash mass load of volcanic plumes: retrievals from a new millimeter-wave radar at Stromboli and Sabancaya volcanoes

In the framework of the French Government Laboratory of Excellence ClerVolc initiative, two experiments using a new millimeter-wave radar were carried out to retrieve various physical properties of the ash plumes, especially the mass loading parameters which are critical for the modelling of ash dispersal, as well as to study the internal dynamics of the plumes and their fallout. First measurements at Stromboli in 2015 using a 95 GHz cloud radar prototype with a fixed beam pointing above the crater characterized the distribution of plume internal reflectivities, plume widths and durations at unprecedented space-time resolutions. Combining radar in situ measurements with data modelling from a disdrometer and ash sampling on the ground further allowed the retrieval of ash concentration and gradients inside the plumes, and sometimes proximal fallout. Plume maximum ash concentration range from 1 mg/m3 to about 1 g/m3. Structuration of ash concentration with variations by a factor of 3 was also found to occur inside the falling ash in correlation with variations in the sedimentation rate measured on the ground by the disdrometer. New results from radar measurements inside stronger plumes and fallout at Sabancaya volcano (Peru, May 2018) using volume scans will also be presented

Ash concentration of Sabancaya volcanic plumes retrieved from a 95 GHz radar and a disdrometer

We have carried out an experiment using a 3.2 mm-wavelength scanning Doppler radar and a laser disdrometer to investigate ash plumes of Sabancaya volcano (Peru) in May 2018. Our main objectives were to retrieve the mass loading parameters (concentration, mass flux) which are critical for the modelling of ash dispersal, as well as to study the dimensions and internal dynamics of the eruptive columns, plumes and fallout. The radar and the disdrometer were respectively located at 4.5 km NNE and 4.5 km E from the vent. Multiple radar sounding configurations were tested either in fixed-pointing mode, generally close to the source, or using scans across various regions of the plumes. Particle Size Distribution, shapes and density were characterized from microphysical analyses, sieving and water pycnometry of ash samples collected on the ground. A Parsivel2 disdrometer also recorded the sizes, and settling velocities of fallout, allowing us to estimate sedimentation rates on the ground and to derive an empirical law relating calculated ash concentrations and reflectivities. Comparing the latter with reflectivities measured by the radar at unprecedented space-time resolutions (down to 12.5 m and 0.25 s) allowed us to obtain the internal mass distribution of eruptive columns, plumes, and fallout at various distances from the emission source

The source term of tephra plumes : radar applications at Etna and Stromboli volcanoes (Italy)

Les panaches volcaniques de téphras constituent un des aléas volcaniques majeurs. Pour prévoir leur dispersion et les zones d'impacts de leurs retombées, des modèles numériques sont utilisés en opérationnel et basés sur des paramètres éruptifs, regroupés sous la notion de terme source, caractérisant l'émission des panaches. L'ensemble du terme source est cependant difficile à mesurer en temps réel. C'est pourquoi les modèles de dispersion sont souvent basés sur des scénarios d'éruptions passées et utilisent des lois empiriques reliant la hauteur des panaches avec les flux massiques à la source. Cependant, les résultats qui découlent de ces modèles sont peu contraints, moyennés sur la durée des éruptions, et souffrent de larges incertitudes. Dans cette optique, les radars Doppler, capables de sonder l'intérieur des colonnes éruptives avec des échelles spatio-temporelles fines, peuvent fournir des contraintes cruciales sur le terme source des panaches en temps réel. Ce travail de thèse traite des applications de radars volcanologiques dédiés, potentiellement transposables aux radars météorologiques communément utilisés, afin de fournir des paramètres éruptifs à la source des panaches de téphras en surveillance opérationnelle mais également pour contraindre la dynamique des colonnes éruptives et les charges internes des panaches et de leurs retombées. Une campagne de mesures au volcan Stromboli a permis de montrer les capacités d'un couplage innovant entre un disdromètre optique (Parsivel2) avec un nouveau radar Doppler à onde millimétrique (Mini-BASTA). Grâce à l'excellente résolution spatio-temporelle de Mini-BASTA (12,5 m et 1 s), des figures intermittentes de sédimentation ont été observées dans les retombées de panaches transitoires dilués. Observées également au disdromètre mesurant la vitesse et la taille des retombées, ces figures ont été reproduites en laboratoire grâce à un modèle analogique. Un modèle conceptuel de formation de thermiques de sédimentation inversés est proposé pour expliquer ces figures et implique que les processus menant à une sédimentation irrégulière typique des panaches soutenus et concentrés peuvent s'appliquer à des panaches dilués, y compris ceux issus d'éruptions Stromboliennes normales en régime transitoire. Ensuite, une caractérisation physique d'un grand nombre de particules de cendres échantillonnées à Stromboli a permis de valider les mesures de tailles et de vitesses terminales de chutes par disdromètre sur le terrain et en laboratoire, justifiant par ailleurs son utilisation opérationnelle. A partir de ces contraintes, une loi reliant les concentrations de cendres avec les facteurs de réflectivité calculés a pu être comparée aux mesures radar in situ. Les concentrations internes modale et maximale des panaches de Stromboli sont respectivement autour de 1 × 10-5 kg m-3 et 7,45 × 10-4 kg m-3, largement supérieures au seuil fixé pour la sécurité aérienne. Les concentrations en cendres des retombées s’étalent entre 1,87 × 10-8 - 2,42 × 10-6 kg m-3 avec un mode vers 4 × 10-7 kg m-3.Finalement, ce travail de thèse montre les applications opérationnelles du radar UHF VOLDORAD 2B dans le cadre de la surveillance de l'activité de l'Etna. Une méthodologie, applicable à tout radar Doppler, a été développée pour obtenir des flux de masse de téphras en temps réel à partir d’un proxy de masse, uniquement basé sur les vitesses d'éjection et puissances mesurées, calibré avec un modèle de colonne tenant compte de l'influence du vent sur les panaches. La gamme de flux trouvée pour 47 paroxysmes entre 2011 et 2015 s’étend de 2.96 × 104 à 3.26 × 106 kg s-1. A partir d’un autre modèle de colonne éruptive, Plume-MoM, les flux radar ont permis de modéliser des hauteurs des panaches de téphras émis lors de quatre paroxysmes de l'Etna cohérentes avec les observations faites en temps réel par imagerie visible et par radar en bande-X. (...)Volcanic tephra plumes are one of the major volcanic hazards. To forecast their dispersion and the impact zones of their fallout, the numerical models used in operational monitoring are based on eruptive parameters, called the source term, characterizing the plume emission. Source term parameters are challenging to measure in real time. This is why dispersion models are often based on past eruptive scenarios and use empirical laws that relate plume heights to source mass fluxes. However, the model outputs are not well constrained, averaged over the eruption duration, and suffer from large uncertainties. In this topic, Doppler radars are capable of probing the interior of eruptive columns and plumes at high space-time resolution and can provide crucial constraints on the source term in real time. This thesis deals with applications in operational monitoring of dedicated volcanological radars, potentially transposable to most common meteorological radars, to provide eruptive parameters at the source of tephra plumes but also to constrain the dynamics and internal mass load of eruptive columns, volcanic plumes and their fallout.A measurement campaign at Stromboli volcano has shown the capabilities of an innovative coupling between an optical disdrometer (Parsivel2) and a new 3-mm wave Doppler radar (Mini-BASTA). Owing to its high spatio-temporal resolution (12.5 m and 1 s), intermittent sedimentation patterns were observed in the fallout of dilute transient plumes typical of normal strombolian activity. These features, also recorded with the disdrometer, measuring the particle settling speeds and sizes, were reproduced in the laboratory using an analog model. A conceptual model for the formation of reversed sedimentation thermals is proposed to explain these features. It implies that processes leading to irregular sedimentation typical of sustained concentrated strong plumes can be applied to dilute weak plumes, including those formed by normal transient Strombolian activity. Then, a physical characterization of a large number of ash particles sampled at Stromboli allowed the validation of particle size and terminal velocity measurements by the disdrometer in the field and in the laboratory, arguing in favor of its operational use. Then, a physical characterization of a large number of ash particles sampled at Stromboli allowed to validate the measurements of size and terminal velocity of falls by disdrometer in the field and in laboratory, justifying also its operational use. From these constraints, a law relating ash concentrations with calculated reflectivity factors was found and compared to in situ radar measurements inside ash plumes and fallout. The modal and maximum internal concentrations of Strombolian plumes are at about 1 × 10-5 kg m-3 and 7.5 × 10-4 kg m-3 respectively, well above the threshold for aviation safety. Ash concentrations in the fallout range from 1.9× 10-8 to 2.4 × 10-6 kg m-3 with a mode at about 4 × 10-7 kg m-3.Finally, this thesis work shows operational applications of the UHF VOLDORAD 2B radar for the monitoring of explosive activity at Etna. A methodology, applicable to any Doppler radar, has been developed to obtain tephra mass eruption rates in real time from a mass proxy, based only on measured ejection velocities and power, and calibrated with an eruptive column model taking crosswinds into account. Tephra mass fluxes found for 47 paroxysms between 2011 and 2015 range from 3 × 104 to over 3 × 106 kg s-1. Then, tephra plumes heights of four Etna paroxysms were simulated using the eruptive column model Plume-MoM from the radar-derived mass eruption rates and were found consistent with real-time observations made by visible imagery and by X-band radar. This last part demonstrates the capabilities of VOLDORAD 2B to provide quantitative input parameters for dispersion models in the case of future Etna paroxysms. (...

Le terme source des panaches de téphras : applications radars aux volcans Etna et Stromboli (Italie)

Volcanic tephra plumes are one of the major volcanic hazards. To forecast their dispersion and the impact zones of their fallout, the numerical models used in operational monitoring are based on eruptive parameters, called the source term, characterizing the plume emission. Source term parameters are challenging to measure in real time. This is why dispersion models are often based on past eruptive scenarios and use empirical laws that relate plume heights to source mass fluxes. However, the model outputs are not well constrained, averaged over the eruption duration, and suffer from large uncertainties. In this topic, Doppler radars are capable of probing the interior of eruptive columns and plumes at high space-time resolution and can provide crucial constraints on the source term in real time. This thesis deals with applications in operational monitoring of dedicated volcanological radars, potentially transposable to most common meteorological radars, to provide eruptive parameters at the source of tephra plumes but also to constrain the dynamics and internal mass load of eruptive columns, volcanic plumes and their fallout.A measurement campaign at Stromboli volcano has shown the capabilities of an innovative coupling between an optical disdrometer (Parsivel2) and a new 3-mm wave Doppler radar (Mini-BASTA). Owing to its high spatio-temporal resolution (12.5 m and 1 s), intermittent sedimentation patterns were observed in the fallout of dilute transient plumes typical of normal strombolian activity. These features, also recorded with the disdrometer, measuring the particle settling speeds and sizes, were reproduced in the laboratory using an analog model. A conceptual model for the formation of reversed sedimentation thermals is proposed to explain these features. It implies that processes leading to irregular sedimentation typical of sustained concentrated strong plumes can be applied to dilute weak plumes, including those formed by normal transient Strombolian activity. Then, a physical characterization of a large number of ash particles sampled at Stromboli allowed the validation of particle size and terminal velocity measurements by the disdrometer in the field and in the laboratory, arguing in favor of its operational use. Then, a physical characterization of a large number of ash particles sampled at Stromboli allowed to validate the measurements of size and terminal velocity of falls by disdrometer in the field and in laboratory, justifying also its operational use. From these constraints, a law relating ash concentrations with calculated reflectivity factors was found and compared to in situ radar measurements inside ash plumes and fallout. The modal and maximum internal concentrations of Strombolian plumes are at about 1 × 10-5 kg m-3 and 7.5 × 10-4 kg m-3 respectively, well above the threshold for aviation safety. Ash concentrations in the fallout range from 1.9× 10-8 to 2.4 × 10-6 kg m-3 with a mode at about 4 × 10-7 kg m-3.Finally, this thesis work shows operational applications of the UHF VOLDORAD 2B radar for the monitoring of explosive activity at Etna. A methodology, applicable to any Doppler radar, has been developed to obtain tephra mass eruption rates in real time from a mass proxy, based only on measured ejection velocities and power, and calibrated with an eruptive column model taking crosswinds into account. Tephra mass fluxes found for 47 paroxysms between 2011 and 2015 range from 3 × 104 to over 3 × 106 kg s-1. Then, tephra plumes heights of four Etna paroxysms were simulated using the eruptive column model Plume-MoM from the radar-derived mass eruption rates and were found consistent with real-time observations made by visible imagery and by X-band radar. This last part demonstrates the capabilities of VOLDORAD 2B to provide quantitative input parameters for dispersion models in the case of future Etna paroxysms. (...)Les panaches volcaniques de téphras constituent un des aléas volcaniques majeurs. Pour prévoir leur dispersion et les zones d'impacts de leurs retombées, des modèles numériques sont utilisés en opérationnel et basés sur des paramètres éruptifs, regroupés sous la notion de terme source, caractérisant l'émission des panaches. L'ensemble du terme source est cependant difficile à mesurer en temps réel. C'est pourquoi les modèles de dispersion sont souvent basés sur des scénarios d'éruptions passées et utilisent des lois empiriques reliant la hauteur des panaches avec les flux massiques à la source. Cependant, les résultats qui découlent de ces modèles sont peu contraints, moyennés sur la durée des éruptions, et souffrent de larges incertitudes. Dans cette optique, les radars Doppler, capables de sonder l'intérieur des colonnes éruptives avec des échelles spatio-temporelles fines, peuvent fournir des contraintes cruciales sur le terme source des panaches en temps réel. Ce travail de thèse traite des applications de radars volcanologiques dédiés, potentiellement transposables aux radars météorologiques communément utilisés, afin de fournir des paramètres éruptifs à la source des panaches de téphras en surveillance opérationnelle mais également pour contraindre la dynamique des colonnes éruptives et les charges internes des panaches et de leurs retombées. Une campagne de mesures au volcan Stromboli a permis de montrer les capacités d'un couplage innovant entre un disdromètre optique (Parsivel2) avec un nouveau radar Doppler à onde millimétrique (Mini-BASTA). Grâce à l'excellente résolution spatio-temporelle de Mini-BASTA (12,5 m et 1 s), des figures intermittentes de sédimentation ont été observées dans les retombées de panaches transitoires dilués. Observées également au disdromètre mesurant la vitesse et la taille des retombées, ces figures ont été reproduites en laboratoire grâce à un modèle analogique. Un modèle conceptuel de formation de thermiques de sédimentation inversés est proposé pour expliquer ces figures et implique que les processus menant à une sédimentation irrégulière typique des panaches soutenus et concentrés peuvent s'appliquer à des panaches dilués, y compris ceux issus d'éruptions Stromboliennes normales en régime transitoire. Ensuite, une caractérisation physique d'un grand nombre de particules de cendres échantillonnées à Stromboli a permis de valider les mesures de tailles et de vitesses terminales de chutes par disdromètre sur le terrain et en laboratoire, justifiant par ailleurs son utilisation opérationnelle. A partir de ces contraintes, une loi reliant les concentrations de cendres avec les facteurs de réflectivité calculés a pu être comparée aux mesures radar in situ. Les concentrations internes modale et maximale des panaches de Stromboli sont respectivement autour de 1 × 10-5 kg m-3 et 7,45 × 10-4 kg m-3, largement supérieures au seuil fixé pour la sécurité aérienne. Les concentrations en cendres des retombées s’étalent entre 1,87 × 10-8 - 2,42 × 10-6 kg m-3 avec un mode vers 4 × 10-7 kg m-3.Finalement, ce travail de thèse montre les applications opérationnelles du radar UHF VOLDORAD 2B dans le cadre de la surveillance de l'activité de l'Etna. Une méthodologie, applicable à tout radar Doppler, a été développée pour obtenir des flux de masse de téphras en temps réel à partir d’un proxy de masse, uniquement basé sur les vitesses d'éjection et puissances mesurées, calibré avec un modèle de colonne tenant compte de l'influence du vent sur les panaches. La gamme de flux trouvée pour 47 paroxysmes entre 2011 et 2015 s’étend de 2.96 × 104 à 3.26 × 106 kg s-1. A partir d’un autre modèle de colonne éruptive, Plume-MoM, les flux radar ont permis de modéliser des hauteurs des panaches de téphras émis lors de quatre paroxysmes de l'Etna cohérentes avec les observations faites en temps réel par imagerie visible et par radar en bande-X. (...

New insights into real-time detection of tephra grainsize, settling velocity and sedimentation rate

Characterizing the size and settling velocity of pyroclastic fragments injected into the atmosphere during volcanic eruptions (i.e., tephra) is crucial to the forecasting of plume and cloud dispersal. Optical disdrometers have been integrated into volcano monitoring networks worldwide in order to best constrain these parameters in real time. Nonetheless, their accuracy during tephra fallout still needs to be assessed. A significant complication is the occurrence of particle aggregates that modify size and velocity distributions of falling tephra. We made the first use of the Thies Clima Laser Precipitation Monitor (LPM) for tephra-fallout detection at Sakurajima volcano (Japan), which is characterized by a lower size detection window with respect to more commonly used disdrometers (e.g., Parsivel 2 ) and can more easily distinguish different falling objects. For the first time, individual particles have been distinguished from most aggregates based on disdrometer data, with the potential to provide useful grain-size information in real time. In case of negligible aggregation, LPM and collected sample-based estimates are in agreement for both grain-size and sedimentation rate. In case of significant aggregation, particle shape analyses and a dedicated drag equation are used to filter out aggregates from LPM data that also provide good agreement with collected tephra samples. </p

Ground-based Remote Sensing and Uncertainty Analysis of the Mass Eruption Rate associated with the 3-5 December 2015 Paroxysms of Mt Etna

During explosive eruptions, the real-time estimation of the mass eruption rate (MER) is challenging although crucial to mitigate the impact of erupted tephra. Microwave radar techniques at L and/or X bands, as well as thermal infrared imagery, can provide a reliable MER estimation in real time. Using lava fountains of 3-5 December 2015 at Mt. Etna (Italy) as test cases, we investigate the differences among all these remote sensing methods and introduce a new approach, called the Near Source Approach (NSA) using only X-band radar data. We also extend the Volcanic Advanced Radar Retrieval (VARR) methodology to estimate the gas-tephra mixture density near the volcanic crater. The analysis of uncertainty is carried out comparing the NSA with the Mass Continuity Approach (MCA), Top Plume Approach (TPA) and Surface Flux approach (SFA), already used to estimate the MER of other Etna explosive events. The analysis allows us to identify the optimal real-time MER retrieval strategy, showing the potential and limitations of each method. We show that the MCA method, entirely based on the Xband radar data processing, is the best strategy with a percentage uncertainty in the MER estimation of 22.3%, whereas other approaches exhibit e a higher uncertainty (26.4% for NSA, 30% for TPA and 31.6% for SFA)

Tephra characterization and multi-disciplinary determination of Eruptive Source Parameters of a weak paroxysm at Mount Etna (Italy)

The determination of Eruptive Source Parameters (ESPs) is a major challenge especially for weak volcanic explosions associated with poorly exposed tephra-fallout deposits. In such a case, the combination of deposit analyses and remote sensing observations can provide fundamental insights. We use the 29 August 2011 weak paroxysm at Mount Etna (Italy) as a case study to discuss some of the challenges associated with multi-disciplinary determination of ESPs of poorly exposed tephra-fallout deposits. First, we have determined the erupted mass from a combination of field and synthetic data to fill a significant gap in data sampling; synthetic data have been derived based on extrapolation of field observations and validated based on comparisons with other tephra deposits at Etna and TEPHRA2 modelling. Second, we have combined the estimates of erupted mass and grain-size distribution as derived both from deposit observations and satellite retrievals. Analytical modelling was applied to characterize the size fractions most likely represented in satellite retrievals and tephra deposits, respectively. In addition, the Rosin-Rammler distribution fitting is shown to inform on missing parts of the grain-size distributions and reproduce a tail of very fine ash (1–20 μm) whose mass proportion is close to the satellite estimates (1.3–1.6% versus 1.9%, respectively). Finally, it was found that this very-fine-ash fraction increases as a function of satellite-derived Mass Eruption Rate for a set of eruptions for which independent estimates are available. This critical combination of field observations, analytical modelling and satellite retrievals demonstrates the potential and importance of multidisciplinary strategies for the derivation of ESPs even for small-size explosive events and poorly exposed deposits such as that of the 29 August 2011 paroxysm of Mt. Etna

Ash concentration of Sabancaya volcanic plumes retrieved from a 95 GHzradar and a disdrometer

International audienceWe have carried out an experiment using a 3.2 mm-wavelength scanning Doppler radar and a laser disdrometerto investigate ash plumes of Sabancaya volcano (Peru) in May 2018. Our main objectives were to retrieve themass loading parameters (concentration, mass flux) which are critical for the modelling of ash dispersal, as wellas to study the dimensions and internal dynamics of the eruptive columns, plumes and fallout. The radar andthe disdrometer were respectively located at 4.5 km NNE and 4.5 km E from the vent. Multiple radar soundingconfigurations were tested either in fixed-pointing mode, generally close to the source, or using scans acrossvarious regions of the plumes.Particle Size Distribution, shapes and density were characterized from microphysical analyses, sieving and waterpycnometry of ash samples collected on the ground. A Parsivel2 disdrometer also recorded the sizes, and settlingvelocities of fallout, allowing us to estimate sedimentation rates on the ground and to derive an empirical lawrelating calculated ash concentrations and reflectivities. Comparing the latter with reflectivities measured by theradar at unprecedented space-time resolutions (down to 12.5 m and 0.25 s) allowed us to obtain the internal massdistribution of eruptive columns, plumes, and fallout at various distances from the emission source