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Abstract:

Satellite instruments play a valuable role in detecting, monitoring and characterising emissions of ash and gas into the atmosphere during volcanic eruptions. This study uses two satellite instruments, the Infrared Atmospheric Sounding Interferometer (IASI) and the Advanced Baseline Imager (ABI), to examine the plumes of ash and sulfur dioxide (SO₂) from the April 2021 eruption of La Soufrière, St Vincent. The frequent ABI data have been used to construct a 14 d chronology of a series of explosive events at La Soufrière, which is then complemented by measurements of SO₂ from IASI, which is able to track the plume as it is transported around the globe. A minimum of 35 eruptive events were identified using true, false and brightness temperature difference maps produced with the ABI data. The high temporal resolution images were used to identify the approximate start and end times, as well as the duration and characteristics of each event. From this analysis, four distinct phases within the 14 d eruption have been defined, each consisting of multiple explosive events with similar characteristics: (1) an initial explosive event, (2) a sustained event lasting over 9 h, (3) a pulsatory phase with 25 explosive events in a 65.3 h period and (4) a waning sequence of explosive events. It is likely that the multiple explosive events during the April 2021 eruption contributed to the highly complex plume structure that can be seen in the IASI measurements of the SO₂ column amounts and heights. The bulk of the SO₂ from the first three phases of the eruption was transported eastwards, which based on the wind direction at the volcano implies that the SO₂ was largely in the upper troposphere. Some of the SO₂ was carried to the south and west of the volcano, suggesting a smaller emission of the gas into the stratosphere, there being a shift in wind direction around the height of the tropopause. The retrieved SO₂ heights show that the plume had multiple layers but was largely concentrated between 13 and 19 km, with the majority of the SO₂ being located in the upper troposphere and around the height of the tropopause, with some emission into the stratosphere. An average e-folding time of 6.07±4.74 d was computed based on the IASI SO₂ results: similar to other tropical eruptions of this magnitude and height. The SO₂ was trackable for several weeks after the eruption and is shown to have circulated the globe, with parts of it reaching as far as 45∘ S and 45∘ N. Using the IASI SO₂ measurements, a time series of the total SO₂ mass loading was produced, with this peaking on 13 April (descending orbits) at 0.31±0.09 Tg. Converting these mass values to a temporally varying SO₂ flux demonstrated that the greatest emission occurred on 10 April with that measurement incorporating SO₂ from the second phase of the eruption (sustained emission) and the beginning of the pulsatory phase. The SO₂ flux is then shown to fall during the later stages of the eruption: suggesting a reduction in eruptive energy, something also reflected in ash height estimates obtained with the ABI instrument. A total SO₂ emission of 0.63±0.5 Tg of SO₂ has been derived, although due to limitations associated with the retrieval, particularly in the first few days after the eruption began, this, the retrieved column amounts and the total SO₂ mass on each day should be considered minimum estimates. There are a number of similarities between the 1979 and 2021 eruptions at La Soufrière, with both eruptions consisting of a series of explosive events with varied heights and including some emission into the stratosphere. These similarities highlight the importance of in-depth investigations into eruptions and the valuable contribution of satellite data for this purpose; as these studies aid in learning about a volcano's behaviour, which may allow for better preparation for future eruptive activity.

Abstract:

Rapid and simple estimation of the mass eruption rate (MER) from column height is essential for real-time volcanic hazard management and reconstruction of past explosive eruptions. Using 134 eruptive events from the new Independent Volcanic Eruption Source Parameter Archive (IVESPA, v1.0), we explore empirical MER-height relationships for four measures of column height: spreading level, sulfur dioxide height, and top height from direct observations and as reconstructed from deposits. These relationships show significant differences and highlight limitations of empirical models currently used in operational and research applications. The roles of atmospheric stratification, wind, and humidity remain challenging to detect across the wide range of eruptive conditions spanned in IVESPA, ultimately resulting in empirical relationships outperforming analytical models that account for atmospheric conditions. This finding highlights challenges in constraining the MER-height relation using heterogeneous observations and empirical models, which reinforces the need for improved eruption source parameter data sets and physics-based models.

Abstract:

A series of explosive eruptions began at La Soufrière, on St Vincent, on 9th April 2021, with the last event occurring on 22nd April. The eruption was observed by the Advanced Baseline Imager (ABI) on the Geostationary Operational Environmental Satellite East (GOES-East) platform. The instrument measures in 16 channels in the visible, near-infrared and infrared, across a large area which includes the Carribean. During the La Soufrière eruption, the ABI instrument made measurements for the full disc every 10-minutes, and for part of the eruption, a mesoscale region was moved over the volcano, providing data every minute. In this study, data from this satellite instrument has been used to construct four images: (a) a true colour image, (b) a false colour image (12.3 - 10.3 µm, 11.2 - 8.4 µm and 10.3 µm channels assigned to red, green and blue respectively), (c) 10.3 - 11.2 µm brightness temperature difference, and (d) 11.2 - 12.3 µm brightness temperature difference. These have been used to study the plumes produced during the April 2021 eruption, allowing the identification of at least 35 explosive events. In this repository, an animation of the created images has been provided for each of the explosive events identified. The animation names are constructed to indicate the volcano (LaSoufriere), the instrument (ABI), the type of data (i.e. full disc or meso), the event number, the start date/time (yyyymmddhhmm) and the end date/time (yyyymmddhhmm). For example: ‘LaSoufriere_ABI_full_event01_202104091250_202104091740.mp4’ Note that the start and end times indicate the start time of measurement period and some adjustment is needed to ascertain the time over the volcano for the full disc (~ +243 seconds). This has been accounted for in the title shown in each frame of the full disc animations. Also note that two animations are provided for event 5 as measurements from the mesoscale region began during this period. We are grateful to the NOAA Big Data Program through which the GOES-16 data are available. These animations accompany a paper: Taylor, I.A., Grainger, R.G., Prata, A.T., Proud, S,R, Mather, T.A., Pyle, D.M.: A satellite chronology of plumes from the April 2021 eruption of La Soufrière, St Vincent, Accepted for publication in Atmospheric, Chemistry and Physics.

Abstract:

Uncertainty-bounded satellite retrievals of volcanic ash cloud properties such as ash cloud-top height, effective radius, optical depth and mass loading are needed for the robust quantitative assessment required to warn aviation of potential hazards. Moreover, there is an imperative to improve quantitative ash cloud estimation due to the planned move towards quantitative ash concentration forecasts by the Volcanic Ash Advisory Centers. Here we apply the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm to Advanced Himawari Imager (AHI) measurements of the ash clouds produced by the June 2019 Raikoke (Russia) eruption. The ORAC algorithm uses an optimal estimation technique to consolidate a priori information, satellite measurements and associated uncertainties into uncertainty-bounded estimates of the desired state variables. Using ORAC, we demonstrate several improvements in thermal infrared volcanic ash retrievals applied to broadband imagers. These include an improved treatment of measurement noise, accounting for multi-layer cloud scenarios, distinguishing between heights in the troposphere and stratosphere, and the retrieval of a wider range of effective radii sizes than existing techniques by exploiting information from the 10.4 µm channel. Our results indicate that 0.73 ± 0.40 Tg of very fine ash (radius ≤ 15 µm) was injected into the atmosphere during the main eruptive period from 21 June 18:00 UTC to 22 June 10:00 UTC. The total mass of very fine ash decreased from 0.73 to 0.10 Tg over ∼ 48 h, with an e-folding time of 20 h. We estimate a distal fine ash mass fraction of 0.73 % ± 0.62 % based on the total mass of very fine ash retrieved and the ORAC-derived height–time series. Several distinct ash layers were revealed by the ORAC height retrievals. Generally, ash in the troposphere was composed of larger particles than ash present in the stratosphere. We also find that median ash cloud concentrations fall below peak ash concentration safety limits (< 4 mg m−3) 11–16 h after the eruption begins, if typical ash cloud geometric thicknesses are assumed. The ORAC height retrievals for the near-source plume showed good agreement with GOES-17 side-view height data (R=0.84; bias = −0.75 km); however, a larger negative bias was found when comparing ORAC height retrievals for distal ash clouds against Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP) measurements (R=0.67; bias = −2.67 km). The dataset generated here provides uncertainties at the pixel level for all retrieved variables and could potentially be used for dispersion model validation or be implemented in data assimilation schemes. Future work should focus on improving ash detection, improving height estimation in the stratosphere and exploring the added benefit of visible channels for retrieving effective radius and optical depth in opaque regions of nascent ash plumes.