Showing papers by "William I. Rose published in 2004"

Re‐evaluation of SO2 release of the 15 June 1991 Pinatubo eruption using ultraviolet and infrared satellite sensors

Abstract: [1] In this study, ultraviolet TOMS (Total Ozone Mapping Spectrometer) satellite data for SO2 are re-evaluated for the first 15 days following the 15 June 1991 Pinatubo eruption to reflect new data retrieval and reduction methods. Infrared satellite SO2 data from the TOVS/HIRS/2 (TIROS (Television Infrared Observation Satellite) Optical Vertical Sounder/High Resolution Infrared Radiation Sounder/2) sensor, whose data sets have a higher temporal resolution, are also analyzed for the first time for Pinatubo. Extrapolation of SO2 masses calculated from TOMS and TOVS satellite measurements 19–118 hours after the eruption suggest initial SO2 releases of 15 ± 3 Mt for TOMS and 19 ± 4 Mt for TOVS, including SO2 sequestered by ice in the early Pinatubo cloud. TOVS estimates are higher in part because of the effects of early formed sulfate. The TOMS SO2 method is not sensitive to sulfate, but can be corrected for the existence of this additional emitted sulfur. The mass of early formed sulfate in the Pinatubo cloud can be estimated with infrared remote sensing at about 4 Mt, equivalent to 3 Mt SO2. Thus the total S release by Pinatubo, calculated as SO2, is 18 ± 4 Mt based on TOMS and 19 ± 4 Mt based on TOVS. The SO2 removal from the volcanic cloud during 19–374 hours of atmospheric residence describes overall e-folding times of 25 ± 5 days for TOMS and 23 ± 5 days for TOVS. These removal rates are faster in the first 118 hours after eruption when ice and ash catalyze the reaction, and then slow after heavy ash and ice fallout. SO2 mass increases in the volcanic cloud are observed by both TOMS and TOVS during the first 70 hours after eruption, most probably caused by the gas-phase SO2 release from sublimating stratospheric ice-ash-gas mixtures. This result suggests that ice-sequestered SO2 exists in all tropical volcanic clouds, and at least partially explains SO2 mass increases observed in other volcanic clouds in the first day or two after eruption.

Weather radar observations of the Hekla 2000 eruption cloud, Iceland

Abstract: The Hekla eruption cloud on 26–27 February 2000 was the first volcanic cloud to be continuously and completely monitored advecting above Iceland, using the C-band weather radar near the Keflavik international airport. Real-time radar observations of the onset, advection, and waning of the eruption cloud were studied using time series of PPI (plan-position indicator) radar images, including VMI normal, Echotop, and Cappi level 2 displays. The reflectivity of the entire volcanic cloud ranges from 0 to >60 dBz. The eruption column above the vent is essentially characterised by VMI normal and Cappi level 2 values, >30 dBz, due to the dominant influence of lapilli and ash (tephra) on the overall reflected signal. The cloud generated by the column was advected downwind to the north-northeast. It is characterised by values between 0 and 30 dBz, and the persistence of these reflections likely result from continuing water condensation and freezing on ash particles. Echotop radar images of the eruption onset document a rapid ascent of the plume head with a mean velocity of ~30 to 50 m s−1, before it reached an altitude of ~11–12 km. The evolution of the reflected cloud was studied from the area change in pixels of its highly reflected portions, >30 dBz, and tied to recorded volcanic tremor amplitudes. The synchronous initial variation of both radar and seismic signals documents the abrupt increase in tephra emission and magma discharge rate from 18:20 to 19:00 UTC on 26 February. From 19:00 the >45 dBz and 30–45 dBz portions of the reflected cloud decrease and disappear at about 7 and 10.5 h, respectively, after the eruption began, indicating the end of the decaying explosive phase. The advection and extent of the reflected eruption cloud were compared with eyewitness accounts of tephra fall onset and the measured mass of tephra deposited on the ground during the first 12 h. Differences in the deposit map and volcanic cloud radar map are due to the fact that the greater part of the deposit originates by fallout off the column margins and from the base of the cloud followed by advection of falling particle in lower level winds.

Scattering matrices of volcanic ash particles of Mount St. Helens, Redoubt, and Mount Spurr Volcanoes

Abstract: [1] We present measurements of the whole scattering matrix as a function of the scattering angle at a wavelength of 632.8 nm in the scattering angle range 3°–174° of randomly oriented particles taken from seven samples of volcanic ashes corresponding to four different volcanic eruptions: the 18 May 1980 Mount St. Helens eruption, the 1989–1990 Redoubt eruption, and the 18 August and 17 September 1992 Mount Spurr eruptions. The samples were collected at different distances from the vent. The samples studied contain large mass fractions of fine particles and were chosen to represent ash that could remain in the atmosphere for at least hours or days. They include fine ashfall samples that fell at a variety of distances from the volcano and pyroclastic flows that retained their fine fractions. Together, they represent a range of ashes likely to remain in the atmosphere in volcanic clouds following eruptions from convergent plate boundary volcanoes, Earth's most important group of explosive sources of ash. All measured scattering matrix elements are confined to rather limited domains when plotted as functions of the scattering angle following the general trends presented by irregular mineral particles. This similarity in the scattering behavior justifies the construction of an average scattering matrix for volcanic ash particles as a function of the scattering angle. To facilitate the use of the average scattering matrix for multiple-scattering calculations with polarization included, we present a synthetic scattering matrix based on the average scattering matrix for volcanic ashes and the assumption that the diffraction forward scattering peak is the same for randomly oriented nonspherical particles and projected-surface-area-equivalent spheres. This synthetic scattering matrix is normalized so that the average of its 1-1 element over all directions equals unity. It is available in the full range from 0° to 180° and can be used, for example, for interpretation of remote-sensing data after a volcanic eruption when the actual properties of the volcanic ash are not known. The measured results for the Mount St. Helens sample have been compared with results of Lorenz-Mie calculations for projected-surface-area-equivalent spheres with the refractive index of the Mount St. Helens particles. We find strong differences between measured and calculated values.

Numerical modeling of geophysical granular flows: 2. Computer simulations of plinian clouds and pyroclastic flows and surges

Abstract: [1] Geophysical granular flows display complex nonlinear, nonuniform, and unsteady rheologies, depending on the volumetric grain concentration within the flow: kinetic, kinetic-collisional, and frictional. To account for the whole spectrum of granular rheologies (and hence concentrations), we have used and further developed for geophysical-atmospheric applications a multiphase computer model initially developed by U.S. Department of Energy laboratories: (Geophysical) Multiphase Flow with Interphase Exchange. As demonstrated in this manuscript, (G)MFIX can successfully simulate a large span of pyroclastic phenomena and related processes: plinian clouds, pyroclastic flows and surges, flow transformations, and depositional processes. Plinian cloud simulations agree well with the classical plume theory and historical eruptions in the upper altitude of the cloud (HT) versus mass flux diagram. At high mass flux (>107 kg/s), plinian clouds pulsate periodically with time because of the vertical propagations of acoustic-gravity waves within the clouds. The lowest undercooled temperature anomalies measured within the upper part of the column can be as low as −18 K, which agrees well with El Chichon and Mt. St. Helens eruptions. Vertical and horizontal speed profiles within the plinian cloud compare well with those inferred from simple plume models and from umbrella experiments. Pyroclastic flow and surge simulations show that both end-members are closely tight together; e.g., an initially diluted flow may generate a denser basal underflow, which will eventually outrun the expanded head of the flow. We further illustrate evidence of vertical and lateral flow transformation processes between diluted and concentrated flows, particularly laterally from a turbulent “maintained over time fluidized zone” near source. Our comprehensive granular rheological model and our simulations demonstrate that the main depositional process is mainly a progressive vertical aggradation.

Particles in the great Pinatubo volcanic cloud of June 1991:The role of ice

Abstract: [1] Pinatubo's 15 June 1991 eruption was Earth's largest of the last 25 years, and it formed a substantial volcanic cloud. We present results of analysis of satellite-based infrared remote sensing using Advanced Very High Resolution Radiometer (AVHRR) and TIROS Operational Vertical Sounder/High Resolution Infrared Radiation Sounder/2 (TOVS/HIRS/2) sensors, during the first few days of atmospheric residence of the Pinatubo volcanic cloud, as it drifted from the Philippines toward Africa. An SO2-rich upper (25 km) portion drifted westward slightly faster than an ash-rich lower (22 km) part, though uncertainty exists due to difficulty in precisely locating the ash cloud. The Pinatubo clouds contained particles of ice, ash, and sulfate which could be sensed with infrared satellite data. Multispectral IR data from HIRS/2 were most useful for sensing the Pinatubo clouds because substantial amounts of both ice and ash were present. Ice and ash particles had peak masses of about 80 and 50 Mt, respectively, within the first day of atmospheric residence and declined very rapidly to values that were <10 Mt within 3 days. Ice and ash declined at a similar rate, and it seems likely that ice and ash formed mixed aggregates which enhanced fallout. Sulfate particles were detected in the volcanic cloud by IR satellites very soon after eruption, and their masses increased systematically at a rate consistent with their formation from SO2, which was slowly decreasing in mass during the same period. The initially detected sulfate mass was 4 Mt (equivalent to 3 Mt SO2) and after 5 days was 12–16 Mt (equivalent to 9–12 Mt SO2).

Explosion dynamics of pyroclastic eruptions at Santiaguito Volcano

Abstract: [1] In Jan. 2003 we monitored explosions at Santiaguito Volcano (Guatemala) with thermal, infrasonic, and seismic sensors. Thermal data from 2 infrared thermometers allowed computation of plume rise speeds, which ranged from 8 to 20 m/s. Rise rates correlated with cumulative thermal radiance, indicating that faster rising plumes correspond to explosions with greater thermal flux. The relationship between rise speeds and elastic energy is less clear. Seismic radiation may not scale well with thermal output and/or rise speed because some of the thermal component may be associated with passive degassing, which does not induce significant seismicity. But non-impulsive gas release is still able to produce a high thermal flux, which is the primary control on buoyant rise speed.

Surface temperature and spectral measurements at Santiaguito lava dome, Guatemala

Abstract: [1] An infrared thermometer, spectroradiometer and digital video camera were used to observe and document short-term evolution of surface brightness temperature and morphology at Santiaguito lava dome, Guatemala. The thermometer dataset shows 40–70 minute-long cooling cycles, each defined by a cooling curve that is both initiated and terminated by rapid increases in temperature due to regular ash venting. The average cooling rate calculated for each cycle range from 0.9 to 1.6°C/min. We applied a two-component thermal mixture model to the spectroradiometer (0.4–2.5 μm) dataset. The results suggest that the observed surface morphology changed from a cool (120–250°C) crust-dominated surface with high temperature fractures (>900°C) in the first segment of the measurement period to an isothermal surface at moderately high temperature (350–500°C) during the second segment. We attribute the change in the thermal state of the surface to the physical rearrangement of the dome's surface during the most energetic of the ash eruptions.

Ice in Volcanic Clouds: When and Where?

Abstract: This paper investigates data that has been published from remote sensing and other sources regarding particles of ice that occur in volcanic clouds. An attempt is made to discover the patterns that occur in the variability of ice particles in volcanic clouds. A number of volcanic eruptions are examined, including some that occurred in the following volcanoes: Rabaul volcano (Papua New Guinea), Hekla volcano (Iceland), Mount Spurr volcano (Alaska), Cleveland volcano (Alaska), Pinatubo volcano (Philippines), Soufriere Hills volcano (Montserrat), and El Chichon volcano (Mexico).

Removal Processes of Volcanic Ash Particles from the Atmosphere

Abstract: This paper presents a review of relevant research and observations from the past and present ongoing studies of volcano monitoring by satellites. Over the past 25 years many infrared, ultraviolet, and satellite sensors have been used to investigate the liquid, gas, and solid species in a variety of volcanic ash clouds. Each sensor offers a different perspective on volcanic ash clouds, depending on their temporal, spectral and spatial resolutions. Combined, these techniques offer crucial constraints on the fates and interactions of species within the clouds.

Short communication Observations of eruptive activity at Santiaguito volcano, Guatemala

Abstract: The study of active vent dynamics is hindered by the difficulty of directly observing features and processes during eruptive periods. Here, we describe some recent observations of the summit dome activity of Santiaguito volcano, Guatemala, from the vantage point of its parent, Santa Maria. We have taken 12 h of digital video of activity over a 3-year period, which includes 28 eruptions and numerous smaller gas exhalations. Santiaguito persistently extrudes a dacitic lava flow, and produces strombolian eruptions on the order of every 0.5 to 2 h; we have documented many of these eruptions as emitting from a ring-shaped set of fractures in the dome surface. The ring has apparently grown from 70 m diameter in 2002 to 120 m in 2004, which could reflect an increasing conduit opening. Eruptions typically consist of 30-60 s of vigorous emissions; measurements of emission exit velocities have ranged from 5 to 30 m/s. The observed ash bursts, correlated with measured extrusion rates, suggest an incremental plug flow through the conduit. Bubble generation and shearing at the conduit boundaries produce the ring-shaped ash and gas pulses. Continued field studies from this unique observation site may help relate summit emission characteristics to conduit geometry and eruption processes. D 2004 Elsevier B.V. All rights reserved.