V31A-0288
A tephra-dispersal model based on 3-D simulations of eruption clouds and experiments on particle settling in turbulent flow
During an explosive volcanic eruption, hot volcanic gases and pyroclasts are ejected from the volcanic vent into the atmosphere, and the mixture of the ejected materials and the air buoyantly rises as an eruption column. After the eruption column reaches the neutral buoyancy level, it flows horizontally to form an umbrella cloud. Pyroclasts generated by the eruption fall out from the umbrella cloud to the ground surface. In previous tephra-dispersal models, it is assumed that (1) pyroclasts are homogeneously distributed in the umbrella cloud because of turbulence, and (2) they fall out at their terminal velocities from the bottom of the umbrella cloud where turbulence diminishes [e.g., Koyaguchi and Ohno, 2001]. The first assumption is appropriate only when turbulent intensity is sufficiently strong relative to the terminal velocities of particles. Here, we attempt to establish a generalized model in which the relationship between the turbulent intensity and the terminal velocity is taken into account on the basis of 3-dimensional (3-D) numerical simulations of eruption clouds and a series of laboratory experiments on particle settling in turbulent flow. The numerical model is designed to simulate the fluid dynamical features of the eruption cloud, such as column height and laterally spreading umbrella cloud as a function of vent conditions such as magma discharge rate. The model correctly reproduces the turbulent mixing as well as the density of the eruption cloud as a function of mixing ratio by applying 3-D coordinates, high order accuracy calculation schemes, and sufficiently fine grid sizes. From the 3-D simulations, we determined the turbulent intensity in the eruption clouds. Laboratory experiments of particle settling in turbulent flow are performed focusing on the effects of turbulent intensity on the process of particle settling. In the experiments spherical glass-bead particles are mixed in stirred water with variable turbulent intensity, and the spatial distribution and the temporal evolution of the particle concentration are measured. The experimental results suggest that, when the root-mean-square (rms) of velocity fluctuation in the fluid is much greater than the particle terminal velocity, the particles are homogeneously distributed in the fluid, while they settle at their terminal velocities from the bottom of the fluid. On the other hand, when the rms of velocity fluctuation is smaller than the particle terminal velocity, the particle concentration increases toward the bottom of the fluid during settling process, which substantially increases the rate of particle settling. The above results of numerical simulations and laboratory experiments imply that small pyroclasts (less than 1/8 mm in diameter) are distributed homogeneously throughout the umbrella cloud, whereas relatively large pyroclasts (more than a few mm in diameter) tend to concentrate around the bottom of the umbrella cloud. The generalized tephra-dispersal model in which the gradient of particle concentration is taken into consideration better explains the granulometric data of the deposits of Pinatubo 1991 eruption.
V31A-0289
Simulation of the 1980 Eruption of Mount St. Helens Using the Ash-tracking Model PUFF
The dispersal of volcanic ash from the May 18, 1980 eruption of Mt. St. Helens (MSH) has been simulated using the Lagrangian ash-tracking model PUFF. Previous applications of the model were limited to smaller, short-lived eruptions with ash dispersal occurring mainly within the troposphere. Two high-resolution atmospheric reanalysis datasets (ERA-40 and NCEP/NCAR-40) allowed MSH ash cloud dispersal to be simulated up to 30 km elevation. The 1980 eruption was divided into two distinct eruptive phases, an initial, relatively short-lived blast/surge phase that injected ash up to 30 km and a subsequent nine-hour plinian phase that maintained an average eruption column height of 16 km. Using PUFF, the two phases of the MSH eruption were modeled separately based on a range of individual input parameters and then combined to produce an integrated model of the entire eruption. The trajectory and areal extent of the modeled atmospheric ash cloud best match the actual distribution of MSH ash when input parameters such as eruption column height and umbrella cloud thickness are set to values inferred from satellite and radar data collected on May 18, 1980. The prevailing wind field exerts the strongest control on the advection and ultimate position of the modeled ash cloud, making the maximum column height and the vertical distribution of ash the most sensitive of the PUFF input parameters for this event. The results indicate that the PUFF model works well at simulating the dispersal of ash injected well into the lower stratosphere generated from a moderate, relatively long-lived eruption such as Mt. St. Helens. However, attempts to use PUFF to recreate some granulometric aspects of the MSH fallout deposit, such as plotting the maximum particle size as a function of distance from source, were not successful. PUFF consistently predicts much greater fallout distances for small ash particles (> 1 φ\)) than actually observed in the MSH deposit. The effective settling velocities used by the PUFF model appear to be too slow to accurately predict fallout distances of small ash particles. As a consequence the PUFF model may overestimate the duration of ash loading in the atmosphere associated with the distal fine ash component of explosive eruptions.
V31A-0290
Plumes and Wind II: Potential Hazards to Air Traffic From Inyo Craters
The Inyo Craters, CA, USA, last erupted explosively approximately 600 yBP when three vents in the chain produced four distinct subplinian eruptions. South Deadman 1, the first of these eruptions, produced a pulsating eruption column that may have reached a maximum height of 9 km in a 30 m/s S ambient wind, though anomalous elongation of the airfall deposit prevents it from being properly modeled using standard inversion models. During the second, and steadier, eruption from South Deadman the column rose to 12 km in a 20 m/s N ambient wind. The Obsidian Flow eruption deposits are distributed to the north, with an initial vent-clearing blast followed by a sustained eruption column of 14 km in a 10 m/s S wind. The final, and largest, eruption came from the Glass Creek vent. The Glass Creek eruption column had a maximum height of 15 km in a 25 m/s ambient wind. Transcontinental flights to San Francisco and Oakland Airports cross directly over the Inyo vent sites. A recurrence of the Inyo eruption sequence today could significantly disrupt California air traffic by injecting tephra into the jet stream. ATHAM is used to simulate the Inyo eruptions and the intersection of their ash clouds with potential flight paths. The use of a Navier-Stokes based eruption simulator allows for more detailed characterization of cloud trajectory and mass loading than has been previously possible through statistical models.
V31A-0291
Volcanic plumes: What is the Realistic Neutral Buoyancy Height?
Atmospheric residence time, interaction with radiation and chemical processes as well as the sedimentation of volcanic ash and sulphate particles depend on the injection height and it is important to calculate this parameter as exactly as possible. Explosive volcanic eruptions form buoyant plumes after sufficient entrainment has taken place. If explosive energy is high, overshooting may take place and the maximum height of a plume therefore may be much higher than the Neutral Buoyancy Height that determines the lateral spread and settling of volcanic material. Further, if a (buoyant) Plinian eruption plume cannot be formed, the eruption column will collapse. A secondary, so called co-ignimbrite plume can be formed from the developing pyroclastic flow at horizontal scales much larger than the initial eruption. In previous work one-dimensional, often stationary plume models based on the assumption that they can be regarded of top hat profile were used to study the behaviour of such Plinian and co-ignimbrite eruptions. Such simulations result in unrealistically deep penetrating columns and the formation of umbrella clouds from which ash is falling out is parameterised using arbitrary assumptions. Here we will present results from the three- dimensional plume model ATHAM of the development of eruption columns under different realistic initial and environmental conditions and compare with results of simpler models. Clearly, Neutral Buoyancy Heights are strongly overestimated when common stationary top hat models are applied. These only in case of small eruption energy deliver reasonable results, however lacking effects of variable entrainment etc., which would lead to higher variability than suggested. Depending on the size of the hot ash-air mixture from which the co-ignimbrite develops, single or multiple plumes develop and the maximum height of neutral buoyancy is reduced considerably. Entrainment and wind shear have strong influence on the plume development.
V31A-0292
Ash cloud detection from satellite data: the strengths and weaknesses as related to eruption source parameters?
The detection of volcanic ash clouds using satellite remote sensing data is an important tool used by volcano observatories and volcanic ash advisory centers. Satellite data may be the first source of information routinely available for a known eruption that can be used to characterize plumes. Therefore, accurate measurements of plume characteristics are critical for hazard assessments. Using satellite data, it is possible to determine plume size, height and location, and the presence of volcanic ash. This satellite remote sensing data also record the dynamics of a plume as it evolves over time. Each technique or observation has strengths, weaknesses or limitations and also requires some approximation or assumptions regarding local conditions. Here, we describe information derived from the satellite data, discuss their limitations and suggest how the plume characteristics can be used to validate or adjust eruption source parameters for volcanic ash tracking and dispersion models.
V31A-0293
The NOAA Near Real-time OMI-SO2 Cloud Visualization and Product Distribution System
The Ozone Monitoring Instrument (OMI) on the NASA EOS/Aura research satellite allows measurement of SO2 concentrations at UV wavelengths with daily global coverage. SO2 is detected from space using its strong absorption band structure in the near UV (300-320 nm) as well as in IR bands near 7.3 and 8.6 mm. Thirty years of UV SO2 measurements with the Total Ozone Mapping Spectrometer (TOMS) and OMI sensors have shown that the highest concentrations of SO2 occur in volcanic clouds produced by explosive magmatic eruptions, which also emit ash. However, icing of ash particles in water-rich eruption clouds, and/or suppression of the IR split- window signal by ambient water vapor or cloud opacity can inhibit direct detection of ash from space. Large SO2 concentrations are therefore a reliable indicator of the presence of airborne volcanic ash. UV SO2 measurements are very robust and are insensitive to the factors that confound IR data. SO2 and ash can be detected in a very fresh eruption cloud due to sunlight backscattering and ash presence can be confirmed by UV derived aerosol index measurements. The lack of other large point sources of SO2 facilitates development and implementation of automated searches for volcanic clouds with a very low false alarm rate. The NASA Earth Sciences Applications Office has funded a cooperative agreement between UMBC, NOAA, GSFC, and USGS to infuse research satellite SO2 data products into volcanic hazard Decision Support Systems (DSSs) operated by the National Oceanic and Atmospheric Administration (NOAA) and the US Geological Survey (USGS). This will provide aviation alerts to the Federal Aviation Administration (FAA), that will reduce false alarms and permit more robust detection and tracking of volcanic clouds, and includes the development of an eruption alarm system, and potential recognition of pre-eruptive volcanic degassing. Near real-time (NRT) observations of SO2 and volcanic ash can therefore be incorporated into data products compatible with Decision Support Tools (DSTs) in use at Volcanic Ash Advisory Centers (VAACs) in Washington and Anchorage, and the USGS Volcano Observatories. In this poster we show the latest NOAA Office of Satellite Data Processing and Distribution (OSDPD) development of an online NRT image and data product distribution system that generates eruption alarms, allows the extraction of volcanic cloud subsets for special processing, and provides access to analysis tools and graphical products. Products are infused into DSTs including the Volcanic Ash Coordination Tool (VACT), under development by the NOAA Forecast Systems Laboratory and the FAAs Oceanic Weather Product Development Team (OWPDT), to monitor and track, drifting volcanic clouds.
V31A-0294
Testing and Adapting a Daytime Four Band Satellite Ash Detection Algorithm for Eruptions in Alaska and the Kamchatka Peninsula, Russia
Volcanic ash is detectable from satellite remote sensing due to the differences in spectral signatures compared to meteorological clouds. Recently a new global daytime ash detection algorithm was developed at University of Madison, Wisconsin. The algorithm is based on four spectral bands with the central wavelengths 0.65, 3.75, 11 and 12 micrometers that are common on weather satellite sensors including MODIS, AVHRR, GOES and MTSAT. The initial development of the algorithm was primarily based on MODIS data with global coverage. We have tested it using three years of AVHRR data in Alaska and the Kamchatka Peninsula, Russia. All the AVHRR data have been manually analyzed and recorded into an observational database during the daily monitoring performed by the remote sensing group at the Alaska Volcano Observatory (AVO). By taking the manual observations as accurate we were able to examine the accuracy of the four-channel algorithm for daytime data. The results were also compared to the current automated ash alarm used by AVO, based on the reverse absorption technique, also known as the split window method, with a threshold of -1.7K. This comparison indicates that the four- banded technique has a higher sensitivity to volcanic ash, but a greater number of false alarms. The algorithm was modified to achieve a false alarm rate comparable to current ash alarm while still maintaining increased sensitivity.
V31A-0295
Monitoring of Ash Ejection from Ecuadorean Volcanoes Using Infrasound
Two infrasound arrays with 100-meter apertures have been deployed in Ecuador in order to monitor volcanic eruptions that may produce ash clouds. Data is collected continuously and streamed via satellite and internet links to processing centers that apply array processing techniques to display acoustic detections of volcanic eruptions in near-real-time. These data products are delivered to the DC Volcanic Ash Advisory Center (VAAC), and have been used as additional confirmation of an eruption in ash advisories provided by the VAAC to commercial aircraft. Considerable research has been put into identifying the types of acoustic signals that indicate ash injection, as well as eliminating false alarms and reducing the latency between data collection and notification. Research conducted to date will be discussed, as well as several case studies wherein infrasound data proved useful for quick detection of eruptions. A brief summary of the advantages gained from use of this technique will also be presented. http://www.isla.hawaii.edu/volcano/ashe.shtml
V31A-0296
Hydrometeor-Enhanced Tephra Sedimentation
New evidence presented here supports hydrometeor formation as a fundamental process in volcanic cloud sedimentation. Distal fallout from several recent eruptions was characterized using laser diffraction particle size analysis for diameters between 0.2-2000 μm. Based on simple modeling, sedimentation rates of most ash particles <100 μm are faster than single particle terminal velocities. In the case of the Mount St. Helens 18 May 1980 (MSH80) eruption, fallout at <300 km involved single particle fall in addition to multiple-particle aggregate fall. Fallout at >300 km was predominantly through the formation of aggregates composed of a particle subpopulation with a mode at 18 μm. Airborne measurements, satellite remote sensing, observations of mammatus clouds and simple model calculations indicate volcanic clouds contain abundant water and hydrometeors. Observational and modeling studies of mammatus clouds on thunderstorm anvils provide insight and constraints on volcanic analogues. In a conceptual model presented here, ash particles initiate hydrometeor formation and subsidence of the cloud deck occurs through mammatus generation. Rapid aggregation and fallout occur as the cloud passes through the melting level in a process analogous to snowflake growth. Cloud particles then settle en masse, forming the distal mass deposition maxima observed in many recent volcanic ash-fall deposits. Based on this new insight, VATDM should include the effects of hydrometeor formation to accurately model distal volcanic fallout.
V31A-0297
The Dispersal of Ash From an Ancient Volcanic Center on Mars
Apollinaris Patera (-8°S, 174°E) is a medium-sized martian volcano thought to have been active during the Hesperian period in Mars history. Extensive deposits near the volcano, mapped as a part of the Medusae Fossae Formation (MFF), have been dated as Amazonian. The deposits surrounding the volcano have been shown to be friable and fine grained, resulting in a wide variety of eolian landforms including long, linear dunes and yardangs. The MFF is found at both high and low elevations. These pieces of evidence have caused some to suspect that the formation is made of windblown, reworked eolian material or loess. Pedestal craters, which are thought to result from impact-induced armoring or cementing of the target material and subsequent erosion by wind or volatile sublimation, are common in the unit, although pedestal craters are usually seen in volatile-rich substrates closer to the poles. The MFF also appears similar to circumpolar deposits. This evidence has suggested that either the deposits are the result of airborne volatile-rich sediments deposited during high obliquity, or that they are themselves paleopolar deposits. The MFF lies in close proximity to Apollinaris, a volcano of considerable size; it is thus an interesting possibility that the MFF is the result of pyroclastic flows, ashfall (or some combination thereof) from Apollinaris. The MFF has been dated as Amazonian, postdating Apollinaris Patera, but there is evidence that a considerable amount of reshaping and removal of material has gone on in the Medusae Fossae region which could cause a crater count to yield an anomalously young age. We present results from a modeling study using a Mars Global Climate Model (Mars GCM) and an eruption and dispersal model to better constrain where deposits of ash erupted from Apollinaris might accumulate according to current ideas regarding eruption dynamics on Mars and modern martian global wind patterns. Mapping of deposits surrounding the volcano provides a general check of modeling results and an assessment of the viability of the ashfall hypothesis. Further work will focus on differences between current wind patterns and those present during the eruptive phase of the volcano, more than three billion years before the present.
V31A-0298
Comparison of 3D Stereo SEM Shape Data With 2D Projections and BET Surface Area Data for Volcanic Ash: When 3D Might Be Advantageous
In this study we present a technique for measuring the shape and surface area of individual grains of volcanic ash using SEM stereo-pairs. The shape and surface area of fine particles is traditionally measured using 2D sections and with BET equipment. However, 2D sections are incomplete shape descriptions and BET analysis is precluded when the amount of sample is limited, for example, in aerial ash cloud sampling. The application we discuss is the stereoscopic surface area analysis of 10 - 300 micrometer ash from the August 1992 Crater Peak/Spurr-eruption. Validation data is presented from measurements of glass micro- spheres in the size range. The stereo results are recast into a single surface area value using the particle size distribution of the ash and area compared to a BET analysis of the ash sample. Differences in surface area values between our technique, 2D shape data and BET are presented and discussed.