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1.
The dispersal of volcanic ash from the May 18, 1980 eruption of Mount 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, (1) an initial, relatively short-lived blast/surge phase that injected ash up to 30 km and (2) 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 simulation 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 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 from a moderate, relatively long-lived eruption, such as MSH. However, attempts to use PUFF to recreate some granulometric aspects of the MSH fallout deposit, such as 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 (< 500 µm) 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.  相似文献   

2.
During volcanic eruptions, volcanic ash transport and dispersion models (VATDs) are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind. Those models use input parameters, called “eruption source parameters”, such as plume height H, mass eruption rate Ṁ, duration D, and the mass fraction m63 of erupted debris finer than about 4 or 63 μm, which can remain in the cloud for many hours or days. Observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected. Moreover, observed plume height may change during an eruption, requiring rapid assignment of new parameters. This paper reports on a group effort to improve the accuracy of source parameters used by VATDs in the early hours of an eruption. We do so by first compiling a list of eruptions for which these parameters are well constrained, and then using these data to review and update previously studied parameter relationships. We find that the existing scatter in plots of H versus Ṁ yields an uncertainty within the 50% confidence interval of plus or minus a factor of four in eruption rate for a given plume height. This scatter is not clearly attributable to biases in measurement techniques or to well-recognized processes such as elutriation from pyroclastic flows. Sparse data on total grain-size distribution suggest that the mass fraction of fine debris m63 could vary by nearly two orders of magnitude between small basaltic eruptions ( 0.01) and large silicic ones (> 0.5). We classify eleven eruption types; four types each for different sizes of silicic and mafic eruptions; submarine eruptions; “brief” or Vulcanian eruptions; and eruptions that generate co-ignimbrite or co-pyroclastic flow plumes. For each eruption type we assign source parameters. We then assign a characteristic eruption type to each of the world's  1500 Holocene volcanoes. These eruption types and associated parameters can be used for ash-cloud modeling in the event of an eruption, when no observational constraints on these parameters are available.  相似文献   

3.
The maximum height attained by a volcanic eruption cloud is principally determined by the convective buoyancy of the mixture of volcanic gas + entrained air + fine-sized pyroclasts within the cloud. The thermal energy supplied to convection processes within an eruption cloud is derived from the cooling of pyroclastic material and volcanic gases discharged by an explosive eruption. Observational data from six recent eruptions indicates that the maximum height attained by volcanic eruption clouds is positively correlated with the rate at which pyroclastic material is produced by an explosive eruption (correlation coefficient r = + 0.97). The ascent of industrial hot gas plumes is also governed by the thermal convection process. Empirical scaling relationships between plume height and thermal flux have been developed for industrial plumes. Applying these scaling relationships to volcanic eruption clouds suggests that the rate at which thermal energy is released into the atmosphere by an explosive eruption increases in an approximately linear manner as an eruption's pyroclastic production rate increases.  相似文献   

4.
Improved prediction and tracking of volcanic ash clouds   总被引：3，自引：1，他引：2  
During the past 30 years, more than 100 airplanes have inadvertently flown through clouds of volcanic ash from erupting volcanoes. Such encounters have caused millions of dollars in damage to the aircraft and have endangered the lives of tens of thousands of passengers. In a few severe cases, total engine failure resulted when ash was ingested into turbines and coating turbine blades. These incidents have prompted the establishment of cooperative efforts by the International Civil Aviation Organization and the volcanological community to provide rapid notification of eruptive activity, and to monitor and forecast the trajectories of ash clouds so that they can be avoided by air traffic. Ash-cloud properties such as plume height, ash concentration, and three-dimensional ash distribution have been monitored through non-conventional remote sensing techniques that are under active development. Forecasting the trajectories of ash clouds has required the development of volcanic ash transport and dispersion models that can calculate the path of an ash cloud over the scale of a continent or a hemisphere. Volcanological inputs to these models, such as plume height, mass eruption rate, eruption duration, ash distribution with altitude, and grain-size distribution, must be assigned in real time during an event, often with limited observations. Databases and protocols are currently being developed that allow for rapid assignment of such source parameters. In this paper, we summarize how an interdisciplinary working group on eruption source parameters has been instigating research to improve upon the current understanding of volcanic ash cloud characterization and predictions. Improved predictions of ash cloud movement and air fall will aid in making better hazard assessments for aviation and for public health and air quality.  相似文献   

5.
A new model is presented which simulates the dispersal and deposition of material from a Hawaiian eruption column. The model treats the Hawaiian column as a coarse-grained Plinian column and uses a modified version of the Wilson and Walker [Wilson, L., Walker, G.P.L., 1987. Explosive volcanic eruptions: VI. Ejecta dispersal in Plinian eruptions: the control of eruption conditions and atmospheric properties. Geophys. J. R. Astron. Soc. 89, 657–679.] Plinian pyroclast dispersal model to simulate the fall out of material during a Hawaiian eruption. The model results are found to be in good agreement with independent estimates of various parameters made for the 1959 Kilauea Iki eruption of Kilauea volcano. The close agreement between the model results and these independent estimates shows that, dynamically, Hawaiian eruptions are indistinguishable from Plinian eruptions. The major differences in the styles and deposits of these two types of eruptions are accounted for by differences in the mass fluxes and gas contents of the erupting magmas and, most fundamentally, by differences in the grainsize distribution of the erupted clasts. Plume heights predicted by the model are greater than those found for previous models of Hawaiian eruptions. This is because previous models did not allow for the progressive fall out of particles from the plume and, more importantly, made no correction for the velocity disequilibrium between gas and clasts when the grainsize distribution is coarse.  相似文献   

6.
Ash clouds are one of the major hazards that result from volcanic eruptions. Once an eruption is reported, volcanic ash transport and dispersion (VATD) models are used to forecast the location of the ash cloud. These models require source parameters to describe the ash column for initialization. These parameters include: eruption cloud height and vertical distribution, particle size distribution, and start and end time of the eruption. Further, if downwind concentrations are needed, the eruption mass rate and/or volume of ash need to be known. Upon notification of an eruption, few constraints are typically available on many of these source parameters. Recently, scientists have defined classes of eruption types, each with a set of pre-defined eruption source parameters (ESP). We analyze the August 18, 1992 eruption of the Crater Peak vent at Mount Spurr, Alaska, which is the example case for the Medium Silicic eruption type. We have evaluated the sensitivity of two of the ESP – the grain size distribution (GSD) and the vertical distribution of ash – on the modeled ash cloud. HYSPLIT and Puff VATD models are used to simulate the ash clouds from the different sets of source parameters. We use satellite data, processed through the reverse absorption method, as reference for computing statistics that describe the modeled-to-observed comparison. With the grain size distribution, the three options chosen, (1) an estimated distribution based on past eruption studies, (2) a distribution with finer particles and (3) the National Oceanic and Atmospheric Administration HYSPLIT GSD, have little effect on the modeled ash cloud. For the initial vertical distribution, both linear (uniform concentration throughout the vertical column) and umbrella shapes were chosen. For HYSPLIT, the defined umbrella distribution (no ash below the umbrella), apparently underestimates the lower altitude portions of the ash cloud and as a result has a worse agreement with the satellite detected ash cloud compared to that with the linear vertical distribution for this particular eruption. The Puff model, with a Poisson function to represent the umbrella cloud, gave similar results as for a linear distribution, both having reasonable agreement with the satellite detected cloud. Further sensitivity studies of this eruption, as well as studies using the other source parameters, are needed.  相似文献   

7.
The Grímsv?tn eruption in November 2004 belongs to a class of small- to medium-sized phreatomagmatic eruptions which are common in Iceland. The eruption lasted 6?days, but the main phase, producing most of the 0.02?km3 of magma erupted, was visible for 33?h on the C-band weather radar of the Icelandic Meteorological Office located in Keflavík, 260?km to the west of the volcano. The plume rose to 8–12?km high over sea level during 33?h. The long distance between radar and source severely reduces the accuracy of the plume height determinations, causing 3.5-km steps in recorded heights. Moreover, an apparent height overestimate of ~1.5?km in the uncorrected radar records occurs, possibly caused by wave ducting or super-refraction in the atmosphere. The stepping and the height overestimate can be partly overcome by averaging the plume heights and by applying a height adjustment based on direct aircraft measurements. Adjusted weather radar data on plume height are used to estimate the total mass erupted using empirical plume models mostly based on magmatic eruptions and to compare it with detailed in situ measurements of the mass of erupted tephra. The errors arising because of the large radar plume distance limit the applicability of the data for detailed comparisons. However, the results indicate that the models overestimate the mass erupted by a factor of three to four. This supports theoretical models indicating that high steam content of phreatomagmatic (wet) plumes enhances their height compared to dry plumes.  相似文献   

8.
Physical parameters of explosive eruptions are typically derived from tephra deposits. However, the characterization of a given eruption relies strongly on the quality of the dataset used, the strategy chosen to obtain and process field data and the particular model considered to derive eruptive parameters. As a result, eruptive parameters are typically affected by a certain level of uncertainty and should not be considered as absolute values. Unfortunately, such uncertainty is difficult to assess because it depends on several factors and propagates from field sampling to the application and interpretation of dispersal models. Characterization of explosive eruptions is made even more difficult when tephra deposits are poorly exposed and only medial data are available. In this paper, we present a quantitative assessment of the uncertainty associated with the characterization of tephra deposits generated by the two largest eruptions of the last 2,000 years of Cotopaxi volcano, Ecuador. In particular, we have investigated the effects of the determination of the maximum clast on the compilation of isopleth maps, and, therefore, on the characterization of plume height. We have also compared the results obtained from the application of different models for the determination of both plume height and erupted volume and for the eruption classification. Finally, we have investigated the uncertainty propagation into the calculation of mass eruption rate and eruption duration. We have found that for our case study, the determination of plume height from isopleth maps is more sensitive to the averaging techniques used to define the maximum clast than to the choice of dispersal models used (i.e. models of Carey and Sparks 1986; Pyle 1989) and that even the application of the same dispersal model can result in plume height discrepancies if different isopleth lines are used (i.e. model of Carey and Sparks 1986). However, the uncertainties associated with the determination of erupted mass, and, as a result, of the eruption duration, are larger than the uncertainties associated with the determination of plume height. Mass eruption rate is also associated with larger uncertainties than the determination of plume height because it is related to the fourth power of plume height. Eruption classification is also affected by data processing. In particular, uncertainties associated with the compilation of isopleth maps affect the eruption classification proposed by Pyle (1989), whereas the VEI classification is affected by the uncertainties resulting from the determination of erupted mass. Finally, we have found that analytical and empirical models should be used together for a more reliable characterization of explosive eruptions. In fact, explosive eruptions would be characterized better by a range of parameters instead of absolute values for erupted mass, plume height, mass eruption rate and eruption duration. A standardization of field sampling would also reduce the uncertainties associated with eruption characterization.  相似文献   

9.
The dispersal of an ash plume as it propagates downwind with the ash settling under gravity is presented. It is shown that wind shear, the initial plume height and the plume grain size distribution have an important role in the dispersal of the ash, in particular in determining the evolution of the upper surface and the leading edge of the ash cloud. Once the ash has thermally equilibrated with the atmosphere, the temperature of the upper surface of the ash plume is directly related to its altitude. As a result we can use the model to interpret satellite images of the temperature of the upper surface of ash plumes. These calculations are compared with new analyses of satellite data from the 18 May 1980 eruptions of Mount St Helens in which both thermal infra-red and visible GOES satellite data were examined. In accord with the data, the model is able to predict the rate at which the Mount St Helens ash plume propagated downwind through a combination of the wind shear and gravitational settling of different size particles. The model is also able to explain the observed thermal structure of the upper surface of the ash plume, in which the temperature initially decreases, but then increases with downwind distance as the ash falls through the tropopause.  相似文献   

10.
Volcanic plumes interact with the wind at all scales. On smaller scales, wind affects local eddy structure; on larger scales, wind shapes the entire plume trajectory. The polar jets or jetstreams are regions of high [generally eastbound] winds that span the globe from 30 to 60° in latitude, centered at an altitude of about 10 km. They can be hundreds of kilometers wide, but as little as 1 km in thickness. Core windspeeds are up to 130 m/s. Modern transcontinental and transoceanic air routes are configured to take advantage of the jetstream. Eastbound commercial jets can save both time and fuel by flying within it; westbound aircraft generally seek to avoid it.Using both an integral model of plume motion that is formulated within a plume-centered coordinate system (BENT) as well as the Active Tracer High-resolution Atmospheric Model (ATHAM), we have calculated plume trajectories and rise heights under different wind conditions. Model plume trajectories compare well with the observed plume trajectory of the Sept 30/Oct 1, 1994, eruption of Kliuchevskoi Volcano, Kamchatka, Russia, for which measured maximum windspeed was 30–40 m/s at about 12 km. Tephra fall patterns for some prehistoric eruptions of Avachinsky Volcano, Kamchatka, and Inyo Craters, CA, USA, are anomalously elongated and inconsistent with simple models of tephra dispersal in a constant windfield. The Avachinsky deposit is modeled well by BENT using a windspeed that varies with height.Two potentially useful conclusions can be made about air routes and volcanic eruption plumes under jetstream conditions. The first is that by taking advantage of the jetstream, aircraft are flying within an airspace that is also preferentially occupied by volcanic eruption clouds and particles. The second is that, because eruptions with highly variable mass eruption rate pump volcanic particles into the jetstream under these conditions, it is difficult to constrain the tephra grain size distribution and mass loading present within a downwind volcanic plume or cloud that has interacted with the jetstream. Furthermore, anomalously large particles and high mass loadings could be present within the cloud, if it was in fact formed by an eruption with a high mass eruption rate. In terms of interpretation of tephra dispersal patterns, the results suggest that extremely elongated isopach or isopleth patterns may often be the result of eruption into the jetstream, and that estimation of the mass eruption rate from these elongated patterns should be considered cautiously.  相似文献   

11.
PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climatic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO2 gas to higher altitudes (~ 25 km), but these pulses represent only a small fraction (~ 1%) of the total erupted material released during the simulation. Comparison with TOMS images of the SO2 cloud after 71 and 93 h indicate that the SO2 gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16–18 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and at a level below the calculated height of neutral buoyancy (~ 25 km). Simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO2 cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.  相似文献   

12.
Tephra fallout from the A-1 (March 29, 0532 UT), B (April 4, 0135 UT), and C (April 4, 1122 UT) 1982 explosive eruptions of El Chichon produced three tephra fall deposits over southeastern Mexico. Bidirectional spreading of eruption plumes, as documented by satellite images, was due to a combination of tropospheric and stratospheric transport, with heaviest deposition of tephra from the ENE tropospheric lobes. Maximum column heights for the eruptions of 27, 32, and 29 km, respectively, have been determined by comparing maximum lithic-clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns. These column heights suggest peak mass eruption rates of 1.1 × 108, 1.9 × 108, and 1.3 × 108 kg/s. Maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits. Sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration. New estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2.19 km3 (1.09 km3 DRE, dense rock equivalent) or roughly twice the amount of the deposit mapped on the ground. Up to one-half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra.  相似文献   

13.
Volcanic gases such as SO 2, H 2S, HCl and COS emitted during explosive eruptions significantly affect atmospheric chemistry and therefore the Earth's climate. We have evaluated the dependence of volcanic gas emission into the atmosphere on altitude, latitude, and tectonic setting of volcanoes and on the season in which eruptions occurred. These parameters markedly influence final stratospheric gas loading. The latitudes and altitudes of 360 active volcanoes were compared to the height of the tropopause to calculate the potential quantity of volcanic gases injected into the stratosphere. We calculated a possible stratospheric gas loading based on different volcanic plume heights (6, 10, and 15 km) generated by moderate-scale explosive eruptions to show the importance of the actual plume height and volcano location. At a plume height of 15 km for moderate-scale explosive eruptions, a volcano at sea level can cause stratospheric gas loading because the maximum distance to the tropopause is 15–16 km in the equatorial region (0–30°). Eruptions in the tropics have to be more powerful to inject gas into the stratosphere than eruptions at high latitudes because the tropopause rises from ca. 9–11 km at the poles to 15–16 km in the equatorial region (0–30°N and S). The equatorial region is important for stratospheric gas injection because it is the area with the highest frequency of eruptions. Gas injected into the stratosphere in equatorial areas may spread globally into both hemispheres.  相似文献   

14.
Hasselblad and Nikon stereographic photographs taken from Skylab between 9 June 1973 and 1 February 1974 give synoptic plan views of several entire eruption clouds emanating from Sakura-zima volcano in Kagoshima Bay, Kyushu, Japan. Analytical plots of these stereographic pairs, studied in combination with meteorological data, indicate that the eruption clouds did not penetrate the tropopause and thus did not create a stratospheric dust veil of long residence time. A horizontal eddy diffusivity of the order of 106 cm2 s?1 and a vertical eddy diffusivity of the order of 105 cm2 s?1 were calculated from the observed plume dimensions and from available meteorological data. These observations are the first, direct evidence that explosive eruption at an estimated energy level of about 1018 ergs per paroxysm may be too small under atmospheric conditions similar to those prevailing over Sakura-zima for volcanic effluents to penetrate low-level tropospheric temperature inversions and, consequently, the tropopause over northern middle latitudes. Maximum elevation of the volcanic clouds was determined to be 3.4 km. The cumulative thermal energy release in the rise of volcanic plumes for 385 observed explosive eruptions was estimated to be 1020 to 1021 ergs (1013 to 1014 J), but the entire thermal energy release associated with pyroclastic activity may be of the order of 2.5 × 1022 ergs (2.5 × 1015 J).Estimation of the kinetic energy component of explosive eruptions via satellite observation and meteorological consideration of eruption clouds is thus useful in volcanology as an alternative technique to confirm the kinetic energy estimates made by ground-based geological and geophysical methods, and to aid in construction of physical models of potential and historical tephra-fallout sectors with implications for volcano-hazard prediction.  相似文献   

15.
Augustine, an island volcano in Lower Cook Inlet, southern Alaska, erupted in January, 1976, after 12 years of dormancy. By April, when the eruptions ended, a new lava dome had been extruded into the summit crater and about 0.1 km3 of pyroclastics had been deposited on the island, mainly as pyroclastic debris avalanches and pumice flows. The ventclearing phase in January was highly explosive and we have been able to document 13 major vulcanian eruptions.The timing, thermal energy, mass loading of fine particles and the horizontal dispersion of these eruption clouds were determined from radar measurements of cloud height, reports of pilots flying in plumes, satellite photography, seismic records and infrasonic detection of air waves. A lower estimate of the mass of fine (r < 68 μm) particles injected into the troposphere from the 13 main eruptions in January is 5.5–18 × 1012 g. The corresponding mass loading of fine particles within individual eruption clouds is 0.3–1 g m−3. We calculated thermal energies of 4 × 1014 to 35 × 1014 J for individual eruptions by applying convective plume rise theory to observed cloud heights and seismically determined eruption durations. This energy range compares favorably with the 4–16 × 1014 J of thermal energy, calculated from the cooling of juvenile material contained in a typical eruption cloud.The vulcanian eruption clouds stayed intact for at least 700 km downwind. Satellite images in both visible and infrared wavebands, showing the Gulf of Alaska just after sunrise on January 23, reveal a series of puffs strung out downwind from the volcano, 20–30 km in diameter and with their tops at altitudes of about 8 km, overlying a continuous plume at altitude 4 km. Each puff corresponded to a seismically and infrasonically timed eruption. A substantial portion of the material injected into the atmosphere between January 22 and 25 was rapidly transported by the subpolar jet stream through southwestern Canada and the western United States, then northeast across the States into the Atlantic. The clouds were observed passing over Tucson, Arizona, on January 25 at an elevation of 7 km.Several of the eruptions penetrated into the stratosphere. Sun photometer measurements, taken at Mauna Loa, Hawaii, six weeks after the eruption, showed an increased stratospheric optical thickness of 0.01 (wavelength 0.5 μm), which decayed in about 5 months. The maximum column mass loading of the veil was 4–10 × 10−7 g cm−2. The mass of the veil, spread-ever a fourth of the earth's surface, is 10 to 100 times larger than can be accounted for by assuming that injected ash and converted sulfate particles from the 13 main Augustine eruptions are the only components contributing to the stratospheric turbidity observed at Mauna Loa.  相似文献   

16.
Plinian plumes erupt with a bulk density greater than that of air, and depend upon air entrainment during their gas-thrust phase to become buoyant; if entrainment is insufficient, the column collapses into a potentially deadly pyroclastic flow. This study shows that strombolian ash plumes can be erupted in an initially buoyant state due to their extremely high initial gas content, and in such cases are thus impervious to column collapse. The high gas content is a consequence of decoupled gas rise in the conduit, in which particles are ultimately incidental. The relations between conduit gas flow, eruption style and plume density are explored here for strombolian scenarios and contrasted with conventional wisdom derived from plinian eruptions. Considering the inherent relation between gas content and initial plume density together with detailed measurements of plume velocities can help unravel ambiguities surrounding conduit processes, eruption styles and hazards at poorly understood volcanoes. Analysis of plume dynamics at Santiaguito volcano, Guatemala adds further support for a model involving decoupled gas rise in the conduit.  相似文献   

17.
We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.  相似文献   

18.
Volcanic eruptions produce ash clouds, which are a major hazard to population centers and the aviation community. Within the North Pacific (NOPAC) region, there have been numerous volcanic ash clouds that have reached aviation routes. Others have closed airports and traveled for thousands of kilometers. Being able to detect these ash clouds and then provide an assessment of their potential movement is essential for hazard assessment and mitigation. Remote sensing satellite data, through the reverse absorption or split window method, is used to detect these volcanic ash clouds, with a negative signal produced from spectrally semi-transparent ash clouds. Single channel satellite is used to detect the early eruption spectrally opaque ash clouds. Volcanic Ash Transport and Dispersion (VATD) models are used to provide a forecast of the ash clouds' future location. The Alaska Volcano Observatory (AVO) remote sensing ash detection system automatically analyzes satellite data of volcanic ash clouds, detecting new ash clouds and also providing alerts, both email and text, to those with AVO. However, there are also non-volcanic related features across the NOPAC region that can produce a negative signal. These can complicate alerts and warning of impending ash clouds. Discussions and examples are shown of these non-volcanic features and some analysis is provided on how these features can be discriminated from volcanic ash clouds. Finally, there is discussion on how information of the ash cloud such as location, particle size and concentrations, could be used as VATD model initialization. These model forecasts could then provide an improved assessment of the clouds' future movement.  相似文献   

19.
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 Keflavík 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.Editorial responsibility: P. Mouginis-Mark  相似文献   

20.
The 2006 eruption of Augustine Volcano, Alaska, began with an explosive phase comprising 13 discrete Vulcanian blasts. These events generated ash plumes reaching heights of 3–14 km. The eruption was recorded by a dense geophysical network including a pressure sensor located 3.2 km from the vent. Infrasonic signals recorded in association with the eruptions have maximum pressures ranging from 13–111 Pa. Eruption durations are estimated to range from 55–350 s. Neither of these parameters, however, correlates with eruption plume height. The pressure record, however, can be used to estimate the velocity and flux of material erupting from the vent, assuming that the sound is generated as a dipole source. Eruptive flux, in turn, is used to estimate plume height, assuming that the plume rises as a buoyant thermal. Plume heights estimated in this way correlate well with observations. Events that exhibit strongly impulsive waveforms are underestimated by the model, suggesting that flow may have been supersonic.  相似文献   

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