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1.
Subaqueous pyroclastic flows and ignimbrites: an assessment 总被引：2，自引：0，他引：2
An assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary, hot gas-rich pyroclastic flows, mass-flows which resulted from the transformation of gassupported flows into water-supported ones, and secondary mass-flows carrying redeposited pyroclastic debris. Based on subaerial pyroclastic flows, the term pyroclastic flow should be restricted to demonstrably hot, gas-rich mass-flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described. In the majority of these cases, the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate. The only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the Ordovician nearshore, shallow marine ignimbrites of Ireland and Wales, and Miocene ignimbrites of southwest Japan, resulting from the passage of subaerially erupted pyroclastic flows into shallow water. Other possible examples are near-vent dense clast deposits in the Donzurobo Formation of Japan, possible submarine intra-caldera ponded ignimbrite successions in California and Wales, and near-vent pumiceous deposits of Ramsay Island, Wales. All other purported cases are either clearly the result of water-supported mass-flow transportation and deposition (debris avalanches, debris flows, turbidity currents), or lack adequate supporting evidence regarding the heat state or the palaeoenvironment. Only the shallow marine ignimbrites of Ireland and Wales show adequate evidence of welding, but even these could have been nearly wholly exposed above sea-level when welding occurred. We conclude that when pyroclastic flows enter water they are generally disrupted explosively and/or ingest water and transform into water-supported mass-flows, and we suggest the various scenarios in which this occurs. There is no evidence to suggest that welding in wholly subaqueous environments is common. 相似文献
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利用HYSPLIT_4模式分析珠海地面SO2浓度的变化规律 总被引：16，自引：1，他引：16
利用HYSPLIT_4传输、扩散模式，模拟大尺度气象场下的1999年10月-2000年1月珠海地面SO2浓度变化规律，结果与实测资料对比表明，模式较好地模拟出珠海地面SO2的浓度水平和变化趋势。模拟结果经过分析还表明，珠海地面S02浓度受到广东省其它地区尤其是珠江三角洲地区城市污染源排放的影响；其浓度变化与广州900m高度（或965hPa等压面）水平风场具有相当好的对应关系。 相似文献
3.
M. Escudero A.F. Stein R.R. Draxler X. Querol A. Alastuey S. Castillo A. Avila 《Atmospheric Research》2011,99(3-4):518-527
A source apportionment technique has been applied to determine the geographical distribution of emissions in Northern Africa contributing to dust outbreaks that yield high PM10 levels at Spanish regional background stations. Seven dust episodes have been analyzed in this study. Total suspended particles have been sampled and chemically analyzed for these events at La Castanya background station (Montseny, NE Spain) and differences in the composition of airborne dust have been studied. The dominant role of northern and western source areas (Tunisia, Algeria, Mauritania and the Western Sahara) contrasted with the negligible contribution of major emission source areas such as the Bodelé depression, Libya, Niger, and Sudan. During the simulated events using the dust module of the HYSPLIT model, material from the latter regions is persistently transported across the Atlantic but not towards Western Europe. As a consequence, the composition of the dust turned out to be quite homogeneous since the mixing of dust occurs from various source areas with similar chemical composition. However, differences in Ca/Al ratios have been found in a number of samples that are mainly explained by vertical transport segregation of clay minerals (relatively richer in Al) from coarser dust particles (Ca-carbonate). 相似文献
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Dispersion Modelling of the Kilauea Plume 总被引：1，自引：0，他引：1
Annette T. Hollingshead Steven Businger Roland Draxler John Porter Duane Stevens 《Boundary-Layer Meteorology》2003,108(1):121-144
Emissions from the Kilauea volcano pose significant environmental and health risksto the Hawaiian community. This paper describes progress toward simulating theconcentration and dispersion of plumes of volcanic aerosol after they emanate from thePu'u O'o vent of the Kilauea volcano.In order to produce an accurate regional forecast of the concentration and dispersionof volcanic aerosol, the Hybrid Single-Particle Lagrangian Integrated Trajectory(HY-SPLIT) model was used. Wind fields and thermodynamic data from the non-hydrostatic Mesoscale Spectral Model (MSM) were employed as input for theHY-SPLIT model. A combination of satellite remote sensing, aircraft, and ground-based observations collected during a field experiment was used to validate the model simulation of aerosol distribution.The HY-SPLIT model shows skill in reproducing the plume shape, orientation, and concentration gradients as deduced from satellite images of aerosol optical depth.Comparison of the modelled and observed values suggests that the model was able to produce reasonable plume concentrations and spatial gradients downwind of the source. Model concentrations were generally less than those observed on the leeward side of the Island of Hawaii. This deficiency may be explained by a lack of (i) background concentrations, (ii) local sources of pollution and/or (iii) sea-breeze circulation in the prognostic input wind field. These results represent early progress toward the goal of future operational application of the HY-SPLIT model to predict volcanic aerosol concentrations in Hawaii. This may help mitigate their negative impacts of plumes respiratory health, agriculture, and general aviation. 相似文献
5.
周边地区对北京地面SO2影响的初步研究 总被引：15，自引：2，他引：15
采用Lagrangian传输、扩散、沉降模式计算了1994年9、10月份(非采暖期)、1995年1月和2000年1月(采暖期)北京地区SO2的变化规律,并分别计算了北京市不同类型的排放源以及周边地区污染源对平均地面SO2浓度的贡献.计算结果与实际观测资料对比表明,模式能够较好地模拟北京地区SO2的浓度量级和逐日变化趋势.北京地区地面SO2浓度受天气条件的影响较大.采暖期,北京当地排放源的影响相对较大;但在大气条件有利输送的背景下,来自周边地区污染排放源的影响不容忽视. 相似文献
6.
本文介绍了断裂引起的应变量计算方法。断裂作用可导致连续应变和非连续应变。连续应变与断裂位移,断裂长度比值及断裂面上有效应力成正相关关系。影响非连续应变的因素有：断裂几何形态、断裂的旋转性、断裂规模。已经提出三种断裂旋转机制：刚性旋转,垂直剪切和斜向剪切。对于这三种机制,我们分别建立了断裂非连续应变的计算公式。这些公式与断裂的旋转角度和位移大小相关。刚性旋转时,断块内部没有任何塑性变形,因此地层的长度没有变化。它引起的非连续应变最小。垂直剪切作用使断块内地层变形,但水平方向的地层长度不变。推算的公式表明,对于相同的原始数据,它引起的非连续应变比刚性旋转机制引起的非连续应变大。斜向剪切也使断块内地层变形,但水平方向的长度也不变。在同等条件下,它引起的非连续应变比垂直剪切机制引起的非连续应变大。 相似文献
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Roland R. Draxler 《Boundary-Layer Meteorology》1988,42(1-2):43-53
Experimental studies have shown that tracers injected over short time intervals can often be measured downwind near the ground for a much longer time than would be expected considering the duration of the release and the wind speed. A two-dimensional advection-diffusion model was used in a numerical study to investigate this effect. The results from the numerical study showed that the rate of decrease in concentrations downwind at a receptor after a source has stopped emitting was more sensitive to magnitude of the wind speed and alongwind mixing coefficient than the rate of increase in concentrations when the source first starts emitting. The lower the wind speed or the greater the mixing, the more slowly the concentration decreased as the region upwind of the receptor continued to contribute tracer until it was depleted by advection and vertical diffusion. These atmospheric processes are able, qualitatively, to account for the slowly decreasing air concentrations. 相似文献
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Annette T. Hollingshead Steven Businger Roland Draxler John Porter Duane Stevens 《Boundary-Layer Meteorology》2003,109(1):107-107