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1.
The dynamics and thermodynamics of large ash flows 总被引:6,自引:6,他引:0
Ash flow deposits, containing up to 1000 km3 of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres
from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres
high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a
collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and
deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical
ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more
voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain,
the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system,
and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control
on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the
flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained
into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into
the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate
further than their radially spreading counterparts.
As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous
ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain
large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the
erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less
air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from
the vent, will develop.
Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai
and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass
fluxes of 109–1010 kg/s over periods of 103–105 s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations,
the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km.
Received: 25 August 1995 / Accepted: 3 April 1996 相似文献
2.
This article explores the possibility of urbanization‐ and irrigation‐induced increases in summer precipitation totals in central Arizona. Maximum precipitation impacts are hypothesized to occur downwind of the Phoenix area in the Lower Verde basin. Results from statistical tests indicate that summer precipitation totals in the Lower Verde basin are greater than totals in nearby basins. Precipitation totals in the basin also appear to be equivalent to totals at more monsoon‐impacted stations in eastern Arizona. While this research is preliminary, the results do provide encouraging evidence of the existence of anthropogenically enhanced summer precipitation in central Arizona. 相似文献
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4.
RESURGENT MEXICAN PHOENIX* 总被引:1,自引:0,他引:1
ABSTRACT. Popular impressions of Phoenix, Arizona perpetuate the notion that this metropolitan area is an overwhelmingly Anglo place. We challenge this assertion and demonstrate that the city has substantial Mexican roots and is presently being shaped by a vibrant, resurgent Mexican population. Employing historical records, surveys, and landscape data, we articulate the Mexican character of early Phoenix and highlight how the revival of Mexican Phoenix has transformed the urban landscape. We then relate how Phoenix's Mexican population is a more nuanced regional subculture formed through both historical and contemporary connections with specific Mexican states. We conclude with a call for greater understanding of the internal heterogeneity of Mexicans in the United States and how this can inform our geographical interpretations of the growing Latinization of American cities. 相似文献
5.
《Urban geography》2013,34(1):61-82
This research investigates the process through which small-town central business districts (CBDs) are transformed into visitor-oriented attractions. A case study of Galena, Illinois, reveals that the dominant characteristics of the downtown redevelopment process are applicable to metropolis and small town alike. Similar economic “tools,” enabling legislation, and other mechanisms are available to essentially all municipalities regardless of size. Further, growth coalitions and interest groups are formed to promote the particular use and exchange values of CBDs throughout the urban hierarchy. What differs between small town and large metropolis, however, is the way in which contemporary CBDs are perceived by the community. While the metropolitan downtown is now largely viewed as a “command center” within a global economy, the small-town CBD is still varyingly perceived as the community's economic and social center. Such contrasting conceptions of downtown hold implications for the ability to successfully transform Main Street into a tourist attraction without generating substantial inter-community conflict. [Key words: small town, historic district, main street, downtown redevelopment, tourism.] 相似文献
6.
The Phoenix Lander landed on Mars on 25 May 2008. It has instruments on board to explore the geology and climate of subpolar Mars and to explore if life ever arose on Mars. Although the Phoenix mission is not a life detection mission per se, it will look for the presence of organic compounds and other evidence to support or discredit the notion of past or present life.The possibility of extant life on Mars has been raised by a reinterpretation of the Viking biology experiments [Houtkooper, J. M., Schulze-Makuch, D., 2007. A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. International Journal of Astrobiology 6, 147-152]. The results of these experiments are in accordance with life based on a mixture of water and hydrogen peroxide instead of water. The near-surface conditions on Mars would give an evolutionary advantage to organisms employing a mixture of H2O2 and H2O in their intracellular fluid: the mixture has a low freezing point, is hygroscopic and provides a source of oxygen. The H2O2-H2O hypothesis also explains the Viking results in a logically consistent way. With regard to its compatibility with cellular contents, H2O2 is used for a variety of purposes in terran biochemistry. The ability of the anticipated organisms to withstand low temperatures and the relatively high water vapor content of the atmosphere in the Martian arctic, means that Phoenix will land in an area not inimical to H2O2-H2O-based life. Phoenix has a suite of instruments which may be able to detect the signatures of such putative organisms. 相似文献
7.
ABSTRACT. In an effort to provide a more complex and multifaceted understanding of the process of spatial assimilation, this article explores alternative paths in understanding racial/ ethnic minority residential patterns. It scrutinizes patterns of contemporary Asian Indian and Chinese settlement in two metropolitan areas: Austin, Texas and Phoenix, Arizona. Though not particularly evolved in terms of their Asian immigrant settlement or dynamics, Austin and Phoenix represent the growing number of newly emergent Asian centers throughout the nation that have developed with the rapid rise of immigration from these two countries in the past several decades. The analysis utilizes records from the 2000 census to document and map Asian Indian and Chinese settlement within each metropolitan area and to investigate whether‐and to what degree‐each group is clustered or dispersed. The article then raises important questions about the consequences of concentration and dispersal for the incorporation of Asian Indian and Chinese residents. 相似文献
8.
为研究台风模拟对不同陆面过程方案的敏感性,本文以0808号登陆台风"凤凰"为例,采用中尺度数值模式WRFv3及NCEP同化资料,进行了48 h的数值研究。结果表明,每一陆面方案与大气模式耦合模拟时台风中心位置及台风最大风速都呈现出该耦合系统独特的模拟特征,如与观测台风中心位置相比,NOAH方案模拟移动路径平均偏差最小,PX模拟偏差最大,NOAH-PX两方案差异可达NOAH方案模拟移动路径平均偏差的40%,可见台风模拟可因陆面方案的不同而呈现系统性的差异,且台风模拟对陆面方案的选择是敏感的;不同陆面方案通过模拟地表通量的差异导致温压场差异,并进一步影响模拟台风路径及台风强度。这些结果都表明了在台风的模式预报中,选择适当陆面方案的重要性。 相似文献
9.
Peter A. Taylor P-Y. Li Diane V. Michelangeli Jagruti Pathak Wensong Weng 《Boundary-Layer Meteorology》2007,125(2):305-328
A time and height dependent eddy diffusion model is used to investigate possible scenarios for the size distribution of dust
in the lower atmosphere of Mars. The dust is assumed to either have been advected from a distant source or to have originated
locally. In the former case, the atmosphere is assumed to initially contain dust particles with sizes following a modified
gamma distribution. Larger particles are deposited relatively rapidly while small particles are well mixed up to the maximum
height of the afternoon boundary layer and are deposited more slowly. In other cases, a parameterization of the dust source
at the surface is proposed. Model results show that smaller particles are rapidly mixed within the Martian boundary layer,
while larger particles (r > 10 μm) are concentrated near the ground with a stronger diurnal cycle. In all simulations we assume
that the initial concentration or surface source depend on a modified gamma function distribution. For small particles (cross-
sectional area weighted mean radius, reff = 1.6 μm) distributions retain essentially the same form, though with variations in the mean and variance of the area-weighted
radius, and the gamma function can be used to represent the particle size distribution reasonably well at most heights within
the boundary layer. In the case of a surface source of larger particles (mean radius 50 μm) the modified gamma function does
not fit the resulting particle size distribution. All results are normalised by a scaling factor that can be adjusted to correspond
to an optical depth for assumed particle optical scattering properties. 相似文献
10.
Physical characteristics of naturally formed convective vortices in the Phoenix Mars lander environment have been investigated on a relatively hot summer Martian arctic day. For this, the NCAR LES has been adapted and developed to conduct three micro-scale simulations of the Martian Convective Boundary Layer (CBL), in situations with and without geostrophic wind, and atmospheric radiative flux divergence. Time series analysis of the vortices’ properties is discussed. The study confirms the decrease of vortex populations in windy conditions and also illustrates that intense but small vortices are expected to be observed in higher geostrophic wind situations. This may lead to more dust migration rather than dust devil formation on windy days. The background (geostrophic) wind causes the vortices to become less cyclostrophically balanced. 相似文献