Comprehensive simulation of the middle atmospheric climate: some recent results     

Kevin Hamilton 《Climate Dynamics》1995,11(4):223-241

This study discusses the results of comprehensive time-dependent, three-dimensional numerical modelling of the circulation in the middle atmosphere obtained with the GFDL SKYHI troposphere-stratosphere-mesosphere general circulation model (GCM). The climate in a long control simulation with an intermediate resolution version (3° in horizontal) is briefly reviewed. While many aspects of the simulation are quite realistic, the focus in this study is on remaining first-order problems with the modelled middle atmospheric general circulation, notably the very cold high latitude temperatures in the Southern Hemisphere (SH) winter/spring, and the virtual absence of a quasi-biennial oscillation (QBO) in the tropical stratosphere. These problems are shared by other extant GCMs. It was noted that the SH cold pole problem is somewhat ameliorated with increasing horizontal resolution in the model. This suggests that improved resolution increases the vertical momentum fluxes from the explicitly resolved gravity waves in the model, a point confirmed by detailed analysis of the spectrum of vertical eddy momentum flux in the winter SH extratropics. This result inspired a series of experiments with the 3° SKYHI model modified by adding a prescribed zonally-symmetric zonal drag on the SH winter westerlies. The form of the imposed momentum source was based on the simple assumption that the mean flow drag produced by unresolved waves has a spatial distribution similar to that of the Eliassen-Palm flux divergence associated with explicitly resolved gravity waves. It was found that an appropriately-chosen drag confined to the top six model levels (above 0.35 mb) can lead to quite realistic simulations of the SH winter flow (including even the stationary wave fields) through August, but that problems still remain in the late-winter/springtime simulation. While the imposed momentum source was largely confined to the extratropics, it produced considerable improvement in the simulation of the equatorial semiannual oscillation, with both the easterly and westerly phases being somewhat more intense than in the control simulation. A separate experiment was conducted in which the SKYHI model was simplified so that it had no topography and so that the seasonal cycle was frozen in perpetual equinox conditions. These changes result in a model that has much reduced interhemispheric asymmetry. This model spontaneously produces a long period mean flow oscillation of considerable amplitude in the tropical upper stratopause. The implication of this result for the general issue of obtaining a QBO in comprehensive GCMs is discussed.  相似文献   

Complex refractive index of Martian dust: Wavelength dependence and composition     

Kevin Pang  Joseph M. Ajello 《Icarus》1977,30(1):63-74

Mariner 9 ultraviolet spectra of the 1971 dust clouds were analyzed to obtain the phase function times single scattering albedo of the dust particles. The phase functions were matched with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. The absorption index of the dust particles was found to increase with decreasing wavelenght from 350 nm down to about 210 nm, and drop off shortward of this wavelength. A structural shoulder occurs in the absorption spectrum between 240 and 250 nm. Titanium dioxide (anatase) has the correct transitions at 210 and 250 nm to match the absorption curve of Martian dust, and is proposed as a candidate constituent in Martian surface material. The spectral neutrality of TiO₂ between 0.5 and 4 μm is consistent with visible and infrared observations of Mars. The high refractive index of TiO₂ can explain the large refringence of Martian dust. The titanium dioxide content of the dust particles is estimated to be a few percent or less. Uncertainties in the results due to limitations in the model and data are discussed.  相似文献   

Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle     

Trond H. Torsvik  Mark A. Smethurst  Kevin Burke  Bernhard Steinberger 《Geophysical Journal International》2006,167(3):1447-1460

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Racism and Intolerance in Eastern Australia: a geographic perspective     

James Forrest  Kevin Dunn 《The Australian geographer》2006,37(2):167-186

Racism has become a fact of life in Australia over the past decade or so, yet there are relatively few studies of its nature or extent, and still fewer on its geography. Using a social constructivist approach, this study draws on a survey of 5056 respondents to investigate attitudes to racism and cultural diversity in New South Wales and Queensland, and of perceptions of out-groups as instances of ‘strangers in our midst’. On racism, results show the presence of a continuum of attitudes ranging from generally tolerant to generally intolerant, a presence which cuts across compositional (social or aspatial) characteristics to emphasise the existence of a distinctive geography, an everywhere different nature to racist and non-racist attitudes which transcends urban–rural and traditional social layers. On the other hand, perceptions of out-groups are not uniformly correlated with presence or absence of cultural diversity. In many cases, the ability to make judgements about significant ‘others’ or out-groups has been shown to relate more to abstract notions of self and national identity, reproduced in public by mainstream news media and political leaders. In particular, it may reflect an Anglo (or Anglo-Celtic) view on nationalism, which is a hallmark of the ‘new racism’: an assimilationist or ethnocultural view of Australian society which is different from the ‘civic nation’ ideal envisaged by multiculturalism. That the geography of attitudes and perceptions people have towards and about different cultural groups is so ‘everywhere different’ has important implications for attempts to address and redress issues of intolerance in Australia.  相似文献   

液态水含量和冰晶浓度对闪电频数影响的数值模拟研究   总被引：4，自引：1，他引：4  

谢屹然  郄秀书  郭凤霞  Marcia Baker 《高原气象》2005,24(4):598-603

假定在软雹和冰晶碰撞的非感应起电机制为主要的起电机制成立的条件下，数值模拟研究了国际上公认的Fletcher和H—M冰晶产生机制以及云中的液态水含量对雷暴云放电过程（区分了云闪和地闪）的影响。结果表明，随着气压和温度的变化，在两种冰晶产生机制假定下，冰晶浓度分布有很大差异，这直接导致了雷暴云内电活动的差异。液态水含量的增加将使得首次放电时间延迟，同时将引起放电位置的下降和闪电频数的减少。  相似文献   

The role of an H<Subscript>2</Subscript>O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region,N California   总被引：2，自引：1，他引：2  

T. Grove  S. Parman  S. Bowring  R. Price  M. Baker 《Contributions to Mineralogy and Petrology》2002,142(4):375-396

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Melting of β-quartz up to 2.0 GPa and thermodynamic optimization of the silica liquidus up to 6.0 GPa     

Pierre Hudon  In-Ho Jung  Don R. Baker 《Physics of the Earth and Planetary Interiors》2002,130(3-4):159-174

Melting relations of β-quartz were experimentally determined at 1.0 GPa (1900±20 °C), 1.5 GPa (2033±20 °C), and 2.0 GPa (2145±20 °C) using a new high-pressure assembly in a piston–cylinder apparatus and substantial differences were found with data previously reported. The new melting data of β-quartz were combined and optimized with all available thermodynamic, volumetric, and phase equilibria data for β-cristobalite, β-quartz and coesite to produce a P–T liquidus diagram for silica valid up to 6.0 GPa. Using the new optimized thermodynamic parameters, the invariant point β-cristobalite+β-quartz+liquid and β-quartz+coesite+liquid were determined to lie at 1687±17 °C and 0.457 GPa, and 2425±25 °C and 5.00 GPa, respectively.  相似文献   

Earth to academia: on the need to reconnect university and school geography     

Kevin Stannard 《Area》2003,35(3):316-322

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Drainage, sediment transport, and denudation rates on the Nanga Parbat Himalaya, Pakistan     

Kevin Cornwell  Doug Norsby  Richard Marston 《Geomorphology》2003,55(1-4):25

The Nanga Parbat Himalaya presents some of the greatest relief on Earth, yet sediment production and denudation rates have only been sporadically addressed. We utilized field measurements and computer models to estimate bank full discharge, sediment transport, and denudation rates for the Raikot and Buldar drainage basins (north slope of Nanga Parbat) and the upper reach of the Rupal drainage basin (south slope).The overall tasks of determining stream flow conditions in such a dynamic geomorphic setting is challenging. No gage data exist for these drainage basins, and the overall character of the drainage basins (high relief, steep flow gradients, and turbulent flow conditions) does not lend itself to either ready access or complete profiling.Cross-sectional profiles were surveyed through selected reaches of these drainage basins. These data were then incorporated into software (WinXSPRO) that aids in the characterization (stage, discharge, velocity, and shear stress) of high altitude, steep mountain stream conditions.Complete field measurements of channel depths were rarely possible (except at several bridges where the middle of the channel could actually be straddled and probed) and, when coupled with velocity measurements, provided discrete points of field-measured discharge calculations. These points were then used to calibrate WinXSPRO results for the same reach and provided a confidence level for computer-generated results.Flow calculations suggest that under near bank full conditions, the upper Raikot drainage basin produces discharges of 61 cm and moves about 11,000 tons day⁻¹ (9980 tons day⁻¹) of sediment through its channel. Bank full conditions on the upper portion of the Rupal drainage basin generate discharges of 84 cm and moves only about 3800 tons day⁻¹ (3450 tons day⁻¹) of sediment. Although the upper Rupal drainage basin moves more water, the lower slope of the drainage basin (0.03) generates a much smaller shear stress (461 Pa) than does the higher slope (0.12) of the upper Raikot drainage basin (1925 Pa).Dissolved and suspended sediment loads were measured from water/sediment samples collected throughout the day and night over a period of 10 days at the height of the summer melt season but proved to be a minor variable in transport flux. Channel bed loads were measured using a pebble count method of bank material and then used to generate ratings curves of bed loads relative to discharge volumes. When coupled with discharge data and basin area, mean annual sediment yield and denudation rates for Nanga Parbat are produced. Denudation rates calculated in this fashion range from 0.2 mm year⁻¹ in the slower, more sluggish Rupal drainage basin to almost 6 mm year⁻¹ in the steeper, faster flowing Raikot and Buldar drainage basins.  相似文献   

Predictive instantaneous optimal control of elastic structures during earthquakes     

Kevin K. F. Wong  Rong Yang 《地震工程与结构动力学》2003,32(14):2161-2177

A predictive instantaneous optimal control (PIOC) algorithm is proposed for controlling the seismic responses of elastic structures. This algorithm compensates for the time delay that happens in practical control applications by predicting the structural response over a period that equals the time delay, and by substituting the predicted response in the instantaneous optimal control (IOC) algorithm. The unique feature of this proposed PIOC algorithm is that it is simple and at the same time compensates for the time delay very effectively. Numerical examples of single degree of freedom structures are presented to compare the performance of PIOC and IOC systems for various time delay magnitudes. Results show that a time delay always causes degradation of control efficiency, but PIOC can greatly reduce this degradation compared to IOC. The effects of the structure's natural periods and the choice of control gains on the degradation induced by the time delay are also analyzed. Results show that shorter natural periods and larger control gains are both more sensitive and more serious to the degradation of control efficiency. Finally, a practical application of PIOC is performed on a six‐story moment‐resisting steel frame. It is demonstrated that PIOC contributes significantly to maintain stability in multiple degree of freedom structures, and at the same time PIOC has a satisfactory control performance. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献