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101.
Charles E. Konrad II 《自然地理学》2013,34(3):227-248
Synoptic data associated with a sample of 554 heavy rainfall events is utilized to carry out a trajectory analysis that identifies the movements of moisture towards regions of heavy rain. Both seasonal and regional variations are found in the moisture trajectories associated with heavy rain events occurring in three regions in the Appalachian study area. Numerous events in the region west of the mountain range are tied to westerly and south-southwesterly circulations that bring moisture from the Mississippi River Valley and the Gulf of Mexico during the warm and cool seasons, respectively. Many events southeast of the mountain range are associated with southerly to southeasterly circulations that advect moisture from the Atlantic Ocean. Because of orographic precipitation enhancement and a good exposure to the Gulf of Mexico and the Atlantic Ocean, the southern and southeastern slopes of the Appalachian mountains display high frequencies of heavy rainfall, particularly during the cool season. The interior portions of the mountain range and the adjacent plateau to the northwest, on the other hand, are sheltered from moisture source regions and, therefore, exhibit low heavy rain frequencies. [Key words: precipitation, synoptic climatology, Appalachia.] 相似文献
102.
Wettability and retention capacity of leaf surfaces are parameters that contribute to interception of rain, fog or dew by forest canopies. Contrary to common expectation, hydrophobicity or wettability of a leaf do not dictate the stickiness of drops to leaves. Crucial for the adhesion of drops is the contact angle hysteresis, the difference between leading edge contact angle and trailing edge contact angle for a running drop. Other parameters that are dependent on the static contact angle are the maximum volume of drops that can stick to the surface and the persistence of an adhering drop with respect to evaporation. Adaption of contact angle and contact angle hysteresis allow one to pursue different strategies of drop control, for example efficient water shedding or maximum retention of adhering water. Efficient water shedding is achieved if contact angle hysteresis is low. Retention of (isolated) large drops requires a high contact angle hysteresis and a static contact angle of 65.5°, while maximum retention by optimum spacing of drops necessitates a high contact angle hysteresis and a static contact angle of 111.6°. Maximum persistence with respect to evaporation is obtained if the static contact angle amounts to 77.5°, together with a high contact angle hysteresis. It is to be expected that knowledge of these parameters can contribute to the capacity of a forest to intercept water. 相似文献
103.
Testing the reliability of pollen-based diversity estimates 总被引:2,自引:2,他引:0
Rarefaction analysis is a common tool for estimating pollen richness. Using modern and fossil pollen data from the Canadian
Arctic and Greenland, we examine the effects of pollen concentration (grains/cc) and evenness (the distribution of species
abundances) on palynological richness. Our results show that pollen richness and concentration have a strong negative correlation
at low pollen concentrations. There is a positive correlation between pollen evenness and richness, although the strength
of this relationship is difficult to determine. Rarefaction analysis on samples of low concentration or high evenness is likely
to lead to pollen richness being less underestimated than on samples of high concentration or low evenness. These findings
corroborate theoretical research on these issues. 相似文献
104.
Beata Dziak-Jankowska Jacek Leliwa-Kopystyński Konrad J. Kossacki 《Acta Geophysica》2009,57(2):509-535
In this paper we intend to analyze how the sublimation of ice from cometary nuclei affects changes of the moments of inertia.
Our aim is to show general trends for different orientations of cometary nucleus’ rotation axis. Thus we apply numerical model
of a hypothetical homogeneous and initially spherical nucleus composed of water ice and dust. As an example we present simulations
for a model comet of the orbital elements and the nucleus size the same as determined for C/1995 O1 Hale-Bopp, a widely analyzed
long-period comet. We calculated water production from the nucleus and changes of the shape (initially spherical) and of the
moment of inertia versus time. Simulations are performed for the full range (0–90°) of inclinations I of the rotation axis. The second paramater related to the orientation of the rotation axis is the argument Φ (0–360°). The heat conductivity of the nucleus spans over the vast range, 0.04–4 W m−1 K−1. 相似文献
105.
The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April
2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured
lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in
this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in
width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus
that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular,
measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the
elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported
for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli.
All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully)
suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers
that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust. 相似文献
106.
A numerical model for convective heat and mass transport of compressible or incompressible gas flows with soil-water phase change is presented. In general, the gaseous phase is considered as compressible and the model accounts for adiabatic processes of compression heating and expansion cooling. The inherently compressible gaseous phase may nevertheless be considered as incompressible by adopting the Oberbeck–Boussinesq approximations. The numerical method used to solve the equations that describe natural convection is based on a Galerkin finite element formulation with adaptive mesh refinement and dynamic time step control. As most existing numerical studies have focused on the behavior of incompressible fluids, model substantiation examines the influence of fluid compressibility on two-widely used benchmarks of steady-state convective heat and mass transport. The relative importance of the effect of pressure-compressibility cooling is shown to increase as the thermal gradient approaches the magnitude of the adiabatic gradient. From these results, it may be concluded that pore-air compressibility cannot be neglected in medium to large-sized enclosures at small temperature differentials. After demonstrating its ability to solve fairly complex transient problems, the model is used to further our understanding of the thermal behavior of the toe drain at the LA2-BSU dam in the province of Quebec, Canada. 相似文献
107.
108.
Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania 总被引:1,自引:0,他引:1
Jörg Keller Jurgis Klaudius Matthieu Kervyn Gerald G. J. Ernst Hannes B. Mattsson 《Bulletin of Volcanology》2010,72(8):893-912
With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO2, with 7–11 wt.% CO2, high alkalies (Na2O 15–19%, K2O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition. 相似文献
109.
Matthieu Kervyn Gerald G. J. Ernst Jörg Keller R. Greg Vaughan Jurgis Klaudius Evelyne Pradal Frederic Belton Hannes B. Mattsson Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2010,72(8):913-931
On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL),
N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in
March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake
swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based
on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite
data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions.
After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns
dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone.
Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter
being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented
in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase
of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions
to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL. 相似文献
110.
Copula-based geostatistical modeling of continuous and discrete data including covariates 总被引:4,自引:3,他引:1
Hannes Kazianka Jürgen Pilz 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(5):661-673
It is common in geostatistics to use the variogram to describe the spatial dependence structure and to use kriging as the spatial prediction methodology. Both methods are sensitive to outlying observations and are strongly influenced by the marginal distribution of the underlying random field. Hence, they lead to unreliable results when applied to extreme value or multimodal data. As an alternative to traditional spatial modeling and interpolation we consider the use of copula functions. This paper extends existing copula-based geostatistical models. We show how location dependent covariates e.g. a spatial trend can be accounted for in spatial copula models. Furthermore, we introduce geostatistical copula-based models that are able to deal with random fields having discrete marginal distributions. We propose three different copula-based spatial interpolation methods. By exploiting the relationship between bivariate copulas and indicator covariances, we present indicator kriging and disjunctive kriging. As a second method we present simple kriging of the rank-transformed data. The third method is a plug-in prediction and generalizes the frequently applied trans-Gaussian kriging. Finally, we report on the results obtained for the so-called Helicopter data set which contains extreme radioactivity measurements. 相似文献