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71.
Vincent Eke † George Efstathiou Lisa Wright 《Monthly notices of the Royal Astronomical Society》2000,315(2):L18-L22
Hydrodynamical simulations of galaxy formation in spatially flat cold dark matter (CDM) cosmologies with and without a cosmological constant (Λ) are described. A simple star formation algorithm is employed and radiative cooling is allowed only after redshift z =1 so that enough hot gas is available to form large, rapidly rotating stellar discs if angular momentum is approximately conserved during collapse. The specific angular momenta of the final galaxies are found to be sensitive to the assumed background cosmology. This dependence arises from the different angular momenta contained in the haloes at the epoch when the gas begins to collapse and the inhomogeneity of the subsequent halo evolution. In the Λ-dominated cosmology, the ratio of stellar specific angular momentum to that of the dark matter halo (measured at the virial radius) has a median value of ∼0.24 at z =0. The corresponding quantity for the Λ=0 cosmology is over three times lower. It is concluded that the observed frequency and angular momenta of disc galaxies pose significant problems for spatially flat CDM models with Λ=0 but may be consistent with a Λ-dominated CDM universe. 相似文献
72.
73.
Kelsey?N.?EllisEmail author Jon?M.?Hathaway Lisa?Reyes?Mason David?A.?Howe Thomas?H.?Epps Vincent?M.?Brown 《Theoretical and Applied Climatology》2017,127(3-4):701-710
The urban heat island (UHI) is a well-documented effect of urbanization on local climate, identified by higher temperatures compared to surrounding areas, especially at night and during the warm season. The details of a UHI are city-specific, and microclimates may even exist within a given city. Thus, investigating the spatiotemporal variability of a city’s UHI is an ongoing and critical research need. We deploy ten weather stations across Knoxville, Tennessee, to analyze the city’s UHI and its differential impacts across urban neighborhoods: two each in four neighborhoods, one in more dense tree cover and one in less dense tree cover, and one each in downtown Knoxville and Ijams Nature Center that serve as control locations. Three months of temperature data (beginning 2 July 2014) are analyzed using paired-sample t tests and a three-way analysis of variance. Major findings include the following: (1) Within a given neighborhood, tree cover helps negate daytime heat (resulting in up to 1.19 °C lower maximum temperature), but does not have as large of an influence on minimum temperature; (2) largest temperature differences between neighborhoods occur during the day (0.38–1.16 °C difference), but larger differences between neighborhoods and the downtown control occur at night (1.04–1.88 °C difference); (3) presiding weather (i.e., air mass type) has a significant, consistent impact on the temperature in a given city, and lacks the differential impacts found at a larger-scale in previous studies; (4) distance from city center does not impact temperature as much as land use factors. This is a preliminary step towards informing local planning with a scientific understanding of how mitigation strategies may help minimize the UHI and reduce the effects of extreme weather on public health and well-being. 相似文献
74.
Allison L. Steiner Jeremy S. Pal Sara A. Rauscher Jason L. Bell Noah S. Diffenbaugh Aaron Boone Lisa C. Sloan Filippo Giorgi 《Climate Dynamics》2009,33(6):869-892
Coupling of the Community Land Model (CLM3) to the ICTP Regional Climate Model (RegCM3) substantially improves the simulation
of mean climate over West Africa relative to an older version of RegCM3 coupled to the Biosphere Atmosphere Transfer Scheme
(BATS). Two 10-year simulations (1992–2001) show that the seasonal timing and magnitude of mean monsoon precipitation more
closely match observations when the new land surface scheme is implemented. Specifically, RegCM3–CLM3 improves the timing
of the monsoon advance and retreat across the Guinean Coast, and reduces a positive precipitation bias in the Sahel and Northern
Africa. As a result, simulated temperatures are higher, thereby reducing the negative temperature bias found in the Guinean
Coast and Sahel in RegCM3–BATS. In the RegCM3–BATS simulation, warmer temperatures in northern latitudes and wetter soils
near the coast create excessively strong temperature and moist static energy gradients, which shifts the African Easterly
Jet further north than observed. In the RegCM3–CLM3 simulation, the migration and position of the African Easterly Jet more
closely match reanalysis winds. This improvement is triggered by drier soil conditions in the RegCM3–CLM3 simulation and an
increase in evapotranspiration per unit precipitation. These results indicate that atmosphere–land surface coupling has the
ability to impact regional-scale circulation and precipitation in regions exhibiting strong hydroclimatic gradients. 相似文献
75.
David G. Barber Matthew G. Asplin Tim N. Papakyriakou Lisa Miller Brent G. T. Else John Iacozza C. J. Mundy M. Gosslin Natalie C. Asselin Steve Ferguson Jennifer V. Lukovich Gary A. Stern Ashley Gaden Monika Pu?ko N.-X. Geilfus Fei Wang 《Climatic change》2012,115(1):135-159
Change and variability in the timing and magnitude of sea ice geophysical and thermodynamic state have consequences on many aspects of the arctic marine system. The changes in both the geophysical and thermodynamic state, and in particular the timing of the development of these states, have consequences throughout the marine system. In this paper we review the ??consequences?? of change in sea ice state on primary productivity, marine mammal habitats, and sea ice as a medium for storage and transport of contaminants and carbon exchange across the ocean-sea-ice-atmosphere interface based upon results from the International Polar Year. Pertinent results include: 1) conditions along ice edges can bring deep nutrient-rich ??pacific?? waters into nutrient-poor surface waters along the arctic coast, affecting local food webs; 2) both sea ice thermodynamic and dynamic processes ultimately affect ringed seal/polar bear habitats by controlling the timing, location and amount of surface deformation required for ringed seal and polar bear preferred habitat 3) the ice edges bordering open waters of flaw leads are areas of high biological production and are observed to be important beluga habitat. 4) exchange of climate-active gases, including CO2, is extremely active in sea ice environments, and the overall question of whether the Arctic Ocean is (or will be) a source or sink for CO2 will be dependent on the balance of competing climate-change feedbacks. 相似文献
76.
Picophytoplankton and bacterioplankton in the Mississippi River plume and its adjacent waters 总被引:3,自引:0,他引:3
Hongbin?LiuEmail author Michael?Dagg Lisa?Campbell Juanita?Urban-Rich 《Estuaries and Coasts》2004,27(1):147-156
Picoplankton abundance and distribution in the Mississippi River plume and its adjacent waters were studied during two cruises
in April (high discharge) and October (low discharge) 2000 using flow cytometry. Concentrations of photosynthetic picoplankton,Synechococcus and picoeukaryotes were low in the turbid plume water but high in the coastal waters—i.e., the green waters resulting from
mixing of river and oceanic waters. In this region, three types ofSynechococcus, characterized by their phycoerythrin chromophore composition, were found:Synechococcus cells with a low phycourobilin to phycoerythrobilin ratio (PUB:PEB) occurred throughout the region and dominated the totalSymechococcus abundance during both seasons; high PUB:PEB cells, which are the dominant strains in the open or blue ocean, occurred only
at the outer shelf stations; and PEB-onlySynechococcus were abundant in most of the surveyed area during april, but were not observed during October.Prochlorococcus cyanobacteria only occurred at the oceanic stations, but extended farther inshore in October compared to April. This was
a consequence of the reduced discharge and plume size during October. Picophytoplankton were a less important component of
total phytoplankton biomass in the turbid river water and more important in the oligotrophic Gulf water. Seasonally, the contribution
of picophytoplankton to total phytoplankton biomass in the surveyed area was higher during low discharge in October than during
high discharge in April, even though the spring 2000 river discharge was unusually low and might not present a typical high
discharge scenario. The abundance of heterotrophic bacteria was weakly correlated to chlorophylla (chla) concentration, but better correlated to picophytoplankton biomass. A higher proportion of High DNA bacteria occurred in
the river-impacted regions during both seasons, with the ratio of High DNA bacteria to Low DNA bacteria significantly higher
in April. 相似文献
77.
Polyorogenic reworking of ore‐controlling shear zones at the South Range of the Sudbury impact structure: A telltale story from in situ U–Pb titanite geochronology
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Konstantinos Papapavlou James R. Darling Peter C. Lightfoot Stephanie Lasalle Lisa Gibson Craig D. Storey Desmond Moser 《地学学报》2018,30(3):254-261
The post‐impact orogenic evolution of the world class Ni–Cu–PGE Sudbury mining camp in Ontario remains poorly understood. New temporal constraints from ore‐controlling, epidote–amphibolite facies shear zones in the heavily mineralised Creighton Mine (Sudbury, South Range) illuminate the complex orogenic history of the Sudbury structure. In situ U–Pb dating of shear‐hosted titanite grains by LA‐ICP‐MS reveals new evidence for shear zone reworking during the Yavapai (ca. 1.77–1.7 Ga), Mazatzalian–Labradorian (1.7–1.6 Ga) and Chieflakian–Pinwarian (1.5–1.4 Ga) accretionary events. The new age data show that the effects of the Penokean orogeny (1.9–1.8 Ga) on the structural architecture of the Sudbury structure have been overestimated. At a regional scale, the new titanite age populations corroborate that the Southern Province of the Canadian Shield documents the same tectonothermal episodes that are recorded along orogenic strike within the accretionary provinces of the Southwestern United States. 相似文献
78.
Since its initial invasion in the late 1980s, the Asian shore crab Hemigrapsus sanguineus has spread over 650 km along the Atlantic Coast of the United States. Larvae of this species can live in the water column
for over 3 weeks, which provides ample time for transport to new locations. Previous research on the settlement and metamorphosis
of H. sanguineus showed that postlarvae (megalopae) accelerate metamorphosis in response to very specific water-soluble cues. However, the
response to substratum cues appears to be much broader. The experiments presented here were designed to test the metamorphic
response to biofilms and substratum texture. We found that nylon mesh with pore size of 100 and 1,000 μm had a greater effect
on mean time to metamorphosis than 10 and 2,000 μm mesh. Results also showed a positive response to natural rock substrata,
and this response was most pronounced when megalopae were exposed to substrata with rough texture. In addition, we found that
a combination of a textured surface and a natural biofilm accelerated metamorphosis more than either an abiotic substratum
or a smooth surface covered with natural biofilm. A final experiment compared the effect of textural cues to that of a water-soluble
cue found in exudate from adult H. sanguineus. Results indicated that both types of cue-induced metamorphosis to the same extent. 相似文献
79.
This paper explores the relationship between interdisciplinary studies and research and researchers’ positionalities, both within and beyond geography. The profound degree to which researchers’ assumptions, expectations and attitudes (which in turn are affected by their personal backgrounds, training, location, etc.) influences the very notion of interdisciplinarity, and what it involves and consists of, is often neither noted nor appreciated. This paper will illustrate, particularly through personal examples, how positionality is part of the circuit of knowledge production, informing academic research, employment and publishing to no small extent. The boundaries of interdisciplinary research are shown to be under constant negotiation, still far from mutual understanding or consensus, a fact which explains the often uneasy identification and negotiation of oneself as an interdisciplinary scholar. The paper concludes by making recommendations at individual and institutional level on how to overcome some of the constraints imposed by researchers’ positionalities to the promotion of interdisciplinary research. 相似文献
80.
Rachel Whitsed Lisa T. Smallbone 《International journal of geographical information science》2017,31(4):717-737
Cellular automata (CA) models are commonly used to model vegetation dynamics, with the genetic algorithm (GA) being one method of calibration. This article investigates different GA settings, as well as the combination of a GA with a local optimiser to improve the calibration effort. The case study is a pattern-calibrated CA to model vegetation regrowth in central Victoria, Australia. We tested 16 GA models, varying population size, mutation rate, and level of allowable mutation. We also investigated the effect of applying a local optimiser, the Nelder?Mead Downhill Simplex (NMDS) at GA convergence. We found that using a decreasing mutation rate can reduce computational cost while avoiding premature GA convergence, while increasing population size does not make the GA more efficient. The hybrid GA-NMDS can also reduce computational cost compared to a GA alone, while also improving the calibration metric. We conclude that careful consideration of GA settings, including population size and mutation rate, and in particular the addition of a local optimiser, can positively impact the efficiency and success of the GA algorithm, which can in turn lead to improved simulations using a well-calibrated CA model. 相似文献