Turbulence Statistics Above And Within Two Amazon Rain Forest Canopies   总被引：1，自引：1，他引：0  

B. Kruijt  Y. Malhi  J. Lloyd  A. D. Norbre  A. C. Miranda  M. G. P. Pereira  A. Culf  J. Grace 《Boundary-Layer Meteorology》2000,94(2):297-331

The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth.  相似文献   

Abundance and distribution of ichthyoplankton in Narragansett Bay, Rhode Island, 1989–1990     

Aimee A. Keller  Grace Klein-MacPhee  Jeanne St. Onge Burns 《Estuaries and Coasts》1999,22(1):149-163

An ichthyoplankton survey (18 stations in seven sampling sectors) was conducted in Narragansett Bay in 1990 to provide information on abundance, distribution, and seasonal occurrence of eggs and larvae of estuarine fishes, including seasonal migrants. An additional goal was to examine changes in species composition, abundance, and distribution occurring since the last baywide survey in 1972–73. The taxonomic composition of eggs and larvae in 1990 (41 species in 25 families from 684 plankton samples) and in 1972–73 (43 species in 28 families from 6900 samples) was similar. Maximum abundance of fish eggs occurred in June and larvae in July, minimum abundance in September to February. Species diversity was greatest in May–July and lowest during January in both surveys. However, egg and larval densities in 1990 were considerably lower than in 1972–73. Bay anchovy, tautog, and cunner accounted for 86% of the eggs and 87% of the larvae in the bay in 1990. These three species accounted for only 55% of the eggs and 51% of the larvae in 1972–73, with menhaden accounting for another 18% of the eggs and 34% of the larvae. Searobins, scup, and butterfish eggs were common in 1973 (19%) but rare in 1990 (2%). Ichthyoplankton abundance for several of the most abundant species was significantly lower (p<0.05) in the Providence River, upper bay, and Greenwich Bay in 1990 than in 1972–73. Density of fish eggs and larvae in the lower portions of the bay was lower in 1990 for some species but not others. Distribution data suggested a general down-bay shift in density in 1990. *** DIRECT SUPPORT *** A01BY085 00015  相似文献   

The Discovery of the Chemical Composition - Kinematics Connection in the 1950's     

Nancy Grace Roman 《Astrophysics and Space Science》1999,267(1-4):37-44

This paper reviews the recognition that kinematic characteristics of stars are related to their spectroscopic appearance on low and intermediate dispersion spectra. These differences result from the differences in the abundances of metals as determined from high dispersion spectra. Presumably, these abundance differences reflect differences in age. The high-velocity carbon stars share the molecular peculiarities of the high-velocity oxygen giants but, because of the greater strength of the bands of carbon-containing molecules, exhibit them to a greater degree. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Controls on Arctic glacier-fed river water temperature     

Christopher J. Mellor  Stephen J. Dugdale  Grace Garner  Alexander M. Milner 《水文科学杂志》2017,62(4):499-514

The impact of climate change on Arctic rivers is expected to be severe. There is therefore a need for greater understanding of Arctic river temperature processes. This study quantifies the spatio-temporal variability of water temperatures in the Kårsa River, Sweden. Water temperature was monitored over two summers within the main proglacial channel and within braids fed by different sources. Longitudinal and lateral temperature patterns were assessed in relation to prevailing hydro-meteorology. Temperature metrics in the main channel increased with distance downstream but were moderated by a large lake, while temperatures in the braids were dependent upon channel source. The high temperature standard deviation and inter-site differences within the braids highlight the importance of braided channels for creating thermal habitat heterogeneity. Temperatures were dependent on hydro-meteorological conditions, with sensitivity to air temperature maximized during cooler, rainy conditions. These results shed new light on Arctic river temperature patterns and their controlling processes.  相似文献   

Long-Term Evaluation of Mulch Biowall Performance to Treat Chlorinated Solvents     

Kenneth L. Walker Jr.  Travis M. McGuire  David T. Adamson  R. Hunter Anderson 《Ground Water Monitoring & Remediation》2020,40(1):35-46

Permeable reactive barriers (PRBs), such as mulch biowalls, have been installed at numerous groundwater cleanup sites, and laboratory and field studies have demonstrated biotic and abiotic processes that degrade chlorinated volatile organic compounds (CVOCs) in groundwater passing through these engineered remedies. However, the longevity of mulch biowalls remains a fundamental research question. Soil and groundwater sampling at seven mulch biowalls at Altus Air Force Base (AFB) approximately 10 years after installation demonstrated the ongoing degradation of CVOCs. Trichloroethene was not detected in five of seven groundwater samples collected from the biowall despite upgradient detections above federal drinking water standards. Microbial sampling established the presence of key dechlorinating bacteria and the abundance of genes encoding specific enzymes for degradation, high methane concentrations, low sulfate concentrations, and negative oxidation-reduction potential, all indicative of highly reducing conditions within the biowalls and favorable conditions for CVOC destruction via microbial reductive dechlorination. High cellulose content (>79%) of the mulch, elevated total organic carbon (TOC) content in groundwater, and elevated potentially bioavailable organic carbon (PBOC) measurements in soil samples further supports an ongoing, long-lived source of carbon. These results demonstrate the ongoing and long-term efficacy of the mulch biowalls at Altus AFB. In addition, concentrations of bacteria, TOC, PBOC, and other geochemical parameters suggest a modest impact of the biowalls downgradient. The continued presence of CVOCs downgradient may be attributable to back diffusion from low-permeability shale. However, the biowalls continue to provide benefits by removing CVOCs in groundwater, thus reducing further CVOC loading to the downgradient, low-permeability strata.  相似文献   

Use of Sonication for Enhanced Sampling of Attached Microbes from Groundwater Systems     

Murray Close  Phil Abraham  Judith Webber  Eliza Cowey  Bronwyn Humphries  Graham Fenwick  Simon Howard  Kimberly Huynh  Travis Grace  Pierre-Yves Dupont  Louise Weaver 《Ground water》2020,58(6):901-912

The vast majority of microorganisms in aquifers live as biofilms on sediment surfaces, which presents significant challenges for sampling as only the suspended microbes will be sampled through normal pumping. The use of a down-well low frequency sonicator has been suggested as a method of detaching microbes from the biofilm and allowing rapid sampling of this community. We developed a portable, easy to use, low-frequency electric sonicator and evaluated its performance for a range of well depths (tested up to 42 m below ground level) and casing types. Three sonicators were characterized in laboratory experiments using a 1 m long tank filled with pea gravel. These included a commercially available pneumatic sonicator, a rotating flexible shaft sonicator, and the prototype electric sonicator. The electric sonicator detached between 56 and 74% of microbes grown on gravel-containing biobags at distances ranging between 2 and 50 cm from the sonicator. The field testing comprises of a total of 55 sampling events from 48 wells located in 4 regions throughout New Zealand. Pre- and post-sonication samples showed an average 33 times increase in bacterial counts. Microbial sequence data showed that the same classes are present in pre- and post-sonicated samples and only slight differences were seen in the proportions present. The sampling process was rapid and the significant increases in bacterial counts mean that microbial samples can be quickly obtained from wells, which permits more detailed analysis than previously possible.  相似文献   

Channel precipitation dynamics in a forested Pennsylvania headwater catchment (USA)     

Travis W. Crayosky  David R. Dewalle  Thomas A. Seybert  Thomas E. Johnson 《水文研究》1999,13(9):1303-1314

Seasonal and event variations in stream channel area and the contributions of channel precipitation to stream flow were studied on a 106‐ha forested headwater catchment in central Pennsylvania. Variations in stream velocity, flowing stream surface width and widths of near‐stream saturated areas were periodically monitored at 61 channel transects over a two‐year period. The area of flowing stream surface and near‐stream saturated zones combined, ranged from 0·07% of basin area during summer low flows to 0·60% of total basin area during peak storm flows. Near‐stream saturated zones generally represented about half of the total channel area available to intercept throughfall and generate channel precipitation. Contributions of routed channel precipitation from the flowing stream surface and near‐stream zones, calculated using the Penn State Runoff Model (PSRM, v. 95), represented from 1·1 to 6·4% of total stream flow and 2·5–29% of total storm flow (stream flow–antecedent baseflow) during the six events. Areas of near‐stream saturated zones contributed 35–52% of the computed channel precipitation during the six events. Channel precipitation contributed a higher percentage of stream flow for events with low antecedent baseflow when storm flow generated by subsurface sources was relatively low. Expansion of channel area and consequent increases in volumes of channel precipitation with flow increases during events was non‐linear, with greater rates of change occurring at lower than at higher discharge rates. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

Hydrogeochemical evaluation of groundwater in the lower Offin basin,Ghana   总被引：3，自引：0，他引：3  

Benony K. Kortatsi  Collins K. Tay  Geophrey Anornu  Ebenezer Hayford  Grace A. Dartey 《Environmental Geology》2008,53(8):1651-1662

Alumino-silicate mineral dissolution, cation exchange, reductive dissolution of hematite and goethite, oxidation of pyrite and arsenopyrite are processes that influence groundwater quality in the Offin Basin. The main aim of this study was to characterise groundwater and delineate relevant water–rock interactions that control the evolution of water quality in Offin Basin, a major gold mining area in Ghana. Boreholes, dug wells, springs and mine drainage samples were analysed for major ions, minor and trace elements. Major ion study results show that the groundwater is, principally, Ca–Mg–HCO₃ or Na–Mg–Ca–HCO₃ in character, mildly acidic and low in conductivity. Groundwater acidification is principally due to natural biogeochemical processes. Though acidic, the groundwater has positive acid neutralising potential provided by the dissolution of alumino-silicates and mafic rocks. Trace elements’ loading (except arsenic and iron) of groundwater is generally low. Reductive dissolution of iron minerals in the presence of organic matter is responsible for high-iron concentration in areas underlain by granitoids. Elsewhere pyrite and arsenopyrite oxidation is the plausible process for iron and arsenic mobilisation. Approximately 19 and 46% of the boreholes have arsenic and iron concentrations exceeding the WHO’s (Guidelines for drinking water quality. Final task group meeting. WHO Press, World Health Organization, Geneva, 2004) maximum acceptable limits of 10 μg l⁻¹ and 0.3 mg l⁻¹, for drinking water.  相似文献   

Investigations in Kenya and Malawi using as-dug laterite as bases for bituminous surfaced roads     

Henry Grace 《Geotechnical and Geological Engineering》1991,9(3-4):183-195

Summary In Kenya 1974/5 and in Malawi 1984/5 trial sections of bituminous-surfaced low volume roads were constructed using as-dug laterite in place of stone or stabilized material as a base course.The laterite did not conform with any accepted specifications but performed equally well when compared with adjoining sections of road using stone or stabilized material as a base.The construction procedures employed are described. A period of 4–6 weeks between the compaction of the base and its surfacing is considered essential if the road is to perform satisfactorily. During this period it is exposed to the weather and traffic or to periodic watering and rolling. This treatment fills any cracks with dust or mud and closes them by the kneading action of rubber-tyred traffic. The bituminous surfacing is applied where the base has dried out to about half the optimum moisture content and is extended about 1.5 m beyond each edge of the carriageway.In 1984, $40000 US per km was saved in Malawi in this way, approximately 70% being in convertible currency without incurring additional maintenance or vehicle operating costs.  相似文献   

Outcomes of hospitalizations for common illnesses associated with a comorbid heat-related illness in the United States, 2001–2010     

Michael T. Schmeltz  Peter J. Marcotullio  David U. Himmelstein  Steffie Woolhandler  Grace Sembajwe 《Climatic change》2016,138(3-4):567-584

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