On the structure of Langmuir turbulence     

M.A.C. Teixeira  S.E. Belcher 《Ocean Modelling》2010,31(3-4):105-119

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Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills     

José Luis Argaín  Pedro M. A. Miranda  Miguel A. C. Teixeira 《Boundary-Layer Meteorology》2009,130(1):15-28

A method is suggested for the calculation of the friction velocity for stable turbulent boundary-layer flow over hills. The method is tested using a continuous upstream mean velocity profile compatible with the propagation of gravity waves, and is incorporated into the linear model of Hunt, Leibovich and Richards with the modification proposed by Hunt, Richards and Brighton to include the effects of stability, and the reformulated solution of Weng for the near-surface region. Those theoretical results are compared with results from simulations using a non-hydrostatic microscale-mesoscale two-dimensional numerical model, and with field observations for different values of stability. These comparisons show a considerable improvement in the behaviour of the theoretical model when the friction velocity is calculated using the method proposed here, leading to a consistent variation of the boundary-layer structure with stability, and better agreement with observational and numerical data.  相似文献   

Lacustrine sediments provide geochemical evidence of environmental change during the last millennium in southeastern Brazil     

Sonia Maria Barros de Oliveira  Soraya Elaine Marques Gouveia Saia  Luiz Carlos Ruiz Pessenda  Deborah Ines Teixeira Favaro 《Chemie der Erde / Geochemistry》2009,69(4):395-405

A 172 cm-long sediment core was collected from a small pristine lake situated within a centripetal drainage basin in a tropical karst environment (Ribeira River valley, southeastern Brazil) in order to investigate the paleoenvironmental record provided by the lacustrine geochemistry. Sediments derived from erosion of the surrounding cambisoils contain quartz, kaolinite, mica, chlorite and goethite. Accelerator mass spectroscopy (AMS) ¹⁴C dating provided the geochronological framework. Three major sedimentary units were identified based on the structure and color of the sediments: Unit III from 170 to140 cm (1030±60–730±60 yr BP), Unit II from 140 to 90 cm (730±60–360±60 yr BP) and Unit I from 90 to 0 cm (360±60–0 yr BP). Results of major and trace element concentrations were analysed through multivariate statistical techniques. Factor analysis provided three factors accounting for 72.4% of the total variance. F1 and F2 have high positive loadings from K, Ba, Cs, Rb, Sr, Sc, Th, light rare earth element (LREE), Fe, Cr, Ti, Zr, Hf and Ta, and high negative loadings from Mg, Co, Cu, Zn, Br and loss on ignition (LOI). F3, with positive loadings from V and non-metals As and Sb, accounts for a low percentage (9.7%) of the total variance, being therefore of little interpretative use. The profile distribution of F1 scores reveals negative values in Units I and III, and positive values in Unit II, meaning that K, Ba, Cs, Rb, Sr, Sc, Th, LREE, Fe, Cr, Ti, Zr, Hf and Ta are relatively more concentrated in Unit II, and Mg, Co, Cu, Zn and Br are relatively more abundant in Units I and III. The observed fluctuations in the geochemical composition of the sediments are consistent with slight variations of the erosion intensity in the catchment area as a possible response to variations of climatic conditions during the last millennium.  相似文献   

Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300?years     

Karin Holmgren  Jan Risberg  Johan Freudendahl  Mussa Achimo  Anneli Ekblom  Joao Mugabe  Elin Norstr?m  Sandra Sitoe 《Journal of Paleolimnology》2012,48(2):311-322

Stratigraphic variations in diatom composition and phytolith abundance in a sediment core from a small, hydrologically isolated waterbody, Lake Nhauhache, Mozambique, provide evidence of water-level fluctuations over the past ~2,300?years. Ten AMS radiocarbon dates on bulk sediment samples show that the lake came into existence about 2,300?years ago and that it has dried out since then, but only for brief time periods. Changes in the diatom assemblage composition indicate that lake level fluctuated in response to shifting humidity conditions. The changes reflect wetter conditions ca. 300 BC?CAD 800, more variable conditions between AD 800 and 1150, a distinct dry phase within the time span AD 1150?C1700 and a return thereafter to more humid conditions until present. There is general agreement between the Lake Nhauhache record and other records from the Southern Hemisphere summer rainfall region. This suggests that sediments from small interdunal lakes, which are abundant along the coast of southern Africa, provide reliable, regional paleoenvironmental information about an area from which more such data are needed.  相似文献   

Climate changes associated with high-amplitude <Emphasis Type="Italic">Sq</Emphasis> geomagnetic variations     

Taha Rabeh  Joao Carvalho  Ahmed Khalil  Esmat A. El-Aal  Ibrahim El-Hemaly 《Acta Geophysica》2011,59(5):1044-1056

When the solar irradiance propagates between the outer magnetospheric regions and the ionosphere, dynamic processes of the magnetosphere-ionosphere-thermosphere system are affected at the lower end of their paths by the interaction of radiation with the neutral troposphere. The main target of this work is to investigate the relationship between the diurnal magnetic field variations resulting from solar activities and the variation in the troposphere temperature. Meteorological and geomagnetic data acquired from different observatories located in Egypt, Portugal and Slovakia in a long-term and daily-term scales were analyzed.  相似文献   

Structure and dynamics of distinct fish assemblages in three reaches (upper,middle and lower) of an open tropical estuary in Brazil     

Leonardo M. Neves  Tatiana P. Teixeira  Francisco G. Araújo 《Marine Ecology》2011,32(1):115-131

We collected fishes and environmental variables in three zones (upper, middle and lower) of a small open tropical estuary during flood tide. The aim was to test for differences in fish assemblages along a gradient from freshwater to marine waters and to detect any seasonal variation in fishes and environmental variables across these zones. A total of 111 species (18 in the upper, 50 in the middle and 66 in the lower estuary) were recorded, forming three distinct fish assemblages, with the family Eleotridae dominating in the upper, Gerreidae in the middle, and Sciaenidae in the lower estuary. Only two species (Geophagus brasiliensis in the upper and the middle zones, and Eucinostomus argenteus in the middle and the lower zones) composed more than 1% of the total number of individuals in more than a single zone. Short‐term (tidal) changes in salinity in the middle estuary were associated with different assemblages in the three estuarine zones, even in winter, when the differences in salinity are lowest between the middle and the lower zones. Seasonal variation in salinity was irrelevant, except in a protected sidewater lagoon in the middle estuary. Low salinity seasonal change may be related to the lack of seasonal variation in the structure of fish assemblages in all estuarine zones.  相似文献   

Assessment of mixing between shallow and thermal waters using geochemical and environmental isotope tracers (N Portugal): a review and reinterpretation     

J.?M.?MarquesEmail author  C.?Matos  P.?M.?Carreira  J.?Espinha Marques  J.?Teixeira  H.?I.?Chaminé 《Environmental Earth Sciences》2014,72(7):2557-2567

Caldas de Moledo thermal (27–46 °C) spring and borehole waters issue in the region of the famous Port Wine vineyards, in the Douro River valley (Northern Portugal). The most abundant lithotypes are lower Cambrian metasedimentary rocks, Variscan granitoids and aplite-pegmatitic veins. The thermal waters are characterised by pH ≈ 9.0, TDS ranging from 200 to 350 mg/L, and belong to the HCO₃–Na facies indicating that the reservoir rock should be mainly granite. Since the local Spa is strongly dependent on water quality, the effects of mixing between local shallow cold groundwaters and deep thermal waters have been properly investigated. In the SO₄ ^2? (mg/L) versus δ¹⁸O (‰ vs. V-SMOW) diagram we can observe that some of the thermal springs show evidences of mixing (higher SO₄ ^2? concentrations) with local meteoric waters infiltrated at lower altitude sites (enriched δ¹⁸O signatures), showing the “altitude effect” in the isotopic composition of the recharge waters. Similar trends can be found in the K⁺, NO₃ ^?, Ca²⁺ and Na⁺ (mg/L) versus δ¹⁸O (‰ vs. V-SMOW) diagrams. It should be stated that SO₄ ^2?, K⁺ and Ca²⁺ are present in the fertilizers and fungicides used in the vineyards in the northern part of the country. Up to now, the thermal waters from boreholes used in the local Spa do not show evidences of mixing with shallow groundwaters contaminated with agrochemicals. The results obtained so far indicate that in the near future, special attention should be put on the possible occurrence of diffuse agricultural contamination (related to the Port Wine vineyards) in the thermal spring waters.  相似文献   

Reply to the Discussion by Dillenburg et al. on “Evidence for a transgressive barrier within a regressive strandplain system: implications for complex response to environmental change” by Hein et al. (2013), Sedimentology 60, 469–502     

Christopher J. Hein  Duncan M. Fitzgerald  Antonio H. Da F. Klein  Marcio B. Albernaz  Joao Thadeu de Menezes  William J. Cleary 《Sedimentology》2014,61(7):2213-2217

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Optical positions of radiostars: The necessity of international cooperation     

Y. Réquième  J. M. Mazurier  S. Rousseau  R. Teixeira 《Astrophysics and Space Science》1991,177(1-2):105-108

Positions of 175 radiostars have been obtained with the Bordeaux automatic meridian circle. Comparisons with VLBI and VLA results are given on the FK4 system. An international observation program of a unified list of radiostars is needed in order to obtain the best link of the FK5 with the extragalactic reference frame. The Astronomisches Rechen-Institut is proposed as compilation center.On leave from the Instituto Atronomico e Geofisico, Caixa Postal 30627, 0105 1 Sao Paulo, SP  相似文献   

Kaimal's isopleths from a closure model     

Osvaldo L. L. Moraes  Joao Goedert 《Boundary-Layer Meteorology》1988,45(1-2):83-92

Kaimal's isopleths for the spectra of the Kansas experiment are derived from a closure model built after Claussen's mechanisms for the spectral transfer of kinetic energy in a steady-state and horizontally homogeneous flow. The connection of the predicted three-dimensional spectrum with the measured one-dimensional one is accomplished via the additional assumption that turbulence in the Kansas experiment was basically isotropic.Partially financed by CAPES, FINEP and CNPq.On leave from Faculdade de Engenharia de Joinville, SC, Brazil.  相似文献