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1.
Long-term changes in summer phytoplankton communities of the open northern Baltic Sea 总被引：16，自引：0，他引：16
Sanna Suikkanen Maria Laamanen Maija Huttunen 《Estuarine, Coastal and Shelf Science》2007,71(3-4):580-592
Changes in the biomass and species composition of phytoplankton may reflect major shifts in environmental conditions. We investigated relationships between the late summer biomass of different phytoplankton taxa and environmental factors, and their long-term (1979–2003) trends in two areas of the Baltic Sea, the northern Baltic proper (NBP) and the Gulf of Finland (GF), with statistical analyses. An increasing trend was found in late summer temperature and chlorophyll a of the surface water layer (0–10 m) in both areas. There was also a significant decrease in summer salinity and an increase in winter dissolved inorganic nitrogen to phosphorus (DIN:DIP) ratio in the NBP, as well as increases in winter DIN concentrations and DIN:SiO4 ratio in the GF. Simultaneously, the biomass of chrysophytes and chlorophytes increased in both areas. In the NBP, also the biomass of dinophytes increased and that of euglenophytes decreased, whereas in the GF, cyanobacteria increased and cryptophytes decreased. Redundancy analysis (RDA) indicated that summer temperature and winter DIN concentration were the most important factors with respect to changes in the phytoplankton community structure. Thus, the phytoplankton communities seem to reflect both hydrographic changes and the ongoing eutrophication process in the northern Baltic Sea. 相似文献
2.
Ü. Rannik T. Markkanen J. Raittila P. Hari T. Vesala 《Boundary-Layer Meteorology》2003,109(2):163-189
Observations of wind statistics within and above a Scots pine forest are comparedwith those predicted from an analytical second-order closure model. The roughnesssublayer (RSL) effects, and the influence of stability on similarity functions, arestudied using observations. The commonly accepted forms of similarity functionsdescribe the influence of diabatic effects above the RSL well. According to earlierstudies they are expected also to apply within the RSL. As an exception, the averagewind speed normalised with friction velocity was found to be invariant with stabilityclose to the canopy top under unstable conditions. Lagrangian stochastic trajectorysimulations were used to evaluate the influence of canopy turbulence profiles onfootprint prediction. The main uncertainty was found to arise from parameterisationof the random forcing term in the Lagrangian velocity equation. The influence ofdiabatic conditions was studied, and it was found that thermal stability affectssignificantly the footprint function above the forest canopy, but significantuncertainty exists because of uncertainties in the formulation of stability functions. 相似文献
3.
The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous
limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of
argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated
marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of
late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite
horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up
of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were
immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although
some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element
ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the
degradation of 25 to 50 m of the argillaceous limestone.
Received: 10 June 1996 / Accepted: 15 April 1997 相似文献
4.
利用步长模拟对青藏高原涡度方差测量法的质量评价 总被引：1，自引：0，他引：1
利用痕迹模拟方法对青藏高原两处地方涡度方差的测量数据进行了质量分析，揭示了其空间和时间结构。分析表明高达1/3的测量没有达到必要的数据正确假设。尽管这样对潜热、CO2、动量通量的测量基本通过测试，可以适用于基础研究，但是经常发现特定的风矢量违背基本假设条件。感热通量的测量允许使用不间断的连续测量法，然而由于局地地形的影响少量评估指数未能合理解释，但能够指示出组织结构及用于导出边界层中尺度流体模型假说。 相似文献
5.
L. M. Reithmaier M. Göckede T. Markkanen A. Knohl G. Churkina C. Rebmann N. Buchmann T. Foken 《Theoretical and Applied Climatology》2006,84(4):219-233
Summary Long-term flux measurement sites are often characterized by a heterogeneous terrain, which disagrees with the fundamental
theoretical assumptions for eddy-covariance measurements. An evaluation procedure to assess the influence of terrain heterogeneity
on the data quality has been developed by G?ckede et al. (2004), which combines existing quality assessment tools for flux
measurements with analytic footprint modeling. In addition to micrometeorological input data, this approach requires information
defining the land use structure and the roughness of the surrounding terrain.
The aim of this study was to improve the footprint based site evaluation approach by using high-resolution land use maps derived
by Landsat ETM+ and ASTER satellite data. The influence of the grid resolution of the maps on the results was examined, and
four different roughness length classification schemes were tested. Due to numerical instabilities of the analytic footprint
routine, as an additional footprint model a Lagrangian stochastic footprint routine (Rannik et al., 2003) was employed. Application
of the approach on two German FLUXNET sites revealed only weak influence of the characteristics of the land use data when
the land use structure was homogeneous. For a more heterogeneous site, use of the more detailed land use maps derived by remote
sensing methods resulted in distinct differences indicating the potential of remote sensing for improving the flux measurement
site evaluation. 相似文献
6.
A. R. Breen P. J. Moran C. A. Varley W. P. Wilkinson P. J. S. Williams W. A. Coles A. Lecinski J. Markkanen 《Annales Geophysicae》1998,16(10):1265-1282
Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii. 相似文献
7.
P. J. Moran A. R. Breen C. A. Varley P. J. S. Williams W. P. Wilkinson J. Markkanen 《Annales Geophysicae》1998,16(10):1259-1264
EISCAT observations of the interplanetary scintillation of a single source were made over an extended period of time, during which the orientation of the baselines between the two observing sites changed significantly. Assuming that maximum correlation between the scintillations observed at the two sites occurs when the projected baseline is parallel to the direction of plasma flow, this technique can be used to make a unique determination of the direction of the solar wind. In the past it has usually been assumed that the plasma flow is radial, but measurements of eleven sources using this technique have indicated conclusively that in at least six cases observed at mid or high heliocentric latitude there is a significant non-radial component directed in four cases towards the heliocentric equator and in two cases towards the pole. 相似文献
8.
Sanna HOLM Carl ALWMARK Walter ALVAREZ Birger SCHMITZ 《Meteoritics & planetary science》2011,46(12):1888-1909
Abstract– The Siljan impact structure in Sweden is the largest confirmed impact structure in Western Europe. Despite this, the structure has been poorly studied in the past, and detailed studies of shock metamorphic features in the target lithologies are missing. Here, we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at Siljan. At 21 localities from an area approximately 20 km in diameter located centrally in the structure, the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter. The most strongly shocked samples, recording pressures of up to 20 GPa, occur at the very central part of the structure, and locally in these samples, higher pressures causing melting conditions in the affected rocks were reached. Pressures recorded in the studied samples decrease outwards from the center of the structure, forming roughly circular envelopes around the proposed shock center. Based on the distribution pattern of shocked quartz at Siljan, the original transient cavity can be estimated at approximately 32–38 km in diameter. After correcting for erosion, we conclude that the original rim to rim diameter of the Siljan crater was somewhere in the size range 50–90 km. 相似文献
9.
Carl ALWMARK Birger SCHMITZ Sanna HOLM Federica MARONE Marco STAMPANONI 《Meteoritics & planetary science》2011,46(8):1071-1081
Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth. 相似文献
10.
A Lagrangian stochastic (LS) model, which is embedded into a parallelised large-eddy simulation (LES) model, is used for dispersion
and footprint evaluations. For the first time an online coupling between LES and LS models is applied. The new model reproduces
concentration patterns, which were obtained in prior studies, provided that subgrid-scale turbulence is included in the LS
model. Comparisons with prior studies show that the model evaluates footprints successfully. Streamwise dispersion leads to
footprint maxima that are situated less far upstream than previously reported. Negative flux footprints are detected in the
convective boundary layer (CBL). The wide range of applicability of the model is shown by applying it under neutral and stable
stratification. It is pointed out that the turning of the wind direction with height leads to a considerable dependency of
source areas on height. First results of an application to a heterogeneously heated CBL are presented, which emphasize that
footprints are severely affected by the inhomogeneity. 相似文献