Experimental and Numerical Modelling of Flow over Complex Terrain: The Bolund Hill     

Boris Conan  Ashvinkumar Chaudhari  Sandrine Aubrun  Jeroen van Beeck  Jari Hämäläinen  Antti Hellsten 《Boundary-Layer Meteorology》2016,158(2):183-208

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Fish assemblages in coastal lagoons in land-uplift succession: The relative importance of local and regional environmental gradients     

Martin Snickars  Alfred Sandström  Antti Lappalainen  Johanna Mattila  Kajsa Rosqvist  Lauri Urho 《Estuarine, Coastal and Shelf Science》2009

The assemblages of young-of-the-year fish were studied in coastal lagoons in an archipelago with post-glacial land-uplift, which affects environmental gradients at local and regional scale, i.e. lagoon habitat isolation and archipelago position, respectively. The categorisation of 40 undisturbed lagoons into nine habitat types based on habitat isolation and archipelago position was supported by clear relationships with spring temperature and total fish abundance. Rutilus rutilus, breams (Abramis/Blicca sp.) and Perca fluviatilis were the most abundant and frequently occurring species. The fish assemblage differed among the nine habitat types. Rutilus rutilus, P. fluviatilis and breams were discriminating species in the majority of habitat types with low physical harshness, whereas Alburnus alburnus and Gasterosteus aculeatus increased their contributions in habitat types with high physical harshness. Rutilus rutilus and breams were thus common in lagoons with high habitat isolation situated in the inner archipelago. These lagoons were characterised by warm water and high vegetation coverage. Gasterosteus aculeatus was restricted to lagoons with low habitat isolation and exposure and low vegetation coverage, situated in the outer archipelago. Perca fluviatilis had the widest distribution of all species. The coverage of two macrophytes, Potamogeton perfoliatus and Zannichellia palustris, and salinity matched best the distance among habitat types. These habitat characteristics, as well as the fish abundances and assemblages differed most across the habitat types in the outer and mid archipelago zones and in the lowest habitat isolation. These patterns suggest that the structuring effect of habitat isolation increases along the archipelago gradient as differences between local and regional conditions increase. In the inner archipelago, overall low physical harshness induces homogeneous conditions and the habitat isolation is less important here than in the other zones. We suggest that this difference in the relative importance of the two gradients depending on the level of respective gradient ultimately forms these heterogeneous coastal habitats in a successional landscape. Rutilus rutilus and P. fluviatilis were responsible for large parts of the assemblage patterns. Although sympatric due to similar habitat requirements, differences in dispersal capability, competitive ability and predation vulnerability may add explanation to detected differences in distribution and abundance in these two species in an open system. Our results also stress the structuring role of vegetation in terms of total coverage and species composition, as these two aspects of macrophyte diversity may act as complementary habitat modifiers across gradients of physical harshness.  相似文献   

A 22 GHz receiver with high phase stability for radioastron space-VLBI-mission     

Petri Piironen  Juha Mallat  Antti Räisänen  Petri Jukkala 《Experimental Astronomy》1994,5(3-4):389-404

A space-qualified low-noise 22 GHz receiver for the international space-VLBI mission Radioastron has been constructed. The microwave electronics is realized by using thermally matched hybrid circuits. The most important properties of the receiver are phase stability, sensitivity and reliability. The high sensitivity is due to a cooled low noise HEMT amplifier (LNA). The measured receiver noise temperature is less than 100 K. The phase stability is achieved by compact structure and thermal stabilization. Phase stabilities of better than 0.13°/°C and 0.2°/°C for the receiver and the LNA units are measured, respectively. The calculated reliability of the receiver exceeds the requirement of 0.97 for the three year mission.  相似文献   

Modelling surface drifting of buoys during a rapidly-moving weather front in the Gulf of Finland, Baltic Sea     

Maria Gstgifvars  Hannu Lauri  Annakaisa Sarkanen  Kai Myrberg  Oleg Andrejev  Cecilia Ambjrn 《Estuarine, Coastal and Shelf Science》2006,70(4):567

The Gulf of Finland is an elongated estuary located in the north-eastern extremity of the Baltic Sea. This semi-enclosed sea-area is subject to heavy sea traffic, and is one of the main risk areas for oil accidents in the Baltic. The continuous development and validation of operational particle drift and oil-spill forecasting systems is thus seen to be essential for this sea-area.Here, the results of a three-day drift experiment in May 2003 are discussed. The field studies were performed using GPS-positioned surface floating buoys. The aim of this paper is to evaluate how well models can reproduce the drift of these buoys. Model simulations, both in forecast and hindcast modes, were carried out by three different 3D hydrodynamic models, the results of which are evaluated by comparing the calculated drifts with observations. These models were forced by HIRLAM (High Resolution Limited Area Model) and ECMWF (European Centre for Medium-Range Weather Forecasts) meteorological forecast fields.The simulated drift of the buoys showed a good agreement with observations even when, during the study period, a rapidly-changing wind situation was observed to affect the investigation area; in this situation the winds turned about 100 degrees in half an hour. In such a case it is a very complicated task to forecast the drifters' routes: there is a need to regularly update the meteorological forcing fields and to use these regularly-updated fields throughout the simulations. It is furthermore recommended that forecasts should be made using several circulation models and several meteorological forecasts, in order to get an overview of the accuracy of the forecasted drifts and related differences in between the forecasts.  相似文献   

Spatial feeding patterns of herring (Clupea harengus L.), sprat (Sprattus sprattus L.), and the three-spined stickleback (Gasterosteus aculeatus L.) in the Gulf of Finland, Baltic Sea     

Peltonen  Heikki; Vinni  Mika; Lappalainen  Antti; Ponni  Jukka 《ICES Journal of Marine Science》2004,61(6):966-971

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Comparison of Monte Carlo simulation methods for the calculation of the nucleation barrier of argon     

Antti Lauri  Joonas Merikanto  Evgeni Zapadinsky  Hanna Vehkamki 《Atmospheric Research》2006,82(3-4):489

We compare two molecular Monte Carlo simulation methods, the discrete summation method and the growth/decay method, which calculate the vapor-liquid nucleation free energy barrier by simulating isolated clusters of fixed size without the surrounding vapor. The methods are applied to calculations of nucleation barriers of Lennard–Jones argon at 60 K and 80 K. Both of these methods are computationally efficient, as only isolated clusters without the surrounding vapor are simulated, and the methods can be applied with any given cluster definition. They give equivalent results to other methods where the vapor phase is also included. The discrete summation method is based on the calculation of the difference in free energies between two systems containing an n-cluster and an (n − 1)-cluster plus one non-interacting (free) molecule. We show that the configurational space is not equivalent in the two systems. Hence, there has to be an additional term in the free energy calculation that accounts for several kT in magnitude. In contrast to previous studies we also show that it is not correct to prevent the overlap of the non-interacting molecule and another molecule by a zero or an arbitrarily small repulsive potential, but with a small excluded space around the free molecule.  相似文献   

Physical properties of Vilppula drill cores and petrographic analysis of associated breccias in Keurusselk? impact structure, central Finland     

Selen Raiskila  Pasi Heikkil?  Ulla Preeden  Tiiu Elbra  Lauri J. Pesonen 《Studia Geophysica et Geodaetica》2012,56(3):659-676

The Mesoproterozoic deeply eroded Keurusselk? impact structure in central Finland is situated within the ??1860?C1890 Ma Central Finland Granitoid Complex. An estimate for the original size of the structure is 30 km, yielding a 5 km wide central uplift with insitu shatter cones and shock metamorphic features in quartz. Petrophysical and rock magnetic properties of the three shallow drill cores (V-001, V-002 and V-003) in the vicinity of the central uplift are determined in order to assess the dimensions of the central uplifts magnetic anomalies. The drill core lithologies consist of schists (metagraywackes), metavolcanic rocks, gneisses and breccia. Petrophysical properties of the drill core rocks show average densities (D) of 2644?C2752 kg/m³, susceptibilities (??) of 160?C761 × 10^?6 SI and natural remanent magnetization (NRM) of 3?C306 mA/m and Koenigsberger Q ratios of 0.1?C10. Rock magnetic measurements with temperature dependence of susceptibility (??-T) curves and hysteresis indicated mostly paramagnetic behaviour. However, a fraction of fine-grained ferromagnetic minerals (pyrrhotite and magnetite) was detected from all lithologies. Breccia veins cutting the parautochthonous subcrater floor show lower values of petrophysical properties (D, ??, NRM, Q) and this could be related to the impact event. Amphiboles and micas in the breccia are strongly altered and replaced by secondary chlorite. Chloritization may indicate widespread impact-induced hydrothermal alteration of the target rocks or it may be related to regional tectonic shearing. However, planar deformation features in quartz, found from shatter cones in the central uplift area, are decorated with fluid inclusions indicating that alteration by post-impact processes was present.  相似文献   

Magnetic properties of asteroids from meteorite data — Implications for magnetic anomaly detections     

Mauri Terho  Lauri J. Pesonen  Ilmo T. Kukkonen 《Earth, Moon, and Planets》1995,71(3):225-231

Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al., 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.  相似文献   

Physiological status of whitefish (<Emphasis Type="Italic">Coregonus lavaretus pallasi</Emphasis>) prior to spawning in lakes of differing acidity     

Pekka?J.?VuorinenEmail author  Marja?Kein?nen  Antti?Lappalainen  Seppo?Peuranen  Martti?Rask 《Aquatic Sciences - Research Across Boundaries》2004,66(3):305-314

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Geophysical research on the Kärdla impact structure,Hiiumaa Island,Estonia     

Jüri Plado  Lauri J. Pesonen  Seppo Elo  Vino Puura  Kalle Suuroja 《Meteoritics & planetary science》1996,31(2):289-298

Abstract— The 4 km wide and 500 m deep circular Kärdla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age (～455 Ma), is buried under Upper Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of ?3 mGal and the magnetic anomaly of ?100 nT and also the surrounding weak positive anomalies revealed by high precision survey data. The unshocked granitic rocks outside the structure have a mean density of ～2630 kgm^?3. Their shocked counterparts have densities of ～2400 kgm^?3 at a depth of ～500 m, increasing up to 2550 kgm^?3 at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kärdla. As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks. A new geophysical and geological model of the Kärdla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores.  相似文献