Submillimetre continuum images of the NGC 2024 star-forming ridge     

Anja E. Visser  John S. Richer  Claire J. Chandler  & Rachael Padman 《Monthly notices of the Royal Astronomical Society》1998,301(2):585-592

We present 450- and 800-μm images, made with the James Clerk Maxwell Telescope, of the NGC 2024 molecular ridge. The seven previously known compact cores, FIR1–7, have been detected, and FIR5 has been resolved into a compact object and an associated extended source to the east. The estimated masses of the dense cores vary between 1.6 and 5.1 M⊙ per 14-arcsec beam, assuming a dust temperature of 30 K and a dust opacity of κ_800 μm = 0.002 m² kg⁻¹. A spectral index map made from the 450- and 800-μm images shows spatial variations, with the spectral index, α ( F _ν ∝ ν^α), being systematically lower towards the dense cores. We interpret this as evidence for a lower value of the frequency dependence of the dust opacity, β, towards the denser cores relative to the surrounding molecular material. This may indicate that grain growth is occurring in the cores, prior to planetesimal formation. By comparing the high-resolution 450-μm image with interferometer maps of the integrated CS(2–1) emission, the previously reported discrepancy between dust continuum emission and molecular line emission is found to be very localized. Depletion and temperature variations are discussed as possible explanations.  相似文献   

Some remarks on the european monsoon     

Prof. Dr. S. W. Visser 《Pure and Applied Geophysics》1953,24(1):135-148

Summary 1. The so called sigularities want all reality and are of no value in monsoon investigations. — 2. In this section a short summary ofChromow's monsoon studies is given (Fig. 1). — 3. The monsoon in Western Europe manifests itself in impulses during some days alternating with the general west circulation. Monthly means of the wind direction give the resultant direction of these two independent wind systems and do by no means elucidate the true behaviour of the wind.Chromow's method, the appliance of monthly means of pressure gradients, is also insufficient. The only reliable method of researching the monsoon in moderate regions is the investigation of the separate wind octants. A month is too long to reveal the rather short impulses and therefore calculating decade means is recommended. The monsoon depends upon the direction and the velocity of the wind. Calculating the product of the frequency of the direction (in %) and the wind velocity is in practice a sufficient approximation. I have called this product the «relative wind vector». — 4. Whereas generally the months of January and July are accepted as the central monsoon months, in the Netherlands and Germany these months appear to be November–December and May–June (Table 1, 2; Fig. 2, 3, 5, 6, 7), respectively with S and NE winds. The summer monsoon seems to back from E to N and perhaps even to NW. This backing may be caused by the form of the european continent. — 5. A research into the maximal development of the monsoon impulses shows that they equal or even surpass the general western ciruclation concerning both their number (Table 1, right-hand side) and their intensity (Table 3).Willett's opinion that the normal geographical distribution of air mass source regions in the spring and the autumn are intermediate between those of the months of Janaury and July cannot be maintained for West Europe. — 6. A research by means ofConrad's monsoon index also shows that the full monsoon months in W-Europe are November–December and May–June (Table 4). The application of his method to the relative wind vector at Den Helder and Maastricht confirms this result (Table 5). — All results arrived at show the activity of the monsoon phenomenon in a good deal of West Europe.

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Zusammenfassung 1. Die Singularitäten entbehren jede Realität und sind wertlos für Monsununtersuchungen. — 2. In diesem Paragraph findet man eine Zusammenfassung vonChromow's Anschauungen (Fig. 1). — 3 Der europäische Monsun äussert sich in Stössen während einiger Tage, abwechselnd mit der allgemeinen Westzirkulation. Montasmittel der Windrichtung geben nur die resultierende Richtung dieser zwei unabhängigen Windsystemen und zeigen in keinem Fall die wahren Windverhältnisse. Die einzige Methode ist die Untersuchung der einzelnen Windoktanten. Eine Monat, ist zu lang um die ziemlich kurzen Stösse zu zeigen. Ein gutes Mass ist das Produkt der Richtungsverteilung in Prozenten mit der Geschwindigkeit, der «relative Windvektor» genannt. — 4. In den Niederlanden und Deutschland sind nicht die allgemein angenommenen Monate Januar und Juli die Zentralmonate des Monsuns, sondern November–Dezember und Mai–Juni (Tab. 1, 2.; Fig. 2, 3, 5, 6, 7), beziehungsweise mit S- und NE-Winden. Der NE-Monsun scheint zurückzudrehen von E nach N, vielleicht selbst, nach NW. Diese Eigenschaft kann verursacht werden von der Form des europäischen Kontinents. — 5. Die Wichtigkeit der Monsunstösse ist gleich, der Westzirkulation oder selbst stärker als diese, sowohl was ihre Zahl (Tab. 1, rechts) als ihre Intensität (Tab. 3) betrifft.Willett's Behauptung, dass die normale geographische Verteilung der Luftmassen im Frühling und im Herbst die mittleren Eigenschaften der Verteilung im Januar und im Juli besitzt kann für Westeuropa nicht aufrecht gehalten werden. — 6.Conrad's Monsunindex zeigt auch dass die vollen Monsunmonate in Westeuropa die oben genannten sind (Tab. 4). Die Anwendung seiner Methode auf dem relativen Windvektor bestätigt dieses Resultat (Tab. 5). Alle gefundenen Resultate zeigen die Aktivität des Monsunphänomens in Westeuropa.

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Résumé 1. Les singularités manquent toute réalité et n'ont pas de valeur pour l'étude des moussons dans les régions modérées. — 2. Un résumé court des études deChromow est donné (Fig. 1). — 3. Le mousson européen se manifeste par des impulsions pendant quelques jours alternant avec la circulation atmosphérique générale. Les moyennes mensuelles de la direction du vent ne donnent que la direction résultante mais ne démontrent rien concernant les particularités réelles du vent. La méthode deChromow, l'application des gradients mensuels de la pression, est aussi insuffisante. En outre la durée d'une mois est trop longue pour révéler ces impulsions courtes. Il faut préférer les décades. Pour conclure il faut tenir compte de la vitesse du vent aussi. J'ai calculé le produit de la fréquence (en pourcentage) et la vitesse, le «vecteur relatif du vent» 4. Tandis que généralement les mois de Janvier et de Juin sont acceptées comme les mois centrales du mousson, dans les Pays Bas et en Allemagne ces mois sont Novembre–Décembre et Mai–Juin (Tabl. 1, 2; Fig. 2, 3, 5, 6, 7) avec resp. le mousson de S et le mousson de NE. Le mousson de NE semble changer de l'E à N et peutêtre même à NW. Ce recul du vent peut être causé par la configuration du continent européen. — 5. Une recherche des impulsions du mousson démontre qu'elles égalent, même surpassent la circulation générale aussi bien concernant leur nombre (Tabl. 1, à droite) que leur intensité (Tabl. 3). L'opinion deWillett que les masses d'air du printemps et de l'automne sont intermédiaires entre celles de Janvier et de Juin n'est pas acceptable pour l'Europe occidentale. — 6. Une recherche au moyen de l'index deConrad montre aussi que les mois essentielles du mousson européen sont les mois mentionnées (Tabl. 4). L'application de la méthode deConrad sur le vecteur du vent relatif confirme ce résultat (Tabl. 5). — Tous les résultats gagnés montrent l'activité du mousson dans une grande partie de l'Europe occidentale.

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Competitive interactions among beam trawlers exploiting local patches of flatfish in the North Sea   总被引：5，自引：4，他引：1  

Rijnsdorp  A. D.; van Mourik Broekman  P. L.; Visser  E. G. 《ICES Journal of Marine Science》2000,57(4):894-902

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GPS-based precise orbit determination of the very low Earth-orbiting gravity mission GOCE   总被引：5，自引：0，他引：5  

P. N. A. M. Visser  J. van den IJssel 《Journal of Geodesy》2000,74(7-8):590-602

 A prerequisite for the success of future gravity missions like the European Gravity field and steady-state Ocean Circulation Explorer (GOCE) is a precise orbit determination (POD). A detailed simulation study has been carried out to assess the achievable orbit accuracy based on satellite-to-satellite tracking (SST) by the US global positioning system (GPS) and in conjunction the implications for gravity field determination. An orbit accuracy at the few centimeter level seems possible, sufficient to support the GOCE gravity mission and in particular its gravity gradiometer. Received: 21 January 2000 / Accepted: 4 July 2000  相似文献   

In other worlds: on the politics of research in a transforming South Africa   总被引：1，自引：1，他引：0  

Gustav Visser 《Area》2000,32(2):231-235

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Anomalously Low Temperature Orthopyroxene, Spinel, and Sapphirine Occurrences in Metasediments from the Bamble Amphibolite-to-Granulite Facies Transition Zone (South Norway): Possible Evidence for Localized Action of Saline Fluids: A Reply     

Nijland TG  Touret JL  Visser D 《The Journal of geology》2000,108(2):247-249

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Modelling water erosion in the Sahel: application of a physically based soil erosion model in a gentle sloping environment     

S. M. Visser  G. Sterk  D. Karssenberg 《地球表面变化过程与地形》2005,30(12):1547-1566

Water is a major limiting factor in arid and semi‐arid agriculture. In the Sahelian zone of Africa, it is not always the limited amount of annual rainfall that constrains crop production, but rather the proportion of rainfall that enters the root zone and becomes plant‐available soil moisture. Maximizing the rain‐use efficiency and therefore limiting overland flow is an important issue for farmers. The objectives of this research were to model the processes of infiltration, runoff and subsequent erosion in a Sahelian environment and to study the spatial distribution of overland flow and soil erosion. The wide variety of existing water erosion models are not developed for the Sahel and so do not include the unique Sahelian processes. The topography of the Sahelian agricultural lands in northern Burkina Faso is such that field slopes are generally low (0–5°) and overland flow mostly occurs in the form of sheet flow, which may transport large amounts of fine, nutrient‐rich particles despite its low sediment transport capacity. Furthermore, pool formation in a field limits overland flow and causes resettlement of sediment resulting in the development of a surface crust. The EUROSEM model was rewritten in the dynamic modelling code of PCRaster and extended to account for the pool formation and crust development. The modelling results were calibrated with field data from the 2001 rainy season in the Katacheri catchment in northern Burkina Faso. It is concluded that the modified version of EUROSEM for the Sahel is a fully dynamic erosion model, able to simulate infiltration, runoff routing, pool formation, sediment transport, and erosion and deposition by inter‐rill processes over the land surface in individual storms at the scale of both runoff plots and fields. A good agreement is obtained between simulated and measured amounts of runoff and sediment discharge. Incorporating crust development during the event may enhance model performance, since the process has a large influence on infiltration capacity and sediment detachment in the Sahel. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

The Dwarskersbos,South Africa local tsunami of August 27, 1969: field survey and simulation as a meteorological event   总被引：1，自引：1，他引：0  

Emile A. Okal  Johan N. J. Visser  Coenraad H. de Beer 《Natural Hazards》2014,74(1):251-268

We investigate the hitherto unexplained wave which inundated the village of Dwarskersbos, South Africa, in the early hours of August 27, 1969, in the absence of any seismic disturbance or major meteorological storm. A field survey, based on the interview of nine elderly witnesses still residing in the community, documented maximum run-up of 2.9 m, concentrated on an extremely short segment of coastline, less than 2 km in length. These characteristics are incompatible with generation by a seismic source (which, at any rate, should have been felt by the population). A landslide source, located at the only canyon featuring a steep enough ocean floor, is also ruled out since a numerical simulation fails to reproduce the concentration of the wave at Dwarskersbos. By contrast, the wave can be explained as a “meteo-tsunami” resulting from resonance between a meteorological squall propagating at 18 m/s in the azimuth \(\hbox {N101}^{\circ }\hbox {E}\) and a gravity wave propagating in the shallow waters off the eastern shore of St. Helena Bay. This is confirmed by numerical simulation under the formalism of Proudman (Dynamical oceanography. Methuen, London, 1953), which provides a satisfactory model of the distribution of run-up along the beach.  相似文献   

Open‐foldable domes with high‐tension textile membranes: The GREGOR dome     

R.H. Hammerschlag  J.N. Kommers  S. Visser  F.C.M. Bettonvil  A.G.M. van Schie  S.J. van Leverink  G. Sliepen  A.P.L. Jgers  W. Schmidt  R. Volkmer 《Astronomische Nachrichten》2012,333(9):830-839

Double layers of high‐tensioned textile membranes were applied to the completely open‐foldable dome for the GREGOR telescope for the first time. Simultaneous climate measurements inside and outside the dome have proven the thermalinsulating capability of this double‐layer construction. The GREGOR dome is the result of the continuation of the ESO research on open‐foldable domes with textile structures, followed by the research for the DOT dome with high‐tensioned textile membranes. It cleared the way to extreme stability required for astronomical practice on high mountain sites with heavy storms and ice formation. The storm Delta with 245 km/h 1‐minute mean maximum at the location of the GREGOR caused no problems, nor did other storms afterwards. Opening and closing experiences up to wind speeds of 90 km/h were without problems. New technical developments were implemented and tested at the GREGOR dome, opening the way for application to much larger domes up to the 30 m diameter‐class range (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献