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101.
102.
103.
Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records 总被引:1,自引:0,他引:1
F. Vimeux V. Masson J. Jouzel J. R. Petit E. J. Steig M. Stievenard R. Vaikmae J. W. C. White 《Climate Dynamics》2001,17(7):503-513
Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the
Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium
excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The
common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming
of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends
at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium
excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation.
The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the
Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased
latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture
to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere
model.
Received: 7 July 1999 / Accepted: 21 July 2000 相似文献
104.
Absolute Calibration of an Ashtech Z12-T GPS Receiver 总被引:3,自引:0,他引:3
Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time
transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the
calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a
GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated.
We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate
uncertainty of order 1 ns in the absolute calibration seems to be attainable. ? 2001 John Wiley & Sons, Inc. 相似文献
105.
This paper describes the design and development of an experimental GIS prototype that may be used to assist experienced pilots, student pilots and flight instructors for post-flight evaluation of navigation performance. The system is developed as a GIS experimental application which employs geographical data derived from the Digital Chart of the World integrated with airspace data derived from the ICAO 1:500,000 Aeronautical Chart. Real-time data are collected and integrated within a monitoring computing environment from external sensors which include GPS, altitude and relative motion sensors. This enables the generation of multi-layered digital maps and navigation data capable of being rapidly re-configured to enhance visualization by abstraction. The integration of real time navigation data with geographical data facilitates airborne decision-making, cockpit management, and makes a valuable contribution to flight safety. 相似文献
106.
A. Gaudichel M. De Angelis S. Joussaume J. R. Petit Y. S. Korotkevitch V. N. Petrov 《Journal of Atmospheric Chemistry》1992,14(1-4):129-142
We have studied the distribution of 327 clay mineral particles retrieved from four Antaretic ice smaples corresponding to present and Last Glacial Maximum (LGM) climate conditions. Illite, chlorite, smectite and kaolinite were identified in all samples. Focusing on kaolinite, because of its use as a possible tracer of low latitude soils, we find a significantly smaller amount for LGM samples while the dust concentration in snow during the LGM was about 30 times higher than for present climate conditions. This can be interpreted as change in the contribution of the Australian source with climate.A second approach was based on the modeling of the desert dust cycle using an Atmospheric General Circulation Model (AGCM) under both present-day and ice age conditions. Unlike mineralogical results, the model suggests the prevalence of the Australian dust source in the deposits over East Antarctica under both present-day and LGM climate conditions. However the model fails to reproduce the strong increase in dust deposits during the LGM. This discrepancy could be partly due to the lack of a higher latitude dust source in the model.The stronger dust input recorded in ice cores for the LGM could be related to an additional active high latitude source (possibly close to South America) overlapping the atmospheric background coming from low latitude areas. 相似文献
107.
F. Petit 《地球表面变化过程与地形》1990,15(2):135-148
Shear stresses were evaluated at different sites on two rivers. The first (the Rulles) is characterized by a pebbly bedload and a meandering bed with riffles and pools. The second (the Rouge Eau) has mainly a sandy rippled bed where meandering is well developed but also flat gravelly sectors without meandering system. Shear stresses calculated from friction velocities (τ*) using a redefined y1 roughness height parameter were compared with total shear stresses calculated from the energy grade line and the hydraulic radius (τ), Divergence between these shear stresses seems to increase in the presence of bedforms and large-scale irregularities of the channel. The τ*/τ ratio is close to 0·5 in the gravelly sector of the Rouge Eau and reaches 0·65 in the riffles of the Rulles (generally located at the inflexion point of the meanders), while it is less than 0·3 in the pools of the same river (located in the loops) and only 0·2 in the sandy rippled sector of the Rouge Eau. Grain and bedform shear stresses were evaluated at these same sites by different methods. The grain shear stress (τ') represents on average 30 per cent of the total shear stress in the riffles of the Rulles and the gravelly sector of the Rouge Eau, but less than 15 per cent in the pools in the Rulles and the sandy sectors of the Rouge Eau. However, it emerges from experiments conducted with marked pebbles and in situ observations of erosion and transport of sandy and gravelly particles, that the grain shear stresses are underestimated and cannot explain the movements and modifications actually observed. Conversely, shear stresses calculated from friction velocities at the sites where erosion actually occurred (or failed to occur despite very high velocities) provide a better explanation of the observed movements. 相似文献
108.
109.
110.
Renaud Tutundjian Christophe Minier Frank Le Foll Franois Leboulenger 《Marine environmental research》2002,54(3-5)
Cellular detoxification by direct processes has been investigated in fish by studying the ability of hepatocytes prepared from juvenile aquarium-reared turbot (Scophthalmus maximus) to actively exclude the fluorescent dye rhodamine B (RB). Cell viability was studied by measurements of non-specific esterase activity using fluorescein diacetate. This revealed that turbot hepatocytes can be cultured for a few days with a viability decreasing to 38% after 24 h. The 24-h cultured cells have been used to study the rhodamine B exclusion activity using confocal laser microscopy. Hepatocytes accumulated the dye in a competitive manner with verapamil, thus suggesting that they express a transport system similar to the P-glycoprotein-mediated multixenobiotic resistance process. Incubation of cells with 1 μM RB and 20 μM verapamil led to a 26% increase of cellular fluorescence as compared to the accumulation in absence of competitor. Rhodamine B accumulated in the whole cytoplasm, with more concentrated areas that might correspond to the lysosomal compartment and the cell membrane. 相似文献