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1951-2002年长江流域降水特征 总被引：2，自引：0，他引：2
The monthly, seasonal, and annual precipitation trends in the Yangtze river catchment have been detected through analysis of 51 meteorological stations‘ data between 1950-2002 provided by National Meteorological Administration. Results reveal that: 1) Summer precipitation in the Yangtze river catchment shows significant increasing tendency. The Poyanghu lake basin, Dongtinghu lake basin and Taihu lake basin in the middle and lower reaches are the places showing significant positive trends. Summer precipitation in the middle and lower reaches experienced an abrupt change in the year 1992; 2) The monthly precipitation in months just adjoining to summer shows decreasing tendency in the Yangtze river catchment. The upper and middle reaches in Jialingjiang river basin and Hanshui river basin are the places showing significant negative trends; 3) Extreme precipitation events show an increasing tendency in most places, especially in the middle and lower reaches of the Yangtze river catchment. 相似文献
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Mid-Palaeocene palaeogeography of the eastern North Sea basin: integrating geological evidence and 3D geodynamic modelling 总被引：1，自引：0，他引：1
The Mid‐Palaeocene palaeogeography of Denmark and the surrounding areas have been reconstructed on the basis of published geological data integrated with 3D geodynamic modelling. The use of numerical modelling enables quantitative testing of scenarios based on geological input alone and thus helps constrain likely palaeo‐water depths in areas where the geological data are inconclusive or incomplete. The interpretation of large‐scale erosional valleys and small‐scale circular depressions at the Mid‐Palaeocene Top Chalk surface in the Norwegian–Danish basin as either submarine or subaerial features is enigmatic and has strong implications for palaeogeographical reconstructions of the eastern North Sea basin. A 3D thermo‐mechanical model is employed in order to constrain the likely palaeo‐water depths of the eastern North Sea basin during the Palaeocene. The model treats the lithosphere as an elasto‐visco‐plastic continuum and models the lithospheric response to the regional stress field and thermal structure. The model includes the effects of sea‐level change, sedimentation and erosion, from the Mid Cretaceous to the present. Modelling results reproduce to first order geological data such as present sediment isopachs and palaeo‐water depths. It is concluded that the Mid Palaeocene water depths in the Norwegian–Danish basin were about 250 m. The erosional valleys and circular depressions at the top of the Upper Cretaceous‐Danian Chalk Group are thus interpreted to have formed in relatively deep water rather than due to subaerial exposure. Likely interpretations of the structures are therefore submarine valleys and pockmarks. 相似文献
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Based on the daily observational precipitation data of 147 stations in the Yangtze River basin for 1960-2005, and the projected daily data of 79 grids from ECHAM5/MPI-OM in the 20th century, time series of precipitation extremes which contain annual maximum (AM) and Munger index (MI) were constructed. The distribution feature of precipitation extremes was analyzed based on the two index series. Research results show that (1) the intensity and probability of extreme heavy precipitation are higher in the middle Mintuo River sub-catchment, the Dongting Lake area, the mid-lower main stream section of the Yangtze River, and the southeastern Poyang Lake sub-catchment; whereas, the intensity and probability of drought events are higher in the mid-lower Jinsha River sub-catchment and the Jialing River sub-catchment; (2) compared with observational data, the averaged value of AM is higher but the deviation coefficient is lower in projected data, and the center of precipitation extremes moves northwards; (3) in spite of certain differences in the spatial distributions of observed and projected precipitation extremes, by applying General Extreme Value (GEV) and Wakeby (WAK) models with the method of L-Moment Estimator (LME) to the precipitation extremes, it is proved that WAK can simulate the probability distribution of precipitation extremes calculated from both observed and projected data quite well. The WAK could be an important function for estimating the precipitation extreme events in the Yangtze River basin under future climatic scenarios. 相似文献
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A great deal of attention has been given to the production and spatial distribution of sodium in comets after the discovery
of the sodium tail, by Cremonese et al. (1997a), on Hale-Bopp. The sodium has been observed in several comets in the past,
but the Hale-Bopp represent the first time where it will be deeply analyzed considering the several data and scientists working
on that. The sodium tail stimulated different studies trying to explain the mechanism source and provided the new lifetime
for photoionization of the neutral sodium atom. We took into account other sodium observations performed in this century and
we focalized our attention to comet Hale-Bopp to understand the main sources responsible for the sodium features observed.
We analyzed the sodium tail observations performed by Cremonese et al. (1997b) and Wilson et al. (1998) finding that the Hale-Bopp
had four different tails. The wide field images and the high resolution spectroscopy performed along the sodium tail provided
very important clues to distinguish the two sodium tails observed and their possible sources. Considering most of the data
reported in several papers has been possible to draw a real sketch on what has occurred to the comet during March and April
1997. We are going to demonstrate that the sodium tail observed by Wilson et al. (1998) was not the same reportedby Cremonese
et al. (1997a) and in the images taken by the European Hale-Bopp Team there were two distinct sodium tails. The observations
allowed us to define “narrow sodium tail” the tail reported by Cremonese et al. (1997a), and “diffuse sodium tail” the tail
overimposed to the dust tail. We suggest that the narrow sodium tail was due to a molecular process instead of the diffuse
one due to the release of sodium atoms by the dust particles. Such a conclusion is supported by the spatial distribution of
sodium on the nucleus and in the coma as reported from other authors.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献