Experimental assessment of N₂O background fluxes in grassland systems     

ALBRECHT NEFTEL  CHRIS FLECHARD  CHRISTOF AMMANN  FRANZ CONEN  LUKAS EMMENEGGER  KERSTIN ZEYER 《Tellus. Series B, Chemical and physical meteorology》2007,59(3):470-482

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REGIONAL SNOWFALL PATTERNS IN THE HIGH, ARID ANDES     

MATHIAS VUILLE  CASPAR AMMANN 《Climatic change》1997,36(3-4):413-423

Since 1990 the project Climate Change in the Arid Andes has been focusing on past climate and environmental conditions in the high mountain range of the north Chilean Andes (18° S–28° S). The extreme aridity of this region is shown by the absence of glaciers, even at the highest altitudes above 6700 m a.s.l. More knowledge of the present climatic situation is needed to interpret the proxy data of different paleoarchives in this transition belt between tropical and extratropical circulation. Precipitation events in this arid region are mainly registered during southern hemisphere summer, when the ITCZ reaches its southernmost position. Winter precipitation (snowfall) has so far not been considered an important factor in the hydrologic system of the area, because snow is seldom accurately registered by climatic stations. To fill this gap in our information, winter snowfall activity was analysed for a period of 6 years using digital NOAA/AVHRR satellite data. The results show that snowfall during winter (May–September) is a quite regular phenomenon, mainly linked to northward displacements or cut-offs of cold air-masses from the Pacific. The areal distribution of snowfall is determined by the synoptic situation that produces precipitation. During cold frontal events, snowfall is most frequent in the southernmost part of the research area and on the western Chilean side of the Andes. Cold air that has been cut off from the westerlies often interacts with warmer and more humid air over the continent and therefore gives rise to a different snowfall distribution, with the greatest snowfall frequency between 23° S–25° S, decreasing polewards as well as towards the equator. These two winter snowfall patterns show that reconstruction of paleoclimate has to take into account the different mechanisms that may cause precipitation in the research area. Intensification of winter precipitation (e.g., the west wind zone) can induce largely different precipitation patterns, depending on which mechanisms (cut-offs, cold-fronts or both) within the west wind zone are strengthened.  相似文献   

Late-Glacial radiocarbon- and palynostratigraphy on the Swiss Plateau   总被引：4，自引：0，他引：4  

BRIGITTA AMMANN  RE F. LOTTER 《Boreas: An International Journal of Quaternary Research》1989,18(2):109-126

A detailed late-glacial radiocarbon stratigraphy for the Swiss Plateau has been established on the basis of over 90 accelerator ¹⁴C dates on terrestrial plant macrofossils. Two plateaux of constant.,¹⁴C age were observed, occurring at 12,700 B.P. and at 10,000 B.P. The consequences of these plateaux for palaeo-ecological investigations are threefold: (1) a more refined ¹⁴C dating within the plateaux is not possible, (2) in teleconnections between different sites (if based on ¹⁴C dating and concerning the periods around 12,700 B.P. and 10,000 B.P.) events are considered synchronous which are only synchronous within a plateau of constant age, and (3) exact time-depth relationship and therefore influx calculations are made impossible during these plateau periods. A comparison of the radiocarbon ages derived from terrestrial, telmatic and limnic material at different sites on the Swiss Plateau yields a proposal for modifying the zonation system of Welten for the Late-Glacial. By retaining the limits of chronozones (at 13, 12, 11 and 10ka B.P.) and by refining the palynostratigraphic criteria for the limits of biozones, a separation between chrono- and biozonation at the beginning of the Belling and at the beginning of the Younger Dryas becomes obvious.  相似文献