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881.
Increased heat fluxes near a forest edge 总被引:1,自引:0,他引:1
W. Klaassen P. B. van Breugel E. J. Moors J. P. Nieveen 《Theoretical and Applied Climatology》2002,72(3-4):231-243
Summary ?Observations of sensible and latent heat flux above forest downwind of a forest edge show these fluxes to be larger than
the available energy over the forest. The enhancement averages to 56 W m−2, or 16% of the net radiation, at fetches less than 400 m, equivalent to fetch to height ratios less than 15. The enhancement
of turbulent energy fluxes is explained by advection and increases with the difference in temperature and humidity of the
air over the upwind area as compared to the forest. The relatively high temperature and humidity of the upwind air are not
caused by high surface heat fluxes, but are explained by the relatively low aerodynamic roughness of the upwind surface. Although
the heat fluxes over forest are enhanced, the momentum fluxes are almost adjusted to the underlying forest. The different
behaviour of heat and momentum fluxes is explained by absorption of momentum by pressure gradients near the forest edge. It
is concluded that fetch requirements to obtain accurate surface fluxes from atmospheric observations need to be more stringent
for scalar fluxes as compared to momentum fluxes.
Received November 23, 2001; accepted May 13, 2002 相似文献
882.
Jacek Stankiewicz S bastien Chevrot Rob D. van der Hilst Maarten J. de Wit 《Physics of the Earth and Planetary Interiors》2002,130(3-4):235-251
The technique of receiver function analysis is applied to the study of crustal and upper mantle structures beneath the Kaapvaal craton in southern Africa and its surroundings. Seismic data were recorded by the seismic array of 82 sites deployed from April 1997 to April 1999 across southern Africa, as well as a dense array of 32 sites near Kimberley, in operation from December 1998 to June 1999. Arrival times for phases converted at the Moho are used to determine crustal thickness. The Moho depth in the south–western section of the craton was found to vary between 37 and 40 km, except for one station that recorded a depth of 43 km (SA23). Farther north along the western block of the craton (into Botswana) the depth increases up to 43 km. The depth increases even further in the north–eastern section of the craton, where results vary from 40 to 52 km. Just north of the Kaapvaal craton, in the neighbouring Zimbabwe craton, the crustal thickness drops significantly. The results obtained there varied from 36 to 40 km. For the Kimberley area, using the dense array, the Moho depth was found to be 37.3 km. Arrivals of the Ps and Ppps phases were used to determine the Poisson’s ratio in the region. This was found to be 0.26±0.01. Arrivals of phases from the 410 and 660 km mantle discontinuities are used to interpret the relative positions of these discontinuities, as well as for comparison of mantle temperatures and seismic velocities in the region with global averages. In the Kimberley area the 410 and 660 km discontinuities were found at their expected depth, implying that mantle temperatures in the region are close to the global average. The seismic velocities above the ‘410’ were found up to 5% faster than the averages from the global iasp91 model, which is fast even by Precambrian standards. In other sections of the Kaapvaal craton, the velocities are also faster than global averages, but not as fast as beneath Kimberley. In these sections, the ‘410’ is also slightly elevated, while the ‘660’ is depressed, which implies a slightly lower mantle temperature relative to the global average. Beneath the Kaapvaal craton we find evidence suggesting the presence of a zone with a reduced wavespeed gradient at an upper bound of approximately 300 km, which may mark the lower chemical boundary of the craton. 相似文献
883.
On the role of subducting oceanic plateaus in the development of shallow flat subduction 总被引:2,自引:0,他引:2
Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600–700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile. 相似文献
884.
885.
Marloes van Amerom 《GeoJournal》2002,58(4):265-273
The notion that Transboundary Protected Areas (TBPAs) will act as `Peace Parks' has become an important argument in their promotion in post-Apartheid Southern Africa. This `Peace Parks Concept' is implicitly based upon the assumption that national sovereignty will not become a constraining factor in the creation and management of TBPAs. However, this assumption is problematic. TBPAs introduce various changes in the landscape with consequences for the ways in which a state can exercise its sovereignty over its borderland and citizens. This situation might evoke state action that could endanger the various environmental, socio-economic and political objectives of TBPAs. A state's behaviour with regard to TBPAs is not just informed by its interests in TBPAs, but also by its strategic and other interests in the wider borderland area. These are not necessarily compatible with the environmental and economic objectives of TBPAs. The ways in which these interests are mediated are highly complex and non-linear. Because of the inter-dependency that TBPAs create, the extent to which a state can pursue certain interests by means of TBPAs will be (partially) constrained by the wishes of the other states involved. In addition, power has to be shared with a wide range of non-state actors. The operation of sovereignty in TBPAs is therefore highly unpredictable and cannot be captured in static zero-sum terms. Actor Network Theory is identified as a possible starting-point to unravel and evaluate these complex political processes in TBPAs and their subsequent outcomes for state sovereignty. 相似文献
886.
Lars Olsen Harald Sveian Klaas van der Borg Bjørn Bergstram Maarten Broekmans 《Polar research》2002,21(2):235-242
The onshore record of Middle to Late Weichselian sediments and glacial history in Norway indicates a succession of four major ice advances alternating with rapid, considerable ice recessions and interstadial conditions. During all the glacial advances the ice sheet expanded from onshore/inland positions to the shelf areas. The basis for visualizing these variations in glaciation curves constructed along nine transects from inland to shelf, and for interpretation of the palaeoclimatic history, is the regional Quaternary stratigraphy, more than 300 datings, fossil content and some palaeomagnetic data. The methods applied in recent years for AMS radiocarbon dating of glacial sediments with low organic carbon content have given promising results with respect to accuracy and precision, and the results of such datings were an important tool for our reconstructions and for timing of the ice oscillations. The rapid and rhythmic ice fluctuations, as reconstructed in our new model, have been fairly synchronous in most parts of Norway. Ice advances commenced and culminated at 40, 30-28, 24-21 and 18-15 (14 C) Kya. We describe three intervening interstadials from inland sites: Hattfjelldal I, Hattfjelldal II and Trofors. Our stratigraphical record also includes many indications of high, pre-Holocene, relative sea levels, suggesting a considerable glacioisostatic depression of western Scandinavia during the interstadials. In our glaciation model we suggest that, in addition to precipitation, the mountainous fjord and valley topography, glacial isostasy and relative sea level changes were probably more important for the size of the glacial fluctuations than were air temperature changes. 相似文献
887.
1. Introduction
The main-hole of the Chinese Continental Scientific Drilling Project (CCSD) in Donghai of the Dabie-Sulu UHP metamorphic belt in East China has reached its final depth of 5118 m in March 2005. As shown in Fig. 相似文献
888.
889.
890.
We thank van Emmerik et al. for their discussion of our opinion paper. We do not fully agree that publication of negative results will be very effective in promoting experimental hydrology. Instead, we think that experimental hydrology is considered to be more risky than modelling studies because of difficulties in publishing inconclusive results and the potential for failure. 相似文献