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961.
Brian?Garvey Ian?P.?CastroEmail author Giles?Wiggs Joanna?Bullard 《Boundary-Layer Meteorology》2005,117(3):417-446
Wind-tunnel measurements of the flow over an isolated valley both normal and at an angle (45°) to a simulated neutrally stable
atmospheric boundary layer are presented. Attention is concentrated on the nature of the flow within the valley itself. The
work formed part of a wider study that included detailed field measurements around an African desert valley and some limited
comparisons with that work are included. A scale of about 1:1000 was used for the laboratory work, in which an appropriate
combination of hot wire and particle image velocimetry was employed. For a valley normal to the upwind flow, it is shown that
the upstream influence of the valley extends to a distance of at least one half of the axial valley width upstream of the
leading edge, whereas differences in mean flow and turbulence could be identified well beyond two valley widths from the downwind
edge. Non-normal wind angles lead to significant along-valley flows within the valley and, even at two valley heights above
the valley ridge level, there remains a significant spanwise flow component. Downwind turbulence levels are somewhat lower
in this case, but are still considerably higher than in the undisturbed boundary layer. At both flow angles, there are significant
recirculation regions within the valleys, starting from mean separation just beyond the leading edge, but the strong spanwise
flow in the 45° case reduces the axial extent of the separated zone. The flow is shown to be in some ways analogous to flow
over an isolated hill. Our results usefully enhance the field data and could be used to improve modelling of saltation processes
in the field. 相似文献
962.
Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer 总被引:1,自引:0,他引:1
Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined
from the standard deviations of longitudinal wind velocity and temperature, σ u and σ T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements
that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather
stations, where they replace the standard cup anemometer–wind vane system. With methods such as the one described in this
note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It
is shown that a ‘variance’ dimensionless height (ζ σ) defined from σ u and σ T is highly related to the ‘conventional’ dimensionless stability parameter ζ=z/L, where z is height and L is the Obukhov length. Empirical functions for ζ σ are proposed that allow direct calculation of heat and momentum fluxes from σ u and σ T. The method performs fairly well also during a night of intermittent turbulence. 相似文献
963.
964.
The importance of sulfur dioxide emissions for climate change is now established, although substantial uncertainties remain.
This paper presents projections for future sulfur dioxide emissions using the MiniCAM integrated assessment model. A new income-based
parameterization for future sulfur dioxide emissions controls is developed based on purchasing power parity (PPP) income estimates
and historical trends related to the implementation of sulfur emissions limitations. This parameterization is then used to
produce sulfur dioxide emissions trajectories for the set of scenarios developed for the Special Report on Emission Scenarios
(SRES). We use the SRES methodology to produce harmonized SRES scenarios using the latest version of the MiniCAM model. The
implications, and requirements, for integrated assessment modeling of sulfur dioxide emissions are discussed. We find that
sulfur emissions eventually decline over the next century under a wide set of assumptions. These emission reductions result
from a combination of emission controls, the adoption of advanced electric technologies, and a shift away from the direct
end use of coal with increasing income levels. Only under a scenario where incomes in developing regions increase slowly do
global emission levels remain at close to present levels over the next century. Under a climate policy that limits emissions
of carbon dioxide, sulfur dioxide emissions fall in a relatively narrow range. In most cases, the relative climatic effect
of sulfur dioxide emissions decreases dramatically to a point where sulfur dioxide is only a minor component of climate forcing
by the end of the century. Ecological effects of sulfur dioxide, however, could be significant in some developing regions
for many decades to come. 相似文献
965.
Rudolf?BráZdilEmail author Christian?Pfister Heinz?Wanner Hans?Von?Storch JüRg?Luterbacher 《Climatic change》2005,70(3):363-430
This paper discusses the state of European research in historical climatology. This field of science and an overview of its development are described in detail. Special attention is given to the documentary evidence used for data sources, including its drawbacks and advantages. Further, methods and significant results of historical-climatological research, mainly achieved since 1990, are presented. The main focus concentrates on data, methods, definitions of the “Medieval Warm Period” and the “Little Ice Age”, synoptic interpretation of past climates, climatic anomalies and natural disasters, and the vulnerability of economies and societies to climate as well as images and social representations of past weather and climate. The potential of historical climatology for climate modelling research is discussed briefly. Research perspectives in historical climatology are formulated with reference to data, methods, interdisciplinarity and impacts. 相似文献
966.
Carbon Sequestration in Arable Soils is Likely to Increase Nitrous Oxide Emissions,Offsetting Reductions in Climate Radiative Forcing 总被引:5,自引:0,他引:5
Strategies for mitigating the increasing concentration of carbon dioxide (CO2) in the atmosphere include sequestering carbon (C) in soils and vegetation of terrestrial ecosystems. Carbon and nitrogen
(N) move through terrestrial ecosystems in coupled biogeochemical cycles, and increasing C stocks in soils and vegetation
will have an impact on the N cycle. We conducted simulations with a biogeochemical model to evaluate the impact of different
cropland management strategies on the coupled cycles of C and N, with special emphasis on C-sequestration and emission of
the greenhouse gases methane (CH4) and nitrous oxide (N2O). Reduced tillage, enhanced crop residue incorporation, and farmyard manure application each increased soil C-sequestration,
increased N2O emissions, and had little effect on CH4 uptake. Over 20 years, increases in N2O emissions, which were converted into CO2-equivalent emissions with 100-year global warming potential multipliers, offset 75–310% of the carbon sequestered, depending
on the scenario. Quantification of these types of biogeochemical interactions must be incorporated into assessment frameworks
and trading mechanisms to accurately evaluate the value of agricultural systems in strategies for climate protection. 相似文献
967.
Article 2 of the UN Framework Convention on Climate Change (UNFCCC), which states the treaty's long-term objective, is the
subject of a growing literature that examines means to interpret and implement this provision. Here we provide context for
these studies by exploring the intertwined scientific, legal, economic, and political history of Article 2. We review proposed
definitions for “dangerous anthropogenic interference” and frameworks that have been proposed for implementing these definitions.
Specific examples of dangerous climate changes suggest limits on global warming ranging from 1 to 4 ∘C and on concentrations ranging from 450 to 700 ppm CO2 equivalents. The implications of Article 2 for near term restrictions on greenhouse-gas emissions, e.g., the Kyoto Protocol,
are also discussed. 相似文献
968.
Over the last four centuries several botanists have analysed the Colosseum flora, and by using modern ecological and biogeographical approaches to these same floras, we have observed similar trends with respect to the climatic phenomena as those of the historic Torre Calandrelli Observatory and by other older historical records. The elements of which can be summarised as follows: 1) a substantial reduction of all the biogeographical components taken as indicators of cooler and damper habitats, such as the Euro-Asiatic and Boreal areas starting from the 17th Century in particular; 2) a decrease in numbers of all the species more dependent on water (independent of their chorology); 3) similar clear-cut increases in the species belonging to phytosociological classes typical to Mediterranean climatic conditions, to the detriment of those typical to Middle-European temperate bioclimates. All of which can be interpreted as clear indicators of non-negligible climatic changes towards hotter and drier conditions independent of local variations due to urbanisation. 相似文献
969.
PNNLs Agriculture and Land Use is used to demonstrate the impact of potential changes in climate on agricultural production and land use in the United States. AgLU simulates production of four crop types in several world regions, in 15-yr time steps from 1990 to 2095. Changes in yield of major field crops in the United States, for 12 climate scenarios, are obtained from simulations of the EPIC crop growth model. Results from the HUMUS model are used to constrain crop irrigation, and BIOME3 model is used to simulate productivity of unmanaged ecosystems. Assumptions about changes in agricultural productivity outside the United States are treated on a scenario basis, either responding in the same way as in the United States, or not responding to climate. 相似文献
970.
Urban Atmospheric Chemistry During the PUMA Campaign 2: Radical Budgets for OH,HO<Subscript>2</Subscript> and RO<Subscript>2</Subscript> 总被引:1,自引:0,他引:1
K.?M.?Emmerson N.?CarslawEmail author M.?J.?Pilling 《Journal of Atmospheric Chemistry》2005,52(2):165-183
A detailed photochemical box model was used to investigate the key reaction pathways between OH, HO2 and RO2 radicals during the summer and winter PUMA field campaigns in the urban city-centre of Birmingham in the UK. The model employed
the most recent version of the Master Chemical Mechanism and was constrained to 15-minute average measurements of long-lived
species determined in situ at the site. The results showed that in the summer, OH initiation was dominated by the reactions of ozone with alkenes, nitrous
acid (HONO) photolysis and the reaction of excited oxygen atoms atoms with water. In the winter, ozone+alkene reactions were
the primary initiation route, with a minor contribution from HONO photolysis. Photolysis of aldehydes was the main initiation
route for HO2, in both summer and winter. RO2 initiation was dominated by the photolysis of aldehydes in the summer with a smaller contribution from ozone+alkenes, a situation
that was reversed in the winter. At night, ozone+alkene reactions were the main radical source. Termination, under all conditions,
primarily involved reactions with NO (OH) and NO2 (OH and RCO3). These results demonstrate the importance of ozone+alkene reactions in urban atmospheres, particularly when photolysis reactions
were less important during winter and at nighttime. The implications for urban atmospheric chemistry are discussed. 相似文献