排序方式： 共有40条查询结果，搜索用时 15 毫秒
21.
Philip C. Farese Giorgio Dall&#x;Oglio Josh Gundersen Brian Keating Slade Klawikowski Lloyd Knox Alan Levy Chris O&#x;Dell Alan Peel Lucio Piccirillo John Ruhl Peter Timbie 《New Astronomy Reviews》2003,47(11-12):1033
COMPASS is an on-axis 2.6-m telescope coupled to a correlation polarimeter. The entire instrument was built specifically for CMB polarization studies. Careful attention was given to receiver and optics design, stability of the pointing platform, avoidance of systematic offsets, and development of data analysis techniques. Here we describe the experiment, its strengths and weaknesses, and the various things we have learned that may benefit future efforts to measure the polarization of the CMB. 相似文献
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In this work, we study the mean tropopause structure from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis in the framework of baroclinic adjustment theories, focusing on the impact of baroclinic eddies on the mean tropopause height. In order to measure the effects of such perturbations, we introduce an appropriate global index that selects events of high baroclinic activity and allows us to distinguish the phases of growth and decay of baroclinic waves. We then composite the tropopause mean structure before and after baroclinic events, finding that baroclinic disturbances cause the zonally averaged midlatitude winter tropopause height to rise. Our results establish the importance of baroclinic adjustment processes for midlatitude tropopause dynamics. 相似文献
25.
Valerio Lucarini Sandro Calmanti Alessandro Dell’Aquila Paolo M. Ruti Antonio Speranza 《Climate Dynamics》2007,28(7-8):829-848
We compare, for the overlapping time frame 1962–2000, the estimate of the northern hemisphere mid-latitude winter atmospheric
variability within the available 20th century simulations of 19 global climate models included in the Intergovernmental Panel
on Climate Change—4th Assessment Report with the NCEP-NCAR and ECMWF reanalyses. We compute the Hayashi spectra of the 500 hPa
geopotential height fields and introduce an ad hoc integral measure of the variability observed in the Northern Hemisphere
on different spectral sub-domains. The total wave variability is taken as a global scalar metric describing the overall performance
of each model, while the total variability pertaining to the eastward propagating baroclinic waves and to the planetary waves
are taken as scalar metrics describing the performance of each model phenomenologically in connection with the corresponding
specific physical process. Only two very high-resolution global climate models have a good agreement with reanalyses for both
the global and the process-oriented metrics. Large biases, in several cases larger than 20%, are found in all the considered
metrics between the wave climatologies of most IPCC models and the reanalyses, while the span of the climatologies of the
various models is, in all cases, around 50%. In particular, the travelling baroclinic waves are typically overestimated by
the climate models, while the planetary waves are usually underestimated, in agreement with what found is past analyses performed
on global weather forecasting models. When comparing the results of similar models, it is apparent that in some cases the
vertical resolution of the model atmosphere, the adopted ocean model, and the advection schemes seem to be critical in the
bulk of the atmospheric variability. The models ensemble obtained by arithmetic averaging of the results of all models is
biased with respect to the reanalyses but is comparable to the best five models. Nevertheless, the models results do not cluster
around their ensemble mean. This study suggests caveats with respect to the ability of most of the presently available climate
models in representing the statistical properties of the global scale atmospheric dynamics of the present climate and, a fortiori,
in the perspective of modeling climate change. 相似文献
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A. Costa F. Dell’Erba M. A. Di Vito R. Isaia G. Macedonio G. Orsi T. Pfeiffer 《Bulletin of Volcanology》2009,71(3):259-273
Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area.
In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative
of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar
to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed
using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection–diffusion–sedimentation
equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and
bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations
using a statistical set of wind profiles, obtained from NOAA re-analysis. Probability maps, relative to the considered scenarios,
were constructed for several tephra loads, such as 200, 300 and 400 kg/m2. These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in
the area. 相似文献
27.
Claire L. A. Dell Guilherme O. Longo Carrie Manfrino Deron E. Burkepile 《Marine Ecology》2020,41(5):e12613
Many Caribbean reefs have shifted from coral dominance to macroalgal dominance, often by brown algae such as Dictyota and Lobophora. However, the north side of Cayman Brac in the Cayman Islands is dominated seasonally by the green macroalga Microdictyon (percent cover of Microdictyon is 4% in the winter and ~40% of the reef in the summer), although it is absent from the south side of the island and the remainder of the country. Indeed, Microdictyon is rare in much of the Caribbean, so this situation on Cayman Brac provides an opportunity to investigate the conditions that facilitate its distribution and dominance. The impact of herbivory, competition, nutrient input and other abiotic conditions were examined as factors that could influence the distribution and dominance of Microdictyon. While herbivory or nutrient input are frequently found to be key drivers of benthic community composition on coral reefs, here consumption of Microdictyon by herbivores was low, and thus, the alga was not subject to strong top-down control by herbivory. So, in this case, neither herbivore abundance nor feeding preference appeared to influence the distribution of Microdictyon. Nutrient input was also similar to both sides of the island suggesting nutrients played little role in differential distribution. But, in a controlled transplant experiment where Microdictyon was protected from herbivory and competition, it experienced almost complete mortality (93.3%) when transplanted to the south side, compared to only 11.8% mortality on the north. The south side was exposed to the strongest wave action 92% of the days in our study and was on average a slight, but significant 0.2°C warmer. Thus, these data suggest physical forcing (i.e. wave exposure) is the most likely factor dictating Microdictyon distribution. Conversely, a combination of reduced herbivory and increased competitive strength may explain the seasonal dominance of Microdictyon on the north side of Cayman Brac. Microdictyon was a competitive inferior to the other common algae in winter but increased in competitive strength to equal other species in summer. These results add to the literature on Microdictyon and on the forces impacting benthic community structure of coral reefs. 相似文献
28.
Vladimir Martinusi Lamberto Dell’Elce Gaëtan Kerschen 《Celestial Mechanics and Dynamical Astronomy》2017,127(4):451-476
The paper offers the fully analytic solution to the motion of a satellite orbiting under the influence of the two major perturbations, due to the oblateness and the atmospheric drag. The solution is presented in a time-explicit form, and takes into account an exponential distribution of the atmospheric density, an assumption that is reasonably close to reality. The approach involves two essential steps. The first one concerns a new approximate mathematical model that admits a closed-form solution with respect to a set of new variables. The second step is the determination of an infinitesimal contact transformation that allows to navigate between the new and the original variables. This contact transformation is obtained in exact form, and afterwards a Taylor series approximation is proposed in order to make all the computations explicit. The aforementioned transformation accommodates both perturbations, improving the accuracy of the orbit predictions by one order of magnitude with respect to the case when the atmospheric drag is absent from the transformation. Numerical simulations are performed for a low Earth orbit starting at an altitude of 350 km, and they show that the incorporation of drag terms into the contact transformation generates an error reduction by a factor of 7 in the position vector. The proposed method aims at improving the accuracy of analytic orbit propagation and transforming it into a viable alternative to the computationally intensive numerical methods. 相似文献
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C. R. O’Dell 《Experimental Astronomy》2009,25(1-3):261-272
The Hubble Space Telescope has been the most successful space astronomy project to date, producing images that put the public in awe and images and spectra that have produced many scientific discoveries. It is the natural culmination of a dream envisioned when rocket flight into space was first projected and a goal set for the US space program soon after NASA was created. The design and construction period lasted almost two decades and its operations have already lasted almost as long. The capabilities of the observatory have evolved and expanded with periodic upgrading of its instrumentation, thus realizing the advantages of its unique design. The success of this long-lived observatory is closely tied to the availability of the Space Shuttle and the end of the Shuttle program means that the end of the Hubble program will follow before long. 相似文献
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C. Dubois S. Somot S. Calmanti A. Carillo M. Déqué A. Dell’Aquilla A. Elizalde S. Gualdi D. Jacob B. L’Hévéder L. Li P. Oddo G. Sannino E. Scoccimarro F. Sevault 《Climate Dynamics》2012,39(7-8):1859-1884
Within the CIRCE project “Climate change and Impact Research: the Mediterranean Environment”, an ensemble of high resolution coupled atmosphere–ocean regional climate models (AORCMs) are used to simulate the Mediterranean climate for the period 1950–2050. For the first time, realistic net surface air-sea fluxes are obtained. The sea surface temperature (SST) variability is consistent with the atmospheric forcing above it and oceanic constraints. The surface fluxes respond to external forcing under a warming climate and show an equivalent trend in all models. This study focuses on the present day and on the evolution of the heat and water budget over the Mediterranean Sea under the SRES-A1B scenario. On the contrary to previous studies, the net total heat budget is negative over the present period in all AORCMs and satisfies the heat closure budget controlled by a net positive heat gain at the strait of Gibraltar in the present climate. Under climate change scenario, some models predict a warming of the Mediterranean Sea from the ocean surface (positive net heat flux) in addition to the positive flux at the strait of Gibraltar for the 2021–2050 period. The shortwave and latent flux are increasing and the longwave and sensible fluxes are decreasing compared to the 1961–1990 period due to a reduction of the cloud cover and an increase in greenhouse gases (GHGs) and SSTs over the 2021–2050 period. The AORCMs provide a good estimates of the water budget with a drying of the region during the twenty-first century. For the ensemble mean, he decrease in precipitation and runoff is about 10 and 15% respectively and the increase in evaporation is much weaker, about 2% compared to the 1961–1990 period which confirm results obtained in recent studies. Despite a clear consistency in the trends and results between the models, this study also underlines important differences in the model set-ups, methodology and choices of some physical parameters inducing some difference in the various air-sea fluxes. An evaluation of the uncertainty sources and possible improvement for future generation of AORCMs highlights the importance of the parameterisation of the ocean albedo, rivers and cloud cover. 相似文献