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We have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles, about 30% of the venusian surface. Each quadrangle was split into 0.5° by 0.5° boxes, which were then identified as corona materials, large volcano materials (>100 km diameter), intermediate volcano materials (10-100 km), small edifice materials (<10 km), flow materials from rifts or fractures, plains without an identifiable source, impact crater materials and highly deformed materials, or data gaps. We find that coronae resurface approximately 21%, small edifices 22% and large volcanoes about 6% of the surfaces analyzed. Plains with no identifiable source account for an average of 35% of the surface assessed. Small edifices resurface on a scale of 10-100 s of km2; large edifices resurface areas of 104-105 km2. Coronae have greatly varying amounts of associated volcanism, with some coronae producing negligible flow deposits and others producing deposits of 104-106 km2. The areas identified as plains with no visible source occur on small scales (102 km2) to large scales (> 105 km2). Our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source, that fields of small edifices contribute more to resurfacing than we had anticipated, and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles. All of the units that we quantified occur throughout the histories of the regions mapped. We favor plains resurfacing to have occurred over at least 100 myr, which implies terrestrially reasonable resurfacing rates. 相似文献
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Vethamony P Babu MT Ramanamurty MV Saran AK Joseph A Sudheesh K Padgaonkar RS Jayakumar S 《Marine pollution bulletin》2007,54(6):697-707
The Gulf of Kachchh (GoK) is situated in the northeastern Arabian Sea. The presence of several industries along its coastal belt makes GoK a highly sensitive coastal ecosystem. In the present study, an attempt is made for the first time to study GoK thermohaline structure and its variability, based on field measurements and model simulations. Though GoK is considered as a well-mixed system, the study reveals that only the central Gulf is well mixed. Vertical gradients in temperature and salinity fields are noticed in the eastern Gulf, where a cold and high saline tongue is observed in the subsurface layers. Salinity indicates the characteristic feature of an inverse estuary with low values (37.20 psu) near the mouth and high values (>40.0 psu) near the head of the Gulf. The model simulated temperature and salinity fields exhibit semidiurnal oscillations similar to that of field observations. Model results show cold, high saline waters advecting from the east during ebb forming a transition zone, which oscillates with tides. A high salinity tongue is seen in the bottom layer, indicating a westward flowing bottom current. The transient zone acts as an dynamic barrier, and plays a vital role in the pollutant transport. 相似文献
117.
Antony Millner 《Climatic change》2012,110(1-2):143-167
The role of short- and long-term climate predictions in determining the success of adaptation to climate change is investigated. A simple theoretical model that captures the relationship between adaptive performance, decision structure, and prediction accuracy at different temporal scales is developed, and its implications deduced. It is shown that users who face high adjustment costs (i.e. are inflexible) depend more heavily on accurate long-term predictions than those who are able to adjust their adaptation strategies at low cost. However, the constraints on the accuracy of long-term predictions in order for them to be ‘accurate enough’ to be recommended unreservedly are strongest for precisely those users who desire them most. There are thus intrinsic structural limitations to the utility of long-term predictions for adaptive decision making. In addition, the heterogeneous dependence of adaptive performance on predictions at different temporal scales gives rise to a resource allocation problem for investments aimed at improving predictions so as to maximize their social benefits. It is suggested that an explicit distributional framework that matches users’ needs with scientific feasibility is needed in order to guide such investments, and key issues any such framework will need to address are discussed. 相似文献
118.
Ian Faloona Stephen A. Conley Byron Blomquist Antony D. Clarke Vladimir Kapustin Steven Howell Don H. Lenschow Alan R. Bandy 《Journal of Atmospheric Chemistry》2009,63(1):13-32
Research flights with the National Center for Atmospheric Research (NCAR) C-130 airborne laboratory were conducted over the
equatorial ocean during the Pacific Atmospheric Sulfur Experiment (PASE). The focused, repetitive flight plans provided a
unique opportunity to explore the principal pathways of sulfur processing in remote marine environments in close detail. Fast
airborne measurements of SO2 using the Drexel University APIMS (Atmospheric Pressure Ionization Mass Spectrometer) instrument further provided unprecedented
insight into the complete budget of this important sulfur gas. In general, turbulent mixing in the marine boundary layer (MBL)
continuously depletes SO2 due to the shallow convection of the tropical trade wind regime by venting the gas into the buffer layer (BuL) above. However,
on nearly one-third of the flights a net import of SO2 into the MBL from the BuL was observed. Concurrent measurements of the DMS budget allowed for a heterogeneous S(IV) oxidation
rate to be inferred from the SO2 budget residual. The average heterogeneous loss rate was found to be 0.05 h−1, which taken in conjunction with the observed aerosol surface area distributions and O3 levels indicates that the supermicron aerosols maintain a near neutral pH. The average dry deposition velocity of SO2 was found to be 0.4 cm s−1, about 30% lower than predicted by standard parameterizations. The yield of SO2 from DMS oxidation was found to be near unity. The mission averages indicate that approximately 57% of the SO2 in the MBL is lost to aerosols, 27% is subject to dry deposition, 7% is mixed into the BuL, and 10% is oxidized by OH. 相似文献
119.
Antony Lewis † 《Monthly notices of the Royal Astronomical Society》2009,398(1):471-476
Statistics of the weak lensing of galaxies can be used to constrain cosmology if the galaxy shear can be estimated accurately. In general, this requires accurate modelling of unlensed galaxy shapes and the point spread function (PSF). I discuss suboptimal but potentially robust methods for estimating galaxy shear by stacking images such that the stacked-image distribution is closely Gaussian by the central limit theorem. The shear can then be determined by radial fitting, requiring only an accurate model of the PSF rather than also needing to model each galaxy accurately. When noise is significant, asymmetric errors in the centroid must be corrected, but the method may ultimately be able to give accurate unbiased results when there is a high galaxy density with constant shear. It provides a useful baseline for more optimal methods, and a test case for estimating biases, though the method is not directly applicable to realistic data. I test stacking methods on the simple toy simulations with constant PSF and shear provided by the GREAT08 project, on which most other existing methods perform significantly more poorly, and briefly discuss generalizations to more realistic cases. In the Appendix , I discuss a simple analytic galaxy population model where stacking gives optimal errors in a perfect ideal case. 相似文献
120.
Antony A. Hill 《Dynamics of Atmospheres and Oceans》2004,38(1):441
A model for convection induced by the selective absorption of radiation in a fluid layer is investigated. The concentration based internal heat source is modelled quadratically. A linear analysis and conditional nonlinear analysis are performed. Due to the presence of significantly large regions of potential subcritical instabilities, the results indicate that linear theory may only be accurate enough to predict the onset of convective motion when the model for the internal heat source is predominantly linear. This lends much credence to the use of a linearly modelled internal heat source as used by Krishnamurti [Dyn. Atmos. Oceans 27 (1997) 367], within the constraints of the parameter ranges. 相似文献