We have carried out a survey for 'giant pulses' in six young, Vela-like pulsars. In no cases did we find single pulses with flux densities more than 10 times the mean flux density. However, in PSR B1706–44 we have detected giant micro-pulses very similar to those seen in the Vela pulsar. In PSR B1706–44 these giant micro-pulses appear on the trailing edge of the profile and have an intrinsic width of ∼1 ms. The cumulative probability distribution of their intensities is best described by a power law. If the power law continues to higher intensities, then 3.7×106 rotations are required to obtain a pulse with 20× the mean pulse flux. This number is similar to the giant pulse rate in PSR B1937+21 and PSR B1821–24 but significantly higher than that for the Crab. 相似文献
The effect of mass balance variations on changes in surface elevation of the Greenland ice sheet is examined in connection with the rise in sea level that will be caused by increased melting. Changes in surface elevation of several metres (of either sign) can occur in the ablation area of the ice sheet over a period of a few years as a result of random ablation forcing without being evidence of change in mean climate. Similar, but smaller, changes can occur in the accumulation area due to accumulation forcing. The ablation area of the ice sheet probably thickened from the mid-1970s to the mid-1980s as a result of lower ablation in that period but thinned again in the late 1980s as a result of higher ablation then. There is no evidence of any present trend of increased melting. Future climate warming will involve an accelerated thinning of the ablation area that could be detected in 1–2 decades against the background of natural fluctuations in surface elevation. 相似文献
The cycling representer algorithm of Xu and Daley (2000) is a weak constraint four-dimensional variational data assimilation algorithm. It was successfully applied to a one-dimensional transport problem and was able to successfully extract the signal from noisy and sparse observations. The algorithm, however, has not previously been applied to a multivariate, multidimensional system with dynamic instability. The algorithm is also very computationally demanding and awaits considerable enhancement in computer power before being practical for operational forecast models. We have two objectives in this paper. The first is to apply the cycling representer algorithm to a two-dimensional, multivariate barotropically unstable linear shallow water system. The second objective is to formulate and test an accelerated representer algorithm that is much more computationally tractable than the cycling representer algorithm itself. A linear shallow water system with a barotropically unstable basic state was used as a test bed to conduct data assimilation experiments. The evolution of a 'neutral' eastward-propagating singular vector was selected as the 'truth', against which all data assimilation experiments were to be evaluated. The results indicated that the cycling representer algorithm was capable of providing satisfying state estimates for a multivariate, multidimensional system. The results from the accelerated representer algorithm were very encouraging because it is sufficiently computationally tractable to be used on present day multi-processor machines for operational applications. 相似文献
The cell dimensions and crystal structures of the fluoroperovskite NaMgF3 (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement
over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These
data show that Pbnm NaMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and
atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF3 at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF3 from the aristotype cubic
structure is described in terms of the tilting of the MgF6 octahedra according to the tilt scheme a−a−c+. With decreasing temperature the antiphase tilt (a−) increases from 14.24° to 15.39°, whereas the in-phase tilt (c+) remains effectively constant at ∼10.7°. Changes in the tilt angles are insufficient to cause changes in the coordination
sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF3 is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those
of Pbnm KCaF3. 相似文献
The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre‐scale and patchy, dome‐like gypsum accumulations are well‐exposed within strongly incised present‐day river valleys along the eastern margin of the Namibe and Benguela basins (south‐west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution‐related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin‐spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the current at‐surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross‐cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics. 相似文献
Much of the Eastern Cape province in South Africa has been experiencing a severe drought since 2015. This drought has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, the ongoing drought is examined in the context of long-term trends and the interannual rainfall variability of the region. Although the region has experienced drought in all seasons since 2015, focus here is placed on the spring (September–November) which shows the most consistent and robust signal. On average, this season contributes between about 25–35% of the annual rainfall total. Based on CHIRPS data, it is found that this season shows a significant decreasing trend in both rainfall totals as well as the number of rainfall days (but not heavy rainfall days) for spring over most of the province since 1981. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. Analysis of mid-century (2040–2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and the summer drier. For the spring season of interest here, the multi-model projections also indicate drying but less pronounced than that projected for the summer.
A global baroclinic primitive equation model using the spectral technique has been constructed for short‐ and medium‐range numerical weather prediction. The spectral technique, which is a special case of the Galerkin method, employs spherical harmonic basis functions in the evaluation of all horizontal derivatives. The use of a transform technique allows all the horizontal operations to be performed efficiently and allows physical processes to be evaluated in real space. The model employs a semi‐implicit algorithm for time integration and finite differencing in the vertical. Physical processes include orography, moist convection, large scale precipitation and boundary layer processes. 相似文献
ABSTRACTAs increasing evidence shows that the risks of climate change are mounting, there is a call for further climate action (both reducing global emissions, and adaptation to better manage the risks of climate change). To promote and enable adaptation, governments have introduced, or are considering introducing, reporting on climate risks and efforts being taken to address those risks. This paper reports on an analysis of the first two rounds of such reports submitted under the UK Climate Change Act (2008) Adaptation Reporting Power. It highlights benefits and challenges for reporting authorities and policymakers receiving the reports that could also inform other countries considering such reporting. For reporting authorities, benefits arise from the reporting process and resulting reports. These benefits include elevating climate risks and adaptation to the corporate level and with stakeholders, alongside facilitating alignment and integration of actions within existing risk management and governance structures. For policymakers, reporting provides enhanced understanding of climate risks and actions from a bottom-up perspective that can be integrated into national-level assessments and adaptation planning processes. The identified challenges are those related to capacity and process. These include limited risk and adaptation assessment capacities; relevance of climate change risks and adaptation in the context of other urgent risks and actions; reporting process effectiveness and robustness; and the provision of effective and sufficiently comprehensive support, including feedback.Key policy insights
Effective adaptation reporting needs to be designed and delivered so as to enhance the value of the reporting process and resulting reports both for those reporting and those receiving the reports, as well as from the broader policy perspective.
Providing a positive and supportive reporting environment is critical to encourage participation and facilitating contiuous learning and improvement, while also facilitating delivery of policy-relevant adaptation reports.
Contributions of adaptation reporting can be enhanced by an inclusive reporting requirement involving a broader organizational mix that enables more effective risk management and reporting that reflects associated (inter)dependencies and consistency with the more comprehensive post-2015 resilience agenda (Paris Agreement, Sendai Framework for DRR and UN Agenda 2030 SDGs).
This study examines the hypothesis that the determinants of denominational switching vary regionally among white American Protestants. Data for the analysis are taken from the National Opinion Research Center's General Social Survey for 1974 through 1984. Using logistic regression, the analysis compares the determinants of switching among respondents residing in the nine Census regions of the U.S. Results reveal significant regional differences in the determinants of switching, especially in the effects of conversion, childhood affiliation and interdenominational marriage. 相似文献
