The high temperature phase fields of a multicomponent titanium alloy system have been determined by the CALPHAD approach to assist in rapid processing of a titanium metal matrix composite by electric-field-assisted reaction sintering. The phase field calculations revealed the existence of a wide composition range (in terms of B, Fe, Mo and Al content) for the flexible synthesis of various beta-Ti-TiB composites. After processing, the uniform dispersion of in-situ formed titanium boride (TiB) whiskers, in the ductile beta titanium (beta-Ti) matrix, has resulted in a composite with a flexural strength of 1622 MPa and a fracture toughness of 23 MPa root m, which are much greater than the levels achieved before in this class of materials.

"预报挑战度"(measure of forecast challenge, MFC)是一种衡量预报难易的新尺度;"可预报性演变指数"(predictability horizon diagram index,PHDX)是一种检验集合预报的新评分.这二种新指标是杜钧等于2019年提出来的,文章发表在美国气象学会英文杂志《天气和预报》上.为了让国内基层台站业务预报和研发人员也能熟悉和应用这些方法,我们在此作一简短的总结介绍.

针对2018年4月22日发生在湖北西部山地的一次特大暴雨过程,采用降尺度方案和显式对流参数化方案模式,开展了高分辨率对流许可尺度(3 km)的集合预报试验,并对全球集合预报(GEFS)和对流尺度集合预报(SSEF)的降水预报进行了对比评估,结果表明:(1)SSEF集合平均的雨量和落区预报均优于GEFS.(2)SSEF各成员的降水离散度分布更合理,因而具有更优的降水区间预报,其"离散度-误差关系"更优,能更好地给出预报误差的分布及其可能的大小.(3)SSEF的概率预报在所有空间尺度上均优于GEFS,且在短历时强降水上的优势更加明显.由此可见,针对此类山地暴雨过程,对流尺度集合预报相对于全球集合预报具有巨大的改进潜力.

由于大气初值与数值模式中物理过程存在不确定性等客观事实,集合预报无疑代表着数值天气预报未来前进与发展的方向,它标志着天气预报的预报范式转变,即用户不仅可以得到未来大气状态的单一现实,还可得到未来大气可能出现的一系列场景.文中扼要地梳理了欧洲全球业务集合预报与有限区域模式高分辨业务集合预报的研究动态与技术发展、基本问题及其未来最新发展方向,包括:1)欧洲中期天气预报中心的业务集合预报系统发展沿革及概况;2)欧洲国家主要业务高分辨率集合预报系统概况;3)当前业务集合预报存在的问题、挑战及未来前进的方向.文中除了关注欧洲中期天气预报中心的集合预报应用,还梳理了目前欧洲高分辨业务集合预报取得的成就,以引起有关研究人员的注意.总之,借鉴欧洲业务集合预报的发展思路,不仅有助于集合预报理论创新,还对发展集合预报业务有重要指导意义.

总结回顾了集合敏感性分析(ESA)在诊断中纬度高影响天气预报不确定性中的应用.作为一个简单高效且不需要大量计算资源的方法,集合敏感性分析主要被应用在中纬度气旋、台风或飓风的温带转换,以及在强对流过程中诊断预报误差和不确定性的来源.集合敏感性方法极有灵活性,可以根据实际需要改变不同的预报变量和初始场.在对2010年美国东岸圣诞节暴风雪的分析中,集合敏感性分析通过三种形式来诊断了预报不确定性的初值敏感性,即基于EOF分析的敏感性、预报差别的敏感性,以及基于短期预报误差的向前积分敏感性回归.三种方法证实气旋路径的不确定性主要和位于美国南部大平原的短波槽初始误差相关.此外,气旋强度的不确定性还和产生于北太平洋向下游延伸的罗斯贝波列相关.集合敏感性分析方法对于分析中纬度气旋的不确定性、诊断初值敏感性、分析误差发展机制都非常有效.集合敏感性分析也被应用于分析台风/飓风的温带气旋转换过程的不确定性.在对2019年美国首个主要登陆台风Dorian的分析中发现,加拿大CMC的集合预报主要不确定性来自于强度的不确定性,而这个不确定性与初始时刻的大尺度环流型有关,较连贯的信号可以追溯至东北太平洋的前倾槽.而NCEP和ECMWF的不确定性主要在于气旋位置的东北—西南向移动,而敏感性主要和飓风系统本身(即其北部低压区和中纬度槽)的锁相有关.分析结果进一步验证了集合敏感性分析对诊断模式之间的不一致性,以及模式成员之间不一致性的不确定性来源和发展过程的可靠性.集合敏感性分析方法综合了集合预报、资料同化和敏感性分析,因此对于资料同化技术改进、诊断模式误差(或者缺陷)、附加(目标)观测最优策略,以及评估观测对预报的影响等都有重要意义.同时可以更有效地利用集合预报信息,帮助预报员提高情景意识,最终减少高影响天气预报中的决策失误.

新型传感技术融合了材料科学、微纳电子技术、生物技术等学科,是人工智能、精准医疗、新能源等战略前沿的先导和基础,也是智慧城市、智慧医疗等物联网应用的技术关键.智能化、微型化、多功能化、低功耗、低成本、高灵敏度、高可靠性是新型传感器件的发展趋势和主要研究方向.新型传感器件的传感性能很大程度上取决于传感材料的化学成分、表面修饰、传感层微观结构和完整性等因素.近年来,新型传感材料与器件的研究方兴未艾,为现代传感技术的深入开发与应用带来了新的机遇.本文综述了新型传感材料,包括硅纳米线、石墨烯、碳纳米管、二维材料、金属有机框架材料、水凝胶材料以及有机半导体材料等在力学传感、声学传感、生物传感、爆炸物监测等方面的应用.讨论了微机电系统(MEMS)和纳机电系统(NEMS)传感器技术,以及柔性可穿戴传感器相关材料、器件结构和制造方法的最新进展.

A CALPHAD-guided alloy design strategy is used here to design and process fully dense titanium boride (TiB) ceramic alloys containing a ductile phase for toughening. This involved the construction of high temperature phase fields of the Ti-B-Fe-Mo quaternary alloy system using the CALPAHD approach. To design the compositions, ternary phase diagrams of Ti-B-Fe, Ti-B-Mo, and Fe-Mo-B systems were first constructed using the thermodynamic databases that were verified to predict well the binary phase diagrams of components. Using this as a basis, the pseudo-ternary diagrams of the Ti-B-Fe-Mo system were constructed to map the high temperature phase fields over a wide range of composition. This made it possible to identify relatively lower reaction sintering temperatures for various alloy compositions. Two compositions of TiB ceramic alloys with the ductile beta-phase (both having similar to 85% TiB and similar to 15% beta-Ti phase), were considered for the demonstration of the present approach. Based on the pseudo-ternary phase fields, these two compositions could be processed using electric-field-activated sintering (EFAS), in a relatively short time, at their respective liquid-phase-forming temperatures. The processing temperature was as low as 1423 K for a Fe/Mo-rich ceramic alloy composition, but the formation of brittle intermetallic phase FeTi here was found to be detrimental to mechanical properties. The second TiB ceramic alloy composition, which was Fe/Mo-lean, could be processed at 1623 K. This composition is shown here to possess a very good combination of high flexural strength (similar to 850 MPa) and high fracture toughness (similar to 7.7 MPa root m), making it attractive for potential applications. The identification of the liquid phase reaction sintering regions in the ternary phase diagrams, and the eventual finding of one composition giving a relatively high strength and toughness, demonstrate that this is an effective approach for alloy design. The microstructural factors that control the strength and toughness have been identified and the ways to increase these properties further, in this class of materials, are suggested. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

美国强风暴实验室(NSSL)正在研发的FACETs系统预报对象包括龙卷、大冰雹和局地强降水,这套系统把预报信息同社会影响、人的心理行为和经济效益等学科相结合,把预报信息转化为同用户直接相关的问题. 目前,美国气象局的龙卷预警系统是基于观测的,即当雷达已经看到有强天气的早期迹象或已观测到强天气现象,才运用短临预报方法外推发布预警.但因龙卷等强天气变化快、生命期很短,所以这种基于观测的预警系统可用时效很短,如目前对龙卷的预警只能提前13 min左右,并且预报是确定性的,即只有出现或不出现龙卷的二分预报,而没有该预报的可信度度量.

Winter irrigation has proven to have positive effects on alleviating the "spring drought" and promoting the growth of crops. In seasonally frozen regions, a special rule of soil hydro-thermal movement is formed during the process of freezing and thawing. However, few studies have focused on the effects of winter irrigation on soil hydro-thermal dynamics in the farmland ecosystem of arid desert areas. Therefore, the aim of this study was to investigate the soil hydro-thermal features during the freezing-thawing period after winter irrigation with simulated soil column experiment. The experiment was conducted in marginal farmlands of oasis in middle Hexi Corridor region, Gansu, China. It was located in arid and semi-arid desert-oasis areas. Two types of soil, i.e. ploughed aeolian sandy soil (newly cultivated sandy cropland) and ploughed grey-brown desert soil (old oasis cropland), were backfilled into the soil columns. Each type of soil had 3 repetitions. Winter irrigation (160 mm) was conducted on October 31, 2016. Then the ECH2O soil moisture-temperature sensors were employed to measure soil moisture content and soil temperature at 10, 20, 40, 60, 80, 100, 120, 140, 160 and 180 cm soil depth from November 11, 2016 to April 11, 2017. In addition, the processes of soil water leakage were simulated and the effect of winter irrigation on deep leakage was analyzed. The results showed that an obvious difference existed in soil hydro-thermal dynamics between newly cultivated sandy cropland and old oasis cropland during the freezing-thawing period. In superficial layer (from 0 to 40 cm) and middle layer (from 40 to 100 cm), soil moisture content of the 2 types of soil reached the lowest level in January, and was basically recovered to the pre-freezing level after the soil thawed completely in April, and the soil water storages in these 2 layers in old oasis cropland were higher than that in newly cultivated sandy cropland. However, the change of soil water storage in deep layer (from 100 to 200 cm) was small for the 2 types of soil during the observation period. On January 18, 2017, the soil freezing depth of newly cultivated sandy cropland and old oasis cropland reached the highest value, and the highest value was 105 and 74 cm, respectively. Meanwhile, the duration of soil thawing process of old oasis cropland (26 d) was longer than that of newly cultivated sandy cropland (12 d). Nevertheless, the less thawing time for newly cultivated sandy cropland was detrimental to the soil water retention in spring. In addition, a significant difference in the deep leakage also existed between the 2 types of soil caused by winter irrigation. The deep leakage amount of newly cultivated sandy cropland was 4.1 times that of old oasis cropland when the winter irrigation amount was 160 mm. Above results suggested that the supplementary irrigation was necessary to ensure the germination of crops in spring in newly cultivated sandy cropland. Moreover, the "water preservation" effect in old oasis cropland was better than newly cultivated sandy cropland. As for different soil types, "water preservation" effect and lower deep leakage loss should be considered to ensure the optimal utilization of irrigation water resources in the desert oasis farmland management.

介绍欧洲、美国和中国的龙卷研究进展及其龙卷的时空分布特征,包括对年变化、月变化及日变化等统计特征进行了回顾和比较分析,并肯定了中国对龙卷的关注度及龙卷研究水平的逐步提升以及对未来龙卷的发展前景的展望.

Since successful exfoliation of a single layer of carbon atoms from graphite in 2004,graphene and other two-dimensional (2D) atomic crystal materials have attracted tremendous attention.Due to the extraordinary electronic,thermal,mechanical,chemical and optical properties,2D atomic crystal materials spur fundamental studies and technological advancements in a wide range of applications in optoelectronics,catalysis,energy storage,biological medicine and new composite materials.2D atomic crystals materials have many advantages in manufacturing electronic and photonic devices and are expected to play an increasing role in the post-Moore era.This paper summarized the recent research findings of the major 2D atomic crystals materials,such as graphene,molybdenum sulfides,phosphorene on synthesis,chemical modification,characterization and devices preparation.The development of van der Waals heterostructures was also reviewed,which could be stacked by different 2D atomic crystal materials and exhibited great potential for enabling a variety of devices with complex functionality.The applications of 2D atomic crystals materials and their van der Waals heterostructures in nanoelectonics and next generation information technology were thoroughly discussed.

A terrain perturbation scheme (ter) has been first incorporated into an ensemble prediction system (EPS) and preliminarily tested in the simulation of the extremely heavy rain event occurred on 21 July,2012 in Beijing.Along with other three perturbation schemes (i.e.,initial condition (ic),multi-physics (phy),and one mixed forms (icphy)),similarities and differences in ensemble spread among these schemes have been compared mainly in terms of spatial correlation,total perturbation energy,and scale decomposition.The results show that:(1) the ensemble spread and probabilistic forecasts have been slightly improved by incorporating terrain uncertainty while the ensemble mean of precipitation forecasts remains similar in quality;(2) the spatial structure evolution of the ensemble spread is closely related to the weather system.Quite different initialpatterns rapidly evolve into a similar pattern (the correlation coefficient reaches 0.6 or above) in the first 6 hours of model integration for all the schemes.Therefore,little extra information was gained about the spread structure by the mixed icphy scheme compared to either the ic or the phy single-source scheme;(3) differences in the ensemble spread magnitude are,however,large among these schemes,i.e.the magnitude of phy is the largest,that of ter is the smallest,and the magnitude of ic is in between.The mixture of ic and phy perturbations can noticeably boost the spread magnitude for upper level variables but not for near-surface variables,which implies that it is more challenging to increase ensemble spread for near-surface variables including precipitation in an EPS;(4) the scale decomposition of ensemble spread shows that the similarity of ensemble spread structure among the schemes generally increases with increases in spatial scale and forecast hours,but the differences remain large over small scales (＜448 km) and for very short forecast range (＜12 h);over small scales the differences in the spread magnitude are also obvious among the schemes.Therefore,the selection of perturbation methods in an EPS is more important for small scale and very short-range forecasts than for large scales and long-range forecasts.This study can be used as a guideline to design an effective EPS.

A relatively new ceramic hard material, nano titanium boride (TiB), containing Fe and Mo in solid solution and which has a very uniform nanostructure, has been synthesized in hulk form by reaction sintering under electric field induced heating. X-ray diffraction and SEM analyses indicate that this material is almost 100% TiB and 100% dense. The titanium boride forms in situ as bundles of whiskers during reaction sintering. It was found that the size, morphology, and distribution of TiB whiskers as well as the residual titanium phase were largely independent of the electrical heating path (powder/die) used in the synthesis. In the sintered material, Fe appears to be non-uniformly distributed within the TiB. However, an interesting finding is that hardness values, measured at random throughout the microstructure do not appear to be greatly affected by the inhomogeneous Fe distribution. This suggestes that the TiB synthesized by reaction sintering under electric field induced heating is largely similar to the TiB material synthesized using a conventional hot -pressing technique. A comparison of the physical and mechanical properties of this material with the Cerbec silicon nitride indicates that the present nanostructured TiB ceramic is superior in performance and could be useful in engineering applications.

Rapid in situ synthesis of bulk nanostructured titanium boride (TiB) ceramic, at the lowest possible processing temperature, is demonstrated. Using electric-field-activated-sintering (EFAS), nanoscale TiB whiskers were formed and densified uniformly throughout the volume, from a mixture of micron-sized Ti, TiB2 and FeMo powders, at 1100°C in 30min and at a relatively low pressure. Microstructure and X-ray diffraction (XRD) analysis, revealing the formation of TiB whiskers in this rapid processing condition, are presented. The formation of a transitional quaternary liquid phase, facilitated by localized joule heating at particle contacts, are hypothesized as contributors to rapid and low-temperature synthesis.

The advantages of the anomalous synoptic systems over the traditional total ones in locating re-gional heavy rain (RHR)have been revealed by recent work of the anomaly-based synoptic chart.Forty-one daily heavy rain cases in eastern China from 1998 are grouped into seven patterns based on the anoma-lous synoptic systems.The seven patterns of anomalous synoptic systems associated with regional heavy rain include the shear in South China,vortex in South China,reversed trough in South China,shear and trough along the Yangtze River,vortex along the Yangtze River,vortex in North China and vortex in Northeast China.The RHR can be well located by the negative regions of anomalous geopotential height, wind anomalous convergence combined with high moisture anomaly,which is evident from both the verti-cal cross-sections and the 850 hPa horizontal distributions.This overcomes the shortage of traditional syn-optic chart in which total field systems and high moisture regions do not collocate with heavy rain areas.It is recommended that anomaly-based synoptic chart should be applied in the practice of short-term and me-dium-range forecasts by using the products of the current state-of-the-art operational numerical weather models.

在全球气候变化背景下,干旱区山地森林及其与水的关系和水文功能表现出较强的敏感性,引起了学术界和管理部门的高度关注.从森林空间格局、生态水文过程及其对气候变化的响应等方面,系统阐述了干旱区山地森林生态水文的研究进展,辨析了森林斑块格局的形成与稳定机理、森林与流域产水量的关系以及森林生态水文对气候变化的响应.此外,结合目前的研究进展,提出了未来的研究重点:加强植被格局与水文过程的耦合研究;借助遥感技术和模型解决尺度问题;提升森林水文功能的认识水平;确定兼顾水文和其他生态效应的适宜森林规模.

This article introduces the operational predictions of hazardous weather elements to aviation both en-route and in TAF. Two visibility algorithms and three fog diagnostic schemes used at NCEP are discussed in particular. The ifrst visibility algorithm is Stoelinga and Warner method. This method requires outputs of four hydrometeors from a model to estimate optical extinction coefifcient and calculate visibility. The second visibility method is an upgrade to the ifrst one by adding more species of hydrometeor. This method can also estimate visibility in high humidity and haze conditions without knowing hydrometers as well as considering the differences between daytime and nighttime. The concept of airport Runway Visibility Range (RVR) is introduced and the relationship between the RVR and horizontal visibility range (HVR) is given. The fog diagnosis includes the UPS method, multi-rule method and physical-process balanced method. The UPS method uses surface-layer stability index. The multi-rule method uses a combination of visibility, cloud, surface humidity and wind speed to diagnose fog occurrence. These two methods do not predict fog intensity. The third method can diagnose both fog occurrence and intensity. It is based on the turbulence condition for fog formation and persistence as well as the explicit formulation of fog liquid water content suggested by a fog’s physical balance theory. Both multi-rule and physical-process balance methods have been implemented operationally in the NCEP’s regional ensemble forecast systems and showed encouraging results. However, the current skill of low visibility and fog forecasts directly derived from a numerical weather prediction model is generally low comparing to the forecasts of other weather elements such as precipitation. The dififculties are discussed and the ways of improvement are also suggested.

基于ARPS和WRF模式的多初值、多物理方案的集合预报系统,检验由高、低空间分辨率模式集成的结果.针对2013年5月8日华南地区强降水过程,设计ARPS模式集合、WRF模式集合和ARPS-WRF多模式集合3组试验,对比分析混合集合预报法与传统方法的降水预报效果.结果表明:ARPS模式集合改善了广东省南部局地强降水预报,该方法在中雨、大雨、暴雨量级改进效果显著.WRF模式集合对广东省北部强降水预报优于ARPS模式集合,但空报、漏报率较大,该方法有一定局限性.ARPS-WRF多模式集合在降水落区和量级预报上均优于传统方法.混合集合预报法利用低分辨率(36 km)集合预报和高分辨率(12 km)控制预报实现了高分辨率(12 km)集合预报,改善了降水预报效果,该方法可为业务高分辨率集合预报提供参考.

Using the historical Beijing “7.21”extreme precipitation event as an example,the six ensemble schemes (the initial condition (IC),multi-physics (MULTI),3 stochastic physics as well as a combination of IC and stochastic-physics (COM) were compared in the following three aspects of heavy precipitation forecasts:the performance of ensemble means,ensemble ranges and probabilities with respect to the control forecast,characteristics of ensemble spreads,and spread-forecast error rela-tions.The results show that:(1)In spite of the existence of large systematic forecast error,all the ensembles,especially the IC,MULTI and COM,are able to noticeably improve torrential-rain prediction over the control forecast in both intensity and location,and provide more complete information including the forecast uncertainty for users to make a better decision.(2) Forecast spreads of the three stochastic physics ensembles are similar to each other,generally much less than those of the IC and MULTI ensembles and mainly concentrated near the center of the severe rainfall area.As a result,the ensemble spread is enhanced in the vicinity of the severe precipitation area but little is changed elsewhere after stochastic physics is employed in ad-dition to IC perturbations,which leads to virtually no improvement to the overall spread over a larger domain comparing to that of the IC ensemble.By decomposing spread over the spatial scales,it further shows that the forecast diversity contributed by the stochastic physics is mainly in the smaller-scale (< 320 km,it could reach to a similar level to those by the IC and MULTI ensembles at scale < 160 km),while the contribution from the IC and MULTI perturbations to spread could extend another 400-500 km reaching to larger scales such as > 1000 km;at smaller scales (< 500 km),multi-physics technique could pro-duce larger precipitation spread than IC perturbation does,another advantage of multi-physics approach over other approaches is that it could partially reduce forecast bias.And,(3)the spread spectrum is similar to the forecast error spectrum over spatial scales for all the ensembles,i.e.,decreasing with the increase of the spatial scale.However,the magnitude of the spread spec-trum is smaller than that of the forecast error spectrum (indicating under-dispersion),this departure increases rapidly with the decrease of spatial scale and becomes large over the small scales.

基于“频率匹配法”的思路,采用两种方法进行了集合降水预报的订正研究,一种方法是利用集合成员降水频率订正简单集合平均平滑效应的“概率匹配平均”法,另一种方法是利用实况降水频率订正集合成员降水预报系统偏差的“预报偏差订正”法,通过个例和批量试验,结果表明:(1)概率匹配平均法可以矫正简单集合平均的平滑作用所造成的小量级降水分布范围增大而强降水被削弱的负作用,这种改进对强降水区更显著,并且集合系统离散度越大这种改进也越大;但该方法对预报区域内总降水量的预报没有改进作用,不能改善预报的系统性偏差.(2)虽然预报偏差订正法对降水落区预报的改进有限,但可以订正模式降水预报的系统性误差,改进雨量预报以及集合预报系统的离散度特征和概率预报技巧;直接对集合平均预报进行偏差订正的效果优于单个成员偏差订正后的简单算术平均.(3)在对每个集合成员的降水预报进行偏差订正后,概率匹配平均仍可改善其简单平均的效果,因此在实际业务中,应该综合采用上述两种方法,以获得在消除系统性偏差的同时各量级降水分布又合理的集合平均降水预报.