海洋的潮汐问答

一、什么叫潮汐? 浩瀚的大海和宇宙一切事物一样,永无息止地运动着。它的运动形势是多种多样的。其中有这样一种形式,就是:在一定的时间里,海面会慢慢地升起来,使岸边大片的海滩和礁石被水淹没,而在达到一定高度的时候,海面又开始下落,逐步把淹没的地方裸露出来,而后,又开始上升,并接连不断地重复着这种起落。它是那样的有规律,好像在呼吸一样。人们把这种现象叫做潮汐,同时又把海面上升的过程称为涨潮,下降的过程称为落潮;海面上升的最高点和下降的最低点分别称为高潮和低潮。     

一类非线性二阶常微分方程解的多重性

考虑一类二阶非线性常微分方程的边值问题（p（t）u′）′＋h（t）f（u）=0，0＜t＜1和u（0）=u（1）=0。通过引入f（s）／s在∞与0处的极限值，并运用打靶法和相应的Sturm比较定理得到解的多重性定理，推广有关文献中的许多重要结果。     

全球粮食安全格局的时空演化

Food security is the primary prerequisite for achieving other Millennium Development Goals(MDGs).Given that the MDG of“halving the proportion of hungers by 2015”was not realized as scheduled,it will be more pressing and challenging to reach the goal of zero hunger by 2030.So there is high urgency to find the pattern and mechanism of global food security from the perspective of spatio-temporal evolution.In this paper,based on the analysis of database by using a multi-index evaluation method and radar map area model,the global food security level for 172 countries from 2000 to 2014 were assessed;and then spatial autocorrelation analysis was conducted to depict the spatial patterns and changing characteristics of global food security;then,multi-nonlinear regression methods were employed to identify the factors affecting the food security patterns.The results show:1)The global food security pattern can be summarized as“high-high aggregation,low-low aggregation”.The most secure countries are mainly distributed in Western Europe,North America,Oceania and parts of East Asia.The least secure countries are mainly distributed in sub-Saharan Africa,South Asia and West Asia,and parts of Southeast Asia.2)Europe and sub-Saharan Africa are hot and cold spots of the global food security pattern respectively,while in non-aggregation areas,Haiti,North Korea,Tajikistan and Afghanistan have long-historical food insecurity problems.3)The pattern of global food security is generally stable,but the internal fluctuations in the extremely insecure groups were significant.The countries with the highest food insecurity are also the countries with the most fluctuated levels of food security.4)The annual average temperature,per capita GDP,proportion of people accessible to clean water,political stability and non-violence levels are the main factors influencing the global food security pattern.Research shows that the status of global food security has improved since the year 2000,yet there are still many challenges such as unstable global food security and acute regional food security issues.It will be difficult to understand these differences from a single factor,especially the annual average temperature and annual precipitation.The abnormal performance of the above factors indicates that appropriate natural conditions alone do not absolutely guarantee food security,while the levels of agricultural development,the purchasing power of residents,regional accessibility,as well as political and economic stability have more direct influence.     

食物系统认知进展及其地理学研究范式探讨

食物关系国计民生,中国食物系统面临诸多挑战,耕地资源趋减、环境压力临界、农业劳动力流失、消费需求快速转型等对食物系统功能提出更高要求,地理学的综合性和系统性思维为应对这些问题提供了有效的工具和视角。尽管食物系统得到世界银行、联合国粮农组织以及其他国际各方的高度关注,但目前国内对食物系统的研究却严重不足。论文对食物系统的认知进展进行了深入分析,归纳了食物系统的概念认知历程、类型、特征,梳理出食物系统的研究脉络,包括从“概念”存在到“方法”存在、从线性认知到系统认知、从经济活动到食物景观、从现象描述到时空嵌入等,以此凝练出食物系统的核心内涵与认知进展;在科学哲学范式、人地关系范式、空间范式和系统科学范式指导下,论文遵从“格局—结构—过程—机理”由表及里的研究脉络,进一步探讨了食物系统的时空格局、要素结构、演化过程和发展机理,尝试构建了食物系统的地理学研究范式,研究结论旨在为推动食物系统视角基础研究和实践应用提供参考和借鉴。     

时间周期哈密顿-雅克比方程的比较定理

在有界区域上研究时间周期哈密顿-雅克比方程的黏性解,将Hitoshi Ishii和Hiroyoshi Mitake关于自治哈密顿-雅克比方程的比较定理推广到适用于时间周期的哈密顿-雅克比方程的比较定理。     

1991年江淮梅雨期欧洲中期天气预报中心降水预报的检验

1 前言1991年5—7月江淮地区出现了异常的梅雨天气,梅雨开始得早,持续时间长,强度大,从降水预报的观点看十分困难,对于降水数值预报难度尤其大。目前国内外的降水预报虽然有一定能力和技巧,但要较准确地预报这次特大暴雨还是有很大困难的。欧洲中期天气预报中心制作的预报,目前放认为     

中国东部新生代火山岩

中国东部新生代火山岩分布甚广,大部隐伏于地下(图1),在环太平洋火山岩带中占有重要位置;对研究岩石圈的组成、结构、发展、演化和板块构造均起着重要作用。虽早在50年代就有专文对其进行探讨,但长期以来在这方面的工作进展不大;只近些年来,对中国东部新生代火山岩及有关超镁岩包体的研究有了较大的发展,已积累了较多资料,现以作者     

A CASE OF MESOSCALE CONVECTIVE COMPLEX EVOLVING INTO A VORTEX

A case of mesoscale convective complex(MCC)which evolved into a vortex is documented in this paper.As theMCC entered into the dissipating phase,a well-defined spirally banded structure became visible in the satellite image.The blackbody temperature(TBB)of the residual cold-cloud-shield indicates the vortex existed in the layer from 400 to250 hPa.According to the upper air analysis,the upper level vortex was an anticyclone.The MCC-generated vortex wasvisualized in the satellite images because it was located in the subtropical high where the wind field was very weak.     

中尺度对流系统产生强降水的临近预报方案

一、前言国家环境卫星、数据和情报服务处(NESDIS)在业务上应用卫星资料估算降水量的工作已有十多年了.在此工作的基础上,NESDIS现正在开发一个暴洪估算模型和临近预报方案(PROFFENS).临近预报方案包括现在天气的详细描述以及未来0-12小时的短期预报两部分(Scofield和Weiss,1977).PROFFENS的研制包括分析降了多少雨以及预报未来0-12小时内将出现多少     

关于农村集体建设用地现状调查及处置方式的研究

正党的十八届三中全会通过的《中共中央关于全面深化改革若干重大问题的决定》提出党要"建立城乡统一的建设用地市场"、"保障农户宅基地用益物权,改革完善农村宅基地制度,选择若干试点,慎重稳妥推进农民住房财产权抵押、担保、转让,探索农民增加财产性收入渠道"。党的十八届三中全会就农村集体建设用地改革指明了方向。如何推进农村集体建设用地改革,调整农村土