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We analyzed dust event occurrence and its seasonal distribution at 16 sites in the Tarim Basin,China.Although the overall frequency of dust events was the highest in spring in this region,its variation in other seasons could be classified into three patterns:(1) frequency of dust events in autumn > that in summer > that in winter(at the Kashi and Kuche sites);(2) frequency in summer > that in winter > that in autumn(at the Ruoqiang site);and(3) frequency in summer > that in autumn > that in winter(at all other areas of the Tarim Basin).The frequency of dust events and their seasonal variations in the Tarim Basin were mainly controlled by wind speed and locally available dust sources;the former was the key control when dust sources did not differ significantly.The seasonal variation in evaporation had a smaller,but still significant effect on the frequency of dust events. 相似文献
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Summary. Horizontal and vertical intensity data, obtained between 1957.0 and 1961.0 at 69 observatories, are analysed to determine the worldwide distribution of the annual variation of the geomagnetic field. Only data observed near local midnight are used, to avoid the small, but significant contamination from Sq. Over most of the world the variation is found to be small, with a clear dependence on latitude, but near the poles it is larger and more erratic. The non-polar variation is subjected to spherical harmonic analysis and separated into parts of internal and external origin. The polar variations are shown to be consistent with a north—south oscillation of the mean position of the auroral electrojets during the year. It is suggested that, with the exception of the polar effect, the annual variation is not due to ionospheric currents (as was hitherto believed), but results from an annual variation in the latitude of the ring current. 相似文献
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喜马拉雅山脉中段的珠穆朗玛峰等地,海拔高差巨大、生境复杂多变、土地覆被类型多样且植被垂直带谱完整,是全球范围内研究土地覆被垂直变化的理想场所。本文基于30 m空间分辨率的土地覆被数据（2010年）和DEM数据,在ArcGIS和Matlab平台的支持下,提出并运用脊线法、样带法和扇区法3种山地南北坡划分方法,研究了喜马拉雅山土地覆被垂直分布与结构差异。结果表明：① 山地土地覆被分布具有明确的垂直地带性结构特征,喜马拉雅中部土地覆被垂直带谱为南六北四式,土地覆被垂直带谱中具有人类活动的特点。② 南北坡之间的土地覆被垂直带谱差异明显,南坡土地覆被类型完整多样,北坡相对简单;对同类型土地覆被而言,南坡较北坡分布高程低、幅度宽。③ 依据各类型分布面积比随海拔变化情况,土地覆被类型在南北坡上的垂直分布可分为4种模式：冰川雪被、稀疏植被和草地为单峰分布型,裸地为南单峰北双峰分布型。④ 3种划分方法中,南坡的土地覆被垂直带结构具有相似性,而北坡的土地覆被垂直带结构存在差异,扇区法较好地反映了土地覆被自然分布格局。 相似文献
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秦岭中部山地降水的垂直变化研究 总被引：1，自引：0，他引：1
明确秦岭高海拔山区降水的变化规律,是深入理解秦岭作为中国南北地理过渡带特征、认识秦岭水资源在南水北调中线工程中重要作用的前提。但秦岭高海拔地区长期缺乏有效的降水观测数据,导致对其降水变化缺乏了解。利用2018年6月1日—2019年5月31日秦岭太白山海拔3760 m实测降水数据,发现在秦岭海拔3760 m处年降水量可达1300 mm,远高于汉江盆地和关中平原600~800 mm的年降水量。在此基础上,检验了克里金（Kriging）、反距离加权（IDW）和薄盘样条（ANUSPLIN）插值方法,以及GPM修正数据（GPM-cal）和ERA5再分析资料对秦岭中部山地年和季节降水空间模态的再现效果,各方案均能揭示秦岭高山区是降水高值中心,且降水随海拔的升高而增大,但利用克里金、反距离加权插值方案不能得到准确的高海拔降水值,与此相比,GPM-cal数据、薄盘样条插值与ERA5资料能较准确刻画秦岭中部山地年降水量随地形的变化。水汽通量分析显示,秦岭凭借高大地形对600 hPa高度以下的南来湿润气流具有明显的阻挡、强迫和拦截作用,使其南坡成为区域降水高值中心。结合高山区降水观测、薄盘样条插值、多源格点资料和数据修正方法,是认识秦岭山地降水形成和变化的有效途径。 相似文献
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Summary. The geomagnetic palaeosecular variation has been studied in terms of two current loops which change their positions and orientations with time. The results broadly agree with the observed data. Comparison with the existing models is made. 相似文献
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Geomagnetic secular variation since 1901 总被引：3，自引：0，他引：3
Barbara M. Hodder 《Geophysical Journal International》1981,65(3):763-776
Summary. All available annual means, from the world-wide network of magnetic observatories, of north intensity ( X ), east intensity ( Y ) and vertical intensity (Z) from 1901 to 1977 are subjected to spherical harmonic analysis to obtain 38 models of the Earth's geomagnetic field at two-year intervals. These models are differenced to give 37 models of secular variation at two-year intervals from 1903.5 to 1975.5. The results show the decreasing trend of the dipole moment and are analysed for possible information on the westward drift of the magnetic field. 相似文献
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G. K. Rangarajan 《Geophysical Journal International》1980,61(2):451-455
Summary. Using an index derived from the observations of horizontal intensity at two stations in the Indian equatorial region, the characteristics of the lunar semi-monthly tide in the equatorial electrojet strength are studied. It is shown that a contamination of the lunar signal by recurrent geomagnetic disturbance is largely eliminated and that the strength of the signal vanes systematically with solar time. Comparison of amplitude between conditions of low and high solar activity indicates a difference in local time progression, while the phase change is independent of the solar activity and season. Results of seasonal subdivision of data indicate that the largest amplitude is associated with the d-season. With increased solar activity there is an increase in the d- and e-seasons and a marginal decrease in the j-season. For all the seasons, the phase progression is fairly consistent with the theoretical considerations of Stening. 相似文献
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Summary. Mean hourly values have been analysed month by month for a number of stations for which long runs of data are available. It is found that there is an annual variation in the M2 lunar tide, there being a global enhancement of the tide around January. There is also some evidence from the phase changes with latitude that, near the solstices, the lunar current system consists not of pairs of vortices, one in each hemisphere, but of single vortices with foci in the summer hemisphere. 相似文献
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雅鲁藏布江流域径流特性变化分析 总被引：8，自引：2，他引：6
本文采用Morlet小波对1956~2000年雅鲁藏布江流域6个站点的径流序列进行了分析,揭示了不同时间尺度下四个季节以及年平均径流量的丰枯交替特性、突变性和周期性,通过小波方差确定各序列存在的主要周期。结果表明:在15年时间尺度上,雅江流域四个季节的径流变化趋势基本一致;发生突变的年份主要在1957、1967、1976、1983、1992年;径流序列第一主周期主要以15年长周期和2年短周期为主,第二主周期以15、6、2~3年的长、中、短周期为主;秋季和冬季径流序列的周期空间分布基本一致,年平均径流序列的周期分布与夏季最为接近;除拉萨河子流域春季在2~3年尺度上处于枯水期外,其他子流域其他时段在任何时间尺度上未来几年里都将处于丰水期。 相似文献
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历史时期罗布泊地区环境演变过程 总被引：6，自引：1，他引：5
利用史学考据的方法之一"二重证据法",从历史时期不同阶段进行纵向比较和从同一时期不同地区进行横向比较入手,参考放射性碳素断代法(即14C测定法)对太阳墓遗址、楼兰遗址进行断代测定的结果,结合应用空间遥感技术,对楼兰故城上游地质滑坡形成堰塞湖卫星图像的解读数据,分析新疆楼兰古城消亡的原因和罗布泊地区环境演变的过程.结果显示,历史时期罗布泊地区的环境演变与人类活动密切相关:土地掠夺性开垦,水资源的超常规开采,植物种群的大面积采伐都是造成生态环境变化的原因.对水土资源的的超常规开采和过度使用,也是导致楼兰故城废弃的原因.需要说明的是,这是一个渐进的过程.没有证据可以直接证明,滑坡崩塌形成堰塞湖使孔雀河突然断流,导致楼兰城供水突然中断. 相似文献
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长江口泥质区的季节性沉积效应 总被引：1，自引：0，他引：1
1IntroductionThe continental shelf of the East China Sea (ECS) is one of the widest shelves in the world. There are several mud areas distributing like patches on a background of widespread coarser ambient sediments of silt and sand on the continental shelf of the ECS. The coastal mud area can be divided into two parts, namely, the Changjiang Estuary mud area (CEMA) in the north and Zhejiang coastal mud area in the south. Between these two parts there is a silt area (Figure 1). The CE… 相似文献
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对巴音布鲁克站点1958—2015年的月降水量、降水日数和平均气温进行集合经验模态分解得到其变化趋势，利用Mann Kendall和累计距平法诊断突变点，并采用Morlet小波和R/S法分析其周期特征和未来变化趋势。结果显示：（1） 巴音布鲁克各月降水量1月、6月和11月增多趋势显著，2月、7月和12月呈“凸”字形变化，减少趋势显著。（2） 降水日数1月和9月呈显著减少趋势，2月和6月呈显著增加趋势，3月和11月呈“凸”字形变化，5月、7月和8月呈不同幅度的“凹”字形变化。（3） 各月平均气温基本呈上升趋势，尤其以5月、7月和10月升温最显著。（4） 年降水量、年降水日数和年平均气温分别在1999年、1993年和1997年发生突变，年降水日数增多早于年降水量增多和年平均气温升高的时间，从90年代中期开始气候由干冷逐渐向暖湿转型。（5） 年降水量、年降水日数和年平均气温的主周期分别为41 a、9 a和30 a。（6） 未来年降水量将增多，年降水日数将减少，年平均气温将升高，极端降水发生的频次将增大，易引发洪涝灾害。 相似文献
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Summary. The age of historical lava flows on the Canary Islands can be traced back at least to 1585 ad . During the last 400 years eruptions occurred at fairly regular intervals. The palaeomagnetic directions of these lavas record the ambient field direction with high precision so that an historical secular variation curve (SVC) can be constructed for the Canaries which closely resembles the curve registered at Paris. The SVC can be used as a dating tool for historical eruptions of unknown age and can probably be extended into the last 4000–5000 yr of recent volcanic activity on the Canaries. 相似文献