Statistical characterization of frost zones: Case of tea freeze damage in the Kenyan highlands     

《International Journal of Applied Earth Observation and Geoinformation》2020

Frequent frost occurrences in the Kenyan highlands have had devastating effects on agricultural productivity. With inadequate management systems to mitigate the impacts, farmers have often had to bear the burden of losses resulting from frost damage. While agriculture in Kenya remains dependent on weather and climate, the agricultural economy of Kenya continues to suffer, underscoring the need for building local knowledge as basis for development of early warning systems. The current paper attempts to delineate frost zones by statistically characterizing them based on known risk factors related to topography (elevation, convexity, aspect, upslope flow length) and Land Surface Temperature (LST) derived from Moderate Resolution Imaging Spectroradiometer (MODIS). Through binary logistic regression, a logistic regression model was developed utilizing observation data (frost occurrence and non-occurrence) as a binary dependent variable to estimate the probability of frost occurrence. Assuming a 0.5 probability cut-off threshold between frost occurrence and non-occurrence, an overall accuracy of 81% with area under Receiver Operating Characteristics (ROC) Curve of 0.88 was obtained. No evidence of lack of model fit was detected. This model outperforms the currently operational model that utilizes MODIS LST alone to detect frost zones in the Kenyan tea plantations. It provides an improved method for effective delineation of frost zones by incorporating local topographic characteristics.  相似文献   

中国中小尺度强对流天气气候学特征          下载免费PDF全文

薛晓颖  任国玉  孙秀宝  任玉玉  余予 《气候与环境研究》2019,24(2):199-213

中小尺度强对流天气具有极强的破坏力,了解其气候学特征对于预测、预报和影响评价都具有实际意义。利用1961~2015年的2332个高密度逐月国家级气象站观测资料,分析了中国大陆3种常见中小尺度强对流天气（雷暴、闪电、冰雹）在年、季、月尺度上发生日数的时间变化规律和空间分布特征。结果表明:全国年平均雷暴、闪电和冰雹发生频率分别为39.23 d/a、20.56 d/a和1.07 d/a;雷暴和闪电主要发生在夏季3个月,雷暴日数7月最多,闪电日数8月最多;冰雹主要发生每年5~9月,6月发生频率最高;雷暴和闪电的高发区分布基本一致,主要集中在华南和西南,青藏高原也是雷暴的高发区域之一;冰雹的高发区主要集中在青藏高原、内蒙古高原东部以及中西部山地,而东南沿海地区发生频率则较低。进一步分析发现,我国雷暴和冰雹出现频率随海拔高度增加而明显增加,冰雹和海拔高度有更好的对应关系,二者增加速率分别为2.87 d/500 m和1.80 d/500 m,表明地势高度对这两种强对流天气形成和发展具有重要影响。  相似文献   

基于激光雷达手段的海南地区重力波与其波谱的季节分布特性研究          下载免费PDF全文

邹旭  杨国韬  王继红  龚少华  程学武  岳川  张铁民  傅军 《地球物理学报》2015,58(7):2274-2282

利用子午工程海南激光雷达对我国海南地区上空进行持续观测,通过3年的累积观测数据对我国低纬度地区重力波活动的季节分布特性进行研究,依据重力波线性理论对海南地区上空的大气密度扰动规律、空间功率谱及时间频率谱进行分析,并通过选择波长在1km至8km范围内具有特定波长以及具有波动周期为60 min至25min的特定频率的重力波辅助研究大气密度扰动的季节变化规律,总结得出海南地区重力波活动具有夏季大、春秋季小、而冬季依然频繁的季节性分布规律.结合海南地区特殊的地理位置与当地季节性气候特征分析得出海南地区上空重力波活动季节性变化的可能原因为青藏高原地形及我国南海地区存在的热带强对流与赤道潜流共同作用的结果.  相似文献   

RZB型深井宽频带地形变综合观测系统的垂向应变测量单元     

陈征  欧阳祖熙  李宏  李涛  吴立恒  范国胜  张钧 《震灾防御技术》2014,9(1):126-132

目前钻孔应变观测一般为水平应变,使用无限岩石平板中的钻孔应力集中模型,可反演出水平应力场分布及变化.如果在此基础上加上垂向应力测量,就可反演出三维应力场分布.本文介绍的RZB型深井宽频带地形变综合观测系统的垂向应变测量单元,外钢筒为波纹管结构,并通过螺旋刻丝、热处理等技术,其垂向等效弹性模量比耦合水泥低一个数量级,同时垂向应变钢筒的水平向等效弹性模量高于垂向弹性模量一个数量级.垂向应变探头的力学结构可以被形象地看成是一个螺旋柱形弹簧,其轴向极易产生弹性形变,而水平向的硬度却得到了极大的加强,因此可在竖井中较为准确地测量轴向应变.  相似文献   

汶川地震山脚震害特征及机理分析1   总被引：1，自引：0，他引：1  

王伟  刘必灯  刘欣  沈超  周正华 《震灾防御技术》2014,9(4):863-871

汶川地震和以往地震现场震害调查表明,局部场地地形对震害的影响明显,其中,山脚房屋震害程度明显低于山顶和水平地表土层场地.针对这一震害特征,本文以汶川主震的地形效应观测强震动记录和人工爆破山体地形地震动观测记录为基础,通过数据分析,结合动力集中质量中心差分有限元法进行数值模拟分析,进一步揭示了山脚房屋震害轻微的特征不仅与其所在场地条件良好有关,更与山脚所处山体底部的几何位置直接相关.  相似文献   

起伏地表弹性波传播的间断Galerkin有限元数值模拟方法   总被引：2，自引：1，他引：1  

薛昭  董良国  李晓波  刘玉柱 《地球物理学报》2014,57(4):1209-1223

间断Galerkin有限元法（DG-FEM）作为一种有效的高阶有限元法受到了国内外学者的广泛关注.本文基于任意高阶间断Galerkin有限元法对弹性波方程进行空间离散，并将离散后所得的非齐次线性常微分方程系统齐次化，最后结合针对齐次问题的强稳定性保持龙格库塔（SSP Runge-Kutta）算法，将DG-FEM推广至时间任意高阶精度.另外，借鉴近最佳匹配层（NPML）的思想，基于复频移（CFS）拉伸坐标变换推导了一种新的PML吸收边界条件（简称为CFS-NPML），该CFS-NPML能够与DG-FEM算法很好地结合，形成有效的起伏地表地震波传播数值模拟技术.数值试验结果表明，DG-FEM具有高阶精度，可以适应任意复杂起伏地表和复杂构造情况下的弹性波传播数值模拟.同时，CFS-NPML对包括面波等震相的人为边界反射都具有良好的吸收效果.  相似文献   

Dynamic mechanisms controlling the topography of Longmenshan area   总被引：1，自引：0，他引：1  

Xie  Yuan  Li  Yongdong  Xiong  Xiong 《中国科学:地球科学(英文版)》2020,63(1):121-131

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.  相似文献   

An experimental investigation into topographic resonance in a baroclinic rotating annulus     

S. D. Marshall  P. L. Read 《地球物理与天体物理流体动力学》2015,109(4):391-421

This paper documents an experimental investigation in which a differentially-heated rotating annulus experiment was used to investigate the effects of topography on fluid flow under conditions similar to the atmospheric and oceanic circulation on Earth and other planets. In particular, the relationship between the effects of topographic resonance and the existence and mechanism for generation of low-frequency variability (LFV) were studied, motivated by outstanding questions in works such as Jin and Ghil (J. Atmos. Sci., 1990, 47) and Read and Risch (Geophys. Astrophys. Fluid Dyn., 2011, 105). Whilst employing sinusoidal wavenumber-3 topography a new regime was encountered within a region of stationary wavenumber-3 structural vacillation. Denoted as the “stationary-transition” regime, it featured periodic oscillations between a dominant stationary wavenumber-3 flow and axisymmetric or chaotic flow. Further investigation found that the “stationary-transition” regime appeared to be a near-resonant region where nonlinear topographic resonant instability led to a 23–42 “day” oscillatory behaviour. Within the regime, a Hopf bifurcation sequence was discovered, and the nonlinear instabilities were found to have terms in both wave-zonal flow and wave–wave interactions, including a notable resonant wave-triad. This report summarises the nature of the “stationary-transition” regime, and also makes comparisons with similar regimes of LFV found in other experimental studies, as well as intraseasonal oscillations in the atmosphere.  相似文献   

台湾地形对台风Meranti(1010)经过海峡地区时迅速增强的影响研究   总被引：3，自引：1，他引：2  

薛霖  李英  许映龙  王蕾  戴高菊 《大气科学》2015,39(4):789-801

台风在趋近大陆过程中强度一般衰减, 但Meranti(1010)北上进入台湾海峡过程中却迅速加强, 且在登陆福建时达到最强。采用中国气象局台风资料、NCEP GFS 0.5°×0.5°再分析资料及台湾雷达资料, 结合中尺度数值模式WRF(The Weather Research and Forecasting Model)开展台湾地形敏感性试验, 研究Meranti进入台湾海峡过程中的结构变化及迅速加强机理。结果表明:台湾地形是Meranti迅速加强的一个重要影响因子。Meranti北上过程中, 一方面通过台湾岛地形分流作用及其背风坡效应在台湾海峡内诱生中尺度涡旋, 形成正负相间的涡度分布, 激发出与台风相关的扰动波列。地形强迫抬升及扰动波列可加强垂直运动和积云对流, 有利于台风对流发展。另一方面, 台湾地形还通过改变环境气流使台风高空辐散场加强, 环境风垂直切变减小, 形成有利于台风发展的环流背景。比较不同高度台湾地形试验中台风动能收支发现, 台湾地形激发的扰动波列和积云对流增强了次网格尺度系统与台风间能量的交换, 成为Meranti登陆前迅速加强的主要动能源。  相似文献   

FEM Using Adaptive Topography in 2-D MT Forward Modeling     

Liu Yun  Wang Xuben 《中国地震研究》2011,25(2):148-157

Complex terrain causes great MT noise.This paper puts forward a FEM model method using adaptive topography and quadratic elements based on studies by previous researchers.This method can model all kinds of complicated terrain and geoelectric bodies preferably.The numeric modeling,calculation of the auxiliary field and definition of resistivity are deduced by electromagnetic equations.Lastly,several examples are presented,which show the method is rapid,effective and has highly accurate.  相似文献