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1.
Recruitment algorithms in forest gap models are examined withparticular regard to their suitability for simulating forestecosystem responses to a changing climate. The traditional formulation of recruitment is found limiting in three areas. First, the aggregation of different regeneration stages (seedproduction, dispersal, storage, germination and seedling establishment) is likely to result in less accurate predictionsof responses as compared to treating each stage separately. Second, the related assumptions that seeds of all species are uniformly available and that environmental conditions arehomogeneous, are likely to cause overestimates of future speciesdiversity and forest migration rates. Third, interactions between herbivores (ungulates and insect pests) and forest vegetation are a big unknown with potentially serious impactsin many regions. Possible strategies for developing better gapmodel representations for the climate-sensitive aspects of eachof these key areas are discussed. A working example of a relatively new model that addresses some of these limitations is also presented for each case. We conclude that better modelsof regeneration processes are desirable for predicting effectsof climate change, but that it is presently impossible to determine what improvements can be expected without carrying outrigorous tests for each new formulation.  相似文献   
2.
    
It is essential to learn the temporal and spatial concentration distributions and variations of seeding agents in cloud seeding of precipitation enhancement. A three–dimensional puff trajectory model incorporating a mesoscale nonhydrostatic model has been formulated, and is applied to simulating the transporting and diffusive characteristics of multiple line sources of seeding agents within super–cooled stratus. Several important factors are taken into consideration that affect the diffusion of seeding materials such as effects of topography and vertical wind shear, temporal and spatial variation of seeding parameters and wet deposition. The particles of seeding agents are assumed to be almost inert, they have no interaction with the particles of the cloud or precipitation except that they are washed out by precipitation. The model validity is demonstrated by the analyses and comparisons of model results, and checked by the sensitivity experiments of diffusive coefficients and atmospheric stratification. The advantage of this model includes not only its exact reflection of heterogeneity and unsteadiness of background fields, but also its good simulation of transport and diffusion of multiple line sources. The horizontal diffusion rate and the horizontal transport distance have been proposed that they usually were difficult to obtain in other models. In this simulation the horizontal diffusion rate is 0.82 m s−1 for average of one hour, and the horizontal average transport distance reaches 65 km after 1 h, which are closely related to the background fields.  相似文献   
3.
In order to study mechanisms of hailstone formation and hail suppression with seeding and toobtain optimum seeding technique for hail cloud,a 3-D compressive numerical seeding model forhail cloud is developed.The water substance in hail cloud is divided into 8 categories,i.e.,watervapor,cloud droplet,raindrop,ice crystal,snow.graupel,frozen drop and hail,and the detailedmicrophysical processes are described in a spectrum with two variable parameters and morereasonable particle number/size distributions.Then,the model is able to predict concentration andwater content of various particles.Especially.it can calculate the number of hailstones whosecores are graupel or frozen drop and apply to study mechanism of hailstone formation.Additionally,a conservative equation of AgI as seeding or glacigenous agent is found andnucleation by condensation of artificial nucleus,and nucleation by freezing of cloud droplet or raindrop which contact with AgI particle are considered.The dynamic energy flux of hail shooting onground is used to verify seeding effect.Therefore the model is also used to study mechanism of hailsuppression with seeding and the seeding technique,  相似文献   
4.
A detailed 3-D hail cloud numerical model and parameterization of mierophysieal processeswere described in Part Ⅰ(Hong 1999)of this study.In this part,a hail cloud occurring in Xunyiarea.Shaanxi Province on July 8,1997 is simulated by the model to analyze mechanisms of hailformation and hail suppression with seeding.The results show that 97% of hail embryos are frozendrops.The seeding experiments with AgI in terms of heights show that if the seeding is madebefore hail formation,the optimum seeding position is located in the maximum updraft area and itscenter,i.e.,AgI is seeded in the zone with high water content to be coordinated with maximumzone of the updraft.The seeding makes concentrations of graupel and frozen drop increase andtheir average mass or size decrease,so that the proportion of conversion from graupel and frozendrop into hail descends greatly,and the mass and concentration of hailstone are decreased toachieve our purpose for hail suppression.  相似文献   
5.
A precipitation enhancement operation using an aircraft was conducted from 1415 to 1549 LST 14 March 2000 in Shaanxi Province. The NOAA-14 satellite data received at 1535 LST soon after the cloud seeding shows that a vivid cloud track appears on the satellite image. The length, average width and maximum width of the cloud track are 301 kin, 8.3 and 11 kin, respectively. Using a three-dimensional numerical model of transport and diffusion of seeding material within stratiform clouds, the spatial concentration distribution characteristics of seeding material at different times, especially at the satellite receiving time,are simulated. The model results at the satellite receiving time axe compared with the features of the cloud track. The transported position of the cloud seeding material coincides with the position of the track. The width, shape and extent of diffusion of the cloud seeding material are similar to that of the cloud track.The spatial variation of width is consistent with that of the track. The simulated length of each segment of the seeding line accords with the length of every segment of the track. Each segment of the cloud track corresponds to the transport and diffusion of each segment of the seeding line. These results suggest that the cloud track is the direct physical reflection of cloud seeding at the cloud top. The comparison demonstrates that the numerical model of transport and diffusion can simulate the main characteristics of transport and diffusion of seeding material, and the simulated results are sound and trustworthy. The area, volume, width, depth, and lateral diffusive rate corresponding to concentrations 1, 4, and 10 L^-1 are simulated in order to understand the variations of influencing range.  相似文献   
6.
用双通道动态阈值对GMS-5图像进行自动云检测   总被引:1,自引:0,他引:1       下载免费PDF全文
该文用双通道动态阈值法对GMS-5图像进行自动的云检测。在红外通道和可见光通道,分别对每32×32个像元组成的像元阵进行直方图统计,求出区别云和地物值域的阈值,然后对每个像元阵内的逐个像元进行云判识。讨论了在进行直方图分析以求取阈值的过程中,像元阵大小的选取和动态平滑间距的选取对云检测结果的影响。分析结果表明:像元阵大小取32×32时,像元阵所占的地域空间尺度足够小,像元阵内的观测像元样本数足够多,保证了在直方图聚类分析时,每一类含有足够多的样本;GMS-5观测图像的红外通道对原始直方图进行二次平滑时小平滑间距取1.6K,可见光通道小平滑间距取1.2%,使得确定动态阈值时步长相对小,保证了分析的精度。用目视图像对分析结果进行真实性检验,在中低纬度地区,可见光和红外两个通道都有资料时,该算法的云判识精度较好。在高纬度地区,由于地表温度低,积雪覆盖多,太阳光照角低,该算法的云判识精度较差。  相似文献   
7.
利用1997年11幅TM影像为遥感信息源,以陕北地区(榆林、延安、铜川)为试验区,采用ERDASIMAGINE提供的计算机监督分类方法,对该地区的植被类型、植被覆盖度、土地利用以及坡耕地进行了大规模的遥感调查,并将调查成果按1:10万比例尺制图。研究表明,利用监督分类方法对分辨率为30m的TM影像进行陕北地区生态环境遥感调查与制图是可行的,能较真实地反映该地区植被和土地利用的基本特征。  相似文献   
8.
尘卷风的形成、结构和卷起沙尘过程的数值研究   总被引:5,自引:0,他引:5       下载免费PDF全文
文中应用数值算法对尘卷风的发展过程进行了模拟计算与分析。通过模拟计算得到了尘卷风的内部详细结构和运动过程。研究表明尘卷风是由于地面局部增热不均匀而形成的一种特殊的旋转对流运动。在尘卷风形成的过程中 ,外围空气通过贴近地面的薄层被地面加热后流向中心部位 ,外围空气的旋转能量在中心部位得到加强形成尘卷风 ,其旋转能量是热泡原来具有的旋转能量的局部集中和一部分势能转化而形成的 ,其旋转方向是由热对流泡的初始旋转方向所决定。尘卷风是一种类兰金涡 ,旋转速度和压力的分布具有兰金组合涡的特点 ;在成熟阶段 ,尘卷风的详细结构可以粗略地分为 4个区域———地面附近的气流汇聚区域、柱状的涡核区域、旋风与地面作用形成的转角风区域以及涡核外部的外围气流区域。转角区域可以细分为两个子区域———外围的方位角风区域(在该区域 ,上升气流运动方向与轴线之间有一定的夹角 ,称为方位角 )和中心的下沉停滞气流区域。尘卷风中心的低压和急速的上升气流可使大量沙尘扬起 ,不同直径的沙尘颗粒在尘卷风的作用下运动轨迹不同 ,因此卷起不同大小沙尘的尘卷风的外形也是不同的。  相似文献   
9.
台湾岛邻近海域台风浪模拟分析   总被引:7,自引:0,他引:7       下载免费PDF全文
本文概略介绍了目前国际上较为先进的第三代近岸海浪数值模式SWAN(SimulationWavesNearshore)及风生浪、底摩擦、白帽耗散、深度诱导波破碎、非线性波-波相互作用等物理过程;在此基础上,以较高的分辨率对袭击台湾岛邻近海域的9015、8616号台风浪过程进行了模拟分析;台风浪高模拟值与实际台风浪资料相符较好,可以为该海域台风浪的模拟提供较好的参考。  相似文献   
10.
周志恩  蒋维楣  胡非  奚文 《气象科学》2003,23(3):292-299
目前 ,我国气象部门相当普遍地拥有气象卫星观测资料 ,但是 ,这些资料如何在城市气象服务以及为城市建设规划等方面的决策研究中发挥作用 ,急待进一步研究并开展工作。本文试图通过合理的反演处理 ,给出卫星观测对地面温度的测量结果 ,由此可比较细致地分析了解地面热状况及其与土地利用类型 ,地面覆盖性质等的关系。本文利用有限的 NOAA- AVHRR气象卫星遥感资料 ,采用修正的 Ulivieri分裂窗方法反演出同期的地表温度场 ,通过对北京地区的卫星观测资料进行算例分析 ,揭示出一个 2 0 0 km× 2 0 0 km范围内的地面温度分布特征。反演结果可以明显地揭示出城市热岛现象的变化规律 ,并且能较为清楚地反映出相对的高温区和低温区  相似文献   
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