中尺度地形对“98.7”鄂东特大暴雨的动力作用

用一个高分辨的 η坐标模式对 1998年 7月 2 1日鄂东沿江特大暴雨过程进行数值模拟 ,得到与实况相吻合的结果。通过敏感性试验和分析 ,研究了局地中尺度地形对这次暴雨过程的影响及其机理。结果表明 ,虽然地形对这次特大暴雨过程的基本面貌并未起到决定性的作用 ,但也有一些重要影响。大别山对边界层南风暖湿气流的阻挡使位于其迎风面上游的暴雨带有所加强。幕阜山对边界层水汽流入的拦截则使位于其背风面下游的暴雨带的初期发展减缓。到了暴雨盛期 ,由于暴雨带南侧有中尺度低空急流出现并正好位于幕阜山区 ,因地形而抬高的摩擦层内的Ekman风矢偏转直指暴雨区 ,加强了暴雨区水汽的辐合。由于强对流暴雨系统盛期近地层气流转为辐散 ,对暴雨维持起关键作用的辐合层升高 ,因此地形抬高的摩擦辐合恰好叠加其上使之增强 ,这可能是此次鄂东沿江特大暴雨过程持续时间长的一个重要原因。     

Measurements Of Thermal Updraft Intensity Over Complex Terrain Using American White Pelicans And A Simple Boundary-Layer Forecast Model

An examination of boundary-layer meteorological and avian aerodynamic theories suggests that soaring birds can be used to measure the magnitude of vertical air motions within the boundary layer. These theories are applied to obtain mixed-layer normalized thermal updraft intensity over both flat and complex terrain from the climb rates of soaring American white pelicans and from diagnostic boundary-layer model-produced estimates of the boundary-layer depth z_i and the convective velocity scale w_*. Comparison of the flatland data with the profiles of normalized updraft velocity obtained from previous studies reveals that the pelican-derived measurements of thermal updraft intensity are in close agreement with those obtained using traditional research aircraft and large eddy simulation (LES) in the height range of 0.2 to 0.8 z_i. Given the success of this method, the profiles of thermal vertical velocity over the flatland and the nearby mountains are compared. This comparison shows that these profiles are statistically indistinguishable over this height range, indicating that the profile for thermal updraft intensity varies little over this sample of complex terrain. These observations support the findings of a recent LES study that explored the turbulent structure of the boundary layer using a range of terrain specifications. For terrain similar in scale to that encountered in this study, results of the LES suggest that the terrain caused less than an 11% variation in the standard deviation of vertical velocity.     

云南一次持续性暴雨过程的非地转湿Q*

应用非地转湿Q^*,对2001年5月31日至6月2日发生在云南的持续性暴雨天气过程进行诊断分析。结果表明,非地转湿Q矢量辐合区是暴雨发生的有利区域;非地转湿Q矢量的垂直分布反映了次级环流的方向和强弱,云南的暴雨落区位于次级环流的上升支附近。     

Geochemistry and genesis of brine emanations from Cretaceous strata of the Mamfe Basin, Cameroon

The geochemistry of 5 salt springs in the southwestern Mamfe Basin was investigated in order to infer the mineral content of their source and to relate the genesis of the springs to the local geology. Field observations revealed that, they are cold springs (23–28 °C), and are composed of secondary brines that are neutral to alkaline with pH values ranging from 7 to 8.7. Results of chemical analysis show that the springs contain major ions that form evaporite minerals, as well as chalcophile elements. The dominant cation is Na⁺ (>96%), and the dominant anion is Cl⁻ (>99%). Based on correlation coefficients between ions that form evaporites and field occurrence of efflorescences of halite, it is suggested that the ancient evaporites in the Mamfe Basin are composed entirely of carbonate and chloride salts. Meteoric and convective fluid flow processes are responsible for the dissolution of ancient evaporites and subsequent migration of brines to the surface from underground. The brines migrate through permeable strata with migration pathways resulting from a combination of fracture porosity created by post––Cretaceous tectonism and intergranular porosity enhanced by the chemically aggressive migrating brines.     

波段位置和宽度对不同生育期水稻NDVI影响研究

通过对不同生育期水稻冠层光谱的分析,探讨了不同波段位置和波段宽度对不同生育期水稻NDVI的影响.以及在一定NDVI精度要求下红光波段最大宽度和波段中心位置的相互作用.结果表明,在所有生育期,近红外波段的位置和宽度对NDVI影响不大;红光波段的位置和宽度对NDVI有较大影响,特别是红光波段位置接近红谷极值(670nm附近)时影响尤为显著.相对而言水稻生育前期的NDVI更容易受到波段位置的影响,但是波段位置对不同生育期之间NDVI的差值影响不大.无论红光波段还是近红外波段,当取值处于红边区域(690-740nm)时,对NDVI有较大影响.当保证NDVI偏差在1%以内时,在水稻生长旺期,构成NDVI的红光波段最大宽度随着波段巾心位置向长波移动而逐渐变窄,当到达690nm附近达到最窄,而后略有变宽;对于生长前期和后期,由于在648nm附近变窄而有波动.     

基于离子光谱特征波段反射率的土壤碱化指标反演模型

以新疆奇台地区碱化土壤为研究对象,通过分析碱化土壤实测光谱反射率曲线与八大离子、pH、碱化指标相互间的相关关系,建立基于离子光谱特征波段反射率的各碱化指标一元及多元光谱反演模型,并对其精度进行验证。结果显示:Na^+、CO_3^2-、HCO_3^-含量与光谱反射率正相关,最高点的相关系数分别为0.710、0.798、0.749,而Ca²⁺、Mg²⁺含量与光谱反射率负相关,相关系数最高均不超过-0.370,反映出前3类离子含量与光谱反射率关系更为密切。SAR(钠吸附比)和ESP(碱化度)与Na+相关系数同为0.954,TA(总碱度)、RSC(残余碳酸钠)、pH与CO_3^2-的相关系数分别为0.946、0.949和0.953,总体上Na^+和CO_3^2-含量对各碱化指标的影响更大。各碱化指标与土壤光谱反射率的相关性TA>RSC>ESP>pH>SAR;其中TA与光谱反射率的相关系数达到0.863。碱化指标TA的离子光谱特征波段反射率反演模型精度最好,其R^2为0.703,比利用实测光谱反射率建立的pH反演模型的R^2高约14%,说明前者精度更高,能更好地反映研究区内土壤的碱化程度。利用离子光谱特征波段反射率实现对土壤碱化的预测会成为今后研究的重点。     

Estimation of regionalized phenomena by geostatistical methods: lake acidity on the Canadian Shield

 This paper describes a geostatistical technique based on conditional simulations to assess confidence intervals of local estimates of lake pH values on the Canadian Shield. This geostatistical approach has been developed to deal with the estimation of phenomena with a spatial autocorrelation structure among observations. It uses the autocorrelation structure to derive minimum-variance unbiased estimates for points that have not been measured, or to estimate average values for new surfaces. A survey for lake water chemistry has been conducted by the Ministère de l'Environnement du Québec between 1986 and 1990, to assess surface water quality and delineate the areas affected by acid precipitation on the southern Canadian Shield in Québec. The spatial structure of lake pH was modeled using two nested spherical variogram models, with ranges of 20 km and 250 km, accounting respectively for 20% and 55% of the spatial variation, plus a random component accounting for 25%. The pH data have been used to construct a number of geostatistical simulations that produce plausible realizations of a given random function model, while 'honoring' the experimental values (i.e., the real data points are among the simulated data), and that correspond to the same underlying variogram model. Post-processing of a large number of these simulations, that are equally likely to occur, enables the estimation of mean pH values, the proportion of affected lakes (lakes with pH≤5.5), and the potential error of these parameters within small regions (100 km×100 km). The method provides a procedure to establish whether acid rain control programs will succeed in reducing acidity in surface waters, allowing one to consider small areas with particular physiographic features rather than large drainage basins with several sources of heterogeneity. This judgment on the reduction of surface water acidity will be possible only if the amount of uncertainty in the estimation of mean pH is properly quantified. Received: 3 March 1997 · Accepted: 16 November 1998     

Spatial patterns of ice storm disturbance on a forested landscape in the Appalachian Mountains, Virginia

Ice storms (major freezing rain events) periodically disturb forests in eastern North America. The damage may vary spatially, especially in complex terrain. This study uses satellite imagery to investigate spatial heterogeneity of forest damage caused by ice storms that affected the Appalachian Mountains, Virginia during 1994. The results display a region-scale (southwest-to-northeast) gradient in damage that apparently corresponds to a gradient in the depth of ice that accumulated during the storms. Damage also varied topographically, particularly by aspect. Damage was most extensive on east-, southeast- and south-facing slopes; at middle elevations; and on slopes of moderate steepness.     

Chemical evolution of acid precipitation in the unsaturated zone of the Pennsylvanian siltstones and shale of central Ohio

The North Appalachian Experimental Watershed in Coshocton, Ohio, USA has recorded average pH of precipitation of 4.7 over a 30-year period. The area lies within the Pennsylvanian siltstones and shale, dominated by aluminosilicates and <5% calcite. A study was conducted to determine the evolution of acid deposition through an unsaturated to saturated zone composed of siltstone and shale in an isolated hill, precluding lateral flow and seepage. The results from water–rock chemical reactions modeled using PHREEQM demonstrate the percolating precipitation water is neutralized to pH 7.5 within the top 1.5 m. The model suggests that, along with calcite, dissolution of albite, illite, and kaolinite are the dominant mechanisms of neutralization. The cation exchange capacity of the siltstone and shale, in the range 54.6–386 meq/100 g, appears to be a function of high organic carbon content of 2.0–3.2%. While cation exchange is responsible for some of the Na⁺ in solution, it is not the primary source of Ca²⁺, Mg²⁺, or K⁺ ions. Exchange onto clays is occurring, but is secondary to exchange on organic matter. Chemical composition of groundwater perched within a coal seam is controlled by oxidation and dissolution of pyrite, returning pH to approximately 4.0.