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981.
利用中国气象局国家气象信息中心1961—2008年夏、冬两季日平均温度资料,基于偏态分布函数提出最概然温度.研究表明,最概然温度比平均温度更能合理地代表气象观测站的背景温度场.就年代际而言,在20世纪90年代中期之前,中国夏季年最概然温度以相对低温为主,随后呈现波动增温趋势,但自2005年增温趋势有所减缓.冬季年最概然温度在1961—1986年这一时段以相对低温为主,1987年到21世纪初显著变暖,但2000年后增温趋势减缓.总体而言,冬季增温幅度较夏季强,且增温时间早于夏季5—10年.本文定义的1961 相似文献
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983.
Stress, fluid and temperature are three of the major factors that impact natural gas migration and accumulation. In order to study the influences of tectonic stress field on natural gas migration and accumulation in low-permeability rocks, we take the Kuqa Depression as an example and analyze the evolution of the structure and tectonic stress field at first. Then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the KL2 gas field. We summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation. Firstly, under the effects of the tectonic stress field, the rock dilation increases with the added stress and strain, and when the shear stress of rock exceeds its shear strength, the shear fractures are well developed. On one hand, the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration. On the other hand, these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation. Secondly, because fluid potentials decrease in these places near the faults where fractures are well developed, natural gas can migrate rapidly along the faults and accumulates. The impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low-permeability rocks in an active tectonic compressive setting. 相似文献
984.
Northeasters are storms that affect the Chesapeake Bay area more frequently, last for longer periods and impact larger areas than hurricanes. Their impacts on storm surge development and the water exchange between estuary and subestuaries (tributaries) in the Bay vary from one event to another. In this study, three different northeaster events were selected based on their tracks when passing through the Chesapeake Bay area. An unstructured grid finite volume model ELCIRC was utilized to examine the response of the water level of the Chesapeake Bay to three selected northeasters, and the barotropic subtidal water exchanges between the tributaries and the estuary in the Bay. Model sensitivity tests were conducted to examine various effects induced by, for example, tide–surge interaction, open boundary condition, river inflow, wetting-and-drying of the low-lying land area and the usage of 2-D or 3-D mode. The results show that excluding tide–surge interaction did not deteriorate the model performance in the lower Bay but it increased the model inaccuracy in the upper Bay and in the tributaries; using radiation boundary condition decreased the sea level variation in the Bay without appropriately specifying incoming wave; excluding wetting-and-drying of low-lying land area reduced the volume flux by approximately 5%; and using 3-D mode generally increased the water level variation in the Bay. The model predicted storm surges well for three northeaster events. Further diagnostic experiments show that the relative importance of the local and remote winds in generating storm surges in the Bay varied with different northeasters. The inverse barometeric effect played an important role in inducing storm surges for two selected northeasters. The interaction between the tributaries and the Bay proper is considerable. The impacts of the remote wind and Bay wind can be much larger than that of the tributary wind and, thus, control the hydrodynamics and mass transport in the tributaries. The Bay wind and tributary wind effects are largely affected by the wind direction and wind phase, and geographic locations of the tributaries in the Bay. The tributary wind can be dominant over the remote wind and Bay wind effects when the local wind stress and barometric pressure changes are large. 相似文献
985.
顾及距离与形状相似性的面状地理实体聚类 总被引:3,自引:0,他引:3
与点状地理实体不同,面状地理实体不仅具有位置特征.还具有形状特征.对于面状地理实体而言,仅考虑距离因素设计聚类准则是不全面的.综合考虑距离和几何形状相似性来设计聚类准则,实现了相应的聚类算法.实验证明,该算法适合面状地理实体的聚类分析. 相似文献
986.
987.
988.
989.
Wind plays an important role in regulating mixing/stratification, estuarine circulation, and transport timescale in estuaries. A three-dimensional model was used to investigate the effect of wind on transport time by using the concept of water age (WA) in the tidal Rappahannock River, a western tributary of the Chesapeake Bay, USA. The model was calibrated for water level, current, and salinity. A series of experiments regarding the effects of wind on WA was conducted under various dynamic conditions. The effect of wind on transport timescale depends strongly on the competition between the wind and buoyancy forcings, and on the pre-status of the circulation. A down-estuary wind generally decreases WA along the estuary. An up-estuary wind increases WA substantially because it changes the vertical mixing and estuarine circulation more significantly. When the buoyancy forcing increases, the up-estuary wind effect decreases whereas the down-estuary wind effect increases. A 2-day period wind pulse with a maximum speed of 15 m s−1 can alter WA for 3 days; but the wind influence on WA lasts up to 40 days in the simulation. Both local and non-local wind forcings alter WA distribution. The local wind enhances vertical mixing and changes the gravitational circulation in the downstream portion of the estuary whereas it enhances transport in the freshwater portion of the estuary. Consequently, the local wind has a significant impact on WA distribution. In contrast, the non-local wind does not change the gravitational circulation significantly by imposing setup (setdown) of water level at the open boundary, resulting in a lesser impact on WA distribution. 相似文献
990.
Tropical Cyclone Isabel of 2003 generated large storm surge, strong waves, and subsequent river flooding in the York River Estuary, USA during its passage across the Chesapeake Bay region. A 3D model was used to investigate the changes of sediment concentration, sediment flux, and the recovery time of the York River Estuary to its naturally evolved condition without the storm. The results showed that two sediment concentration peaks appeared during the storm event. The first one was induced by the large upstream flow and waves during the storm surge rising period, and the later one was caused by the strong downstream flow during the descent of the storm surge. The advection, which was induced by the barotropic gradient, dominated the sediment flux during the storm event. The sediment fluxes increased by a factor of 100 during the rise and descent of the storm surge. A large amount of sediment that was transported into the estuary and eroded from the seabed during the rising of the storm surge was quickly transported out of the estuary during the descent of the storm surge. Waves played a key role in stirring the seabed and increasing the sediment concentration during the storm. Subsequent high freshwater inflow changed the sediment loading and hydrodynamics in the estuary, and thus, influenced the estuarine turbidity maximum (ETM) dynamics profoundly. The ETM moved downstream with the river flooding initially and returned upstream with the waning of river flooding and the re-establishment of gravitational circulation. The effect of river flooding on sediment concentration varied spatially and depended on the changes of ETM locations and vertical mixing. The model results suggest that a large amount of sediment was transported out of the estuary during the storm event and the subsequent river flooding had a larger impact on recovery time of the estuary. 相似文献