A new approach is proposed to use the covariant scalar equations of the a-coordinate （the covariant method）, in which the pressure gradient force （PGF） has only one term in each horizontal momentum equation, and t...A new approach is proposed to use the covariant scalar equations of the a-coordinate （the covariant method）, in which the pressure gradient force （PGF） has only one term in each horizontal momentum equation, and the PGF errors are much reduced in the computational space. In addition, the validity of reducing the PGF errors by this covariant method in the computational and physical space over steep terrain is investigated. First, the authors implement a set of idealized experiments of increasing terrain slope to compare the PGF errors of the covariant method and those of the classic method in the computational space. The results demonstrate that the PGF errors of the covariant method are consistently much-reduced, compared to those of the classic method. More importantly, the steeper the terrain, the greater the reduction in the ratio of the PGF errors via the covariant method. Next, the authors use geometric analysis to further investigate the PGF errors in the physical space, and the results illustrates that the PGF of the covariant method equals that of the classic method in the physical space; namely, the covariant method based on the non-orthogonal a-coordinate cannot reduce the PGF errors in the physical space. However, an orthogonal method can reduce the PGF errors in the physical space. Finally, a set of idealized experiments are carried out to validate the results obtained by the geometric analysis. These results indicate that the covariant method may improve the simulation of variables relevant to pressure, in addition to pressure itself, near steep terrain.展开更多
A terrain-following coordinate （a-coordinate） in which the computational form of pressure gradient force （PGF） is two-term （the so-called classic method） has significant PGF errors near steep terrain. Using the ...A terrain-following coordinate （a-coordinate） in which the computational form of pressure gradient force （PGF） is two-term （the so-called classic method） has significant PGF errors near steep terrain. Using the covariant equations of the a-coordinate to create a one-term PGF （the covariant method） can reduce the PGF errors. This study investigates the factors inducing the PGF errors of these two methods, through geometric analysis and idealized experiments. The geometric analysis first demonstrates that the terrain slope and the vertical pressure gradient can induce the PGF errors of the classic method, and then generalize the effect of the terrain slope to the effect of the slope of each vertical layer （φ）. More importantly, a new factor, the direction of PGF （a）, is proposed by the geometric analysis, and the effects of φ and a are quantified by tan φ.tan a. When tan φ.tan a is greater than 1/9 or smaller than -10/9, the two terms of PGF of the classic method are of the same order but opposite in sign, and then the PGF errors of the classic method are large. Finally, the effects of three factors on inducing the PGF errors of the classic method are validated by a series of idealized experiments using various terrain types and pressure fields. The experimental results also demonstrate that the PGF errors of the covariant method are affected little by the three factors.展开更多
Metaphor, a rhetoric device which involves both stylistics as well as cognition, is really a hot theme in recent years of academic research. Thus this paper thinks that metaphor may help the language learners to write...Metaphor, a rhetoric device which involves both stylistics as well as cognition, is really a hot theme in recent years of academic research. Thus this paper thinks that metaphor may help the language learners to write creatively by a brief introduction and some personal experiences in language teachi ng.展开更多
基金supported by the National Basic Research Program of China(973 Program)[grant number 2015CB954102]the National Natural Science Foundation of China[grant number41305095],[grant number 41175064]
文摘A new approach is proposed to use the covariant scalar equations of the a-coordinate （the covariant method）, in which the pressure gradient force （PGF） has only one term in each horizontal momentum equation, and the PGF errors are much reduced in the computational space. In addition, the validity of reducing the PGF errors by this covariant method in the computational and physical space over steep terrain is investigated. First, the authors implement a set of idealized experiments of increasing terrain slope to compare the PGF errors of the covariant method and those of the classic method in the computational space. The results demonstrate that the PGF errors of the covariant method are consistently much-reduced, compared to those of the classic method. More importantly, the steeper the terrain, the greater the reduction in the ratio of the PGF errors via the covariant method. Next, the authors use geometric analysis to further investigate the PGF errors in the physical space, and the results illustrates that the PGF of the covariant method equals that of the classic method in the physical space; namely, the covariant method based on the non-orthogonal a-coordinate cannot reduce the PGF errors in the physical space. However, an orthogonal method can reduce the PGF errors in the physical space. Finally, a set of idealized experiments are carried out to validate the results obtained by the geometric analysis. These results indicate that the covariant method may improve the simulation of variables relevant to pressure, in addition to pressure itself, near steep terrain.
文摘城市化进程的加快以及人口持续增长,造成土地利用急剧变化,致使城市生态系统和居民生活质量受到了巨大的挑战,因此对土地利用驱动分析的研究显得尤为迫切。以2013年、2016年和2019年3期Landsat 8遥感影像为数据源,依托遥感技术(remote sensing,RS)和地理信息系统(geographic information system,GIS)空间分析技术,对西安市主城区土地利用变化进行动态分析,并从景观格局角度对西安市主城区城市景观格局演化进行定量分析,最后利用地理探测器模型定量分析不同驱动因子对土地利用程度的影响。结果表明:①西安市中心城区的土地利用以建设用地为主,六年来西安市六个中心城区的绿地面积先减少后增加,建筑面积先增加后小幅减小,而耕地面积和水体面积变化不大。②绿地斑块个数逐年增加,建筑用地斑块个数不断减少,说明绿地景观的破碎化程度逐年加重,建筑用地的布局已经呈大面积发展。③2013年和2016年驱动因子中地区温度的解释能力最强,2019年驱动因子中海拔的解释能力最强;2013年和2016年驱动因子间的交互作用均表现为双因子增强,2019年的交互作用表现为其他驱动因子与地区温度的交互效果为非线性增强。基于城市景观格局动态分析和驱动力分析,以期为西安市主城区及其他城区的土地规划及城市生态环境建设提供参考。
基金jointly supported by the National Basic Research Program of China[973 Program,grant number 2015CB954102]National Natural Science Foundation of China[grant numbers41305095 and 41175064]
文摘A terrain-following coordinate （a-coordinate） in which the computational form of pressure gradient force （PGF） is two-term （the so-called classic method） has significant PGF errors near steep terrain. Using the covariant equations of the a-coordinate to create a one-term PGF （the covariant method） can reduce the PGF errors. This study investigates the factors inducing the PGF errors of these two methods, through geometric analysis and idealized experiments. The geometric analysis first demonstrates that the terrain slope and the vertical pressure gradient can induce the PGF errors of the classic method, and then generalize the effect of the terrain slope to the effect of the slope of each vertical layer （φ）. More importantly, a new factor, the direction of PGF （a）, is proposed by the geometric analysis, and the effects of φ and a are quantified by tan φ.tan a. When tan φ.tan a is greater than 1/9 or smaller than -10/9, the two terms of PGF of the classic method are of the same order but opposite in sign, and then the PGF errors of the classic method are large. Finally, the effects of three factors on inducing the PGF errors of the classic method are validated by a series of idealized experiments using various terrain types and pressure fields. The experimental results also demonstrate that the PGF errors of the covariant method are affected little by the three factors.
文摘Metaphor, a rhetoric device which involves both stylistics as well as cognition, is really a hot theme in recent years of academic research. Thus this paper thinks that metaphor may help the language learners to write creatively by a brief introduction and some personal experiences in language teachi ng.
