城市气象学课件：06_2城市风场_.pptVIP

流线 流线 建筑群流场 2m高度处流场 每一个城市都有其个性！ Northern Hemisphere atmospheric stilling partly attributed to an increase in surface roughness Robert Vautard1, Julien Cattiaux1, Pascal Yiou1, Jean-No?l Thépaut2 Philippe Ciais1 Nature Geoscience Published online: 17 October 2010 | doi:10.1038/ngeo979 Abstract: Surface winds have declined in China, the Netherlands, the Czech Republic, the United States and Australia over the past few decades. The precise cause of the stilling is uncertain. Here, we analyse the extent and potential cause of changes in surface wind speeds over the northern mid-latitudes between 1979 and 2008, using data from 822 surface weather stations. We show that surface wind speeds have declined by 5–15% over almost all continental areas in the northern mid-latitudes, and that strong winds have slowed faster than weak winds…… In contrast, upper-air winds calculated from sea-level pressure gradients, and winds from weather reanalyses, exhibited no such trend. Changes in atmospheric circulation that are captured by reanalysis data explain 10–50% of the surface wind slowdown. In addition, mesoscale model simulations suggest that an increase in surface roughness—the magnitude of which is estimated from increases in biomass and land-use change in Eurasia—could explain between 25 and 60% of the stilling. Moreover, regions of pronounced stilling generally coincided with regions where biomass has increased over the past 30?years, supporting the role of vegetation increases in wind slowdown. Why winds are slowing? Afforestation and climate change are blamed for stilling surface winds in the Northern Hemisphere. [Joseph Milton] Afforestation? Climate change? Urbanization? Not the final answer! The end of Ch 06Thank YOU Pigment 把…加色 * Ch6 The Urban Wind Field Part 2 内 容 三、城市冠层内流场的局地差异 气流在建筑物或建筑群的阻挡和干扰下，将在建筑群周围形成十分复杂的流场 单个建筑物周围流场 对称街谷流场 非对称街谷流场 有谷底加热时的流场 建筑群周围流场 Flow patterns around a sharp-edged building. Side view of (a) streamlines and flow zones. Notice that the f