Local Winds In A Valley City

Local winds were studied around a valley city, by using a high resolution two-dimensional mesoscale model forced by surface temperatures from a measurement campaign around Lanzhou City, China, during stagnant conditions. In the simulations nighttime winds are purely katabatic downslope winds without urban effects, despite the fact that the city is 6–7°C warmer than its surroundings all night. In contrast, daytime near-surface winds result from upslope flow resisted by an opposing simultaneous urban heat-island circulation (UHIC). Hence winds remain weak and variable around a city in a narrow valley during daytime. These conditions may lead to severe air quality problems day and night.The local circulations are sensitive to the widths of the valley and/or city,and also latitude, as is demonstrated by model experiments. Interestingly, in a flat and calm environment an extratropical daytime UHIC cell may turn into a weak `anti-UHIC' by the morning, due to frictional decoupling after sunset and subsequent inertial oscillation during the night, analogously tothe land breeze and nocturnal low-level jet formation.     

Numerical Research on Effects Upon Precipitation Forecast of Doppler-Radar Estimated Precipitation and Retrieved Wind Field Under Different Model Initial Schemes

On the basis of the joint estimated 1-h precipitation from Changde, Jingzhou, and Yichang Doppler radars as well as Wuhan digital radar, and the retrieved wind fields from Yichang and Jingzhou Doppler radars, a series of numerical experiments with an advanced regionalη-coordinate model (AREM) under different model initial schemes, i.e., Grapes-3DVAR, Barnes objective analysis, and Barnes-3DVAR, are carried out for a torrential rain process occurring along the Yangtze River in the 24-h period from 2000 BT 22 July 2002 to investigate the effects of the Doppler-radar estimated rainfall and retrieved winds on the rainfall forecast. The main results are as follows: (1) The simulations are obviously different under three initial schemes with the same data source (the radiosounding and T213L31 analysis). On the whole, Barnes-3DVAR, which combines the advantages of the Barnes objective analysis and the Grapes-3DVAR method, gives the best simulations: well-simulated rain band and clear mesoscale structures, as well as their location and intensity close to observations. (2) Both Barnes-3DVAR and Grapes-3DVAR schemes are able to assimilate the Doppler-radar estimated rainfall and retrieved winds, but differences in simulation results are very large, with Barnes-3DVAR's simulation much better than Grapes-3DVAR's. (3) Under Grapes-3DVAR scheme, the simulation of 24-h rainfall is improved obviously when assimilating the Doppler-radar estimated precipitation into the model in compared with the control experiment; but it becomes a little worse when assimilating the Doppler-radar retrieved winds into the model, and it becomes worse obviously when assimilating the Doppler-radar estimated precipitation as well as retrieved winds into the model. However, the simulation is different under Barnes-3DVAR scheme. The simulation is improved to a certain degree no matter assimilating the estimated precipitation or retrieved winds, or both of them. The result is the best when assimilating both of them into the model. And (4) Barnes-3DVAR is a new and efficient initial scheme for assimilating the radar estimated rainfall and retrieved winds.     

山东大风天气的低频特征及机理分析

利用山东省122个国家级地面气象观测站的风速数据与欧洲中期天气预报中心（ECMWF）提供的ERA- Interim再分析数据,采用小波分析、带通滤波等方法对2015年9月—2020年9月山东的大风天气及相应的低频大气环流形势进行分析。结果表明,近几年山东的大风天气有増加的趋势,春季大风发生频次最多,秋季最少;山东半岛东部大风频次最多,鲁南地区最少;全年只有7月偏南大风站次较偏北大风多,其余月份多以偏北大风为主。山东大风具有显著的11~13 d与20~23 d的低频振荡周期。其中,春季大风以11~13 d的振荡周期为主,秋、冬季以20~23 d的振荡周期为主,夏季大风的振荡周期不明显。振荡周期的演变与大范围的大风过程有对应关系,大范围的大风过程大致发生在振荡的波峰处。春季偏北大风盛行时,多伴有经向风自北向南的传播。秋季大约以35°N为界,对流层中高层在35°N以北,经向风自南向北传播,35°N以南,则是自北向南传播,对流层中低层反之。山东春季大风产生之前,乌拉尔山东侧低频气旋与黄海上空低频反气旋同时出现并东移,之后衍生出华北低频反气旋与渤海低频气旋,这两个系统的加强促使华北上空偏北风加大,为山东大风的产生提供了可能。同时,华北地区经向风正距平逐渐被负距平所代替,是山东大风天气产生的又一先兆。     

纳木错湖夏季典型大气边界层特征的数值模拟

为了进一步检验分析纳木错湖对当地地方性环流、湖气能量交换及大气边界层的影响,在本文中使用美国NCAR新版MM5V3.7非静力中尺度模式,设计了有湖面、3/4湖面、1/2湖面及无湖四组试验,以NCEP再分析资料做初、边值条件,做了48 h三重嵌套模拟试验,对比中国科学院"纳木错圈层相互作用"综合观测站实测资料,说明该模式模拟性能良好。白天在纳木错湖风与念青唐古拉山北坡谷风共同作用下,在念青唐古拉山脊处汇合,形成一强水平切变及辐合上升运动,造成了该地夏季白天复杂多变的天气;夜间由于南岸湖风与山风叠加,使得整个区域为强大南风气流控制,这也补给了水汽和热量,也为白天不稳定运动提供了能量。对大气边界层特征的模拟结果表明:由于白天(夜晚)纳木错湖的存在有很好的降温(保温)作用,该湖表现出了明显的冷(暖)湖效应;纳木错湖对感热和潜热的影响有很强的日变化,白天湖面感热、潜热均小于周围陆地,夜间湖面有强潜热通量;纳木错湖使得白天湖区边界层顶低,陆区边界层顶高,夜间相反。这样的地方性环流和大气边界层特征的配合,是该地中小尺度天气剧烈变化的重要原因。     

冬夏季海面风场和海洋表层流的数值计算

设计了一个海面风场的诊断模式和海洋表层流的数值计算模式。只要输入海面气压场并利用迭代方法，即可求得海洋表面的风场和海洋场。利用１月和７月多年平均的海平面气压场，对这两个模式的性能进行了检验。结果表明，无论冬夏，用海平面气压场诊断出的海面风场与观测到的海面风场很接近，而与地转风场有很明显的区别。用海面风场和海面气压场启动的海洋表面流，与实况也较一致。本文的试验结果还表明，计算海流时，不应忽略海平面气     

NGC 3603星团中的大质量逃逸双星

非团环境中的大质量恒星以及大质量双星的起源是理解大质量恒星形成和演化的关键问题之一. 年轻大质量星团内的动力学交会过程是产生大质量逃逸星的重要途径之一. 选取了银河系内年轻大质量星团NGC 3603外围的两个碰撞星风系统候选体MTT68A和MTT71作为研究对象, 通过分析\emphChandra X射线观测以及\emphGaia第2批数据发布(DR2)中的天体测量结果, 研究它们作为相互作用的大质量双星系统的起源. X射线数据的分析表明, 它们的X射线能谱中存在Fe XXV发射线成分; 相较于普通O型星, 以双温等离子体模型拟合得到偏高的高温成分温度($\gtrsim$2.0keV), 并且X射线光度与热光度的比值也较高($\gtrsim10^{-5.8