Numerical Simulations of Sheltering in Valleys: The Formation of Nighttime Cold-Air Pools

Numerical simulations of flow over two-dimensional valleys are conducted in order to study the occurrence of pools of cold air that form at the bottom of valleys during stable nighttime conditions. The results show that during strong surface radiative cooling and light-wind events, the near-surface potential temperatures that occur at the bottom of valleys can be several kelvin below the environmental mean. This is true for quite shallow valleys with depths and widths of 50 m and 1 km, respectively, and is a result of in situ sheltering at the valley bottom. For windier conditions or less rapid cooling, the cold-pool temperature contrasts are reduced. For shallow valleys the magnitude of the difference between the potential temperature at the bottom of the valley and the mean value increases with increasing valley depth. However there is a critical valley depth, beyond which the valley flow becomes decoupled from that aloft and there are no further increases in the potential temperature difference. This critical valley depth depends on the wind speed and radiative cooling rate and the results indicate it is a function of a non-dimensional valley depth (or inverse Froude number), which is itself a property of the undisturbed profiles of wind and stability.     

冬季台风“南玛都”结构性质的初步研究

利用1982～2001年NCEP/NCAR再分析的周平均SST场、逐日表面热通量场及近地层10米高度风场资料,分析了南海地区季风爆发前后几周南海多年平均SST随时间的变化和空间分布特征及其影响因子.结果表明,南海季风爆发前,SST急剧升高,季风爆发后,SST的变化呈现比较明显的空间差异,南海北部SST继续上升,而南部SST持续下降.南海季风爆发前,海面净得热,这是季风爆发前南海SST上升的主要原因.季风爆发后几周,海面净得热减少,此时的海表净热通量收支与SST无显著相关.而季风爆发期和爆发后几周,南海SST变化的不均匀性与西南气流具有很好的相关性,南海的降温区呈东北-西南走向,与低层西南气流的方向一致.因而,在季风爆发后的一段时间内,近地层风场导致的海洋表面及内部动力过程是影响南海SST变化的另一重要因子.     

台风凤凰形成发展过程中对流凝结潜热和感热的作用

应用NCEP再分析资料,计算分析了台风凤凰发生和发展过程中的积云对流潜热加热和海面感热通量.表明:感热释放通过海气相关作用使海面风及对流层涡度增强,可能是台风初始低压的形成机制;积云对流潜热释放不但使台风中心增暖并使台风中间层上升运动增强,从而促使台风加强和发展,因此,对流凝结潜热是台风凤凰维持和发展的主要热力和动力因子.     

中国风蚀起沙研究进展

风蚀起沙是开展沙尘气溶胶和沙尘天气研究的基本问题.从风蚀起沙的机制、粒子运动形态、起沙条件、沙尘通量等几个方面,回顾了我国在风蚀起沙方面的主要研究结果,对开展这方面研究的难点问题开展讨论.介绍了计算临界摩擦速度的不同方法,并对不同研究者在不同地点观测到的临界摩擦速度进行比较.针对提高观测精度和观测研究结果的可比较性问题,提出了三点建议:一是要建立比较详细的地理、地貌和土壤信息系统;二是要利用卫星观测的优势,建立地面对比观测系统,标校卫星遥感结果;三是要规范观测方法和流程,充分利用业务化的观测资料,提高观测的可对比性.     

A Re-examination of Density Effects in Eddy Covariance Measurements of CO_2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and ...     

青藏高原东坡理塘地区近地层湍流通量与微气象特征研究

简要介绍青藏高原东坡理塘大气综合观测站长期观测试验,并利用2006年1、7月资料分别代表该站冬季和夏季,初步分析和比较该地区冬、夏季近地层微气象特征和湍流通最输送情况,得到了以下结论:(1)风、温、湿均表现出明显的日变化特征.冬季风速值平均大于夏季,风速极大值均出现在下午;冬季温度梯度早晚大,白天小,而夏季均较小;湿度梯度早晚大于白天.(2)中件条件下风速廓线对数关系表现为一条直线而非中件条件下略偏离对数关系,晚上均有逆温现象出现.在一定高度能观测到较弱的逆湿现象.(3)冬季以感热为主,潜热值较小,夏季以潜热为主,但感热也较大,且冬季通量值要远小于夏季;冬季动量通量平均大于夏季,二氧化碳通量远小于夏季;浅层(地面以下2和5 cm)土壤热通量也具有明显的日变化特征,白天从土壤吸收热量,夜间则放出热量.(4)地面热源强度具有显著的日变化特征:白天为强热源,夜间冷热源特征不明显.冬季和夏季全天平均表现为热源,但夏季强度远大于冬季,平均达到134 W/m2左右,冬季仪约35.3 W/m2.     

ANALYSIS OF RAINSTORMS ASSOCIATED WITH SIMILAR TRACK TROPICAL CYCLONES HAITANG (0505) AND BILLS (0604)

It is generally thought that the influence of comparable track typhoons is approximately similar,but in fact their wind and especially their rainstorm distribution are often very different. Therefore,a contrastive analysis of rainstorms by tropical cyclones (TCs) Haitang (0505) and Bilis (0604),which are of a similar track,is designed to help understand the mechanism of the TC rainstorm and to improve forecasting skills. The daily rainthll of TC itaitang (0505) and Bills (0604) is diagnosed and compared.The result indicates that these two TCs have similar precipitation distribution before landtall but different precipitation characteristics after landthll. Using NCEP/GFS analysis data,the synoptic situation is analyzed; water vapor transportation is discussed regarding the calculated water vapor flux and divergence.The results show that the heavy rainfall in the Zhejiang and Fujian Provinces associated with Haitang (0505) and Bitis (0604) before landlhll results ti'om a peripheral easterly wind,a combination of the tropical cyclone and the terrain. After landthll and moving far inland of the storm,the precipitation of Haitang is caused by water vapor convergence carried by its own circulation; it is much weaker than that in the coastal area. One of the important contributing thctors to heavy rainstorms in southeast Zhejiang is a southeast jet stream,which is maintained over the southeast coast. In contrast,the South China Sea monsoon circulation transports large amounts of water vapor into Bills - when a water-vapor transport belt south of the tropical cyclone significantly strengthens - which strengthens the transport. Then,it causes water vapor flux to converge on the south side of Bilis and diverge on the north side. Precipitation is much stronger on the south side than that on the north side. After Bilis travels far inland,the cold air guided by a north trough travels into the TC and remarkably enhances precipitation. In summary,combining vertical wind shear with water vapor transportation is a good way to predict rainstorms associated with landing tropical cyclones.     

DIAGNOSIS OF WAVE ACTIVITY OVER RAINBAND OF LANDFALL TYPHOON

A generalized wave-activity density, which is defined as an absolute value of production of three-dimensional vorticity vector perturbation and gradient of general potential temperature perturbation, is introduced and its wave-activity law is derived in Cartesian coordinates. Constructed in an agoestrophic and nonhydrostatic dynamical framework, the generalized wave-activity law may be applicable to diagnose mesoscale weather systems leading to heavy rainfall. The generalized wave-activity density and wave-activity flux divergence were calculated with the objective analysis data to investigate the character of wave activity over heavy-rainfall regions. The primary dynamical processes responsible for disturbance associated with heavy rainfall were also analyzed. It was shown that the generalized wave-activity density was closely correlated to the observed 6-h accumulative rainfall. This indicated that the wave activity or disturbance was evident over the frontal and landfall-typhoon heavy-rainfall regions in middle and lower troposphere. For the landfall-typhoon rainband, the portion of generalized wave-activity flux divergence, denoting the interaction between the basic-state cyclonic circulation of landfall typhoon and mesoscale waves, was the primary dynamic process responsible for the evolution of generalized wave-activity density.     

2008年河南黄淮地区暴雨过程个例分析

利用常规气象资料和NCEP资料对2008年7月22日河南黄淮地区的暴雨过程进行了分析.结果表明:这次过程是在500 hPa槽前西南气流引导下,高低空急流耦合区内西南涡沿切变线移出,弱冷空气侵入暖倒槽触发不稳定能量释放造成的.垂直螺旋度计算结果显示:中低层正垂直螺旋度中心与降水落区有很好的对应关系,大暴雨中心位于正垂直螺旋度中心附近.湿位涡演变分析发现,这次过程有"干侵入"发生,暴雨区中低层对流不稳定和对称不稳定共存,有利于降水增幅.水汽条件分析表明:这次过程的水汽源地在孟加拉湾和南海,主要是低层和近地层的水汽辐合.     

RS485总线在近地层通量观测系统中的应用

RS485接口具有良好的抗噪声干扰性、长传输距离和接收多站数据能力等优点,使其成为分布式测量控制系统首选的串行接口。介绍了RS485总线的特点、工作方式、组网及在张掖近地层通量观测系统中的具体应用,同时探讨了RS485总线使用中的一些问题及采取的应对措施。