Turbulence characteristics of flow region over a series of 2-D dune shaped structures

This paper addresses the development of a flow region associated with turbulence and stress characteristics over a series of 2-D asymmetric dunes placed successively at the flume surface. Experiments were conducted over twelve asymmetric dunes of mean length 32 cm, crest height 3 cm and the dune width almost as wide as width of the flume, using 3-D Micro-ADV at the Indian Statistical Institute, Calcutta. The variations of turbulence statistics along the flow affected by the wavy bottom roughness have been studied. Quadrant decomposition of the instantaneous Reynolds shear stress has been adopted to calculate the contribution of ejection and sweeping events in shear stress generation. The relative dominance of two events are found to contribute in a cyclic manner (spatially) in the near bed region, whereas such phenomenon seems to be disappeared towards the main flow.     

Conditional simulations of water–oil flow in heterogeneous porous media

This study is an extension of the stochastic analysis of transient two-phase flow in randomly heterogeneous porous media (Chen et al. in Water Resour Res 42:W03425, 2006), by incorporating direct measurements of the random soil properties. The log-transformed intrinsic permeability, soil pore size distribution parameter, and van Genuchten fitting parameter are treated as stochastic variables that are normally distributed with a separable exponential covariance model. These three random variables conditioned on given measurements are decomposed via Karhunen–Loève decomposition. Combined with the conditional eigenvalues and eigenfunctions of random variables, we conduct a series of numerical simulations using stochastic transient water–oil flow model (Chen et al. in Water Resour Res 42:W03425, 2006) based on the KLME approach to investigate how the number and location of measurement points, different random soil properties, as well as the correlation length of the random soil properties, affect the stochastic behavior of water and oil flow in heterogeneous porous media.     

初始扰动振幅和集合样本数对CNOPs集合预报的影响

初始扰动振幅的大小和集合样本数对于集合预报取得更高预报技巧具有重要意义。本文将正交条件非线性最优扰动方法（orthogonal conditional nonlinear optimal perturbations,简称CNOPs）应用于概念模型Lorenz-96模式探讨了初始扰动振幅和集合样本数对集合预报技巧的影响,从而为使用更复杂模式进行集合预报提供指导。结果表明,由于CNOPs扮演了非线性系统中的最优初始扰动,从而使得当初始扰动振幅小于初始分析误差的大小时,CNOPs集合预报获得更高的预报技巧,并且CNOPs集合预报的最高预报技巧总是高于奇异向量法（singular vectors,简称SVs）集合预报的最高预报技巧。结果还表明,CNOPs集合预报倾向于具有一个合适的样本数时,达到最高技巧。更好的集合离散度——预报误差关系和更为平坦的Talagrand图（Talagrand diagram）进一步证明了CNOPs集合预报系统的可靠性,从而夯实了上述结果的合理性。因此,针对CNOPs集合预报,本文认为采用一个适当小于初始分析误差的初始扰动振幅和一个合适的集合样本数,有利于CNOPs集合预报达到最高预报技巧。     

陕西一次远距离台风持续性暴雨的成因分析

利用MICAPS实况观测、卫星云图TBB、地面气象自动观测资料,对2010年7月16—18日陕西一次持续性暴雨天气过程进行诊断分析,结果表明:500 hPa大陆高压,副热带高压稳定维持使得两高之间低值系统稳定少动,宽广的低压底部不断有短波槽携带冷空气东移南下,与两高之间的低值区合并,Conson台风在登陆和减弱的过程中在南海及附近地区维持较长的时间,低层从台风东侧至陕西建立了温湿能通量的能量输送通道,把台风东侧的温湿能沿副热带高压外围偏南风向陕西输送,暴雨位于低层温湿能等值线密集处;条件性对称不稳定与对流不稳定是此次暴雨发展与维持的重要机制,高层湿位涡大值区向下伸展,对暴雨起到增幅的作用。暴雨产生在地面湿静力温度线密集区略偏向于高能中心一侧。     

Ensemble Forecast for Tropical Cyclone Based on CNOP-P Method: A Case Study of WRF Model and Two Typhoons

In this paper, we set out to study the ensemble forecast for tropical cyclones. The case study is based on the Conditional Nonlinear Optimal Perturbation related to Parameter (CNOP-P) method and the WRF model to improve the prediction accuracy for track and intensity, and two different typhoons are selected as cases for analysis. We first select perturbed parameters in the YSU and WSM6 schemes, and then solve CNOP-Ps with simulated annealing algorithm for single parameters as well as the combination of multiple parameters. Finally, perturbations are imposed on default parameter values to generate the ensemble members. The whole proposed procedures are referred to as the Perturbed Parameter Ensemble (PPE). We also conduct two experiments, which are control forecast and ensemble forecast, termed Ctrl and perturbed-physics ensemble (PPhyE) respectively, to demonstrate the performance for contrast. In the article, we compare the effects of three experiments on tropical cyclones in aspects of track and intensity, respectively. For track, the prediction errors of PPE are smaller. The ensemble mean of PPE filters the unpredictable situation and retains the reasonably predictable components of the ensemble members. As for intensity, ensemble mean values of the central minimum sea-level pressure and the central maximum wind speed are closer to CMA data during most of the simulation time. The predicted values of the PPE ensemble members included the intensity of CMA data when the typhoon made landfall. The PPE also shows uncertainty in the forecast. Moreover, we also analyze the track and intensity from physical variable fields of PPE. Experiment results show PPE outperforms the other two benchmarks in track and intensity prediction.     

"02.6"陕南大暴雨的结构及成因分析

通过诊断分析发现:(1)“02.6”强降水与6月上旬越赤道气流和季风爆发密切相关,携带大量水汽的偏南气流与冷空气于6月8日交汇在西北地区东部,导致了这次强降水的发生;(2)与暴雨区相联系,存在横越低空急流的经向垂直环流,暴雨区处于该垂直环流的上升支;(3)偏南和偏东气流水汽通道在西北地区东部交汇,水汽的辐合积聚主要在对流层低层和行星边界层内完成;(4)整层的视热源高值区在暴雨区附近呈东北—西南向分布,与切变线走向非常一致,降水产生的凝结潜热释放是强降水区大气的主要热源。同时,在大尺度上升运动区中低层存在一个条件对称不稳定建立的机制,使得在暴雨区,既存在深厚的热力不稳定机制,又存在水汽输入机制和热力不稳定的触发机制,从而形成强暴雨。     

Source identification of PM2.5 in an arid Northwest U.S. City by positive matrix factorization

Spokane, WA is prone to frequent particulate pollution episodes due to dust storms, biomass burning, and periods of stagnant meteorological conditions. Spokane is the location of a long-term study examining the association between health effects and chemical or physical constituents of particulate pollution. Positive matrix factorization (PMF) was used to deduce the sources of PM_2.5 (particulate matter ≤2.5 μm in aerodynamic diameter) at a residential site in Spokane from 1995 through 1997. A total of 16 elements in 945 daily PM_2.5 samples were measured. The PMF results indicated that seven sources independently contribute to the observed PM_2.5 mass: vegetative burning (44%), sulfate aerosol (19%), motor vehicle (11%), nitrate aerosol (9%), airborne soil (9%), chlorine-rich source (6%) and metal processing (3%). Conditional probability functions were computed using surface wind data and the PMF deduced mass contributions from each source and were used to identify local point sources. Concurrently measured carbon monoxide and nitrogen oxides were correlated with the PM_2.5 from both motor vehicles and vegetative burning.     

上级指导预报订正方法研究

从适应现代预报业务需求出发,针对中央台、省台2010年指导预报进行检验,并认真分析降水空漏报原因,结合本地天气形势及地形特点,建立降水、气温及灾害性天气预报指标,并对历史资料进行反查。采用条件概率将消空率≥80％的预报指标作为消空指标,将预报准确率≥90％的预报指标作为补漏指标,对指导预报建立消空、补漏预报方法,提高市级订正预报准确率。该订正方法在2011年业务应用中预报效果显著。     

引起舟曲特大泥石流灾害的"8·8"暴雨过程中尺度特征分析

利用常规气象资料以及FY-2E气象卫星云图、多普勒天气雷达资料和NCEP每6h一次的1°×1°格点资料,采用天气学诊断分析的方法,对2010年8月7—8日出现在甘肃省舟曲县的一次局地突发性致泥石流暴雨进行了诊断分析。结果表明:此次暴雨是在高空短波槽、东风倒槽、低涡切变线、副热带高压和地面冷空气的共同作用下发生的。对流云团发展生成MCS是暴雨发生的直接原因。对流云降水回波的发展和增强与降水强度的变化有很好的对应关系。来自孟加拉湾和东海的暖湿空气是此次暴雨的主要水汽来源。中低层辐合,高层辐散的配置和垂直涡度的显著增大,为本次暴雨的发生提供了必要的动力条件。暴雨中心位于700 hPa等假相当位温线密集带的西南侧边缘,暴雨是伴随着对流有效位能和大量不稳定能量的有效释放而发生的。倾斜涡度的活跃发展,条件性对称不稳定机制的形成可能是导致此次暴雨发生的主要触发机制。     

登陆台风内中尺度强对流系统演变机制的湿位涡分析

2005年05号台风"海棠"登陆福建后,在外围云系里有个明显发展的中尺度对流云团经过温州东部及北部地区,引起了强降水,从而造成比热带风暴环流本身更具破坏力的强烈天气,因此研究台风内中尺度对流系统(M(2S)的发展机制能够为预报台风灾害提供依据.文中使用中尺度静力模式WRF对台风"海棠"登陆过程进行了模拟,模式很好地模拟了台风登陆过程的路径、强度变化趋势和降水分布,尤其是模拟出了台风环流内的一次中尺度对流系统的发展过程,并利用模拟结果对台风环流内的这次中尺度对流系统进行了与之相关联的湿位涡分析,从而揭示了台风环流内中尺度对流系统发展演变的湿位涡特征.结果表明,在对流形成阶段,MPV1即对流不稳定为MCS的形成提供背景不稳定条件,由MPV2即湿等熵面的倾斜和水平风的垂直切变而引起的涡旋发展作为强迫机制:MCS形成的区域及东南区域中低层是强对流不稳定层,蕴含丰富的不稳定能量,倾斜上升运动把对流不稳定区具有强不稳定能量的暖湿卒气向西北中层的中性层结区输送.由于θep的减小,气旋性涡度增强,有利于形成对流,另一方面,由于湿等熵面倾斜和低空急流加强而引起的涡旋发展作为一种强迫机制激发对流不稳定能晕得到释放,从而形成对流;在对流系统的发展阶段,由于低层的对流不稳定性进一步减弱,θep一步减小,气旋性涡度进一步增强,有利于MCS的增强,中层等θe线的倾斜度比绝对动量M等值线的倾斜度大,对应有条件对称不稳定区域,满足条件对称不稳定(CSI)条件,在湿等熵面倾斜和台风低空急流作用下引起的涡旋发展强迫对称不稳定能量释放,从而使得对流得以维持和加强.通过以上的分析给出了台风环流内中尺度对流系统发生发展的概念模型.