Circulation cells topology and their effect on migration pattern of different multi-bend meandering rivers

In meandering rivers, a cross-stream flow, referred to as a secondary current, has important effects on broad spectra of hydraulic/environmental characteristics, running the gamut from river hydrodynamics and geomorphology to stream ecology. The transport equation for vorticity and kinetic energy transfer should be analyzed to specify terms involved in generation of secondary currents. However, there is limited research on scrutinizing these terms in meandering rivers. On the other hand, while rivers are mostly multi-bend, previous studies have been limited to single bends. In the current paper, three physical multi-bend channels representing a strongly curved bend, a mild bend and an elongated symmetrical meander loop are designed in order to unravel mechanisms responsible for forming circulation cells in cross sections. Experiments are carried out in the middle bend of these models. Cross-stream turbulence anisotropy considerably strengthens almost all near bank cells. Moreover, contrary to single sharp bends, multi bend effects hinder the transfer of the kinetic energy in both directions in the entrance section of the strongly curved bend.     

一种基于温度历史和瞬态地温场模拟的剥露历史反演方法

根据低温热年代学数据,提取岩石从深部剥露到地表的信息,对理解诸多地质问题（如造山带演化、地表过程及其相互作用等）具有重要意义.本文提出一种基于岩石温度历史（可利用古温标、热年代计等方法制约）,并考虑剥露过程对地温场扰动的剥露历史反演计算方法.基于假定的与真实数据的正反演模拟和参数敏感性分析表明：热扩散率的变化对剥蚀量计算影响不大,在常规岩石热扩散率变化范围内（20~35 km²/Ma）,总剥蚀量变化小于10%;传统计算方法低估了剥蚀总量,对于现今地温梯度小于20℃/km、冷却速率大于2~3℃/Ma,或现今地温梯度大于30℃/km、冷却速率大于5~10℃/Ma的地区,需要考虑热平流对剥蚀量计算的影响;匀速冷却的热历史指示剥蚀速率持续减小,而非匀速剥蚀.本文将该方法应用到龙门山南段和四川盆地,反演计算显示龙门山南段15 Ma以来的剥蚀总量为8 km,四川盆地中部80 Ma以来剥蚀总量为约3 km、东部约5 km.

     

西伯利亚高压与冬季欧亚大陆热力变化的关系

主要应用EOF方法和相关、合成、小波等分析方法,初步分析了冬季欧亚大陆北部区域地面热力变化特征和西伯利亚高压(SH)的相互联系,结果表明:冬季欧亚大陆北部与南部不同区域间存在反向的热力差异,其中北部区域变化最为强烈,其地面气温存在2～4a为周期的年际振荡和准10 a为周期的年代际变化特征.SH与欧亚大陆北部区域热力变化有着紧密联系,当SH位置偏南(北),则该区域地面温度偏高(低),即SH平均纬度位置南北移动1.0°时,该区域的平均气温就变化0.65℃.进一步分析表明欧亚大陆北部区域热力异常和SH中心纬度位置的南北摆动与中高纬度环流经向度异常所激发的冷、暖平流的活动有密切联系.     

APPLICATION OF ADVECTIVE VORTICITY EQUATION TO TYPHOON FUNG-WONG

The momentum advection vorticity equation in the form of cross multiplication is introduced, in which the divergence term in the classic vorticity equation does not appear explicitly. This equation includes the rotation effect of the horizontal wind advection, which are not explicitly included in the classic vorticity equation. The vorticity and its tendency of Typhoon Fung-Wong (0808) that occurred in July 2008 are analyzed. The computed results show that the rotation effect of the advection of the horizontal wind is a leading factor in determining the change of vertical vorticity for Fung-Wong during its life cycle, especially in the period leading up to landfall. The advection term represents the tendency variation of the vertical vorticity, and the positive-value region of the vertical vorticity tendency is almost in accord with the track of Fung-Wong, which may be taken as a factor to locate the key observational region of Fung-Wong. The equation provides a supplementary diagnostic tool for the systems related with strong advection of horizontal wind.     

Two‐dimensional solute transport for periodic flow in isotropic porous media: an analytical solution

In this article, a mathematical model is presented for the dispersion problem in finite porous media in which the flow is two‐dimensional, the seepage flow velocity is periodic, and dispersion parameter is proportional to the flow velocity. In addition to these, first‐order decay and zero‐order production parameters have also been considered directly proportional to the velocity. Retardation factor is taken into account in the present problem. First‐type boundary condition of periodic nature is considered at the extreme end of the boundary. Mixed‐type boundary condition is assumed at the origin of the domain. A classical mathematical substitution transforms the original advection–dispersion equation into diffusion equation in terms of other dependent and independent variables, with constant coefficients. Laplace transform technique is used to obtain the analytical solution. Copyright © 2011 John Wiley & Sons, Ltd.     

风廓线雷达反演温度平流的应用

利用北京延庆站风廓线雷达水平风廓线资料进行大气温度平流的反演,详细分析2014年11月15日夜间冷空气入侵过程,并统计分析2015年9—11月6次冷空气入侵过程,同T639L60模式的预报风场及温度平流预报产品进行对比。结果表明:在一定预报时效内 (约6~12 h),风廓线雷达获取的水平风廓线与模式给出的预报风场有较好的一致性;由风廓线雷达反演的温度平流与模式给出的温度平流量级相同,温度平流属性一致;风廓线雷达6 min完成1次垂直高度分辨率为120 m的探测,高时空分辨率使风廓线雷达获取的温度平流较T639L60模式更能反映大气温度平流的细节。     

一次平流雾的形成和传播特征研究

2013年3月18-19日,江苏出现了一次罕见的平流雾过程,雾区先在长江北岸形成,继而向苏北传播。本文对这次平流雾的形成机理及特征进行了分析,并对雾区的传播机制进行了研究。结果表明:此次平流雾的形成主要受冷空气控制后暖湿平流的影响,冷锋过境后的影响是平流雾形成的基础,东海变性冷高压北抬是其形成的必要条件;地面东南风和低空东南水汽输送共同推动雾区向北传播;长江、洪泽湖、高邮湖等水域对平流雾增强起着重要作用;强浓雾呈渐进式发展,爆发性增强不明显以及云雾共存结构等是本次平流雾的主要特征。本论文的研究结果对平流雾的预报具有重要的实用价值。     

2018年8月22日宁夏北部局地浓雾天气形成机制分析

利用宁夏972个地面自动站的10 m风场、2 m温度及露点温度,银川、大武口、平罗、贺兰4个常规地面观测站的能见度、温度及相对湿度逐时观测资料和欧洲中期天气预报中心（ECMWF）ERA-Interim逐6 h再分析资料（0.125°×0.125°）,对2018年8月22日宁夏北部局地突发浓雾天气过程的环流形势、逆温结构及其热力、动力条件和形成维持机制进行分析。结果表明：8月22日宁夏北部局地浓雾发生在地面冷高压、高空暖脊及近地面微风的静稳天气条件下;较为深厚的暖平流和近地面冷空气侵入所形成的稳定大气层结,是大雾形成的必要条件;弱上升运动使混合层扩展至840 hPa左右,是该地区雾的发展和维持的重要原因。在本次过程中,深厚的弱水汽辐合较水汽自身的饱和与否更为重要,是大武口站浓雾形成的关键性因素。浓雾呈现平流-辐射雾特征,地面热通量在过程前期促进浓雾的发生,后期对浓雾起到抑制作用。     

安顺一次罕见秋季特大暴雨天气分析

利用FNL 1o×1o再分析资料、FY-2F卫星TBB资料及常规气象观测资料对2020年9月13日夜间到14日白天发生在贵州安顺的一次罕见秋季暴雨天气过程进行分析,结果表明：副热带高压东退,引导短波槽东移和低涡切变线南压,为暴雨的发生提供了有利的天气尺度背景条件;强水汽辐合中心稳定维持在安顺,暴雨中心与水汽辐合中心对应;暴雨区低层暖平流和中层冷平流的叠加,利于上升运动的发展,为能量和潜热的释放提供有利条件;中尺度对流云团在生成阶段发展迅速,成熟以后减弱缓慢,并在安顺上空稳定少动,是导致全市范围出现暴雨到特大暴雨的主要原因,且暴雨区与TBB≤55℃的区域对应较好;在对流云团生成的地方大气水汽、能量和抬升条件均较好,在其东移影响的区域,大气层结不稳定,湿层深厚,但能量条件较差,因此此次暴雨过程的对流性不是很强,在降雨最强的时段小时雨强大多在20-40mm·h-1之间     

The Use of Pre-Storm Boundary-Layer Baroclinicity in Determining and Operationally Implementing the Atlantic Surface Cyclone Intensification Index

The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low-level baroclinicity within the coastal zone during periods of offshore cold advection. The ralative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period often precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advection period and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24–48 h prior to the initial cyclonic intensification) are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting.The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982–April 1990), 116-storm study conducted previously. During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham (RAH) forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.