海南岛海风雷暴结构的数值模拟

本文利用高分辨率WRF模式对2012年7月20日发生在海南地区的一次海风雷暴过程进行模拟,探讨了海南岛复杂地形下海风雷暴的结构、发展演变过程及其触发机制.结果表明,海南岛北部向内陆传播的海风与南部受地形阻挡的海风相遇后会形成海风辐合带,辐合带能影响当地的散度和涡旋特征,为雷暴的发生发展提供有利的动力和热力学条件.海南岛受热带海洋的影响较大,当地的水汽条件和对流潜势长期保持着有利于对流发展的状态,自由对流高度始终处于较低的位置,一旦海风辐合带来的抬升运动克服对流抑制到达自由对流高度后,对流就能自主地发展起来,所以单纯的海风辐合也常常能触发当地的强雷暴.雷暴发生发展过程中对流参数存在明显的变化,其演变曲线的突变位置对雷暴的发生有一定的指示作用.海南岛的海风雷暴过程与当地的复杂地形密切相关,地形的动力阻挡作用影响着低层海风的辐合以及对流的发展.     

Wind erosion of blanket peat during a short period of surface desiccation (North Pennines,Northern England)

Strong winds are a characteristic feature of UK upland areas. Despite this, understanding of aeolian processes in upland environments of the UK is limited. This paper presents direct measurements and observations of blanket peat erosion by wind action during a two week period of desiccation in the North Pennines, Northern England. A circular configuration of mass flux sediment samplers was used to collect peat eroded by wind action from 16 cardinal compass directions. Meteorological conditions (wind speed, wind direction, precipitation and temperature) were recorded by an automatic weather station set up adjacent to the site. Surface desiccation led to peat crust erosion and dust deflation. During short (≤1 hour) periods of precipitation, wind‐driven rainfall also caused erosion. Typically, dust flux rates were up to two orders of magnitude lower than recorded during periods of sustained wet weather. Measurements demonstrate the hitherto unreported rapid switch in process regime between wind‐driven rainfall and dry blow deflation in blanket peat environments. Dry blow processes of blanket peat erosion may become more important in UK upland areas if climate change promotes more frequent surface desiccation. Copyright © 2006 John Wiley & Sons, Ltd.     

双模态风垂直切变对台风结构和强度的作用分析

本文应用WRF(v 3.4)模式输出资料,揭示了风垂直切变(Vertical Wind Shear:VWS)在垂直方向上的波状变化特征,这种波状变化在台风不同发展时期又有不同形态,其中在持续强盛期呈双模态分布.应用VWS引起的次级垂直环流影响台风对流分布和强度变化的基本原理,用模式资料分析发现:对流层中层具有的VWS是整层VWS的主要部分,台风强度变化滞后VWS的形态突变6h左右;双模态波状变化的VWS产生的次级环流和台风垂直环流的配置不同使台风强对流带结构变得不对称及眼墙区对流强度在垂直分布上变得不均匀,随着持续强盛期涡旋运动的增强,强对流带分布又趋于对称.又根据VWS形成的垂直方向上涡度力分布不均匀引起台风内中尺度滚轴状对流带不稳定发展原理,分析表明:对流层中、低层的涡度力有利于对流不稳定增强,垂直速度的最大值与风垂直廓线的拐点在同一高度上,这与理论模型的结论一致.因此,VWS的波状变化分布特征不仅影响台风强对流带中尺度结构的改变,也对台风持续强盛具有重要作用;同时也是台风内滚轴状对流带不稳定的可能启动机制.     

Indirect air–sea interactions simulated with a coupled turbulence-resolving model

A turbulence-resolving parallelized atmospheric large-eddy simulation model (PALM) has been applied to study turbulent interactions between the humid atmospheric boundary layer (ABL) and the salt water oceanic mixed layer (OML). The most energetic three-dimensional turbulent eddies in the ABL–OML system (convective cells) were explicitly resolved in these simulations. This study considers a case of shear-free convection in the coupled ABL–OML system. The ABL–OML coupling scheme used the turbulent fluxes at the bottom of the ABL as upper boundary conditions for the OML and the sea surface temperature at the top of the OML as lower boundary conditions for the ABL. The analysis of the numerical experiment confirms that the ABL–OML interactions involve both the traditional direct coupling mechanism and much less studied indirect coupling mechanism (Garrett Dyn Atmos Ocean 23:19–34, 1996). The direct coupling refers to a common flux-gradient representation of the air–sea exchange, which is controlled by the temperature difference across the air–water interface. The indirect coupling refers to thermal instability of the Rayleigh–Benard convection, which is controlled by the temperature difference across the entire mixed layer through formation of the large convective eddies or cells. The indirect coupling mechanism in these simulations explained up to 45 % of the ABL–OML co-variability on the turbulent scales. Despite relatively small amplitude of the sea surface temperature fluctuations, persistence of the OML cells organizes the ABL convective cells. Water downdrafts in the OML cells tend to be collocated with air updrafts in the ABL cells. The study concludes that the convective structures in the ABL and the OML are co-organized. The OML convection controls the air–sea turbulent exchange in the quasi-equilibrium convective ABL–OML system.     

Observing Convective Aggregation

Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.     

Numerical modelling of pollutant dispersion in sea breeze conditions

The characteristics of air pollution in Tarragona (Spain) were investigated. Tarragona has an important petrochemical industry in a coastal region with a complex terrain. The numerical study was made in sea breeze conditions with a three-dimensional mesoscale model. Temporal and spatial variations of the wind fields have been used in the Eulerian equation for a non-reactive pollutant. The results of this study reveal the complexity of the dispersion patterns due to the combined effects of the sea breeze circulation and the orography. This work presents a comparison between the model output and the observed wind data by sodar and surface wind measurements. The evaluation shows that the model is capable of providing very realistic wind fields within this domain.     

Internal airflow of a convective storm from dual-Doppler radar measurements

The dual-Doppler radar coplane method of scanning and data reduction has been used to determine the internal airflow and radar reflectivity structure of a convective storm. Cumulus convection growing in a moderately sheared wind environment resulted in a nonsteady, moderate intensity thunderstorm. Precipitation fallout and downward moving air are found downshear of an updraft inclined in the downshear direction. Rapid storm translation, vertical shear of the ambient wind, and slow subcloud ascent of inflow air act to establish this observed draft configuration. The absence of significant cold air outflow and its attendant gust front at the surface is attributed to (i) appreciable inflow of slow moving air into the downdraft at the middle layers and (ii) the fact that the potentially coldest air was located too low to contribute significantly to a deep downdraft circulation.     

Cumulus dynamics: Local compensating subsidence and its implications for cumulus parameterization

Observations of air flow in and around convective clouds are summarized and discussed in light of the requirements for parameterization of midlatitude convection. Both theory and observation indicate that a substantial portion of the compensating subsidence occurs as dry downdrafts in the immediate vicinity of convective clouds, which suggests that an additional physical mechanism is necessary in linking convection to the large scale. A conceptual three-dimensional model of midlatitude organized convection along with its implications for parameterization, particularly in mesoscale modeling, is presented. The effects of moist downdrafts are also considered and some differences between midlatitude and tropical convection are discussed.     

Impact of South China Sea Cold Surges and Typhoon Peipah on Initiating Cyclone Sidr in the Bay of Bengal

In the present study, an attempt was made to understand the role of South China Sea (SCS) convection associated with northerly cold surges and Typhoon Peipah in initiating Cyclone Sidr in the Bay of Bengal (BoB). The variation of air sea fluxes during the entire history of Cyclone Sidr tracking before its landfall over Bangladesh was also studied. The presence of cold surges in the north SCS associated with heavy rainfall episodes has been noticed at the southern Gulf of Tonkin coast prior to the formation of Typhoon Peipah. Subsequently, these surges migrated south, which resulted in intensification of a deep convection on reaching the Vietnamese coast. During the same period in the western Pacific, Typhoon Peipah developed, propagating in the westward direction and entering the SCS. Analysis of geostationary water vapour images, mean sea level pressure, and surface wind maps clearly depicted the transport of convective cloud clusters, moisture, and westward momentum from Typhoon Peipah to the deep convection cells over the SCS. Consequently, the existing deep convection over the Vietnamese coast resulted in a westward direction and entered the Gulf of Thailand and Andaman Sea. The availability of higher latent heat fluxes, warmer sea surface temperatures, and suitable atmospheric conditions over this region favoured the formation of a tropical depression in the Andaman Sea. This depression further intensified in the southeast BoB, resulting in the formation of Cyclone Sidr. NCEP/NCAR wind fields and air-sea fluxes revealed left asymmetry surface winds and higher latent heat flux on the left side of the track during the intensification phase of Sidr.     

Influence of low viscosity asthenosphere on mantle flows

Numerical simulation in recent years has revealed that the cold lithosphere, whose viscosity is three to four orders of magnitude higher than that of the underlying mantle, behaves during mantle convection as a stagnant lid. On the basis of model calculations, this paper shows how convection changes over to this regime with increasing viscosity. Spatially fixed high viscosity inclusions and those moving with the convective flow have fundamentally different effects on the structure of convective flows. The model calculations indicate that anomalously low viscosity asthenospheric regions also lead to a specific regime of convection. With a decrease in the viscosity by more than three orders of magnitude, a further reduction in the viscosity of the region ceases to influence the structure of convection in the outer region. The boundary with this region behaves as a freely permeable boundary. In the low viscosity asthenospheric region itself, autonomous convection can arise under certain conditions.     

Pollutant transport in coastal areas with and without background wind

A three-dimensional meteorological model and a Lagrangian particle dispersion model are used to study the effects of a uniform large-scale wind on the dispersion of a non-reactive pollutant in a coastal region with complex terrain. Simulations are carried out both with and without a background wind. A comparison between model results and measured data (wind and pollutant concentrations) indicates that the coupled model system provides a useful mechanism for analyzing pollutant dispersion in coastal regions.     

Impact of Doppler Radar Wind in Simulating the Intensity and Propagation of Rainbands Associated with Mesoscale Convective Complexes Using MM5-3DVAR System

Pre-monsoon rainfall around Kolkata (northeastern part of India) is mostly of convective origin as 80% of the seasonal rainfall is produced by Mesoscale Convective Systems (MCS). Accurate prediction of the intensity and structure of these convective cloud clusters becomes challenging, mostly because the convective clouds within these clusters are short lived and the inaccuracy in the models initial state to represent the mesoscale details of the true atmospheric state. Besides the role in observing the internal structure of the precipitating systems, Doppler Weather Radar (DWR) provides an important data source for mesoscale and microscale weather analysis and forecasting. An attempt has been made to initialize the storm-scale numerical model using retrieved wind fields from single Doppler radar. In the present study, Doppler wind velocities from the Kolkata Doppler weather radar are assimilated into a mesoscale model, MM5 model using the three-dimensional variational data assimilation (3DVAR) system for the prediction of intense convective events that occurred during 0600 UTC on 5 May and 0000 UTC on 7 May, 2005. In order to evaluate the impact of the DWR wind data in simulating these severe storms, three experiments were carried out. The results show that assimilation of Doppler radar wind data has a positive impact on the prediction of intensity, organization and propagation of rain bands associated with these mesoscale convective systems. The assimilation system has to be modified further to incorporate the radar reflectivity data so that simulation of the microphysical and thermodynamic structure of these convective storms can be improved.     

我国中东部地区夏季中尺度对流系统形成前物理量诊断分析

利用2008—2010年夏季(6—8月)FY-2地球静止卫星红外云图资料识别出我国中东部地区(110°E—124°E,27°N—40°N)共208个中尺度对流系统(mesoscale convective system,MCS)和174个不能增长发展成为MCS的普通雷暴群(widespread convections,WCS).提取MCS形成前约6h和WCS成熟时(个数最多)的NCEP再分析资料(时间间隔6h,空间分辨率1°×1°),通过对表征水汽、动力和热力等条件的基本物理量和一些常用衍生物理量采用平均值、标准差等常用统计方法、动态合成和评估方法逐步筛选和分析诊断两种系统环境物理量场,最终从众多物理量中挑选出了能显著区别两种系统的物理量(即MCS形成的关键物理量),分别为强天气威胁指数、修正的K指数、地面抬升指数、2m比湿和0~3km垂直风切变,希望对预报我国中东部地区MCS发生与否提供一定的科学依据.     

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.     

风垂直切变对中尺度地形对流降水影响的研究

针对长江中下游中尺度地形特点以及暴雨过程发生发展期间风垂直切变的主要观测特征,设计了一系列中尺度地形的三维理想数值试验,分析了干大气地形流和重力波特征,探讨了条件不稳定湿大气地形对流降水的模态分布,在此基础上研究了圆形、直线风垂直切变和切变厚度对中尺度地形对流降水强度和模态分布的影响.结果发现:在 F_r≈1的干大气条件下,气流遇到地形后分支、绕流和爬升现象同时存在,地形激发的重力波在水平和垂直方向上传播,其在迎风坡、背风坡、地形上游和下游的振幅不同,并组织出不同强度的垂直上升运动.在F_r > 1的条件不稳定湿大气下,地形对流降水主要存在三种模态,即迎风坡和背风坡准静止对流降水以及地形下游移动性对流降水,地形对流降水的形成与重力波在低层组织的上升运动密切相关.风垂直切变对地形对流降水的强度和模态分布有重要作用,其中圆形风垂直切变(风随高度旋转)不仅影响地形下游对流降水系统的移动方向,而且影响迎风坡和背风坡山脚处对流降水中心的分布和强度;直线风垂直切变(风随高度无旋转)主要影响地形对流降水的移动速度和强度.风随高度自下而上顺(逆)时针旋转,地形对流系统向下游传播时向右(左)偏移.风垂直切变主要通过影响地形重力波的结构和传播以及对流系统的形成、移动方向和速度,来影响地形对流降水的模态分布,其中对流层中低层的风垂直切变对地形对流降水强度和模态分布有重要影响.     

日本琵琶湖流域大气边界层的三维数值模拟

本文在地形坐标下引入一种水平导数差分近似方法,建立一个适合日本琵琶湖复杂地形、满足静力平衡条件的三维非定常大气边界层数值模式。在弱的背景风场下,模式加入了琵琶湖流域的实际地形,计算结果表明:模式运行十分稳定,且占机内存小,节省计算时间,同时模式还做了流域地形、湖泊本身存在与否等因子对边界层风场影响的数值试验。在强的背景风场下,模式加入了经过适当平滑的琵琶湖流域地形,得出了一些有益的看法。     

大气边界层研究进展

大气边界层对云和对流的发展、演变有重要作用.本文回顾了在大气边界层高度计算方法,边界层的时空分布特征、结构和发展机理,以及边界层参数化方案等方面的主要研究进展.大气边界层高度计算方法主要分为基于大气廓线观测数据计算和基于模式参数化方案计算两大类;大气边界层高度频率分布形态具有明显的日变化特征,并且稳定、中性和对流边界层...     

Case studies on mesoscale structures of heavy rainfall system in the Yangtze River generated by Meiyu front

The heavy rainfall generated by Meiyu front in the Yangtze River is popular topics in recent years. The heavy rainfall is formed in advantageous weather con-dition and interaction between different mesoscale processes. The previous studies included formation and maintenance of heavy rainfall, atmospheric circu-lation, relationship between the Meiyu frontal and mesoscale disturbance[1]. In recent years, the numer-ous studies and filed experiments are focus at the mesoscale structures of heavy…     

Small-scale upper mantle convection and orogeny of Tianshan Mountains in China

In this study, from the travel time data recorded in the Tianshan passive seismic array experiment, we present the P-wave velocity structure of the upper mantle down to 660 km along the Kuqa-Kuitun pro-file in terms of seismic tomography technique. Based on the P-wave velocity model, we derive the corresponding 2D upper mantle density model. The 2D small-scale convection of the upper mantle underneath the Tianshan Mountains in China driven by the density anomalies is simulated using the hybrid finite element method combining with the marker-in-cell technique. The main features of the upper mantle convection and the reciprocation between the convection and mountain building are in-vestigated. The results manifest that (1) in the upper mantle underneath the Junggar basin and North Tianshan exists a counterclockwise convection, which scale is ~ 500 km; (2) underneath the Tarim ba-sin and South Tianshan exists a clockwise northward convection, which is relatively weak; (3) the convective velocity at the top of the upper mantle underneath the Tianshan Mountains in China should not be less than 20 mm/a, while considering the dependent of convective velocity on the viscosity; (4) the northward extrusion of the Tarim block plays a key role in the Cenozoic Tianshan mountain building and the present-day tectonic deformation of the Tianshan range is related closely to the upper mantle convection; and (5) the northward subduction of the Tarim block does not influence obviously the up-per mantle convection.