浙江天目山背风坡对流触发个例的对比分析

针对夏季副热带高压背景下浙北天目山附近的强对流天气个例,利用中尺度实况资料,分析了天目山对触发对流的作用。结果表明:浙江省夏季位于副热带高压边缘时,低层处于西南背景风时,在低Froude数条件下,气流经过黄山、天目山后在背风侧形成一段辐合线,在有利的热力条件配合下,容易触发对流。山地的热力强迫作用使地形上空新生了很多积云,积云分布基本与地形一致。同时,天目山背风侧出现一条积云线,其形成的原因是天目山背风侧辐合线的辐合抬升作用。背风侧辐合线尺度有几十千米,方向随环境风向转变。对流触发的位置位于这条辐合线上靠近山地的一端。这可能是由于山地热力强迫作用产生的积云移到辐合线上继续发展产生对流云,即山地的动力和热力作用共同触发了对流。     

复杂地形对对流混合层的影响

用发展后的混合层模式研究下垫面（地形和地面热力）的不均匀性对混合层结构的影响。数值实验表明,在保持了混合层模式简便和实用等特点的同时,发展后的混合层模式能够很好地反映复杂地形对混合层厚度和风速的影响。     

Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley

Based on the previous statistical analysis of mesoscale convective systems(MCSs)over the second-step terrain along Yangtze-Huaihe River Valley,eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas.We subject each MCS to a semi-idealized(CNTL)simulation and a sensitivity(NOLH)simulation that neglects condensational heating in the formation region.The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere.Upon merging with other convective systems,moist convection develops into an MCS,which propagates eastward under the influence of mid-tropospheric westerlies,and moves out of the second-step terrain.The MCS then merges with pre-existing local convection over the plains;the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa.While this vortex moves eastward to regions with local vortex at 850 hPa,another vortex at 925 hPa is also intensified.Finally,the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex(MCV).In contrast,MCSs fail to form and move eastward in the NOLH experiment.In the absence of eastward-propagating MCSs,moist convection and mesoscale vortices still appear in the plains,but the vortex strength and precipitation intensity are significantly weakened.It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.     

A High-Resolution Modeling Study of the 19 June 2002 Convective Initiation Case during IHOP 2002: Localized Forcing by Horizontal Convective Rolls

The initiation processes of one of the initial convective cells near and on the east side of a dryline on 19 June 2002 during the IHOP 2002 field experiment in the central United States is analyzed in detail based on a high-resolution numerical simulation. Prominent horizontal convective rolls and associated near-surface moisture convergence bands [called roll convergence bands(RCBs) here] develop within the convective boundary layer(CBL) due to surface heating, in the hours leading to convective initiation(CI). The RCBs east of the dryline are advected toward the primary dryline convergence boundary(PDCB) by the southerly moist flow as the CBL deepens with time. Backward trajectories of air parcels forming the initial precipitating updraft of the convective cell are found to primarily originate at about 1–1.5 km above ground, within the upper portion of the shallower CBL earlier on. The representative air parcel is found to follow and stay on top of a surface RCB as the RCB moves toward the PDCB, but the RCB forcing alone is not enough to initiate convection. As this RCB gets close to the PDCB, it moves into a zone of mesoscale convergence and a deeper CBL that exhibits an upward moisture bulge associated with the PDCB. The combined upward forcing of the RCB and the mesoscale PDCB convergence quickly lifts the representative air parcel above its level of free convection to initiate convection. A conceptual model summarizing the CI processes is proposed.     

The Forced Secondary Circulation of the Mei-yu Front

Using National Centers for Environmental Prediction reanalysis data for the period 28 June to 12 July during 2001 to2013, the secondary circulation(SC) associated with the mei-yu front was quantitatively diagnosed by numerically solving a primitive version of the Sawyer-Eliassen equation. Results demonstrate that a direct SC exists near the mei-yu front zone during mid-summer and the synoptic-scale geostrophic deformations are the main factors determining SC structures. About94% of the sinking s...     

The Structure of a Typical Mei-yu Front Identified by the Equivalent Temperature

In mainland China, the summer monsoon rainy band is referred to as the mei-yu precipitation front, which extends northward from South China to the Yangtze River, Huaihe River, and Yellow River, depending on the season. This paper describes the structure of the mei-yu front associated with a persistent heavy rainfall event that occurred in the summer of 2007. The mei-yu front occurs when the subtropical oceanic warm, moist air mass and the extra tropical continental dry, cold air mass converge on the lee side of the Tibetan Plateau. The authors defined the equivalent temperature using two terms of dry-air temperature and the specific humidity and calculated its horizontal gradient to indicate the mei-yu front. The vertical structure of the mei-yu front and the moist thermal winds surrounding it were examined based on the equivalent temperature.     

An Analysis of a Meso-β System in a Mei-yu Front Using the Intensive Observation Data During CHeRES 2002

The conventional and intensive observational data of the China Heavy Rain Experiment and Study (CHeRES) are used to specially analyze the heavy rainfall process in the mei-yu front that occurred during 20-21 June 2002, focusing on the meso-β system. A mesoscale convective system (MCS) formed in the warm-moist southwesterly to the south of the shear line over the Dabie Mountains and over the gorge between the Dabie and Jiuhua Mountains. The mei-yu front and shear line provide a favorable synoptic condition for the development of convection. The GPS observation indicates that the precipitable water increased obviously about 2 3 h earlier than the occurrence of rainfall and decreased after that. The abundant moisture transportation by southwesterly wind was favorable to the maintenance of convective instability and the accumulation of convective available potential energy (CAPE). Radar detection reveals that meso-β and -γ systems were very active in the Mα CS. Several convection lines developed during the evolution of the MαCS, and these are associated with surface convergence lines. The boundary outflow of the convection line may have triggered another convection line. The convection line moved with the mesoscale surface convergence line, but the convective cells embedded in the convergence line propagated along the line. On the basis of the analyses of the intensive observation data, a multi-scale conceptual model of heavy rainfall in the mei-yu front for this particular case is proposed.     

Statistical Characteristics of Convective Initiation in the Beijing-Tianjin Region Revealed by Six-Year Radar Data

Characteristics of convective initiation (CI) in the Beijing-Tianjin region during the warm season of2008-2013 are examined. A total of 38877 CI cases are identified by a thunderstorm identification, tracking, analysis, and nowcasting algorithm. CI cases are evaluated in the context of associated terrain, weather systems, and land cover properties. The spatial distribution of all CI cases shows that there are dense CI activities around the 200-m elevation, which means that convective storms are more easily triggered over foothills. From 1500-1800 to 0300-0600 BT (Beijing Time), the high-occurrence CI region tends to propagate southeastward (i.e., from mountains to plains, then to ocean). Among the four local weather systems, the Mongolian cold vortex has the highest CI frequency while the after-trough system has the lowest CI frequency. For the land cover relationships with CI, the urban land cover has the highest CI density and the forest-type land cover has the second highest CI density; these two types of land cover are more conducive to CI formation.     

对流边界层中过山气流的数值模拟

采用ARPS4.0非静力中尺度气象模式模拟了对流边界层中气流过山引起的地形波,讨论了地形及大气条件改变对其的影响.模拟表明,当大气边界层是对流边界层时,气流过山引起的地形强迫,仍能在上部稳定层结中造成足够的垂直扰动,产生向上传播的重力内波,重力内波引起的波动阻力仍不可忽略.     

Numerical Study on the Initiation of the Severe Convective Weather in Chongqing on 6 May 2010

As a follow-up of a previously published article on the synoptic background of the development of the severe convective weather that happened in Chongqing on 6 May 2010, this study further examines the initiation of the severe convective weather via a better high-resolution simulation with the Weather Research and Forecasting (WRF) model. It is found that the cold front approaching Chongqing from the northwest played a critical role in the initiation of the severe convective weather. As the cold front approached Chongqing, the low-to-mid level updrafts ahead of the front acted to increase the atmospheric lapse rate via the stretching effect, which in combination with the low-level diabatic heating induced by the sensible heat fluxes and infrared radiation emitted from the ground surface led to the continuous decrease of the low-level static stability and the increase of the convective available potential energy (CAPE) in Chongqing area. This provided necessary unstable energy for the development of deep moist convection. Furthermore, along with the reaching of a nearly east-west-oriented mesoscale convergence line from the southeast of Chongqing, the outflow right above the cold front began to interact with that above the mesoscale convergence line and induced distinct convergence at the altitude of approximately 1-2 km in the triangular area sandwiched by the cold front and the mesoscale convergence line. It is found that the updrafts associated with this convergence provided lifting necessary for the initiation of the severe convection. The sensitivity experiment without the terrain west of Chongqing indicates that the local topography did not play an important role in the initiation of this severe convective weather.     

修正WRF次网格地形方案及其对风速模拟的影响

复杂地形区域风场模拟的准确率一直是风能研究领域的难点和重点。WRF模式是目前风能评估领域应用最广泛的天气数值模式之一,但该模式在复杂地形区域存在对平原、山谷风速高估且对山顶风速低估的系统性误差,并有研究建立次网格地形方案以订正误差。而次网格地形方案在不同水平分辨率下常出现错误的修正结果,该文基于高精度地形高程数据分析了方案失效的主要原因,发现其方程组中判断山体形态特征的阈值-20在过低和过高水平分辨率下均失去参考性。针对这一原因,将方案中影响关键参数C_t的地形高度算子与模式水平分辨率进行拟合,形成地形高度算子与水平分辨率相依赖的线性关系,获得不同分辨率下更适合的山体形态阈值。通过与自动气象站10 m风速对比分析了修正前后WRF对低层风速的模拟效果,结果显示:修正后的次网格地形方案能够分别在较低和较高分辨率下,部分矫正原方案错误的订正结果,使低层风速模拟更接近实况。修正后的次网格地形方案可为复杂地形区域开展高分辨率风场模拟提供参考。     

Adjoint-based Sensitivity Analysis of a Mesoscale Low on the Mei-yu Front and Its Implicationsfor Adaptive Observation

An adjoint sensitivity analysis of one mesoscale low on the mei-yu Front is presented in this paper.The sensitivity gradient of simulation error dry energy with respect to initial analysis is calculated. And after verifying the ability of a tangent linear and adjoint model to describe small perturbations in the nonlinear model, the sensitivity gradient analysis is implemented in detail. The sensitivity gradient with respect to different physical fields are not uniform in intensity, simulation error is most sensitive to the vapor mixed ratio. The localization and consistency are obvious characters of horizontal distribution of the sensitivity gradient, which is useful for the practical implementation of adaptive observation. The sensitivity region tilts to the northwest with height increasing; the singular vector calculation proves that this tilting characterizes a quick-growing structure, which denotes that using the leading singular vectors to decide the adaptive observation region is proper. When connected with simulation of a mesoscale low on the mei-yu Front, the sensitivity gradient has the following physical characters: the obvious sensitive region is mesoscale, concentrated in the middle-upper troposphere, and locates around the key system; and the sensitivity gradient of different physical fields correlates dynamically.     

渤海湾对流风暴的个例分析

对１９９８年５月２０日渤海湾上空一次向西移动的对流风暴进行个例分析,讨论了其特点及成因。     

华北平原对流风暴的个例分析

应用较详细的卫星云图,多普勒雷达和探空资料对一次强对流风暴进行了综合分析研究,得出了具有一定特色的对流单体组合与垂直结构图象。     

一次大范围强对流天气的中尺度分析

运用１小时间隔的数字红外云图和常规气象资料，分析了１９９２年４月２０－２２日造成我国中南部地区一次较大范围强风雹天气中的中尺度对流系统活动，展示了卫星资料与常规气象资料相结合，中尺度天气分析和临近预报中的潜力。     

Numerical Simulation of Torrential Rainfall and Vortical Hot Towers in a Midlatitude Mesoscale Convective System

A cloud-resolving model simulation of a mesoscale convective system （MCS） producing torrential rainfall is performed with the finest horizontal resolution of 444 m. It is shown that the model reproduces the observed MCS, including its rainfall distribution and amounts, as well as the timing and location of leading rainbands and trailing stratiform clouds. Results show that discrete convective hot towers, shown in Vis5D at a scale of 2-5 kin, are triggered by evaporatively driven cold outflows converging with the high-θe air ahead. Then, they move rearward, with respect to the leading rainbands, to form stratiform clouds. These convective towers generate vortical tubes of opposite signs, with more intense cyclonic vorticity occurring in the leading convergence zone. The results appear to have important implications for the improvement of summertime quantitative precipitation forecasts and the understanding of vortical hot towers, as well midlevel mesoscale convective vortices.     

低纬高原地区一次中尺度对流复合体个例研究

利用地球静止气象卫星(GMS)资料和有限区域暴雨预报模式(HLAFS)分析场,对1999年6月6日低纬高原地区发生的一次中尺度对流复合体(MCC)进行了分析.研究结果表明,MCC发生在副热带高压与滇缅高压之间辐合区中;受两高之间辐合区及云贵高原地形的影响,MCC位于500hPa一个明显的中尺度辐合线上,垂直速度中心强度比其他地区的MCC约大1倍,无辐散层位于400～500hPa之间,北侧准静止锋锋区很强.     

Study on a Mesoscale Convective Vortex Causing Heavy Rainfall during the Mei-yu Season in 2003

The strong heavy rainfall on 3-5 July 2003 causing the severe flooding in Huaihe River basin (HRB), China is studied. It is noted that there are sometimes mesoscale convective vortex (MCV) in East Asia during the mei-yu season. Simulation results from the ARPS (Advanced Regional Prediction) data analysis system (ADAS) and WRF model were used to study the development of the mesoscale convective system (MCS) and mesoscale convective vortex (MCV). It is confirmed that the MCV formed during the development of a...     

Using Satellite Data to Analyze the Initiation and Evolution of Deep Convective Clouds

In this study, two deep convective cloud cases were analyzed in detail to study their initiation and evolution. In both cases, all deep convective clouds were positioned at the rear of the cold front cloud bands and propagated backward. Satellite data showed that prior to initiation of the deep convective clouds, thermodynamic and moist conditions were favorable for their formation. In the morning, a deep convective cloud at the rear of cold front cloud band propagated backward, the outflow boundary of which created favorable conditions for initiation. An additional deep convective cloud cluster moved in from the west and interacted with the outflow boundary to develop a mesoscale convective system（MCS） with large, ellipse-shaped deep convective clouds that brought strong rainfall. The initiation and evolution of these clouds are shown clearly in satellite data and provide significant information for nowcasting and short-term forecasting.     

地形对昆明准静止锋影响的数值模拟研究

利用2008年1月NCEP/NCAR全球1°×1°再分析资料和MM5中尺度数值模拟结果,分析研究了昆明准静止锋的结构和位置.结果表明:在中国南方地区持续、频繁受冷空气影响下,昆明准静止锋的形成和维持是高原地形的阻挡和特定天气系统配置共同作用的结果;通过对地形减半的敏感性数值试验,定量分析得到高原地形阻挡作用是昆明准静止锋形成和维持的必要条件.