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1.
近30 a江苏夏季降水日变化的气候学特征   总被引：2，自引：1，他引：1  
基于1980—2013年江苏省61站小时降水资料,分析了江苏省夏季降水日变化的特点及小时极端降水、不同级别雨日的日变化特征。结果表明,江苏省夏季降水日变化具有显著的双峰分布特征,然而江苏省北部和南部降水的主峰时段并不一致。从降水频次、累积降水量来看,江苏省北部降水以清晨至早上时段为主峰、午后至傍晚时段为次峰,南部降水与之相反。长持续性降水占夏季降水的2/3左右,且江苏北部占比多于南部,均为清晨至早上的单峰分布;短持续性降水占夏季降水的1/3,在江苏北部呈现出以午后至傍晚为主峰,清晨至早上为次峰的双峰分布,而在江苏南部呈现出以午后至傍晚的单峰分布特点。小时极端降水,阈值分布南低北高,虽然频次较少,但占夏季降水的40%左右。小时极端降水日变化的双峰分布和夏季总体降水分布类似,但主峰大都出现在午后至傍晚。不同级别雨日的日变化分布各有不同,但全省各区无显著差异。累积降水量贡献主要来自于暴雨和大雨。暴雨无论是从降水频次、累积降水量还是降水强度都呈现清晨至早上的单峰分布。  相似文献   

2.
Based on continuous observations of the planetary boundary layer (PBL) with anL-band (1357.5 MHz) boundary-layer radar (BLR) at a hilly location in Japan, we have discovered that on clear days, a thin enhanced echo layer corresponding to the top of the PBL (or mixed layer) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward velocities were observed below the echo layer (or inside the PBL), reaching 1500 m in the afternoon.  相似文献   

3.
In general, the overall differences in activity and timing of convection are a result of the influence of large-scale regional and synoptic flow patterns on the local mesoscale environment. The linkage between the space?Ctime variability of observed clouds and rainfall, with large-scale circulation patterns and mesoscale variables over north India during the pre-monsoon season (March to May) is the focus of this paper. We use harmonic analysis to identify the first hour of rainfall for 42 stations spread over the north Indian region during the pre-monsoon summer season (March to May), from 1980 to 2000. The variability is observed to be systematic, with large regions having similar timing for occurrence of rainfall. The stations located in the foothills of the Himalayas have a late night to early morning maximum of first hour rainfall. In the northwestern plains, the first hour of rainfall mostly starts in the early afternoon to evening hours. Further eastward, the rainfall occurs in the late evening hours. Overall, there is a gradient in the occurrence of first rainfall events from late afternoon hours in the southern sections of the north Indian region to nocturnal maxima in the higher altitude regions. Five of these stations, located in different regions of homogenous timing of rainfall occurrence, were selected to analyze in detail the variable trigger for convection. Our results indicate that convective episodes occur mostly in association with the passage of westerly troughs over this region. These upper atmosphere troughs enable moisture to flow from the surrounding oceanic regions to the dry inland regions and also provide some dynamic support to the episodes of convection. However, the actual occurrence of convection is triggered by local factors, giving rise to the mesoscale structure of the weather systems during this season. Specifically, over the plains of northwest India, convection is triggered in a moistened environment by diurnal solar heating. The late night to early morning convection over the foothills is triggered by the orography, when the moistened airflow is normally incident on the mountain slopes. Further eastward, the primary trigger for localized moist convection is downdrafts from south-eastward propagating convective systems that originate at a north?Csouth dry line over north India. These systems propagate with a speed of about 15?m?s?1. The above results are supported by geostationary satellite brightness temperature data for March to May 2008.  相似文献   

4.
江苏南部汛期降水日变化特征分析   总被引：2，自引：1，他引：1  
利用江苏南部20个气象观测站2008—2012年汛期(5—10月)逐小时降水资料,应用降水频率来分析了江苏南部地区降水日变化基本特征和区域差异。研究表明:降水日变化特征地域性差异较强,西部站、东部站和东北沿海站都存在一定的特征差异。东部站降水量的最大值主要出现在下午和傍晚;西部站降水量主峰值出现在下午,并且在清晨和夜间还有两个次峰值;东北沿海站呈现出午前、午后的双峰值形式。2008—2011年降水量下午高值区有先减弱后增强并提前的趋势,而上午的高值区有总体减弱并推迟的特征。2011年后有明显减弱的趋势。江苏南部总体来说,短时强降水(大于20和25 mm/h)在16—19时出现主峰值,07—09时也有相对较小的次峰值。  相似文献   

5.
中国大陆日降水峰值时间位相的区域特征分析   总被引：5，自引：0，他引：5       下载免费PDF全文
利用高密度的中国国家级地面气象站逐时降水数据,系统分析和比较了中国大陆地区暖季降水量、降水频次和降水强度的日变化峰值位相的整体特征、空间分布差异及典型区域平均的日变化演变特征。研究指出,中国大陆暖季降水日变化峰值时间主要表现为下午、清晨、夜间3类典型位相,且整体而言降水频次的清晨峰值更凸出,降水强度以下午峰值为主。综合考虑降水量和降水频次的日变化峰值位相,发现中国大陆地区降水日变化峰值位相在空间分布上存在7个典型区域:下午峰值区(东北至华北山区、东南内陆地区)、夜间峰值区(四川盆地西部至云贵高原东部、华北平原西部贴近山地的区域)和清晨峰值区(华北平原东部、秦巴山区至华中西南部)各两个,以及傍晚至夜间峰值位相的青藏高原区。各典型区域内部具有较一致的降水量和频次的日峰值时间位相,而区域边缘或交界处降水量和频次的峰值位相则相反,主要是降水量的下午主峰值时段与降水频次的清晨主峰值时段的错位。从降水量、降水频次和降水强度的日变化的演变特征来看,午后峰值区、夜间峰值区和青藏高原的傍晚至夜间峰值区的多数台站,都存在降水量位相滞后于降水强度而超前于降水频次的特征,这应是降水演变过程中时间演变不对称性和对流云系发展演变的具体表现。  相似文献   

6.
长江下游梅汛期中尺度涡旋特征分析   总被引：6，自引：3，他引：3  
利用2006～2009 年日本再分析资料对长江下游地区梅汛期间(5～7 月)边界层内中尺度涡旋进行普查,并分类统计分析了边界层内中尺度涡旋与暴雨、低空急流的关系。研究结果表明:每年的5～7 月该地区经常在对流层低层或(和)边界层内出现中尺度扰动涡旋,根据中尺度涡旋最初生成的高度不同,可划分为边界层中尺度涡旋、对流层低层中尺度涡旋和对流层低层—边界层中尺度涡旋三类。边界层中尺度涡旋中与暴雨有密切关系的中尺度涡旋称为边界层中尺度扰动涡旋(PMDV),根据涡旋前或后6 小时累积雨量,可以进一步将其分成两类:第一类是暴雨的直接制造者中尺度对流系统(MCS)先于边界层中尺度扰动涡旋发生(MCS-PMDV);第二类是边界层中尺度涡旋产生后,激发了中尺度对流,造成了暴雨过程(PMDV-MCS)。PMDV-MCS 类涡旋暴雨的特点是在对流层低层850 hPa 是一条切变线,其南侧有一支西南低空急流,边界层925 hPa 则是一个闭合的涡旋,暴雨区主要落在涡旋的东北面和东南面。  相似文献   

7.
彭莉莉  邓剑波  谢傲 《湖北气象》2020,39(2):201-206
利用南岳山南坡不同海拔高度上的3个气象观测站2015年9月1日-2018年8月31日逐时降水资料,分析了南岳山降水日演变特征。结果表明:从山底到山顶总降水量逐渐增加,存在3个降水峰值时段,分别在清晨、午后和傍晚,清晨雨量峰值主要由该时段降水频次较高所致,午后与傍晚雨量峰值主要与该时段降水强度较大有关,山顶高山站与山底站降水量差异主要体现在午后与傍晚时段;小时最大降水量主要出现在午后至傍晚,山底站短时强降水出现时段较分散,山腰和山顶高山站短时强降水主要集中在午后至傍晚时段;持续时间小于等于6 h的短持续降水频次多于持续时间大于6 h长持续降水频次,其主要出现在午后至傍晚,长持续降水过程多出现在凌晨至中午,其对总降水量的贡献大于短持续降水。  相似文献   

8.
The influence of roof-edge roughness elements on airflow, heat transfer, and street-level pollutant transport inside and above a two-dimensional urban canyon is analyzed using an urban energy balance model coupled to a large-eddy simulation model. Simulations are performed for cold (early morning) and hot (mid afternoon) periods during the hottest month of the year (August) for the climate of Abu Dhabi, United Arab Emirates. The analysis suggests that early in the morning, and when the tallest roughness elements are implemented, the temperature above the street level increases on average by 0.5 K, while the pollutant concentration decreases by 2% of the street-level concentration. For the same conditions in mid afternoon, the temperature decreases conservatively by 1 K, while the pollutant concentration increases by 7% of the street-level concentration. As a passive or active architectural solution, the roof roughness element shows promise for improving thermal comfort and air quality in the canyon for specific times, but this should be further verified experimentally. The results also warrant a closer look at the effects of mid-range roughness elements in the urban morphology on atmospheric dynamics so as to improve parametrizations in mesoscale modelling.  相似文献   

9.
Diurnal variations of two mountain-plain solenoid （MPS） circulations associated with ＂first-step＂ terrain [Tibetan Plateau （TP）] and ＂second-step＂ terrain （high mountains between the TP and ＂east plains＂） in China and their influence on the south west vortex （SWV） and the mei-yu front vortex （MYFV） were investigated via a semi-idealized mesoscale numerical model [Weather Research and Forecasting （WRF）] simulation integrated with ten-day average fields （mei-yu period of 1-10 July 2007）. The simulations successfully reproduced two MPS circulations related to first and second-step terrain, diurnal vari- ations from the eastern edge of the TP to the Yangtze River-Huaihe River valleys （YHRV）, and two precipitation maximum centers related to the SWV, MYFV. Analyses of the averaged final seven-day simulation showed the different diurnal peaks of precipitation at different regions： from the aftemoon to early evening at the eastern edge of the TP; in the early evening to the next early morning in the Sichuan Basin （SCB）; and in the late evening to the next early morning over the mei-yu front （MYF）. Analyses of individual two-day cases confirmed that the upward branches of the nightlime MPS circulations enhanced the precipitation over the SWV and the MYFV and revealed that the eastward extension of the SWV and its con vection were conducive to triggering the MYFVs. The eastward propagation of a rainfall streak from the eastern edge of the TP to the eastern coastal region was primarily due to a series of convective activities of several systems from west to east, including the MPS between the TP and SCB, the SWV, the MPS between second-step terrain and tile east plains, and the MYFV.  相似文献   

10.
The Growth of the Planetary Boundary Layer at a Coastal Site: a Case Study   总被引：2，自引：1，他引：1  
A lidar system is used to determine the diurnal evolution of the planetary boundary layer (PBL) height on a summer day characterized by anticyclonic conditions. The site is located some 15 km distant from the sea, on a peninsula in south-east Italy. Contrary to expectations, the PBL height, after an initial growth consequent to sunrise, ceases to increase about 2 h before noon and then decreases and stabilizes in the afternoon. An interpretation of such anomalous behaviour is provided in terms of trajectories of air parcels towards the lidar site, which are influenced by the sea breeze, leading to a transition from a continental boundary layer to a coastal internal boundary layer. The results are analyzed using mesoscale numerical model simulations and a simple model that allows for a more direct interpretation of experimental results.  相似文献   

11.
Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.  相似文献   

12.
Precipitation measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite indicate that the southeastern area of Kalimantan (Borneo) Island receives much less rainfall than elsewhere on the island during the period from July to October.Results from sur-face meteorological observations show that the diurnal cycle of rainfall differs greatly between the eastern and western coasts of the island.Rainfall on the western coast of the island is frequent in the afternoon and evening,whereas almost all rainfall on the eastern coast occurs in the morning.Meanwhile,the Global Positioning System (GPS)-derived precipitable water (PW) on the eastern coast shows a substantial decrease in moisture in the af-ternoon and evening.Numerical experiments with a mesoscale model reveal that gravity waves driven by di-urnal heating of the elevated land surface of the moun-tains on Sulawesi Island,which lies approximately 300 kilometers to the east of Kalimantan Island,significantly affect the diurnal cycle of rainfall over southeast Kali-mantan Island.  相似文献   

13.
Observations and numerical modelling of Lake Ontario breezes   总被引：1，自引：0，他引：1  
Abstract

Analysis of two years of land‐based data shows that the Lake Ontario breeze develops on 30% of the days during summer. It typically develops in mid‐morning and persists until the late evening when it is replaced by a well developed land‐breeze regime. Simulations of 4 cases with the Colorado State University mesoscale model show good agreement with observations and suggest that local lake breezes are strongly influenced by adjacent water bodies (e.g. Lake Erie), the elongated shape of the lake, and the large‐scale wind direction. With gradient flows across the long axis of the lake, a broad band of along‐lake flow develops during the afternoon (easterly winds during southerly gradient flows and westerly winds during northerly gradient flows). Furthermore, during west‐to‐northwesterly gradient flow a nocturnal cyclonic eddy is predicted at the western end of the lake. These results imply that wind‐field models applied in the vicinity of Lake Ontario should incorporate the entire lake in their modelling domain.  相似文献   

14.
A mei-yu front process in the lower reaches of the Yangtze River on 23 June 1999 was simulated by using the fifth-generation Pennsylvania State University-NCAR （PSU/NCAR） Mesoscale Model （MM5） with FDDA （Four Dimension Data Assimilation）. The analysis shows that seven weak small mesoscale vortexes of tens of kilometers, correspondent to surface low trough or mesoscale centers, in the planetary boundary layer （PBL） in the mei-yu front were heavily responsible for the heavy rainfall. Sometimes, several weak small-scale vortexes in the PBL could form a vortex group, some of which would weaken locally, and some would develop to be a meso-α-scale low vortex through combination. The initial dynamical triggering mechanism was related to two strong currents： one was the northeast flow in the PBL at the rear of the mei-yu front, the vortexes occurred exactly at the side of the northeast flow; and the other was the strong southwest low-level jet （LLJ） in front of the Mei-yu front, which moved to the upper of the vortexes. Consequently, there were notable horizontal and vertical wind shears to form positive vorticity in the center of the southwest LLJ. The development of mesoscale convergence in the PBL and divergence above, as well as the vertical positive vorticity column, were related to the small wind column above the nose-shaped velocity contours of the northeast flow embedding southwestward in the PBL, which intensified the horizontal wind shear and the positive vorticity column above the vortexes, baroclinicity and instability.  相似文献   

15.
何晓凤  蒋维楣  刘红年 《大气科学》2008,32(6):1445-1457
用南京大学区域边界层模式NJU-RBLM, 通过对一组理想试验的模拟, 研究了TEB方案 (town energy balance) 和SVAT方案 (soil－vegetation－atmosphere transfer) 模拟城市热岛现象的差异及本质原因, 发现TEB方案对城市热岛 (UHI) 尤其是夜间UHI模拟效果更优, 这是由于TEB方案具备较强模拟城市储热项的能力形成的。此外, 深入探讨UHI对大气边界层热力结构的影响, 发现UHI现象使城市和郊区的近地层位温廓线在清晨和傍晚都存在明显差异, 同时使城市区域气温全天高于郊区, 且日间城乡温差能达到的高度明显高于夜间。分析人为热源和建筑物冠层对UHI的影响时发现: 人为热源对UHI的影响在夜间强于白天, 而建筑物对白天城市湍能的影响强于人为热源的作用。  相似文献   

16.
A novel approach for upscaling land-surface parameters based on inverse stochastic surface-vegetation-atmosphere transfer (SVAT) modelling is presented. It allows estimation of effective parameters that yield scale invariant outputs e.g. for sensible and latent heat fluxes and evaporative fraction. The general methodology is used to estimate effective parameters for the Oregon State University Land-Surface Model, including surface albedo, surface emissivity, roughness length, minimum stomatal resistance, leaf area index, vapour pressure deficit factor, solar insolation factor and the Clapp–Hornberger soil parameter. Upscaling laws were developed that map the mean and standard deviation of the distributed land-surface parameters at the subgrid scale to their corresponding effective parameter at the grid scale. Both linear and bi-parabolic upscaling laws were obtained for the roughness length. The bi-parabolic upscaling law fitted best for the remaining land-surface parameters, except surface albedo and emissivity, which were best fitted with linear upscaling laws.  相似文献   

17.
The effects of uncertainty in the specification of surface characteristics on simulated atmospheric boundary layer (ABL) processes and structure were investigated using a one-dimensional soil-vegetation-boundary layer model. Observational data from the First International Satellite Land Surface Climatology Project Field Experiment were selected to quantify prediction errors in simulated boundary-layer parameters. Several numerical 12-hour simulations were performed to simulate the convective boundary-layer structure, starting at 0700 LT 6 June 1987.In the control simulation, measured surface parameters and atmospheric data were used to simulate observed boundary-layer processes. In the remaining simulations, five surface parameters – soil texture, initial soil moisture, minimum stomatal resistance, leaf area index, and vegetation cover – were varied systematically to study how uncertainty in the specification of these surface parameters affects simulated boundary-layer processes.The simulated uncertainty in the specification of these five surface parameters resulted in a wide range of errors in the prediction of turbulent fluxes, mean thermodynamic structure, and the depth of the ABL. Under certain conditions uncertainty in the specifications of soil texture and minimum stomatal resistance had the greatest influence on the boundary-layer structure. A lesser but still moderately strong effect on the simulated ABL resulted from (1) a small decrease (4%) in the observed initial soil moisture (although a large increase [40%] had only a marginal effect), and (2) a large reduction (66%) in the observed vegetation cover. High uncertainty in the specification of leaf area index had only a marginal impact on the simulated ABL. It was also found that the variations in these five surface parameters had a negligible effect on the simulated horizontal wind fields. On the other hand, these variations had a significant effect on the vertical distribution of turbulent heat fluxes, and on the predicted maximum boundary-layer depth, which varied from about 1400–2300 m across the 11 simulations. Thus, uncertainties in the specification of surface parameters can significantly affect the simulated boundary-layer structure in terms of meteorological and air quality model predictions.  相似文献   

18.
An analytical framework is proposed for studying variations in the diurnal wind structure in the planetary boundary layer (PBL) and the evolution of the low-level nocturnal jet. A time-dependent eddy-diffusivity coefficient corresponding to solar input is proposed, and an appropriate coordinate transformation ensures that mixing height varies continuously with ground heat-flux changes. The solution exhibits the receding character of the daytime PBL as evening approaches, thereby dividing the PBL into two regimes — the one just above the ground, representing the nocturnal boundary layer, and the region above it. It is assumed that inertial oscillations (IO) are triggered in the upper layer at about the time of sunset when the reversal in the direction of ground heat flux is felt in the upper layer. Two approaches are adopted to determine the characteristic features of IO and the evolution of the nocturnal low-level jet. The first one is based on the physical principle that release of horizontal momentum due to deviation from the geostrophic wind gives rise to the IO. The solution captures all the characteristic features of the IO, such as phase shift and decreasing amplitude of the IO with increasing height. According to this analysis the IO is triggered at a level as soon as the top of the receding boundary layer leaves that level. The solution is discontinuous with respect to the vertical coordinate. In the second approach we solve an initial-value problem to determine the solution in the upper layer, assuming that at about the time of sunset there is a rapid collapse of the daytime PBL to the steady, nocturnal boundary layer. The assumption is based on the mixing-height profiles prepared from climatological data collected at Delhi. The solution for the nocturnal boundary-layer regime is then obtained as a boundary-value problem. The solutions so obtained are continuous throughout the domain of interest and exhibit the characteristic features of an IO. The analysis leads to the conditions under which a low-level nocturnal jet is produced and provides quantitative estimates of the parameters, such as length of night, latitude, mixing height at sunset and nocturnal mixing height, that are conducive to the generation of a jet. The nocturnal wind profile produced by this approach compares well both with typical atmospheric data observed at Delhi and with output from a mesoscale numerical model. There is still some uncertainty related to the time of initiation of the IO as a function of latitude.  相似文献   

19.
A comparison of two separate MM5 land-use datasets (i.e., ‘US Geological Survey (USGS)’ and ‘Pollutants in the Atmosphere and their Transport over Hong Kong (PATH)’, each with different parameter values and different spatial distributions) was performed to understand the importance of land-surface processes and land-atmosphere interactions in the evolution of mesoscale weather phenomena during a high pollution episode in Hong Kong from 28 December 1999 through 1 January 2000. Also, a series of high resolution mesoscale numerical experiments was performed to investigate the possible roles of various surface characteristics or land-use parameters in this high pollution episode. Specifically, the relative importance of six land-use parameters including the roughness length, thermal inertia, soil moisture availability, albedo, surface heat capacity and surface emissivity are studied. Results from this study suggest that the soil moisture availability is the most important controlling parameter on the flow pattern and on surface fluxes. Sensitivity tests also show that the general flow pattern is insensitive to the other five land-use parameters  相似文献   

20.
This study investigates diurnal variations of precipitation during May–August, 1998–2012, over the steep slopes of the Himalayas and adjacent regions(flat Gangetic Plains–FGP, foothills of the Himalayas–FHH, the steep slope of the southern Himalayas–SSSH, and the Himalayas-Tibetan Plateau tableland–HTPT). Diurnal variations are analyzed at the pixel level utilizing collocated TRMM precipitation radar and visible infrared data. The results indicate that rain parameters(including rain frequency, rain rate, and storm top altitude) are predominantly characterized by afternoon maxima and morning minima at HTPT and FGP, whereas, maximum rain parameters at FHH typically occur in the early morning. Rain parameters at SSSH are characterized by double peaks;one in the afternoon and one at midnight. Over HTPT and FGP,convective activity is strongest in the afternoon with the thickest crystallization layer. Over FHH, the vertical structure of precipitation develops most vigorously in the early morning when the most intense collision and growth of precipitation particles occurs. Over SSSH, moist convection is stronger in the afternoon and at midnight with strong mixing of ice and water particles. The results of harmonic analysis show that rain bands move southward from lower elevation of SSSH to FHH with apparent southward propagation of the harmonic phase from midnight to early morning. Moreover, the strongest diurnal harmonic is located at HTPT, having a diurnal harmonic percentage variance of up to 90%. Large-scale atmospheric circulation patterns exhibit obvious diurnal variability and correspond well to the distribution of precipitation.  相似文献   

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