Wintertime winds in and around the Ulaanbaatar metropolitan area in the presence of a temperature inversion

Temperature inversions are frequently observed in mountainous urban areas and can cause severe air pollution problems especially in wintertime. This study investigates wintertime winds in and around the Ulaanbaatar, the capital of Mongolia, metropolitan area in the presence of a temperature inversion using the Weather Research and Forecasting (WRF) model coupled with the Seoul National University Urban Canopy Model (SNUUCM). Ulaanbaatar is located in complex terrain and in a nearly east-west-oriented valley. A wintertime scenario with clear skies, weak synoptic winds, and a temperature inversion under the influence of a Siberian high-pressure system is selected. Local winds are weak in the presence of the temperature inversion. In the daytime, weak mountain upslope winds develop, up-valley winds appear to be stronger in the urban area than in the surrounding areas, and channeling winds are produced in the main valley. The bottom of the temperature inversion layer rises up in the urban area, and winds below the bottom of the temperature inversion layer strengthen. In the nighttime, mountain downslope winds and down-valley winds develop. Urban effects in the presence of the temperature inversion are examined by comparing the results of simulations with and without the city. It is shown that in the daytime the urban area acts to elevate the bottom of the temperature inversion layer and weaken the strength of the temperature inversion layer. Winds east of the city weaken in the afternoon and down-valley winds develop later in the simulation with the city.     

Interactions of an urban heat island and sea-breeze circulations during winter over the metropolitan area of São Paulo,Brazil

The Town Energy Budget (TEB) model, a detailed urban parameterisation using a generalised canyon geometry, coupled with the Regional Atmospheric Modelling System (RAMS) is used to simulate the wintertime local circulation in the megacity environment of the metropolitan area of Sao Paulo (MASP) in Brazil. Model simulations are performed using actual topography and land-use fields. Comparison with a simple urban parameterisation based on the LEAF-2 scheme is also shown. Validation is based on comparison between model simulations and observations. Sensitivity tests with TEB reveal an important interaction between the sea breeze and the MASP heat island circulation. Even though topography is known to play an important role in the MASP region’s weather, in these tests the simulations were performed without topography in order to unambiguously identify the interaction between the two local circulations. The urban heat island (UHI) forms a strong convergence zone in the centre of the city and thereby accelerates the sea-breeze front toward the centre of the city. The presence of the urban region increases the sea-breeze front propagation mean speed by about 0.32 m s⁻¹ when compared with the situation of no city. After its arrival in the urban region, the sea-breeze front stalls over the centre of the city for about 2 h. Subsequently, the sea breeze progresses beyond the city when the heat island dissipates. Thereafter, the sea breeze propagates beyond the urban area at a decelerated rate compared to a simulation without an UHI.     

渭河流域中尺度暴雨时空分布及环流特征

制定渭河流域中尺度暴雨天气个例标准,筛选符合标准且资料完整的62个中尺度暴雨历史个例作为研究对象,分别对其时空分布特点、发展移动路径和环流场的结构配置特征进行分析和研究。结果表明,渭河流域中尺度暴雨主要出现在7—8月;空间分布上渭河主干道,黄河、洛河、渭河三河交界处和泾河、洛河、渭河三河交界处出现的次数较多;渭河流域中尺度暴雨的日变化与冰雹完全不同,夜间出现的频率高于白天,共有5条移动路径,由西北向东南方向(或由西向东)移动的个例占所有个例数的91.9%;中尺度暴雨有4种概念模型,干侵入型、西南气流型、干锋生型和暖干型。     

东亚和太平洋上空平均垂直环流----(一)夏季

本文利用中央气象局出版的《北半球高空气候图集》中的风场资料,计算了50°E—130°W,0°—50°N范围内,七月平均850mb—100mb各标准层上的垂直运动,分析了不同区域的经向和纬向垂直环流的特点。指出:夏季130°E以西的低纬度为巨大的西南季风环流区,此外在高原的南北两侧还各存在一较小的经向环流圈,影响着高原附近的天气和气候;160°E以东的中低纬度的海洋上空为经典的Hadley环流所控制,其强度和影响的范围自西向东递增;由于青藏高原地形及夏季的热源作用,使其与西半球,南半球和中东太平洋的天气系统产生遥相关;平均直接从青藏高原上升的气流对西太平洋副高的贡献不大,而平均从西边直接下沉到西太平洋副高的气流是从高原以东的大陆上升的。最后根据计算和分析结果给出了青藏高原及其邻近地区三维空间的垂直环流示意图。     

Variation of regional circulations due to long-term change of land use in the Daegu metropolitan region for 100 years

The urbanization of Daegu metropolitan region has progressed rapidly over the last 100 years, but changing land-use resulting from that urbanization has also caused the regional circulations to evolve. The effect of 100 years of urbanization on the regional circulations was simulated for the analysis using an A2C (Atmosphere to Computational Fluid Dynamics) model. Calm synoptic conditions and a stable lapse rate were assumed for the initial model atmosphere, with a horizontal grid resolution of 1 km. The simulation demonstrated that land-use changes have affected the Daegu metropolitan area’s regional circulations by deepening the convective boundary layer (CBL) over the urbanized area, resulting in deviations from mean surface temperature and surface wind fields. The last 100 years have seen the following changes in the region: 1) the urbanized area has increased by around 15 times, while the number of forests and paddy fields has rapidly decreased; 2) surface wind speeds have decreased by about 25% and 35% during the day and at night, respectively, and severe changes in wind direction have been noted over the urbanized areas; 3) regional warming has been around 1.5°C and 4°C in the daytime and early morning hours, respectively (in particular, warming in the early morning hours suggests the presence of overnight heat islands), and 4) sensible heat flux and CBL height have increased by about 40 W m^?2 and 500 m for the period, respectively.     

Urban climate and clues of heat island events in the metropolitan area of Rio de Janeiro

This paper aims to map the thermal field in the metropolitan region of Rio de Janeiro (MARJ) considering the atmospheric characteristics and the land use that contribute to understanding the urban heat island. Three thermal maps are defined through the use of Landsat5-TM satellite images for three winter events chosen for the decades of 1980, 1990, and 2000, respectively. The results reveal a concentration of warmer cores in urban central areas as well as some local warmer areas in suburban region. Sites with lower temperatures correspond to vegetated areas which are away from the central part of the MARJ, including points of suburban areas. This work emphasizes the importance of the combined analysis of surface temperature with land use and atmospheric conditions, depicting a distinct pattern of heat islands for tropical climate.     

Role of advection and penetrative convection in affecting the mixing-height variations over an idealized metropolitan area

This study deals with the variability of mixing height during daylight hours in the summer months for weak wind regimes. A two-dimensional model was employed using simulated input variables which are quite representative of conditions found over the midwestern United States in late summer and early fall. With the aid of this model and various analytical techniques, the dependence of the urban mixing height on such factors as horizontal advection, downward heat flux across the stable mixing-layer interface, lapse rate in the stable layer, etc., was delineated and compared with actual mixing height variations observed in St. Louis, Missouri during selected days for August, 1972.The experiment indicated the following: (1) A spatially symmetric surface heating profile over a city is accompanied by a similarly symmetric mixing-height profile in the absence of vertical wind shear; (2) When the same heating assumption is invoked and vertically variable wind profiles are introduced, the model-generated mixing-height contours become increasingly asymmetric with vertical wind shear; (3) The modelled mixing heights are more sensitive to temperature fluctuations than to those of wind over the range of speeds studied (wind speeds 4ms^–1); (4) Present operational methods of predicting the time of erosion of an inversion (based upon forecast surface temperature ranges and adiabatic diagram considerations) underestimate breakup time by a factor which is proportional to the amount of available downward heat flux from the stable layer into the mixed layer below.     

Rainfall spatial organization and areal reduction factors in the metropolitan area of Barcelona (Spain)

The rainfall spatial organization in the metropolitan area of Barcelona (Spain) has been studied from records of an urban rain gauge network in the period 1994–2009. Using statistical and regional analysis techniques, correlation between data recorded by the different rain gauges has been calculated, and the effective number of independent stations (n _eq) equivalent to the used network has been determined. It has been found out that for durations longer than 20 min, the areal rainfall return period observed for a storm registered by the network approximately decreases by a factor of 1/n _eq in relation to the current point rainfall intensity–duration–frequency relationships for the metropolitan area of Barcelona. Using objective analysis techniques, continuous precipitation fields have been generated on a regular grid with a spatial resolution of 300?×?300 m for the storms registered by the rain gauges from 1994 to 2009, for durations from 10 min to 24 h. The precipitation fields obtained have been useful to estimate the characteristic areal reduction factors in the metropolitan area of Barcelona. A direct relationship has been found between the areal reduction factor for all the area corresponding to the urban rainfall network of Barcelona and the effective number of n _eq for every duration considered.     

气温变幅较大 南方发生冻害 大部雨雪稀少 局地干旱持续——1999年12月——

1999年 1 2月 ,全国大部地区气温接近常年 ,但气温起伏变化较大 ,南方因受强冷空气袭击发生较大范围冻害 ;全国大部雨雪稀少 ,局地干旱持续或发展。1　大部气温偏高 ,气温变幅较大本月 ,气温起伏变化较大。上半月 ,全国大部地区仍呈偏暖态势 ,中旬后期至下旬初受强冷空气影响 ,全国出现大范围剧烈降温 ,此后气温又有所回升。从各旬平均气温与常年同期相比 ,上旬 ,全国大部地区接近常年或偏高 ,其中华北大部、河套地区大部、东北南部等地偏高 2～ 3℃ ,东北中部和内蒙古东北部偏高 4～ 6℃ ;中旬 ,除东北大部及内蒙古东部偏低 1～ 3℃、部分…     

环太湖地区气候变化特征研究

根据1971—2010年环太湖地区苏州、常州、长兴等9个气象台站日平均气温和日降水量资料,采用EOF正交经验分析法、线性倾向率法、小波分析法和Mann-Kendall检验法研究了环太湖地区近40 a来的气候变化特征。结果表明:1) 1971—2010年间,环太湖地区整体上呈增暖趋势,环太湖地区的季节性增暖存在空间差异,西北部的气温在春、夏季明显升高,而东南部则在秋、冬季明显增暖,1990年前后该地区的增暖率存在完全相反的空间分布。2)该40 a中,降水表现为北部增加,南部减少。整个环太湖地区的降水在冬季普遍呈现增加趋势,春、夏季降水的空间分布差异性大于秋、冬季。3) M orlet小波分析结果表明,环太湖地区年平均温度存在16～17 a和6 a、26 a左右的变化周期;年降水量存在15～16 a和24 a的强显著性变化周期,各地区在年均温、年降水量周期振荡的强度上存在一定的差异。4) Mann-Kendall突变检验显示,1971—2010年环太湖地区各站点均表现为气温由低向高的突变,突变发生在1992—1993年。     

Diagnostic analysis of future climate scenarios applied to urban flooding in the Chicago metropolitan area

Past heavy precipitation events in the Chicago metropolitan area have caused significant flood-related economic and environmental damages. A key component in flood management policies and actions is determining flood magnitudes for specified return periods. This is a particularly difficult task in areas with a complex and changing climate and land-use, such as the Chicago metropolitan area. The standard design storm methodology based on the NOAA Atlas 14 and ISWS Bulletin 70 has been used in the past to estimate flood hydrographs with variable return periods in this region. In a changing climate, however, these publications may not be accurate. This study presents and illustrates a methodology for diagnostic analysis of future climate scenarios in the framework of urban flooding, and assesses the corresponding uncertainties. First, the design storms are calculated using data downscaled by a regional climate model (RCM) at 30-km spacing for the present and 2050s under the IPCC A1Fi (high) and B1 (low) emissions scenarios. Next, the corresponding flood discharges at six watersheds in suburban Chicago are estimated using a hydrologic event model. The resulting scenarios in flood frequency were first assessed through a set of diagnostic tests for precipitation timing and frequency. The study did not reveal any significant changes in the 2050s in the average timing of heavy storms, but their regularity decreased. The average timing did not exhibit any significant spatial variability throughout the region. The precipitation frequency analysis revealed distinct differences between the northern and southeastern subregions of the Chicago metropolitan area. The quantiles in the northern subregion averaged for 2-year, 5-year, and 10-year return periods exhibited a 20% and 16% increase in daily precipitation for scenarios B1 and A1Fi, respectively. The southeastern subregion, however, exhibited a decrease of 12% for scenario B1 and a minor increase of 3% for scenario A1Fi. The hydrologic effects of changing precipitation on the flood quantiles were illustrated using six small watersheds in the region. The relative increases or decreases in precipitation translated into even larger relative increases or decreases in flood peaks, due to the nonlinear nature of the rainfall-runoff process. Simulations using multiple climate models, for longer periods, finer spatio-temporal resolution, and larger areal coverage could be used to more accurately account for numerous uncertainties in the precipitation and flood projections.     

Effects of atmospheric circulation and boundary layer structure on the dispersion of suspended particulates in the Seoul metropolitan area

Summary A three-dimensional non-hydrostatic numerical model and lagrangian particle model (random walk model) were used to investigate the effects of the atmospheric circulation and boundary layer structure on the dispersion of suspended particulates in the Seoul metropolitan area. Initially, emitted particulate matter rises from the surface of the city towards the top of the convective boundary layer (CBL), owing to daytime thermal heating of the surface and the combined effect of an onshore wind with a westerly synoptic-scale wind. A reinforcing sea-valley breeze directed from the coast toward the city of Seoul, which is enclosed in a basin and bordered by mountains to its east, disperses the suspended particulate matter toward the eastern mountains. Total suspended particulate concentration (TSP) at ground level in the city is very low and relatively high in the mountains. Radiative cooling of the surface produces a shallow nocturnal surface inversion layer (NSIL) and the suspended particulate matter still present near the top of the CBL from the previous day, sinks to the surface. An easterly downslope mountain wind is directed into the metropolitan area, transporting particulate matter towards the city, thereby recycling the pollutants. The particulates descending from the top of the NSIL and mountains, combine with particulates emitted near the surface over the city at night, and under the shallow NSIL spread out, resulting in a maximum ground level concentration of TSP in the metropolitan area at 2300 LST. As those particles move toward the Yellow Sea through the topographically shaped outlet west of Seoul city under the influence of the easterly land breeze, the maximum TSP concentration occurs at the coastal site. During the following morning, onshore winds resulting from a combined synoptic-scale westerly wind and westerly sea breeze, force particulates dispersed the previous night to move over the adjacent sea and back over the inland metropolitan area. The recycled particulates combine with the particulates emitted from the surface in the central part of the metropolitan area, producing a high TSP and again rise towards the top of the CBL ready to repeat the cycle.     

秦岭山区近50 年降水差异及可能局地成因探讨

利用秦岭山区54个气象站50年以上日降水资料,分析了复杂地形下小区域降水变化差异,探讨了可能的局地成因,结果表明:(1)年均降水量总体为南高北低、西高东低,反映出秦岭阻挡作用和山谷东风回流影响;(2)近50年来区域年降水以减为主,9个增加站位于东部,夏季降水以增加为主,减少站集中在秦岭山上和区域西部,降水向夏季集中倾向明显;(3)日雨量小于5 mm的年雨量和雨日数减少趋势明显,雨量≥50 mm的年雨量和雨日数增加趋势明显,即小雨减少大雨增多;(4)降水变化的多个方面及其与海拔高度和经度对应关系的分析结果,反映出气溶胶抑制地形降水以及成冰作用恢复被抑制降水的作用,说明局地成因中气溶胶起了不可忽视作用;(5)地形作用和区域能量平衡也是重要局地成因.     

Multifractal analysis of the rainfall time distribution on the metropolitan area of Barcelona (Spain)

In most of the studies on scale properties in the rainfall process, multifractal behavior has been investigated without taking into account the different rain generation mechanisms involved. However, it is known that rain processes are related to certain scales, determined by climatological characteristics as well as regional and local meteorological features. One of the implications derived from these correspondences is the possibility that the multifractal parameters of the rainfall could depend on the dominant precipitation generation mechanism. Fractal analysis techniques have been applied in this work to rainfall data recorded in the metropolitan area of Barcelona in the period 1994–2001, as well as to a selection of synoptic rainfall events registered in the same city in the period 1927–1992. The multifractal parameters obtained have been significantly different in each case probably showing the influence of the rain generation mechanisms involved. This influence has been revealed also in the analysis of the effects of seasonality on the multifractal behavior of rainfall in Barcelona.     

Determination of O3-, CO- and PM10-transport in the metropolitan area of São Paulo, Brazil through synoptic-scale analysis of back trajectories

Summary This study is aimed to qualitatively analyze the impact of remote sources on air pollution in the Metropolitan Area of S?o Paulo (MASP). Air-mass back trajectories from June to August of 1999 were calculated using a three-dimensional kinematic trajectory model and grouped into trajectory clusters. Correlations of individual trajectory clusters with O₃, CO and PM₁₀ concentrations were determined. In this model, trajectories were obtained using the means of the three wind velocity components (U, V and W). The three-dimensional wind field was derived from the Regional Atmospheric Modeling System, and downscaling was employed. Coarse and fine nested grids (64-km and 16-km horizontal resolution, respectively) were used. Every 12 h (at 00 and 12 UTC), a back-trajectory ensemble, using the 64-km grid, was calculated for five defined endpoints at intervals of 0.5° N, S, E and W of the MASP (λ = 23° 33′S, ϕ = 46° 45′W), that last endpoint being centered in the MASP. To analyze cluster trajectories, the five trajectory ensembles from each day were allocated into one of four clusters (northeast, southeast, southwest or northwest quadrant) based on the origin of the trajectory over 4 days. Days on which all five trajectories originated from the same quadrant were classified as “core” days. Core day concentrations of CO, O₃ and PM₁₀ during the study period were evaluated. The results show that, during the study period, air-mass back trajectories in the MASP originated from all four quadrants: northeast (32%), southeast (12%), southwest (19%) and northwest (37%). Our analysis of back-trajectory clusters in the MASP suggests a transport to ambient air of O₃ precursors and O₃ from the northeast region, which is associated with agricultural activities involving biomass burning.     

Changes in the Walker and Hadley circulations associated with the Southern Annular Mode

Extratropical impacts on tropical climates are one of the most exciting areas of meteorological investigation in recent times. The present study elucidates the seasonal impact of the Southern Annular Mode (SAM) on predominant tropical circulations such as Hadley and Walker. The velocity potential at 200 hPa is used to understand the spatio-temporal variability in tropical circulations in the boreal summer and winter seasons. The results show an intensification of seasonal velocity potential in the composite of the low phase of the SAM. The seasonal climatological values of velocity potential observed for the period from 1979 to 2012 are of lesser magnitudes than earlier findings. The convergence/divergence locations of tropical circulation have shifted from their mean positions in the alternative phase of the SAM. The low-level convergence in the southern hemispheric Hadley circulation (HC) is enhanced in the composite of the positive phase of the SAM; however, the SAM’s effect on the HC is no stronger in the summer. Another interesting feature noted in the present study is the weakening of the Walker circulation associated with the positive phase of the SAM, which can influence the basic state of the tropical Pacific Ocean. The SAM’s interannual variability exhibits a significant positive trend in winter. The study reveals that the positive phase of the SAM could be a possible explanation for the recent changes in the tropical circulation patterns; however, the variability in tropical circulation anomalies associated with the SAM should be noted on seasonal and monthly scales to understand the dynamical mechanism behind the relationship. The impact of the SAM on tropical circulation may continue in future decades, as this southern extratropical vacillation is predicted to remain in a positive phase due to the increase in greenhouse gases and the variability in ozone.     

Simulation of seasonal circulations and thermohaline variabilities in the Gulf of Thailand

Based on the Princeton Ocean Model （POM）, the seasonal thermohaline feature and the ocean circulation in the Gulf of Thailand （GOT）, situated between 6°N to 14°N latitude and 99°E to 105°E longitude, were studied numerically with 37 × 97 orthogonal curvilinear grid and 10 vertical sigma levels conforming to a realistic bottom topography. A spin-up phase of the first model run was executed using wind stress calculated from climatological monthly mean wind, restoring-type surface heat and salt, and climatological monthly mean fresh water flux data. In this paper, the temperature and salinity fields taken from Levitus94 data sets and the calculated temperature and salinity from the model run for 12-month mean and for each season are presented where the winter, summer, rainy, and end of the rainy seasons of Thailand are represented by the months January, April, July, and October, respectively. The simulated circulations are also described. The results show that the temperature in the GoT is warmer than the temperature of the other parts connected to the South China Sea （SCS）. At any depth of inflow from SCS into the GoT, the salinity is high, but in the outflow from the GoT at the surface, the salinity is low. The strong circulations are clockwise during summer and the rainy seasons of Thailand, which are the East Asian monsoon periods, northeasterly and southwesterly during summer. They occur near Pattani and Narathiwat provinces during summer and in the central GoT during the rainy seasons. Sensitivity experiments were designed to investigate the effects of wind forcing and open boundary conditions. Wind forcing is shown to be the important factor for generating the circulation in the GoT. The lateral velocity at the open boundaries is of considerable importance to current circulation for the rainy and end of the rainy seasons, with insignificant effect for the winter and summer seasons of Thailand.