The Adaptation of the Atmospheric Boundary Layer to a Change in Surface Roughness

The adaptation of the atmospheric boundary layer to a change in the underlying surface roughness is an interesting problem and hence much research, theoretical, experimental, and numerical, has been undertaken. Within the atmospheric boundary layer an accurate numerical model for the turbulent properties of the atmospheric boundary layer needs to be implemented if physically realistic results are to be obtained. Here, the adaptation of the atmospheric boundary layer to a change in surface roughness is investigated using a first-order turbulence closure model, a one-and-a-half-order turbulence closure model and a second-order turbulence closure model. Perturbations to the geostrophic wind and the pressure gradients are included and it is shown that the second-order turbulence closure model, namely the standard k - model, is inferior to a lower-order closure model if a modification to limit the turbulent eddy size within the atmospheric boundary layer is not included within the model.     

Turbulent Statistics of Neutrally Stratified Flow Within and Above a Sparse Forest from Large-Eddy Simulation and Field Observations

Turbulent statistics of neutrally stratified shear-driven flow within and above a sparse forest canopy are presented from a large-eddy simulation (LES) and compared with those from observations within and above a deciduous forest with similar height and foliage density. First- and second-order moments from the LES agree with observations quite well. Third-order moments from the LES have the same sign and similar vertical patterns as those from the observations, but the LES yields smaller magnitudes of such higher-order moments. Turbulent spectra and cospectra from the LES agree well with observations above the forest. However, at the highest frequencies, the LES spectra have steeper slopes than observations. Quadrant and conditional analyses of the LES resolved-scale flow fields also agree with observations. For example, both LES and observation find that sweeps are more important than ejections for the transport of momentum within the forest, while inward and outward interaction contributions are both small, except near the forest floor. The intermittency of the transport of momentum and scalar increases with depth into the forest. Finally, ramp structures in the time series of a passive scalar at multiple levels within and above the forest show similar features to those measured from field towers. Two-dimensional (height-time cross-section) contours of the passive scalar and wind vectors show sweeps and ejections, and the characteristics of the static pressure perturbation near the ground resemble those deduced from field tower-based measurements. In spite of the limited grid resolution (2 m × 2 m × 2 m) and domain size (192 m × 192 m × 60 m) used in this LES, we demonstrate that the LES is capable of resolving the most important characteristics of the turbulent flow within and above a forest canopy.     

边界层和陆面过程对中国暴雨影响研究的进展

总结了近年来我国学者关于边界层和陆面过程对中国暴雨影响的研究成果。此类研究主要应用中尺度数值模式,有的对边界层过程和陆面过程做敏感性试验,有的则对模式中边界层和陆面过程参数化作改进。结果表明：边界层和陆面过程对我国暴雨有明显影响。主要表现在影响暴雨的强度以及使暴雨中心位置有一定的变动,但决定暴雨发生发展的主要因子是大中尺度动力过程。对边界层和陆面过程的改进能有效改善数值模式对暴雨的模拟。     

Turbulence Structure of the Unstable Atmospheric Surface Layer and Transition to the Outer Layer

We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z _s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z _s = u _* ³ /k_s, where u _* is friction velocity, k is the von Kármán constant and _s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z _s, so z _s is a new basis parameter for similarity models of the surface layer.     

一次暴雪过程前后近地层物理量场特征分析

利用铁塔风梯度观测资料和超声风温仪观测资料,对2008年1月18—21日暴雪前后,湖北黄石长江岸边近地层风场和湍流作了计算分析,探索其异常变化特征,为认识黄石地区暴雪近地层发生发展的物理过程提供依据。结果表明,暴雪前,风向转变,水平风速和垂直风速明显增大,湍流通量的输送较活跃,湍流动能和湍流强度有显著峰值出现;降雪过程结束后,湍流动能再次增大后缓慢减弱。可见此次暴雪过程前后近地层物理量场有异常变化     

The impact of convergence zones on the initiation of deep convection: A case study from COPS

During the ‘Convective and Orographically-induced Precipitation Study’ (COPS) performed in summer 2007, deep convection developed on July 15, although convective available potential energy was only moderate and convective inhibition was high. Convection was restricted to an area east of the Black Forest crest. Data analysis revealed that the convection was triggered by different mechanisms. Due to a surface high which was situated east of the Black Forest and a surface low which approached the investigation area from the west, a mesoscale convergence zone was established between the two regions and moved eastwards. Secondly, high insolation favoured the development of slope and valley winds and high evapotranspiration resulted in an increase of moisture in the planetary boundary layer (PBL). The thermally driven circulation systems formed a convergence zone along the mountain crest. When the synoptically induced mesoscale convergence zone reached the Black Forest, the different convergence zones superimposed optimally, such that strong updraughts were observed above the mountain. These updraughts penetrated the PBL-capping inversion and nearly reached the level of free convection. About 15 min after the convergence zone had passed the Black Forest crest, first clouds developed east of it. While moving further eastwards, the convergence zone intensified and became visible as a north-south oriented cloud line in the satellite images. Some deep convective cells with precipitation formed within the cloud line. The dense COPS network allowed the capture of the position and characteristics of the convergence zone and explains why convection developed in some restricted areas only.     

夏季金塔边界层风、温度和湿度结构特征的初步分析

利用2004年6～7月在河西走廊金塔陆-气相互作用试验的观测资料,分析了该地区夏季夜间和中午风、温、湿的垂直结构特征,结果表明：夏季夜间,当地面风较小时,金塔绿洲高空可能为偏西风气流,夜间稳定层高度大致在100～190m。夏季中午,当低空为偏东风时,风速随高度的变化比较复杂。总的来说,存在着东风急流,急流高度在1000-4000m之间,大气边界层顶盖（即逆温层底）约在3000-3600m高度,在500-800m高度以下存在绿洲内边界层;当低空为偏北风或西北风时,高空都为偏西风或西北风气流,低空风速随高度的变化比较平缓,风速有时存在极大值,大气边界层顶盖（即逆温层底）在3500m左右,在1200m以下可能存在绿洲内边界层,绿洲内边界层高度有时会很低。     

Turbulence Characteristics of the Stable Boundary Layer Over a Mid-Latitude Glacier. Part I: A Combination of Katabatic and Large-Scale Forcing

Profile and eddy-correlation (heights of 4 and 10 m) measurements performed on the Pasterze glacier (Austria) are used to study the characteristics of the stable boundary layer under conditions of katabatic and large-scale forcing. We consider cases where large-scale forcing results in a downslope (or following) ambient wind. The analysis of averaged spectra and cospectra reveals low frequency perturbations that have a large influence on the variances of temperature and horizontal wind components and also alter the cospectra of momentum and sensible heat flux. Only the spectrum of the vertical wind speed is comparable to universal spectra. The low frequency perturbations occur as brief intermittent events and result in downward entrainment of ambient air thereby producing enhanced downward sensible heat fluxes and downward as well as upward momentum fluxes with various magnitudes and timescales. After the variances were high pass filtered, the normalised standard deviations of wind speed and temperature compare favourably to findings in the literature within the range 0>z/L>0.5. For larger z/L they deviate as a result of an increased influence from low frequency perturbations and thus non-stationarity. In line with this, the turbulent kinetic energy budget (at 4 m height) indicates that production (shear) is in balance with destruction (buoyancy and dissipation) within the range 0>z/L>0.3. Non-dimensional gradients of wind speed within the range 0>z/L>0.3 have a slope of about 3.5. The scatter for the dimensionless temperature gradient is quite large, and the slope is comparable to that for wind speed gradients. For z/L>0.3 the imbalance in the turbulent kinetic energy budget grows and non-dimensional gradients for wind speed and temperature deviate considerably from accepted values as a result of increased non-stationarity. Average roughness lengths for momentum and sensible heat flux derived from wind speed and temperature profiles are respectively 1 × 10^-3 m and 6 × 10^-5 m, consistent with the literature. The ratio (z_0h/z_0m) compares to those predicted by surface renewal models. A variation of this ratio with the roughness Reynolds number is not indicated by our data.     

1961-2009年辽河流域气候变化特征及其对水资源的影响

辽河流域是我国七大流域之一,长期以来一直存在水资源严重不足的问题。采用1961-2009年辽河流域境内水文、气象观测数据,研究气象、水文要素历史变化特征,并采用同期和滞后相关分析,建立气象要素与水文要素的最优相关关系。结果表明：辽河流域气候变暖明显,增温幅度远高于全球和中国的同期增温幅度;辽河流域降水量增减趋势不明显,总体上为略减少趋势,但存在明显的少-多-少-多-少5个阶段性变化。辽河流域蒸发量为略减少趋势,春季、夏季是蒸发量较大季节;辽河流域近50年来径流量为减少趋势,经历了偏多-偏少-偏多-偏少4个阶段的变化,最近的1996-2009年经历了年径流量最少阶段,平均年径流量仅为16.2亿 m³,只达到多年平均径流量的58 %、径流量最多年代的32 %。7月、8月是流量最为集中的月份,2个月流量就占到全年的50.24 %,超过全年的一半;辽河流域降水量与径流量有较好的相关关系。在年尺度,径流量与铁岭、法库等周边地区相关系数基本达到0.6左右;在日尺度,日降水量与降水发生后第二日流量相关程度最好,在所有等级上两者相关系数均在0.7以上,在日降水量大于等于25 mm等级上,相关系数最高可达到0.85。     

1961-2005年嫩江流域右岸气候变化及对水资源的影响

利用累积距平法和气候倾向率对1961－2005年嫩江流域右岸气温、降水量和径流量资料进行分析,研究嫩江流域右岸气候变化及其对水资源的影响。结果表明：近45 a来嫩江流域右岸气温显著增高,平均以0.52 ℃/10 a的速率上升,而且四季均为上升趋势, 不同季节增温幅度以冬、春、秋、夏季依次递减,1986年以来为气温升高最显著的时段;降水变化可分为3个阶段： 1961－1982年降水量呈减少趋势,1982－1998年处于增加时期,1998年以来降水量又呈现减少趋势。夏季降水量变化趋势与年降水量变化趋势趋于一致, 降水量总趋势是在波动中微弱上升;嫩江流域右岸主要控制站年径流量与年降水量保持同步变化。     

1961—2009年辽河流域气候变化特征及其对水资源的影响

辽河流域是中国七大流域之一,长期以来一直存在水资源严重不足的问题。采用1961—2009年辽河流域境内水文、气象观测数据,研究气象、水文要素历史变化特征,并采用同期和滞后相关分析,建立气象要素与水文要素的最优相关关系。结果表明：辽河流域气候变暖明显,增温幅度远高于全球和中国的同期增温幅度;辽河流域降水量增减趋势不明显,总体上为略减少趋势,但存在明显的少—多—少—多—少5个阶段性变化。辽河流域蒸发量为略减少趋势,春季、夏季是蒸发量较大季节。近50 a辽河流域径流量为减少趋势,经历了偏多—偏少—偏多—偏少4个阶段的变化,1996—2009年经历了年径流量最少阶段,平均年径流量仅为16.2亿m3,只达到多年平均径流量的58%、径流量最多年代的32%。一年之中,7月和8月径流量最大,两个月径流量占全年的50%。辽河流域降水量与径流量有较好的相关关系。在年尺度,径流量与铁岭、法库等地区降水量相关系数为0.60;在日尺度,日降水量与降水发生后第2日经流量相关程度最好,在所有等级上两者相关系数为0.70或以上;在日降水量大于等于25 mm等级上,相关系数最高为0.85。     

基于BJ-RUCv2.0预报系统对大雾形成和发展关键条件的数值分析

为了弄清北京地区持续性雾-霾天气过程的演变规律、揭示大雾形成和发展的关键条件,利用常规气象观测资料、高速路自动气象站观测资料和大气成分观测资料分析了2013年1月26~31日雾-霾天气过程的演变特征和有利于大雾形成和发展的天气形势。在此基础上,采用先进的北京快速循环同化中尺度数值预报系统（BJ-RUC v2.0）开展数值模拟,分析大雾形成的水汽、动力和热力条件,得出:模式对1月30日夜间至31日前半夜的雾区模拟较好,但对28日夜间至29日白天（大雾天气伴严重大气污染）雾区的模拟偏差较大。发现近地层的持续性东南风使950 hPa以下湿度增大是大雾形成的关键条件。上层（975~800 hPa）的明显暖平流导致逆温层的加强和维持,使大气层结稳定度增强,是大雾天气发展和维持的重要条件。另外,近地层950 hPa以下为风场辐合、其上层为风场辐散的结构有利于雾的进一步发展。     

Vulnerability of water supply from the Oregon Cascades to changing climate: Linking science to users and policy

Despite improvements in understanding biophysical response to climate change, a better understanding of how such changes will affect societies is still needed. We evaluated effects of climate change on the coupled human-environmental system of the McKenzie River watershed in the Oregon Cascades in order to assess its vulnerability. Published empirical and modeling results indicate that climate change will alter both the timing and quantity of streamflow, but understanding how these changes will impact different water users is essential to facilitate adaptation to changing conditions. In order to better understand the vulnerability of four water use sectors to changing streamflow, we conducted a series of semi-structured interviews with representatives of each sector, in which we presented projected changes in streamflow and asked respondents to assess how changing water availability would impact their activities. In the McKenzie River watershed, there are distinct spatial and temporal patterns associated with sensitivity of water resources to climate change. This research illustrates that the implications of changing streamflow vary substantially among different water users, with vulnerabilities being determined in part by the spatial scale and timing of water use and the flexibility of those uses in time and space. Furthermore, institutions within some sectors were found to be better positioned to effectively respond to changes in water resources associated with climate change, while others have substantial barriers to the flexibility needed to manage for new conditions. A clearer understanding of these opportunities and constraints across water use sectors can provide a basis for improving response capacity and potentially reducing vulnerability to changing water resources in the region.     

宁南退耕还草区近地面的湍流通量特征研究

根据Monin—Obukhov相似性理论，运用空气动力学方法和能量平衡法，结合2002—2003年小气候资料，分别分析了宁夏南部山区（下称宁南）退耕还草区湍流通量和地表通量（净辐射、感热、潜热和土壤热通量）的分布和季节变化特征，为宁南退耕还草区陆面过程的认识、数值模拟和气候分析提供理论参考依据。     

新龙站近48年气温、降水量变化特征分析

新龙县位于青藏高原向四川盆地过渡面上,平均海拔3500m,研究新龙站的气候变化,对揭示青藏高原的气候有一定的参考价值。本文对新龙站1960~2007年气温和降水量资料进行了统计分析。结果显示:新龙站近48年平均气温略有上升的趋势,近10年处在一个较高的水平;冬季气温有上升趋势,夏季气温变化基本平稳;年平均气温有显著的15年左右的周期变化。降水量变化略有增加的趋势,无显著的年际和年代际变化特征,近年来强降水出现的频率增大。     

新龙站近48年气温、降水量变化特征分析

新龙县位于青藏高原向四川盆地过渡面上,平均海拔3500m,研究新龙站的气候变化,对揭示青藏高原的气候有一定的参考价值。本文对新龙站1960～2007年气温和降水量资料进行了统计分析。结果显示：新龙站近48年平均气温略有上升的趋势,近10年处在一个较高的水平;冬季气温有上升趋势,夏季气温变化基本平稳;年平均气温有显著的15年左右的周期变化。降水量变化略有增加的趋势,无显著的年际和年代际变化特征,近年来强降水出现的频率增大。     

阿尔泰山南坡1821-2014年树轮汞浓度变化及其对气候的响应

研究汞的历史环境水平,对于认识人为汞的再排放和评估汞污染治理的有效性有重要意义。然而全球大气汞的监测站点和监测数据有限,因此需要利用代用资料来表征大气汞的长期变化。树木年轮是记录大气汞变化趋势的潜在优质档案。本研究建立了阿尔泰山南坡青河地区1821-2014年的树轮汞浓度变化序列,分析了气候对树轮汞浓度的影响,并从全球和区域角度探讨了汞浓度变化的原因。结果表明：1821—2014年青河地区平均树轮汞浓度为1.36±0.28 ng·g-1。相关分析显示树轮汞浓度变化与年平均最低气温（r=0.268, p<0.05, n=57）和平均水汽压（r=0.326, p<0.05, n=57）呈显著正相关,与平均2分钟风速呈显著负相关（r=-0.356, p<0.01, n=57）,表明气候可能会影响树木年轮中汞的积累。1820s—1980s树轮汞浓度呈缓慢上升趋势,在此期间出现1850s—1860s中后期和1980s中后期两个峰值。1990s—2000s初期树轮汞浓度相对平稳。自2000s中后期,树轮汞浓度逐渐下降,可能与各国相继实施空气污染控制法规有关。     

A Typical Mode of Seasonal Circulation Transition: A Climatic View of the Abrupt Transition from Drought to Flood over the Middle and Lower Reaches of the Yangtze River Valley in the Late Spring and Early Summer of 2011

In this study, the seasonal transition of precipitation over the middle and lower reaches of the Yang-tze River Valley (YRV) from late spring to early summer is investigated. The results show that the seasonal transition of precipitation exhibits multi-modes. One of these modes is characterized by an abrupt transition from drought to flood (ATDF) over the middle and lower reaches of the YRV in the seasonal transition of precipitation. It is shown that the ATDF event from May to June 2011 is simply one prominent case of the ATDF mode. The ATDF mode exhibits an obvious decadal variability. The mode has occurred more frequently since 1979, and its amplitude has apparently strengthened since 1994. From the climatic view, the ATDF mode configures a typical seasonal circulation transition from winter to summer, for which the winter circulations are prolonged, and the summer circulations with the rainy season are built up early over the YRV.     

The Development and Initial Tests of an Atmospheric Model Based on a Vertical Coordinate with a Smooth Transition from Terrain Following to Isentropic Coordinates

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A Two-Dimensional Numerical Study of the Impact of a City on Atmospheric Circulation and Pollutant Dispersion in a Coastal Environment

The urban impact on the sea breeze is studied by means of a mesoscale model with a detailed urban parameterisation. Four simulations are carried out on an idealised two-dimensional flat domain. In the base case, half of the domain is characterised by seaand the other half by rural land. In the urban case, an urban area 10 km wide is added near the shoreline. Simulations are performed for a moist rural soil (weak sea breeze) and for a dry rural soil (strong sea breeze). Results are analysed in order to evaluate the impact of the city on the wind, temperature and turbulent kinetic energy fields. The dispersion of a passive tracer emitted near the coastline is, also, used in the comparison. Results show that the city accelerates the sea-breeze formation in the morning (combinations of urban circulation and sea breeze), but it slows thesea-breeze front penetration. Moreover, the presence of the city enhances the recirculation processes and strongly modifies the pollutant dispersion. These effects are enhanced for a moist rural soil.