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1.
Helicity Dynamics of Atmospheric Flow 总被引：17，自引：0，他引：17
Helicity is an important physical variable which is similar to the energy and enstrophy in three-dimensional fluid. It can be used to describe the motion in the direction of fluid rotation and also can be regarded as a new physi-cal variable in turbulence theory. In recent years, it has been used in atmospheric dynamics. In this paper, helicity of atmospheric flow, especially helicity in the boundary layer and in the vicinity of front was discussed. These results show that helicity is usually positive in the boundary layer due to the effect of friction. The helicity of boundary layer flow is larger in anticyclone than that in cyclone, resulting from the different wind structures of boundary layers in an-ticyclone and cyclone under the geostrophic momentum approximation. It is possible that the helicity is negative at certain height in the baroclinic boundary layer. The influences of nonlinearity and baroclinity on the helicity are im-portant. The so called “Cloud Street” in the boundary layer is related to the dynamics of helicity. Helicity in the at-mosphere can be expressed as the temperature advection under some conditions, so helicity would be allowed to des-cribe the frontogenesis and development of frontal structure. The amplitude of helicity increases with time in the frontogenesis. A large gradient of helicity is generated in the region located to the northeast of the surface low and in which the front is formed. In warm frontal region, as well as behind the trough of temperature, the helicity is positive, while the helicity is negative in cold frontal sector and in the ahead ridge of temperature. The largest helicity occurs in the boundary. 相似文献
2.
本文首先利用变分方法,考察了边界层运动能量的变化,指出经典Ekman流是在不可压缩条件下,能量积分达最小值时的一种平衡运动。这对Ekman层运动的物理本质有了进一步的认识。其次,讨论了Ekman动量近似下的Ekman层的平衡风场特征,研究了该平衡风场附近扰动的稳定性问题,结果表明,在自由大气气压场不发生扰动条件下,Ekman层中存在一类新的与惯性不稳定相类似的动力不稳定,且其不稳定性可与Ekman抽吸相联系,还讨论了一般性条件下的扰动不稳定性问题。 相似文献
3.
In this paper, a numerical study has been carried out to assess changes in Atmospheric Boundary Layer (ABL) height under changing hydro-climatic conditions. ABL height is estimated by using the data from experiments conducted at three different sites- LAnd Surface Process EXperiment (LASPEX-97) in Sabarmati river basin, Gujarat, India, during 1997; Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) in Lannemezan, France, during 2011 and Integrated Global Radiosonde Archive (IGRA) in Tucson at Arizona, USA, during 2011. Here ABL height is estimated, during Intensive Observation Period (IOP), using 1-D turbulence closure model at all three site locations as well as following the Bulk Richardson Number (BRN) Approach. For LASPEX-97 only, the 1-D model with module for sub-surface processes coupled with upper-surface model is used for the estimation and compared with the only upper-surface model. ABL height, estimated by the coupled model, is found to be slightly higher than that by the un-coupled model. A copula-based analysis indicates that ABL height estimated by the 1-D model and same by BRN-based approach are statistically similar within 90% confidence interval (CI) under similar stratification condition. Thus, it validates the 1-D turbulence closure model for ABL height estimation. The sensitivity analysis of height of ABL, derived by 1-D model, against critical parameters reveals that surface condition (soil temperature) is more critical than atmospheric condition (net radiation) in forcing change on height of ABL in summer. 相似文献
4.
Effective action taken against climate change must find ways to unite scientific and practice-based knowledges associated with the various stakeholders who see themselves as invested in the global delivery of climate governance. Political decision-makers, climate scientists and practitioners approach this challenge from what are often radically different perspectives and experiences. While considerable work has been done to develop the idea of ‘co-production’ in the development of climate action outputs, questions remain over how to best unite the contrasting epistemological traditions and norms associated with different stakeholders. Drawing on the existing literatures on climate action co-production and from translational perspectives on the science-policy interface, in this paper we develop the concept of ‘boundary agency’. Defining this as the agency ‘possessed’ when willing and able to translate between different epistemological communities invested in a similar policy and governance challenge such as climate change, we offer it as a useful means to reflect on participants’ understanding of the ‘co’ in co-production. This is in contrast to the more established (often academic-led) focus on what it is that is being produced by co-production processes. We draw from two complementary empirical studies, which explicitly encouraged i) engagement and ii) reflection on cross-boundary co-production between climate action stakeholders from different backgrounds. Reflecting on the two studies, we discuss the benefits of (and barriers to) encouraging more active and sustained engagement between climate action stakeholders so as to try to actively blur the boundaries between science and policy and, in doing so, invent new epistemological communities of practice. 相似文献
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6.
“黄蜂”边界层风速分析——
台风平均风速的时距选取与强度估计 总被引：1，自引：0，他引：1
通过对热带风暴(TC)"黄蜂"登陆的一次边界层观测所得到风速资料的分析计算,取得不同时距下平均风速序列.比较不同时距选取时登陆TC的平均风速、风向和台风强度,发现1 min平均比10 min平均水平最大风速大25.30%,风向变化不大,TC强度指数1 min与10 min相差2%～4%.时距选取较小时,TC平均风速和强度变大,所测得风速区域也变大,而风向与时距的选取没有明显关系.只是在阵风变化比较大的情况下选取小的时距可以更好的表示这种特征.所以时距选取需要对所研究问题的需要进行分析,对不同的问题选取不同的时距以得到更好的结果. 相似文献
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本文主要利用第一次“中美热带西太平洋海气相互作用联合调查”所获得的海洋水文、气象及大气探测资料,分析了热带西太平洋暖水区边界层大气层结特征,指出热带西太平洋边界层大气具有多层结构和多层送温现象,边界层内各层厚度及气温、位温、混合比随高度的变化趋势如下:超绝热层位于地面以上50m左右,气温、位温、混合比迅速减小,系绝热不稳定层;第一层为混合层,平均厚度为351m,位温、混合比变化不明显;混合层上又是逆温层大多为30—100m厚,位温迅速增大,混合比迅速减小,系绝对稳定层;700—1000m为浅对流云活跃层, 相似文献
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Wind-tunnel measurements of the flow over an isolated valley both normal and at an angle (45°) to a simulated neutrally stable
atmospheric boundary layer are presented. Attention is concentrated on the nature of the flow within the valley itself. The
work formed part of a wider study that included detailed field measurements around an African desert valley and some limited
comparisons with that work are included. A scale of about 1:1000 was used for the laboratory work, in which an appropriate
combination of hot wire and particle image velocimetry was employed. For a valley normal to the upwind flow, it is shown that
the upstream influence of the valley extends to a distance of at least one half of the axial valley width upstream of the
leading edge, whereas differences in mean flow and turbulence could be identified well beyond two valley widths from the downwind
edge. Non-normal wind angles lead to significant along-valley flows within the valley and, even at two valley heights above
the valley ridge level, there remains a significant spanwise flow component. Downwind turbulence levels are somewhat lower
in this case, but are still considerably higher than in the undisturbed boundary layer. At both flow angles, there are significant
recirculation regions within the valleys, starting from mean separation just beyond the leading edge, but the strong spanwise
flow in the 45° case reduces the axial extent of the separated zone. The flow is shown to be in some ways analogous to flow
over an isolated hill. Our results usefully enhance the field data and could be used to improve modelling of saltation processes
in the field. 相似文献
11.
I.C.A. da Silveira J.A.M. Lima A.C.K. Schmidt W. Ceccopieri A. Sartori C.P.F. Franscisco R.F.C. Fontes 《Dynamics of Atmospheres and Oceans》2008,45(3-4):187
Temporally-growing frontal meandering and occasional eddy-shedding is observed in the Brazil Current (BC) as it flows adjacent to the Brazilian Coast. No study of the dynamics of this phenomenon has been conducted to date in the region between 22° S and 25°S. Within this latitude range, the flow over the intermediate continental slope is marked by a current inversion at a depth that is associated with the Intermediate Western Boundary Current (IWBC). A time series analysis of 10-current-meter mooring data was used to describe a mean vertical profile for the BC-IWBC jet and a typical meander vertical structure. The latter was obtained by an empirical orthogonal function (EOF) analysis that showed a single mode explaining 82% of the total variance. This mode structure decayed sharply with depth, revealing that the meandering is much more vigorous within the BC domain than it is in the IWBC region. As the spectral analysis of the mode amplitude time series revealed no significant periods, we searched for dominant wavelengths. This search was done via a spatial EOF analysis on 51 thermal front patterns derived from digitized AVHRR images. Four modes were statistically significant at the 95% confidence level. Modes 3 and 4, which together explained 18% of the total variance, are associated with 266 and 338-km vorticity waves, respectively. With this new information derived from the data, the [Johns, W.E., 1988. One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras. Dyn. Atmos. Oceans 11, 323–350] one-dimensional quasi-geostrophic model was applied to the interpolated mean BC-IWBC jet. The results indicated that the BC system is indeed baroclinically unstable and that the wavelengths depicted in the thermal front analysis are associated with the most unstable waves produced by the model. Growth rates were about 0.06 (0.05) days−1for the 266-km (338-km) wave. Moreover, phase speeds for these waves were low compared to the surface BC velocity and may account for remarks in the literature about growing standing or stationary meanders off southeast Brazil. The theoretical vertical structure modes associated with these waves resembled very closely to the one obtained for the current-meter mooring EOF analysis. We interpret this agreement as a confirmation that baroclinic instability is an important mechanism in meander growth in the BC system. 相似文献
12.
This article focuses on the science–policy interaction in international negotiations in the context of the United Nations Economic Commission for Europe's Convention for Long Range Transboundary Air Pollution (CLRTAP). It addresses the question how participants in the assessment process divide and co-ordinate work between science and policy and how this enhances credibility, legitimacy and relevance with multiple audiences. For this purpose the article combines an analytical framework to approach effectiveness of scientific assessment in policy making, with the notion of boundary work and co-production of science and policy. The article argues that knowledge produced within the CLRTAP process and the institutional setting in which this knowledge production takes place cannot be separated from each other. Furthermore credibility, legitimacy and relevance are to a large extent determined by boundary work in an early stage of the process. At the same time boundary work has to take place continuously in order keep the assessment process credible, legitimate and relevant for new audiences. The application of a combined framework for analysing credibility, legitimacy and relevance and for analysing boundary work turns out to be helpful in describing in detail what happens in practice at the science–policy interface. In particular it helps to address the question of the way participants in the assessment process divide and co-ordinate work, how this shapes design elements and how this enhances credibility legitimacy and relevance of an assessment. 相似文献
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14.
不同尺度绿洲环流和边界层特征的数值模拟 总被引：12，自引：11，他引：1
使用美国NCAR新版MM5V3．4非静力平衡模式,通过三重嵌套,模拟研究了西北地区不同尺度绿洲环流及边界层特征,发现不同绿洲系统地面能量和水分的输送是不同的,尺度较小的绿洲其地面潜热大,感热相对小。R度在15km以上的绿洲可以形成绿洲一沙漠环流和绿洲的小气候,有较低的边界层,同时在绿洲边缘的沙漠形成湿气柱。尺度在儿公里的绿洲不能形成绿洲一沙漠环流和绿洲边缘的湿气柱。尺度较大的绿洲形成的温度和湿度边界层结构和环流配合,使绿洲形成具有自我保护的绿洲小气候环境,有利于绿洲生态的发展。 相似文献
15.
M. Bottema 《Boundary-Layer Meteorology》1998,89(2):285-316
The aim of this work is to present experimentally evaluated effective roughnesses (zoe) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75–200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate zoe, but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height zd, rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths zop obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate zof for which flux-derived profile parameters u* and * were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter Tu = u/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range. 相似文献
16.
基于单多普勒天气雷达产品的强对流天气预报预警方法 总被引：15，自引：10，他引：15
本文对发生在常德新一代天气雷达探测区内的强对流天气(雷雨大风、冰雹、龙卷)在雷达回波强度场及速度场上的表现特征进行了归类分析,既用实例证实了美国强对流天气在多普勒天气雷达产品上的表现特征同样适用于中国,又对其表现特征进行了完善。在此基础上建立了适用于当地的基于单多普勒天气雷达产品的强对流天气预报方法,并根据雷达实时体扫资料对强对流天气进行语音、文字自动报警以及对风暴移向、移速进行预报作了实用性的探讨。在多普勒天气雷达产品应用还处在探索、起步阶段的我国,该方法为制作强对流天气预报提供了较系统的预报思路。 相似文献
17.
In this paper, we consider boundary-layer effects on atmospheric fronts. For this purpose we developed a dry, hydrostatic, two-dimensional numerical model with turbulence parameterized with the 1/2 -order eddy viscosity closure of Mellor and Yamada. In the surface layer we use Monin–Obukhov similarity and there is an interactive soil model to obtain lower boundary conditions for the atmospheric model. First, effects of turbulence are studied without thermal forcing at the surface and comparisons are made between inviscid and turbulent conditions. Sensitivity studies with respect to surface roughness are also performed. Subsequently we study effects of surface thermal forcing on fronts. The results show that, during summer, fronts display strong diurnal variation. Vertical velocities increase significantly during the day while gradients in temperature and vorticity across the front strengthen in the evening and at night. In winter the diurnal variation is much less pronounced. We attempt to find physical mechanisms responsible for such frontal behaviour and we compare our results with available observations. 相似文献
18.
2008年12月20—21日和2010年1月3日天津地区分别出现了历史同期罕见的暴雪天气。为了提高对这种极端天气发生机理的认识,利用多种资料对这两次天气过程进行了分析。结果表明：两次暴雪过程均属于回流型降雪,但环流形势和影响系统的演变却不尽相同。影响系统分别为高空横槽（高空槽）、850 hPa切变（850 hPa低涡切变）和地面倒槽（地面气旋）,水汽源自700 hPa西南气流和边界层东风的水汽输送。由于两次过程均与边界层东风相伴,特别对渤海西岸边界层东风对降雪天气的影响和作用进行探讨,表明偏东风不仅为本地输送一定量级的水汽,同时这种具有冷湿特征的东风还会与内陆具有暖湿结构的偏南风形成地面辐合线,加强地面的动力抬升作用,产生上升运动,有利于雨雪天气的加强和维持,因此可以认为边界层东风对暴雪的发生发展起到了显著的作用。 相似文献
19.
A. M. Reynolds 《Boundary-Layer Meteorology》1998,86(2):345-352
Rotach, Gryning and Tassone constructed a two-dimensional Lagrangian stochastic model to describe the dispersion of passive tracers in turbulent boundary layers with stabilities ranging from ideally-neutral (w* = 0) to fully-convective (u* = 0). They found that the value of the Kolmogorov constant, C0, as determined by optimizing model agreement with the measured spread of passive tracers, was dependent upon stability. Here, it is shown that the non-uniqueness, associated with satisfaction of the well-mixed condition, can be exploited to construct an alternative version of the model of Rotach et al. for which C0 = 3 is universally applicable over the entire range of stabilities under consideration. This alternative model is shown to be in very close agreement with predictions, obtained in large-eddy simulations, for the dispersion of passive tracers in turbulent boundary layers with stabilities ranging from ideally-neutral to fully-convective. 相似文献
20.
登陆台风边界层风廓线特征的地基雷达观测 总被引：2，自引：0，他引：2
为了分析登陆台风边界层风廓线特征,利用2004—2013年中国东南沿海新一代多普勒天气雷达收集的17个登陆台风资料,采用飓风速度体积分析方法,反演登陆台风的边界层风场结构特征。与探空观测对比表明,利用雷达径向风场可以准确地反演登陆台风的边界层风场结构,其风速误差小于2 m/s,风向误差小于5°。所有登陆台风合成的边界层风廓线显示,在近地层(100 m)以上,边界层风廓线存在类似急流的最大切向风,其高度均在1 km以上,显著高于大西洋观测到的飓风边界层急流高度(低于1 km)。陆地边界层内低层入流强度也明显大于过去海上观测,这主要是由陆地上摩擦增大引起。越靠近台风中心,边界层风廓线离散度越大,其中,径向风廓线比全风速以及切向风廓线离散度更大。将风廓线相对台风移动方向分为4个象限,分析边界层风廓线非对称特征显示,台风移动前侧入流层明显高于移动后侧。最大切向风位于台风移动左后侧,而台风右后侧没有显著的急流特征,与过去理想模拟的海陆差异导致的台风非对称分布特征一致。 相似文献