Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.     

我国西部高原大气边界层中的对流活动

利用 1 998年第 2次青藏高原野外试验中的多普勒声雷达探测、低空探测观测以及卫星观测资料对高原大气边界层内的对流现象进行分析研究。声雷达探测到了高原边界层内有强烈的对流活动。这种对流泡中心的垂直速度可超过 1m/s,并存在尺度为 1个多小时的周期性 ,表现为中小尺度的有组织的湍流活动。高原边界层强对流得以发展和维持的物理机制是 :强辐射加热、复杂的地形地貌形成的下垫面不均一性造成边界层斜压性、边界层内的平流活动等 ,这些现象都有利于对流的发展。在这些条件的作用下 ,边界层内可以产生一系列有组织的强湍流大涡旋活动 ,这些大涡旋形成的热泡在向上发展的过程中有的能够发生合并 ,变得更大也更为猛烈 ,达到凝结高度以上可形成对流云 ,并发生充分的对流混合。成云过程凝结潜热释放更有利于对流运动进一步发展 ,使对流云逐步发展成更大的对流云团 ,从而产生卫星云图中显示的云团发展过程。     

湍流能量闭合法用于大气模式的初探

湍流能量（TKE）闭合方案是近年来大气边界层（PBL）模式中技推崇的方法,它具有11－2阶精度。大气环流模式的计算容量和垂直网络限制,不允许PBL方案过份精细。本文对TKE方案进行简化及网络稀疏化试验。并用Wangara资料验证。验证结果表明：此方案引入PBL内仅设置5层的大气环流模式是可行的。地面以上400米内模式结果与实测吻合。但中、上部位温计算值偏高。关于本方案在大尺度模式中的应用尚须进一步研究。     

Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies,emphasizing the Kuroshio-Oyashio Extension(KOE)region.Oceanic perturbations of interest consist of sharp oceanic fronts,temperature anomalies associated with mesoscale eddies,and to some extent even higher-frequency submesoscale variability.The focus is on the free atmosphere above the boundary layer.As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track,the response of both the time-mean flow and the storm track is assessed.The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations.Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed,followed by the role of oceanic fronts in maintaining the storm track as a whole.KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track.Relevant studies are summarized and intercompared.Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors.Recent modeling studies highlighted the importance of mesoscale eddies and probably even submesoscale processes in maintaining the storm track but confirmation and validation are still needed.Moreover,the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate.By reviewing the above aspects,we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes(fronts,eddies,submesoscale features)and atmospheric processes(fronts,extratropical cyclones etc.),in an integrated way,within the context of different climate background states.     

The wind structure in planetary boundary layer

The investigations on the dynamics of the PBL have been developed in recent years. Some authors emphasized macro-dynamics and others emphasized micro-structure of the PBL. In this paper, we study and review some main characteristics of the wind field in the PBL from the view point connecting the macro-dynamics and micro-structure of the PBL, thus providing the physical basis for the further research of the dynamics and the parameterization of the PBL.     

Turbulence Characteristics of the Shear-Free Convective Boundary Layer Driven by Heterogeneous Surface Heating

Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection. At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values of λ/z _i (λ is the dominant horizontal scale of the turbulent eddies, z _i is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL.     

Recent Advance in Understanding the Dynamics of the Madden-Julian Oscillation

The Madden-Julian oscillation （MJO） is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The effect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher- frequency eddies are delineated.     

春季黄海海雾WRF参数化方案敏感性研究

利用2005—2011年10次春季黄海海雾个例开展WRF模式参数化方案敏感性研究。结果表明:边界层方案对WRF模式雾区模拟结果起决定作用,而微物理方案影响较小,它主要影响海雾浓度和高度。边界层与微物理方案的最佳组合为YSU与Lin方案,最差为Mellor-Yamada与WSM5方案;Mellor-Yamada和QNSE方案模拟的近海面湍流过强,导致边界层过高,不利于海雾的发展与维持;而MYNN与YSU方案刻画的湍流强度与边界层高度合适,有利于海雾发展与维持。MYNN方案虽与YSU方案相当,但在大多数海雾个例中,后者明显优于前者,而在有些个例中却刚好相反。因此对于某一具体海雾个例而言,所用边界层方案仍需在它们之中选择最优者。这些信息可为黄海海雾WRF模式边界层与微物理方案的选择与改进提供参考。     

A possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on TRMM and MERRA reanalysis

Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases.     

A comparison of local and non-local turbulence closure methods for the case of a cold air outbreak

Numerical experiments have shown that large-eddy-simulation models (LES) are able to reproduce the common features of convective boundary layers (CBL) quite well. Models which cannot resolve the convective motions due to their grid structure (1D-models or models with coarse horizontal and/or vertical resolution) have to take into account the effects of large eddies within their subgrid diffusion terms. Turbulent fluxes are frequently parameterized through first-order-closure methods (K-theory). Recently, non-local closure schemes have also been developed. In this paper we compare 1D-and 2D-models using different local and non-local first-order closure methods. The analysis is carried out for the case of an idealized cold air outbreak (CAO). One of the non-local closures is based on the so-called transilient turbulence theory. The reference states are given by a bulk-model and a 2D-model which resolves the large eddies explicitly. A comparison of the results is presented for characteristic quantities such as evolution of boundary-layer height and surface heat flux as well as mean wind and temperature profiles. It is found that simple local first-order closure does not give good agreement with the reference models. The results of the transilient turbulence model shows that a non-local closure is able to parameterize the effects of the large eddies. Comparable results are produced by a local closure where eddy diffusivities are parameterized by dimensionless gradient-functions.     

Innovating the IPCC review process—the potential of young talent

There is significant potential in young talent for enhancing the credibility of the scientific assessments such as the IPCC’s by contributing to quality assurance and quality control. In this essay, we reflect on an experiment that was done by the Dutch government as part of its government review of a contribution to the IPCC’s Fifth Assessment Report (AR5). In an effort to review the entire Working Group II contribution to the AR5 within the official review period for the Second Order Draft (SOD), the PBL Netherlands Environmental Assessment Agency turned to PhD students. This article shows that a systematic review focusing on transparency and errors of a large scientific assessment document using young talented scientists can be successful if certain conditions are met. The reviewers need to have intrinsic motivation to conduct the review. There needs to be a communication plan that fosters engagement and a clear methodology to guide the reviewers through their task. Based on this experiment in review, we reflect on the wider potential for openness and crowdsourcing in scientific assessment processes such as the IPCC’s.     

Wavelet Analysis of Intermittent Turbulent Transport in the Atmospheric Surface Layer over a Monsoon Trough Region

The structure of the turbulence in the atmospheric surface layer over a monsoon trough region has been studied using structural analysis based on wavelet transform. The observational site is located at the eastern (wet) end of the monsoon trough region, characterized by high moisture in the atmospheric surface layer. On the average relative humidity varied from 70% to 100% during the experiment. The wind and temperature data, collected at Kharagpur (22°25' N, 87°18' E) at six observational hours of a day in June 1990 during the Monsoon Trough Boundary Layer Experiment (MONTBLEX), have been utilized in the study. The wind and instantaneous momentum flux time series were decomposed into 12 scales using the Haar wavelet transform. The eddies exhibited a large temporal variability generating intermittency in the energy and flux distributions. A criterion based on the isotropy has been suggested for separating the large eddies from the small eddies. At the separation scale the isotropy coefficient drops sharply. It is shown that the intermittency in the small eddies resulted from the spatial variation of energy, and deviation of velocity statistics from the Gaussian distribution known as flatness. The deviation from the -5/3 power law has been attributed to the increased mean values of, (i) the coefficient of variation of energy, and (ii) the flatness factor, in the inertial subrange. The decomposition of the instantaneous momentum flux time series reveals that the major contribution to the total flux arises from the large eddies. The quadrant analysis of the momentum flux shows that ejections and sweeps account for a substantial part of the total flux, and quantifies the relative importance of the various spatial scales that contribute to the transport of momentum.     

基于社会多媒体内容的用户建模应用研究

随着互联网和社会媒体平台的发展,社会媒体吸引着数以亿计的用户参与其中进行创造和分享信息,产生了海量的文本、图像、音频和视频数据.面对这些数量巨大、异构多源、模态复杂的社会多媒体内容数据,如何对其进行有效的内容理解和知识表示,从而为用户提供更高效、优质的服务,成为实现社会媒体大数据价值的关键.本文对近年来在社会多媒体内容分析、知识提取和表示以及用户建模应用的相关研究展开综述,并针对社会多媒体特征融合、跨模态知识提取与表示,以及基于社会媒体的用户建模相关应用研究三个方面进行详细总结.随后对社会多媒体内容的知识表示和用户建模的研究与应用的发展趋势进行介绍,最后对多媒体知识表示与用户建模研究进行了总结和展望.     

A statistical representation of sub-grid variation in mixing-length models of the boundary layer

In conventional gradient-transfer models of the atmospheric boundary layer, the fluxes are calculated using the gradients of buoyancy and momentum over the grid interval. When using finite-differences, these gradients are approximated using what are essentially the grid-volume means of the quantities involved. This is unrealistic, as within a large volume of the atmosphere fluxes are supported by an ensemble of small-scale motions, which may be organized by the presence of large coherent eddies. In this paper, a new method of computing the flux is developed, in which the gradients of grid-volume mean buoyancy and momentum are replaced by grid-volume ensembles. The distributional parameters of the ensembles are derived using dimensional arguments, augmented by the results of a large-eddy simulation.     

A Case Study of Rayleigh-Bénard Convection with Clouds

A turbulence data set collected by the research aircraft Hercules and Falcon in the planetary boundary layer (PBL) over the North Sea during Rayleigh-Bénard convection (RBC) is analysed. Altogether nearly three hundred cell passages at different levels and in two different flight directions were sampled.The convective boundary-layer height (H) was about 1 km, and the RBC cells had a diameter D of roughly 2–3 km, resulting in an aspect ratio A = D/H 2–3. This value is also found in the case of RBC in laboratory-scale flows, whereas most of the recent PBL experimental work reports convection PBL rolls with A 3 and mesoscale cellular convection (MCC) with A 10–40 over the oceans.The large number of RBC cell passages made it possible to composite their average structure. Due to the more complex three-dimensional structure and the importance of thermals to the RBC dynamics, spectral, temporal and spatial decompositions and model calculations were necessary to illuminate structure, dynamics, energetics and organisation. The final impression is that the structure of RBC in the PBL is given by a honeycomb-like arrangement of short-lived mixed-layer thermals with more passive downward motions in between. The regularity of the Cu-cloud cover results partly from the more stationary flow in the cloud-free cell centres. On the other hand it is shown that active as well as inactive clouds contribute to the cloud cover. Thus, the PBL flow and the cloud cover are decoupled, at least temporarily and locally.Due to sparse observational and measured information about RBC occurrence and structure in the PBL, additional material was gathered, resulting in the impression that RBC is one additional realised mode of organised convection in the PBL, as has already been clarified for PBL rolls and MCC by recent investigations.     

Nonuniform Distribution Of High-Frequency Turbulence In The Unstable Boundary Layer

Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes.     

Review of non-local mixing in turbulent atmospheres: Transilient turbulence theory

Some of the larger eddies in a turbulent region can be coherent structures that turbulently advect air parcels across large vertical distances before smaller eddies mix the parcels with the environment. Such a process is nonlocal rather than diffusive. Transilient turbulence theory, named after a Latin word maaning jump over, provides a framework for considering the ensemble-averaged effect of many eddies of different sizes on the net nonlocal mixing in the vertical. Nonlocal turbulence statistics can then be examinated, and nonlocal first-order closure can be formulated.     

A high resolution numerical study of Gulf of Mexico fronts and eddies

Summary The Gulf of Mexico (GOM) circulation is simulated using the DieCAST ocean model, with a horizontal resolution of 1/12° and 20 vertical layers. The results compare well with observations of both large and small scale features, including Loop Current frontal occlusions associated with frontal eddies. The simulation is carried out without any data assimilation. The frontal eddies tend to be spaced at about 90° intervals around the Loop Current, leading to a Loop Current head shaped like a square with rounded corners. The pattern rotates as the eddies circle the Loop, and frontal eddies elongate as they squeeze through the Florida Strait. Major warm core eddies separate regularly from the Loop Current and propagate to the western GOM. Old warm core eddies in the western Gulf dissipate through bottom drag effects, which also generate cyclonic parasitic eddies. Newly arrived warm core eddies merge with old ones in the western GOM. Recently separated elongated Loop Current eddies can rotate and reattach temporarily to the Loop Current. The barotropic flow component develops eddies between the main separated warm core eddy and the Loop Current due to eastward dispersion, as the main eddy itself propagates westward into the Gulf.With 10 Figures     

Observations of longitudinal roll vortices during arctic cold air outbreaks over open water

An evolving convective Arctic planetary boundary layer (PBL) containing longitudinal roll vortices (rolls) was observed with aircraft data during the 1983 Marginal Ice Zone Experiment and the 1984 Arctic Cyclone Experiment.The PBL is observed to grow rapidly as the very cold and dry air flows off the ice over the relatively warm water. There is very large sensible heat flux, a result of the large surface-air temperature differences. Coherent structures were identified in these PBL's by use of power, coherence squared and phase spectra of the data. A systematic method of separating the rolls from organized thermal plumes was devised, based on theoretical characteristics for roll circulations and the resulting modified mean wind profile. The rapid mixing by the rolls aids in the establishment of equilibrium and an observed adiabatic modified mean Ekman layer. Rolls that form in a thermally neutral atmosphere over ice have different characteristics than those that appear in the unstable stratification over water. The rolls become increasingly more convective in character with distance from the ice edge. They have aspect ratios (wavelength/PBL height) that decrease with distance from the ice edge in agreement with linear theory. This is in contrast to the cloud street wavelength to inversion height ratio which is observed to increase downwind from the ice edge.     

Fluctuations of line integrated concentrations across plume diffusion in grid generated turbulence and in shear flows

The fluctuations of the instantaneous values of line integrated concentrations across plumes from point sources diffusing in turbulent shear flows, and in grid generated turbulence, have been studied experimentally using a fast response system which measured the attenuation of the intensity of an infrared beam crossing the plume. Analysis of the measurements show that the dimensionless statistical properties of the fluctuations at different distances from the source at each flow are approximately similar, in the sense that they depend primarily on the relative off-center location of the line of integration and almost independent of the distance from the source and the nature of the turbulence in the flows, as long as the characteristic length of the mean plume is not large compared to the size of the large eddies. The characteristic time of the fluctuations, on the other hand, was found to grow with the distance from the source and the autocorrelations of the fluctuations, particularly in the case of a plume diffusing in grid generated turbulence, were it found to be proportional to the lateral size of the mean plume. A—5/3 decay law of the power spectrum of the fluctuations was observed in the low frequency range which corresponds to the scale of the large eddies. The decay of the fluctuations caused by smaller eddies was much faster, as expected.