Dispersion of a Passive Scalar Within and Above an Urban Street Network

The transport of a passive scalar from a continuous point-source release in an urban street network is studied using direct numerical simulation (DNS). Dispersion through the network is characterized by evaluating horizontal fluxes of scalar within and above the urban canopy and vertical exchange fluxes through the canopy top. The relative magnitude and balance of these fluxes are used to distinguish three different regions relative to the source location: a near-field region, a transition region and a far-field region. The partitioning of each of these fluxes into mean and turbulent parts is computed. It is shown that within the canopy the horizontal turbulent flux in the street network is small, whereas above the canopy it comprises a significant fraction of the total flux. Vertical fluxes through the canopy top are predominantly turbulent. The mean and turbulent fluxes are respectively parametrized in terms of an advection velocity and a detrainment velocity and the parametrization incorporated into a simple box-network model. The model treats the coupled dispersion problem within and above the street network in a unified way and predictions of mean concentrations compare well with the DNS data. This demonstrates the usefulness of the box-network approach for process studies and interpretation of results from more detailed numerical simulations.     

Estimating the Number of Cloud Layers through Radiosonde Data from Russian Aerological Stations for 1964–2014

Radiosonde data are used for the period of 1964–2014 and the method that determines the boundaries and cloud amount based on the profiles of temperature and humidity [23]; long-period statistical characteristics are computed for the cloud layer number for different altitude ranges from the ground to 10 km. The study is performed for the Russian aerological stations located at different latitudes and climate zones. To specify the spatiotemporal features of the atmosphere layering into cloud layers and cloudless layers between them, the estimates of monthly mean, seasonal mean, and annual mean values of cloud layer number as well as of their standard deviations are computed, and the amplitude of their variations is determined. The results qualitatively agree with the data of aircraft-based sounding of the atmosphere as well as with the data of radars and experiments with free balloons.     

The differential method for calculation of cloud motion vectors

The problem ofcloud data retrieval for the periods between the fixed terms ofobservations from geostationary satellites is solved using the differential methods for the optical flow. To solve the problem, the possibility of using different methods for computing cloud motion vectors is investigated. The estimates of the accuracy of computed vectors based on artificially generated test sequences are presented. The test results reveal that the accuracy of the optical flow calculation with the modt fied Brox’s method excels the other mentioned methods. The software is implemented for creating the smooth animation of cloud movement using the modified Brox’s method.     

Biomass burning in the tropical savannas of Ivory Coast: An overview of the field experiment Fire of Savannas (FOS/DECAFE 91)

FOS/DECAFE 91 (Fire of Savannas/Dynamique et Chimie Atmosphérique en Forêt Equatoriale) was the first multidisciplinary experiment organized in Africa to determine gas and aerosol emissions by prescribed savanna fires. The humid savanna of Lamto in Ivory Coast was chosen for its ecological characteristics representative of savannas with a high biomass density (900 g m^–2 dry matter). Moreover the vegetation and the climate of Lamto have been studied for more than twenty years. The emission ratios (X/CO₂) of the carbon compounds (CO₂, CO, NMHC, CH₄, PAH, organic acids and aerosols), nitrogen compounds (NO_x, N₂O, NH₃ and soluble aerosols) and sulfur compounds (SO₂, COS and aerosols) were experimentally determined by ground and aircraft measurements. To perform this experiment, 4 small plots (100×100 m) and 2 large areas (10×10 km) were prepared and burnt in January 1991 during the period of maximum occurrence of fires in this type of savanna. The detailed ecological study shows that the carbon content of the vegetation is constant within 1% (42 g C for 100 g of vegetal dry matter), the nitrogen content (0.29 g N for 100 g of dry matter) may vary by 10% and the sulfur content (0.05 g S/100 d.m.) by 20%. These variations of the biomass chemical content do not constitute an important factor in the variation of the gas and particle emission levels. With the emission ratios characteristic of humid savanna and flaming conditions (CO/CO₂ of 6.1% at the ground and 8% for airborne measurements), we propose a set of new emission factors, taking into account the burning efficiency which is about 80%: 74.4% of the carbon content of the savanna biomass is released to the atmosphere in the form of CO₂, 4.6% as CO, 0.2% as CH₄, 0.5% as NMHC and 0.7% as aerosols. 17.2% of the nitrogen content of the biomass is released as NO_x, 3.5% as N₂O, 0.6% as NH₃ and 0.5% as soluble aerosols.     

NOx or VOC Limitation in East German Ozone Plumes?

The ozone forming potential of VOCs and NO_x for plumes observed from several cities and a power plant in eastern Germany was investigated. A closed box model with a gas phase photochemical reaction mechanism was employed to simulate several scenarios based upon aircraft observations. In several of the scenarios, the initial concentrations of NO_x, VOCs, and SO₂, were reduced to study the factors limiting the O₃ production. Ozone production was limited by the initial VOC concentrations for all of the simulated plumes. Higher O₃ concentrations were produced with reduced initial NO_x. In one sample with high SO₂ mixing ratios (>100 ppb), SO₂ was also identified as a significant contributor to the production of O₃.     

作为热机的自然对流:CAPE理论

在许多行星上,不断有热量供给地面,且热量也不断通过大气向太空发出红外辐射.由于在高压区是热源不是热汇,因此,该系统能够做机械功.大气对流是可以在此系统中运转的自然热机.基于这种热机结构,提出了一个大气对流的简单理论,预报在统计平衡状态下干、湿对流的浮力、垂直速度及其覆盖区域.在对流热机一次循环期间,来自地面层(热源,热区)的热,其中一部分被报废到空间自由对流层(热汇,冷区),并从那里辐射到太空.其余部分变为机械功.此机械功用于克服机械耗散维持对流运动上.最终,机械能耗尽,转变成热能.消耗掉的部分能量被辐射到太空,而其余的通过对流气块进入再循环.在温度较低能量消耗部分降低的情况下,温度较高能量消耗部分增加,提高了对流热机的视效率.对流热机所做的功对气柱积分,给出了存在于行星大气中克服粘性耗散、维持对流运动的对流有效位能(CAPE)统计平衡量的大小.此积分被认为是准平衡条件下行星状态的一个全球性数字.对于地球目前的气候,热机结构预报热带地区CAPE值的大小为1000 J·kg-1,该预报值与观测值一致.从结果也可得出,存在于对流大气的CAPE总量应随着全球地表温度的升高(或大气对红外辐射的暗度)的增大而增大.     

Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis

Subfossil pollen and plant macrofossil data derived from ¹⁴C-dated sediment profiles can provide quantitative information on glacial and interglacial climates. The data allow climate variables related to growing-season warmth, winter cold, and plant-available moisture to be reconstructed. Continental-scale reconstructions have been made for the mid-Holocene (MH, around 6 ka) and Last Glacial Maximum (LGM, around 21 ka), allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change. The synthesis of the available MH and LGM climate reconstructions and their uncertainties, obtained using modern-analogue, regression and model-inversion techniques, is presented for four temperature variables and two moisture variables. Reconstructions of the same variables based on surface-pollen assemblages are shown to be accurate and unbiased. Reconstructed LGM and MH climate anomaly patterns are coherent, consistent between variables, and robust with respect to the choice of technique. They support a conceptual model of the controls of Late Quaternary climate change whereby the first-order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance.     

Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions

The dispersion of a point-source release of a passive scalar in a regular array of cubical, urban-like, obstacles is investigated by means of direct numerical simulations. The simulations are conducted under conditions of neutral stability and fully rough turbulent flow, at a roughness Reynolds number of Re _τ  = 500. The Navier–Stokes and scalar equations are integrated assuming a constant rate release from a point source close to the ground within the array. We focus on short-range dispersion, when most of the material is still within the building canopy. Mean and fluctuating concentrations are computed for three different pressure gradient directions (0°, 30°, 45°). The results agree well with available experimental data measured in a water channel for a flow angle of 0°. Profiles of mean concentration and the three-dimensional structure of the dispersion pattern are compared for the different forcing angles. A number of processes affecting the plume structure are identified and discussed, including: (i) advection or channelling of scalar down ‘streets’, (ii) lateral dispersion by turbulent fluctuations and topological dispersion induced by dividing streamlines around buildings, (iii) skewing of the plume due to flow turning with height, (iv) detrainment by turbulent dispersion or mean recirculation, (v) entrainment and release of scalar in building wakes, giving rise to ‘secondary sources’, (vi) plume meandering due to unsteady turbulent fluctuations. Finally, results on relative concentration fluctuations are presented and compared with the literature for point source dispersion over flat terrain and urban arrays.