Transition to two-dimensional turbulent convection in a rapidly-rotating annulus

Abstract

We study a semi-analytical model of convection in a rapidly-rotating, differentially-heated annulus with sloping top and bottom lids. Rapid rotation leads to a preservation of relatively simple, two-dimensional (2-D) structure in the experimentally-observed flow, while temporal complexity increases with the Rayleigh number. The model is, therefore, two-dimensional; it exhibits a sequence of bifurcations from steadily-drifting, azimuthally-periodic convection columns, also called thermal Rossby waves, through vacillation and a period-doubling cascade, to aperiodic, weakly-turbulent solutions.

Our semi-analytical results match to within a few percent previous numerical results with a limited-resolution 2-D model, and extend these results, due to the greater flexibility of the model presented here. Two types of vacillation are obtained, which we call, by analogy with classical nomenclature of the baroclinic annulus with moderate rotation rates, amplitude vacillation and tilted-trough vacillation. Their properties and dependence on the problem's nondimensional parameters are investigated. The period-doubling cascade for each type of vacillation is studied in some detail.     

集水区尺度景观格局与过程的时空动态分析:美国俄勒冈州塞勒支盆地1977-2000年的变化

在集水区尺度利用景观生态学的原理进行分析和管理是现代生态学与地理科学的一个重要课题.在人口增长和经济发展的压力下,集水区的格局和过程己受到人类活动越来越强烈地干扰.赛勒支盆地是一个典型的美国太平洋西北部沿海地区的集水区,本文以此为例,研宄了美国俄勒冈州中部集水区尺度的景观格局和过程在人类活动干扰下的时空动态.通过卫星遥感影像的应用,我们对1977年到2000年间赛勒支盆地土地覆盖的变化进行了检测.我们用陆地资源卫星1977年的多光谱影像(MSS), 1988年的专题影像(TM)' 2000年的增强专题影像(ETM+)高精度地定量分析了森林演替系列(如,演替后期的老针叶林和成熟针叶林,演替前期的年轻针叶林,以及更新的幼林）和其它土地覆盖类型的变化.景观的空间格局通过多种格局指数,例如,缀块指数、缀块形状复杂性指数、以及连接指数等进行了分析.同时,基于美国太平洋西北部主要森林类型和其它土地覆盖类型碳通量和碳贮量的空间数据库和文献资料,我们对 1977-2000年赛勒支盆地中的碳库及其在人类活动干扰下的变化作了测定.研宄结果揭示出,因为森林皆伐,老针叶林和成熟针叶林在1977-2000年间显著地减少,分别由占整个盆地土地覆盖面积的23%和12%, 减少为12%和7%;与此相反,年轻针叶林和无林地则分别从24%和5%增加为43%和14%.同时,因为采伐等干扰,留存的老针叶林和成熟针叶林空间分布格局的破碎度也迅速增加.集水区的碳收支在 1977-2000期间发生了巨大的变化.在收获干扰的压力下,在1977-2000年间,整个集水区的生态系统碳贮量从 17640797t 减少到 13405720t;净生态系统生产力(Net Ecosystem Production, NEP)由每年 100462tC 减少为每年76800tC.     

Experimental investigation of the velocity of a sand cloud blowing over a sandy surface

The velocity of a wind‐blown sand cloud is important for studying its kinetic energy, related erosion, and control measures. PDA (particle dynamics analyser) measurement technology is used in a wind tunnel to study the probability distribution of particle velocity, variations with height of the mean velocity and particle turbulence in a sand cloud blowing over a sandy surface. The results suggest that the probability distribution of the particle velocity in a blowing sand cloud is stochastic. The probability distribution of the downwind velocity complies with a Gaussian function, while that of the vertical velocity is greatly complicated by grain impact with the bed and particle–particle collisions in the air. The probability distribution of the vertical velocity of ?ne particles (0·1–0·3 mm sands) can be expressed as a Lorentzian function while that of coarse particles (0·3–0·6 mm sands) cannot be expressed by a simple distribution function. The mean downwind velocity is generally one or two orders greater than the mean vertical velocity, but the particle turbulence in the vertical direction is at least two orders greater than that in the downwind direction. In general, the mean downwind velocity increases with height and free‐stream wind velocity, but decreases with grain size. The variation with height of the mean downwind velocity can be expressed by a power function. The particle turbulence of a blowing sand cloud in the downwind direction decreases with height. The variations with height of the mean velocity and particle turbulence in the vertical direction are very complex. It can be concluded that the velocity of a sand cloud blowing over a sandy surface is mainly in?uenced by wind velocity, grain impact with the bed and particle–particle collisions in the air. Wind velocity is the primary factor in?uencing the downwind velocity of a blowing sand cloud, while the grain impact with the bed and particle–particle collisions in the air are the primary factors responsible for the vertical velocity. Copyright © 2004 John Wiley & Sons, Ltd.     

Dynamics and fluxes of organic carbon and nitrogen in two Guiana Shield river basins impacted by deforestation and mining activities

Deforestation and mining activities have proven to be very damaging to rivers because these activities disturb the environmental characteristics of rivers. Thus, the concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), particulate nitrogen (PN), and Chlorophyll‐a (Chl‐a) were measured monthly during 2 hydrological years in the Maroni and Oyapock Rivers to assess the dynamics and fluxes of organic carbon and nitrogen in these 2 Guiana Shield basins, which have been strongly (Maroni) and weakly (Oyapock) impacted by deforestation and mining activities. The 2‐year time series show that DOC, POC, PN, and Chl‐a concentrations vary seasonally with discharge in both rivers, indicating a hydrologically dominated control. Temporal patterns of DOC, POC, and PN indicate that these variables show maximum concentrations in rising waters due to the yield of organic matter and nitrogen accumulated in soils, which are incorporated into the rivers during rainfall. However, the Chl‐a concentrations were at a maximum during low‐water stages. The C/N and C/Chl‐a ratios also showed a seasonal trend, with lower values during the low water periods due to an increase in algal biomass. During high water, the POC in both rivers is the result of terrestrial organic matter, whereas during low‐water autochthonous organic matter can reach up to 34% of the POC. The mean annual fluxes of TOC and PN were higher (4.56 × 10⁵ tonC year^?1 and 1.77 × 10⁴ tonN year^?1, respectively) in the Maroni River than those (1.84 × 10⁵ tonC year^?1 and 0.54 × 10⁴ tonN year^?1, respectively) in the Oyapock River. However, the specific fluxes of DOC, POC, and PN from both basins were nearly the same. Although gold mining activities are performed in both basins, there is no conclusive evidence regarding the impact of these activities on the dynamics of organic matter and particulate nitrogen in the Maroni and Oyapock Rivers.     

Benthic biogeochemical cycling of mercury in two contaminated northern Adriatic coastal lagoons

Previous research recognized most of the Northern Adriatic coastal lagoon environments as contaminated by mercury (Hg) from multiple anthropogenic sources. Among them, the Pialassa Baiona (P.B.) Lagoon, located near the city of Ravenna (Italy), received between 100 and 200 tons of Hg, generated by an acetaldehyde factory in the period 1957-1977. Further east, the Grado Lagoon has been mainly affected by a long-term Hg input from the Idrija mine (western Slovenia) through the Isonzo River since the 16th century. Hg cycling at the sediment-water interface (SWI) of the two lagoons was investigated and compared by means of an in situ benthic chamber, estimating diffusive Hg and Methyl-Hg fluxes in the summer season. Major chemical features in porewaters (Fe, Mn, H₂S, dissolved inorganic (DIC) and organic carbon (DOC), nutrients) and in the solid phase (C_org, N and S) were also explored to understand the general biogeochemical conditions of the system in response to benthic respiration. The daily integrated flux for the methylated Hg form was extremely low in P.B. Lagoon, accounting for only 7% of the corresponding flux calculated for the Grado Lagoon. Despite a higher sedimentary Hg content in the P.B. Lagoon (14.4-79.0 μg g⁻¹) compared to the Grado Lagoon (10.7-12.5 μg g⁻¹), the in situ fluxes of Hg in the two experimental sites appeared similar. A selective sequential extraction procedure was applied to the solid phase, showing that the stable crystalline mineral phase cinnabar (HgS) is the predominant Hg fraction (about 50%) in the Grado Lagoon surface sediments. Conversely, Hg mobilization and sequestration in the P.B. Lagoon is related to the extremely anoxic redox conditions of the system where the intense sulfate reduction, by the release of sulfur and the formation of sulfides, limits the metal recycling at the SWI and its availability for methylation processes. Thus, the environmental conditions at the SWI in the P.B. Lagoon seem to represent a natural “barrier” for the potential risk of Hg transfer to the aquatic trophic chain.     

2009年环太湖入出湖河流水量及污染负荷通量

通过对2009年环太湖水文巡测及同步水质监测数据整理,得到2009年环太湖河流入出湖水量以及污染负荷,并将之与前期文献资料数据进行对比.结果表明,2009年环太湖河道入出湖水量分别为88.40×108 m3、93.27×108m3.入湖水量超过5×108m3的依次为陈东港、大浦港、梁溪河、太滆运河、望虞河.出湖水量最大...     

在EBEX-2000实验资料中湍流谱和局地各向同性特征

采用2000年8月在美国加州棉花地两个高度上应用超声三分量仪、快速响应050206 温度和湿度仪进行的EBEX-2000 (International Energy Balance Experiment,2000,EBEX-2000)风速三分量、温度和湿度湍流实验观测数据,计算分析了湍流速度、温度和湿度谱在不同稳定度下的特征.对湍流的局地各向同性进行了讨论并与Kansas和长白山原始森林湍流实验得到的结果进行了比较,得到了一些湍流特征量在不同下垫面情况下的一些有意义的特征.     

边界层急流型重力波——飞机颠簸的一种形成机制

采用线性化的Boussinesq流体边界层绝热流动方程，比较一维边界层急流型重力波的垂直运动量级，讨论边界层急流型重力波中的湍流发展，认为边界层急流型重力波是造成边界层飞机颠簸的一种机制。     

用大涡模式研究对流边界层湍流

利用三维对流边界层的大涡模拟模式研究对流边界层湍流统计特征量,模拟结果与前人的同类模拟工作及实验观测结果相比较,得到了一致的结论.       

Aeolian processes across transverse dunes. I: Modelling the air flow

This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright © 1999 John Wiley & Sons, Ltd.