渤海弱层化期湍流混合观测特征分析

利用2017年9月在渤海共享航次中取得的湍流混合直接观测数据,本文研究了渤海海域湍流混合的空间分布特征及有关的影响因素。9月观测海区水体垂向层结较弱,莱州湾受黄河冲淡水影响出现高温低盐结构,位于渤海中央浅滩南北两侧洼地的双中心冷水结构依旧存在。湍流观测结果表明湍动能耗散率在10~(-9)～10~(-5)W/kg之间变化,统计上满足对数正态分布。耗散率强值区出现在辽东湾及渤海湾湾口近岸处,相应的垂向湍扩散系数约为10~(-6)～10~(-2)m~2/s。垂向上,水体表、底层混合较强,进一步研究发现弱层化水体的平均湍动能耗散率〈ε〉与风速和正压潮流速的大小存在正相关关系。另一方面,耗散率ε与浮性频率N近似满足ε=2.0×10~(-8)+3.0×10~(-7)(N~2/N_0~2)~(-5)的拟合函数关系,反映了层化对水体垂向混合的抑制作用。     

夏季黄东海硝酸盐垂向扩散通量的分析

湍流扩散过程导致的硝酸盐垂向输运对海水表层的浮游植物生长和初级生产力的大小有着重要影响。本文基于2018年夏季黄、东海水文环境、硝酸盐浓度和湍动能耗散率的同步、原位数据，分析了海域温度、盐度和硝酸盐的空间分布特征，结果表明营养盐含量丰富的黄海冷水团、长江冲淡水、东海北部底层混合水与黑潮次表层水是影响研究海域硝酸盐分布的主要水团。利用垂向湍扩散硝酸盐通量公式，计算了三个选定断面上的硝酸盐垂向扩散通量，其高值区与湍流扩散系数的高值区的位置基本一致。针对存在明显硝酸盐跃层的站位，计算得到跨硝酸盐跃层的垂向通量F_ND的范围在-9.78—36.60mmol/（m²·d）之间，在黄海冷水团区，夏季温跃层限制了该区营养盐向近表层的湍流垂向扩散；东海北部底层混合水区，湍流垂向扩散向上层补充了大量硝酸盐，促进了跃层之上浮游植物的生长；黑潮次表层水影响海区，夏季中层水体混合较弱，跨跃层的垂向通量也普遍偏低。开展硝酸盐垂向扩散通量的计算与分析，对进一步明确营养盐的输运机制有着重要研究意义。     

2012年夏季海南岛东岸上升流区的混合观测

The turbulent mixing in the upwelling region east of Hainan Island in the South China Sea is analyzed based on in situ microstructure observations made in July 2012. During the observation, strong upwelling appears in the coastal waters, which are 3℃ cooler than the offshore waters and have a salinity 1.0 greater than that of the offshore waters. The magnitude of the dissipation rate of turbulent kinetic energy ε in the upwelling region is O(10–9 W/kg), which is comparable to the general oceanic dissipation. The inferred eddy diffusivity K_ρ is O(10–6 m~2/s), which is one order of magnitude lower than that in the open ocean. The values are elevated to K_ρ≈O(10–4 m~2/s) near the boundaries. Weak mixing in the upwelling region is consistent with weak instability as a result of moderate shears versus strong stratifications by the joint influence of surface heating and upwelling of cold water.The validity of two fine-scale structure mixing parameterization models are tested by comparison with the observed dissipation rates. The results indicate that the model developed by Mac Kinnon and Gregg in 2003 provides relatively better estimates with magnitudes close to the observations. Mixing parameterization models need to be further improved in the coastal upwelling region.     

Transport properties of high albite crystals,near-endmember feldspar and pyroxene glasses,and their melts to high temperature

Thermal diffusivity (D) was measured using laser-flash analysis (LFA) from oriented single-crystal albite and glasses near LiAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈, LiAlSi₂O₆ and CaMgSi₂O₆ compositions. Viscosity measurements of the supercooled liquids, over 2.6 × 10⁸ to 8.9 × 10¹² Pa s, confirm strongly non-Arrhenian behavior for CaAl₂Si₂O₈, and CaMgSi₂O₆, and near-Arrhenian behavior for the others. As T increases, D _glass decreases, approaching a constant near 1,000 K. Upon crossing the glass transition, D decreases rapidly. For feldspars, D for the melt is ~15% below D of the bulk crystal, whereas for pyroxenes, this difference is ~40%. Thermal conductivity (k _lat = ρC _P D) of crystals decreases with increasing T, but k _lat of glasses increases with T because heat capacity (C _P ) increases with T more strongly than density (ρ) and D decrease. For feldspars, k _lat for the melt is ~10% below that of the bulk crystal or glass, whereas this decrease for pyroxene is ~50%. Therefore, melting substantially impedes heat transport, providing positive thermal feedback that could promote further melting.     

Surface layer mixing during the SAGE ocean fertilization experiment

Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF₆ tracer. During the experiment the mixed layer depth z_mld ranged between 50 and 80 m, with several re-stratifying events that brought z_mld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating z_mld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, L_c, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10⁻³ m² s⁻¹ in the pycnocline rising to over 10⁻² m² s⁻¹ higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m²/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.     

祁连山冻土区天然气水合物储层岩石热物性实验研究

采用扫描电镜测试了祁连山冻土区天然气水合物储层泥岩和砂岩微观结构。在显微镜下观察到,泥岩由微小均匀的片状多边形块体构成,这种微观结构使得泥岩中水合物以浸染状赋存;而砂岩内部存在分散状孔隙,砂岩中水合物为孔隙赋存。采用瞬变平面热源法测试了含甲烷水合物储层岩石的导热系数和热扩散率。在温度为-941~941 ℃时,干泥岩导热系数为0577~0853 W·m-1·K-1,含甲烷水合物储层泥岩导热系数为0704~1050 W·m-1·K-1。在温度为-811~928 ℃时,干砂岩导热系数为0828~1271 W·m-1·K-1,含甲烷水合物储层砂岩导热系数为3850~4555 W·m-1·K-1。在温度为-941~941 ℃时,干泥岩热扩散率为0712~0894 mm2·s-1,含甲烷水合物储层泥岩热扩散率为0792~1006 mm2·s-1,干砂岩热扩散率为1198~1674 mm2·s-1,含甲烷水合物储层砂岩热扩散率为1403~1769 mm2·s-1。测试数据表明,对于孔隙型水合物,测定导热系数是一种较好的辨识水合物存在的辅助手段。     

A method used to determine the upper thermal boundary of subgrade based on boundary layer theory

In the numerical simulation of long-term subgrade temperature fields, the daily variation of soil temperature at a certain depth h is negligible. Such phenomenon is called the "boundary layer theory."Depth h is defined as the boundary layerthickness and the soil temperature at h is approximately equal to a temperature increment plus the average atmosphere temperature. In the past, the boundary layer thickness and temperature increment were usually extracted from monitored data in the field. In this paper, a method is proposed to determinate the boundary layer thickness and temperature increment. Based on the typical designs of highway or railway, the theoretical solution of boundary layer thickness is inferred and listed. Further, the empirical equation and design chart for determining the temperature increment are given in which the following factors are addressed, including solar radiation, equivalent thermal diffusivity and convective heat-transfer coefficient. Using these equations or design charts, the boundary layer thickness and temperature increment can be easily determined and used in the simulation of long-term subgrade temperature fields. Finally, an example is conducted and used to verify the method. The result shows that the proposed method for determining the upper thermal boundary of subgrade is accurate and practical.     

Nutrient transport from an artificial upwelling of deep sea water

The transport of nutrient-rich, deep sea water from an artificial upwelling pipe has been simulated. A numerical model has been built within a commercial Computational Fluid Dynamics (CFD) package. The model considers the flow of the deep sea water after it is ejected from the pipe outlet in a negatively buoyant plume (densimetric Froude number = −2.6), within a stably stratified ocean environment subject to strong ocean current cross flow. Two cross-flow profiles were tested with momentum flux ratios equal to 0.92 and 3.7. The standard k-ε turbulence model has been employed and a range of turbulent Schmidt and Prandtl numbers tested. In all cases the results show that the rapid diffusion of heat and salinity at the pipe outlet causes the plume to attain neutral buoyancy very rapidly, preventing strong fountain-like behavior. At the higher momentum flux ratio fountain-like behavior is more pronounced close to the pipe outlet. The strong cross-current makes horizontal advection the dominant transport process downstream. The nutrient plume trajectory remains largely within one relatively thin stratified layer, making any ocean cross-flow profile less important. Very little unsteady behavior was observed. The results show that the nutrient is reduced to less than 2% of its inlet concentration 10 meters downstream of the inlet and this result is largely independent of turbulent Prandtl or Schmidt number. Initial results would suggest that if such an artificial upwelling were to be viable for an ocean farming project, a large number of upwelling pipes would be necessary. Further work will have to determine the minimum nutrient concentration required to sustain a viable phytoplankton population and the required spacing between upwelling pipes.     

Mixing processes relevant to phytoplankton dynamics in lakes

Active turbulence in lakes is confined to the surface mixed layer, to boundary layers on the lake sides and bottom, and to turbulent patches in the interior. The density stratification present in most lakes fundamentally alters the pathways connecting external mechanical energy inputs, for example by wind, with its ultimate fate as dissipation to heat; the density stratification supports internal waves and intrusions that distribute the input energy throughout the lake. Intrusions may be viewed as internal waves with zero horizontal wavenumber and are formed each time localised mixing occurs in a stratified fluid. Intrusions are also formed in the epilimnion by differential heating or cooling and by differential deepening. The fraction of lake volume below the diurnal mixed layer that is subject to active turbulence is very small, probably of the order of 1% or less in small to medium‐sized lakes. By contrast, in the surface mixed layer, turbulence is less intermittent and maintains phytoplankton in suspension and controls their exposure to the underwater solar flux. Nutrient transport to individual cells depends not only on the cell Reynolds number but also on the Peclet number, which, if large, implies enhanced mass transfer above purely diffusive values.     

Thermal Properties of a Supraglacial Debris Layer with Respect to Lithology and Grain Size

This paper focuses on the impacts of debris cover on ice melt with regards to lithology and grain size. Ten test plots were established with different debris grain sizes and debris thicknesses consisting of different natural material. For each plot, values of thermal conductivity were determined. The observations revealed a clear dependence of the sub‐debris ice melt on the layer thickness, grain size, porosity and moisture content. For the sand fraction the moisture content played a dominant role. These test fields were water saturated most of the time, resulting in an increased thermal conductivity. Highly porous volcanic material protected the ice much more effectively from melting than similar layer thicknesses of the local mica schist. However, the analysis of thermal diffusivities demonstrated that the vertical moisture distribution of the debris cover must be taken into consideration, with the diffusivity values being significantly lower in deeper layers.