A multi-model study of the restratification phase in an idealized convection basin

The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model).In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.     

Discrepancies in Simulated Ocean Net Surface Heat Fluxes over the North Atlantic

The change in ocean net surface heat flux plays an important role in the climate system. It is closely related to the ocean heat content change and ocean heat transport, particularly over the North Atlantic, where the ocean loses heat to the atmosphere, affecting the AMOC(Atlantic Meridional Overturning Circulation) variability and hence the global climate.However, the difference between simulated surface heat fluxes is still large due to poorly represented dynamical processes involving multisca...     

The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results

The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments. In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined by LAI was explored.     

Impacts of land surface and sea surface temperatures on the onset date of the South China Sea summer monsoon

The present study analyzes the differences in spatial and temporal variations of surface temperatures between early and late onset years of the South China Sea summer monsoon (SCSSM). It is found that when the land surface temperature north of 40^oN is lower (higher) and the sea surface temperature over the South China Sea-western North Pacific (SCS-WNP) is higher (lower) in winter, the onset of the SCSSM begins earlier (later). When the land surface temperature north of 40^oN is higher (lower) and the sea surface temperature over the SCS-WNP is lower (higher) in spring, the onset of the SCSSM occurs earlier (later). The reason why the anomalies of the land surface temperatures north of 40^oN can influence the atmospheric circulation is investigated by analysis of the wind and temperature fields. In order to verify the mechanisms of influence over the land and sea surface temperature distribution patterns and test the ability of the p-σ regional climate model (p-σ RCM9) to simulate the SCSSM onset, three types of years with early, normal, and late SCSSM onset are selected and the SCSSM regimes are numerically simulated. According to the results obtained from five sensitive experiments, when the land surface temperature is higher in the eastern part, north of 40^oN, and lower in the western part, north of 40^oN, and it rises faster in the eastern coastal regions and the Indian Peninsula, while the sea surface temperatures over the SCS-WNP are lower, the early onset of the SCSSM can be expected.     

基于引射器原理的投砾装置喷嘴CFD优化仿真分析

地浸钻孔成井工艺中,目前常用的投砾设备主要有渣浆泵与各类自制投砾装置。基于引射器原理的投砾装置是一种现场应用较广的投砾装置,为了提高该投砾装置的工作效率和投砾质量,本文基于计算流体力学的数值模拟软件Fluent对引射器原理的投砾装置的引射器结构进行了数值模拟的优化分析。分析结果表明,引射器喷嘴直径对引射效果具有较大影响,引射器的引射系数随喷嘴直径的增大而逐渐减小,但较小的喷嘴直径会导致对泥浆泵的泵压负荷增大,对于现场使用的BW250型泥浆泵,要求最高压力≯6 MPa,因此引射器喷嘴的直径为5 mm时为最优值;混合室直径增大会降低引射器性能,根据砾料通过能力等确定最优参数为18 mm;混合室长度有利于提高引射器性能,本文优化值为120 mm长度时为较佳混合室管长。优化后的投砾装置较原有基于大喷嘴直径引射器的投砾装置,投砾效率和质量均有大幅提高。     

内喷孔式反循环钻头结构优化设计及CFD模拟分析

为解决贯通式潜孔锤反循环连续取心（样）技术在钻进“硬、脆、碎”等复杂地层时反循环排不出渣或者排渣不彻底的现象,在现有贯通式潜孔锤反循环连续取心钻头结构的基础上,优化设计一种新型内喷孔式反循环钻头,能在孔底形成强有力的多股引射抽吸流场,并模拟分析了内喷孔轴线水平偏角和内喷孔轴线与钻头中心的偏距2个结构参数对新型反循环钻头结构抽吸能力的影响。通过模拟分析的验证,此种反循环钻头的结构更加简单,反循环效果比较优良。     

黄骅坳陷中区明下段末期构造应力场三维数值模拟

以翔实的地震、钻孔资料为基础建立了三维数值模型。对黄骅坳陷中区明下段末期的古构造应力场进行的模拟,获得成藏关键时刻凹陷内部空间的应力状态。指出挤压应力场中的相对低值区是油气聚积的有利区域,因为区内不仅存在开放的断裂系统,而且是最小主应力的汇聚中心,是油气运移的指向。该模拟结果可为分析油气运移、聚积规律提供重要的参考资料,为油气藏的进一步勘探开发提供依据。     

基于CFD动力降尺度的复杂地形风电场风电功率短期预测方法研究

复杂地形导致近地层风场时空变化大,是影响风电场短期风电功率预测准确率的重要因素。为此,基于中尺度数值预报模式和微尺度计算流体力学模式,建立了风电场短期风电功率动力降尺度预测系统。该系统由中尺度数值预报模式、微尺度风场基础数据库、风电功率预测集成系统组成,能够预测复杂地形风电场中每台风电机组未来72 h逐15 min的发电量。提高了复杂地形风场发电功率预测准确率,同时还可以在上报电网的风电功率预测结果中考虑运行维护计划和限电等因素对实际并网功率的影响。2014年7月-2015年1月的业务预测试验表明,风电场短期风电功率动力降尺度预测系统的月预测相对误差均小于0.2,满足中国国家电网对风电功率预测误差和时效性的业务要求。动力降尺度技术不受具体项目地形复杂程度和历史观测数据样本量的限制,可以在新建风电场中推广应用,具备实际的可操作性。      

21世纪中国极端降水事件预估

摘 要：全球变暖背景下极端降水事件的变化一直受到广泛关注,本文从观测、理论及模拟预估等方面对近十多年来国内外极端降水气候事件的研究作一综述,并给出IPCC第四次评估报告对我国21世纪极端降水指数变化的预估结果。     

一种全局优化的隐式交错网格有限差分算法及其在弹性波数值模拟中的应用

在数值模拟中,隐式有限差分具有较高的精度和稳定性.然而,传统隐式有限差分算法大多由于需要求解大型矩阵方程而存在计算效率偏低的局限性.本文针对一阶速度-应力弹性波方程,构建了一种优化隐式交错网格有限差分格式,然后将改进格式由时间-空间域转换为时间-波数域,利用二范数原理建立目标函数,再利用模拟退火法求取优化系数.通过对均匀模型以及复杂介质模型进行一阶速度-应力弹性波方程数值模拟所得单炮记录、波场快照分析表明:这种优化隐式交错网格差分算法与传统的几种显式和隐式交错网格有限差分算法相比不但降低了计算量,而且能有效的压制网格频散,使弹性波数值模拟的精度得到有效的提高.