"96.1"高原暴雪过程三维条件性对称不稳定的数值研究

利用“96．1”青藏高原东北部暴雪过程的较成功的中尺度模拟的输出资料,用非纬向非平行基流中的对称不稳定模式,对“96．1”暴雪发生发展过程的动力学机制进行了三维基流中二维非线性对称不稳定数值模拟试验。结果表明,ψ场和ω场的三维配置与切变线的演变过程相当一致,说明用三维基流中二维非线性对称不稳定可能能较好地描述SI在切变线东伸南移过程中所起的作用。     

A laboratory simulation of mesoscale flow interaction with the Alps

A series of laboratory experiments, aimed at the simulation of some aspects of Alpine lee cyclogenesis has been carried out in the rotating tank of the Coriolis Laboratory of LEGI-IMG in Grenoble. Dynamic and thermodynamic processes, typical of baroclinic development triggered by the orography, were simulated. The background flow simulating the basic state of the atmosphere consisted of a stream of intermediate density fluid introduced at the interface between two fluid layers. The structure of the intermediate current was established by mixing fluid obtained from the upper layer of fresh water with fluid removed from the heavier salty layer below.The dynamical similarity parameters are the Rossby (Ro), Burger (Bu) and Ekman (Ek) numbers, although this last, owing to its small values, need not be matched between model and prototype, since viscous effects are not important for small time scales. The flow in both the prototype and laboratory simulation is characterized by hydrostatics; this requires (Ro²δ²/Bu)1 (where δ=H/L is the aspect ratio of the obstacle) which is clearly satisfied, in the atmosphere and oceans, and for the laboratory experiment.A range of experiments for various Rossby and Burger numbers were conducted which delimited the region of parameter space for which background flows akin to that found to the northwest of the Alps prior to baroclinic cyclogenesis events, were observed.One such experiment was carried out by placing a model of the Alps at the appropriate place in the flow field. The subsequent motion in the laboratory was observed and dye tracer motions were used to obtain the approximate particle trajectories. The density field was also analyzed to provide the geopotential field of the simulated atmosphere. Using standard transformations from the similarity analysis, the laboratory observations were related to the prototype atmosphere. The flow and the geopotential fields gave results compatible with the particular atmospheric event presented.     

Cases-97: Late-Morning Warming And Moistening Of The Convective Boundary Layer Over The Walnut River Watershed

Aircraft, radiosonde, surface-flux, and boundary-layer windprofiler data from the Cooperative Atmosphere Surface Exchange Study's 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layerpotential temperature []and mixing-ratio [Q] onthree nearly-cloudless days from 1000 CST to 1200CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency (`budget')equations for []and [Q]. We estimate three of the terms –local time rate of change, vertical flux divergence, andhorizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, buthorizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the -budget,vertical flux divergence accounts for most of the morningwarming, with horizontal advection of secondary importance.The residual in the -budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the []-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [];-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient andhence horizontal advection.     

Equilibrium Evaporation and the Convective Boundary Layer

A theory is developed for surface energy exchanges in well-mixed, partlyopen systems, embracing fully open and fully closed systems as limits.Conservation equations for entropy and water vapour are converted intoan exact rate equation for the potential saturation deficit D in a well-mixed, partly open region. The main contributions to changes in D arise from (1) the flux of D at the surface, dependent on a conductance g_q that is a weighted sum of the bulk aerodynamic and surface conductances; and (2) the exchange flux of D with the external environment by entrainment or advection, dependent on a conductance g_e that is identifiable with the entrainment velocity when the partly open region is a growing convective boundary layer (CBL). The system is fully open when g_e/g_q , and fully closed when g_e/g_q 0. The equations determine the steady state surface energy balance (SEB) in a partly open system, the associated steady-state deficit, and the settling time scale needed to reach the steady state. The general result for the steady-state SEB corresponds to the equations of conventional combination theory for the SEB of a vegetated surface, with the surface-layer deficit replaced by the external deficit and with g_q replaced by the series sum (g_q ^-1 + g_e ^-1)^-1. In the fully open limit D is entirely externally prescribed, while in the fully closed limit, D is internally determined and the SEB approaches thermodynamic equilibrium energy partition. In the case of the CBL, the conductances g_q and g_e are themselves functions of D through short-term feedbacks, induced by entrainment in the case of g_e and by both physiological and aerodynamic (thermal stability) processes in the case of g_q. The effects of these feedbacks are evaluated. It is found that a steady-state CBL is physically achievable only over surfaces with at least moderate moisture availability; that entrainment has a significant accelerating effect on equilibration; that the settling time scale is well approximated by h/(g_q + g_e), where h is the CBL depth; and that this scale is short enough to allow a steady state to evolve within a semi-diurnal time scale only when h is around 500 m or less.     

新疆-甘肃北山金矿南带的成矿流体演化和成矿机制

北山金矿南带是西北5省区规模最大的金矿带。选择北山南带的新金厂、老金厂和小西弓金矿床，在矿床地质和岩相学研究的基础上，对脉石英的流体包裹体进行了显微温度计和激光拉曼探针气体成分测定；对石英和矿石黄铁矿的包裹体H2O，CO2和CH4进行了H和C同位素组成测定，对石英和黄铁矿分别做了O和S同位素组成测定。3个金矿床的脉石英含有富CO2＋CH4、H2O溶液以及H2O-CO2＋CH4包裹体。小西弓金矿床流体包裹体的均一化温度主要介于270℃-450℃，一部分H2O溶液包裹体圈闭了高盐度流体（16．43—18．63wt．％NaCl equiv．），大部分H2O溶液包裹体和全部富CO2＋CH。包裹体代表了中-低盐度（2．8％-13．6％）流体。新金厂金矿床流体包裹体的均一化温度主要介于210℃-346℃；一部分流体包裹体圈闭了高盐度（10．98％～14％NaCl equiv．）流体，一部分H2O溶液包裹体和绝大多数富CO2＋CH4包裹体圈闭了中-低盐度（2．9％-8．81％NaCl equiv．）流体。老金厂金矿床H2O溶液包裹体的均一化温度主要分布于141℃-400℃，含盐度介于1．4％-8．28％，属于中-低盐度流体。进行了大气降水-围岩^18O／^16O、D／H交换反应模拟。小西弓矿床早期硫化物-石英脉金矿成矿流体对应较高的水／岩比（=0．01—0．05），其^18O／^16O和D／H组成更受钾长花岗岩者控制，硫化物的δ^34S值也接近钾长花岗岩的黄铁矿者，指示热液流体围绕着钾长花岗岩的对流淋滤。成矿晚期，围绕着花岗岩侵入体的热液对流崩溃，矿区围岩内发育更大尺度的彼此分离的弥漫性流体渗透淋滤；相应地，小西弓矿床晚期蚀变岩金矿成矿流体的8D值对应低水／岩比（0．005-≈0．01），其δ^18O值变化范围较宽，受当地中元古界变质岩控制，蚀变岩型金矿黄铁矿的δ^34S值也接近中元古界长英质片岩的黄铁矿者。新金厂金矿和老金厂金矿成矿流体的δD值和δ^18O值对应的水／岩值分别介于0．004—0．01和0．007～0．02，与岩浆流体或者下二叠统哲斯群辉绿岩和英安岩围岩具有更密切的关系。新金厂金矿和老金厂金矿黄铁矿样品的δ^34S值介于-2．58‰和-6．32‰，指示S来源于下二叠统哲斯群辉绿岩、英安岩和碳质板岩围岩。3个金矿的石英包裹体CO2（δ^13C=-2．20‰--9．14‰），以及石英和黄铁矿包裹体CH4（δ^13C=013．10‰--27．40‰）不平衡；前者来源于幔源岩浆去气，后者来源于哲斯群碳质板岩或者中元古界长英质片岩中的还原碳。3个金矿黄铁矿包裹体的CO（δ^13C=-10．79‰--23．62‰）主要来源于哲斯群碳质板岩或中元古界长英质片岩中的还原碳，但是，也混合了较少的岩浆CO2。包裹体CO2和CH4δ^13C值的系统变化，也反映了从岩浆侵位和去气、流体对流，到围岩中流体大面积弥漫性渗透淋滤的演化过程。CH4介入成矿流体，导致流体不混溶和金的沉淀。北山金矿南带的形成既不同于典型的造山带型金矿床，也不同于与侵入岩有关的金矿床。我们提出北山金矿南带的成矿模式为：岩浆去气和流体对流、岩石挤压破碎、流体弥漫性渗透淋滤。     

“96·8”华北暴雨数值模拟与稳定性分析

分析1996年8月发生在华北地区的台风暴雨过程的环流形势，发现：副热带高压与台风低压之间的气压梯度很大，宽广的偏南急流源源不断地向北输送水汽和能量，而太行山一带正处于汇合区，构成十分有利的暴雨天气形势。应用MM5数值预报方法对1996年8月4—5日的降雨天气过程进行数值模拟，并依据天气学原理和位涡理论对此过程的稳定性进行分析认为：（1）MM5数值预报模式较好地模拟出了台风暴雨的物理过程。（2）此次降雨的不稳定层结有南高北低现象，同时有对称不稳定和对流不稳定存在；条件性对称不稳定可使环流加速，对降水有一定的增幅作用。     

2005年3月22日华南飑线的综合分析

2005年3月22日华南地区发生了一次飑线天气过程。利用常规观测、雷达回波、自动气象站资料及NCEP1°×1°的逐6小时资料,从天气形势、雷达回波、物理量场等多角度综合诊断分析了该飑线过程。结果表明:该过程具有低层增温、增湿,中高层降温、低湿的特征。飑线发生在快速东移的高空槽前上干冷、下暖湿的不稳定区域,华南地区700hPa低空急流的爆发及低层急流核向东传输对不稳定能量的突然释放有很大的触发作用。飑线系统在低空增温、增湿与对流层中层干侵入的相互作用下形成,产生大范围的雷雨大风、冰雹等强对流天气。     

人工增雨降温机理的数值模拟研究: 对流云个例试验

为减轻夏季用电负荷,2004年我国许多省市实施了碘化银催化增雨降温作业。本文利用数值模拟方法讨论了强对流云(冰雹云)催化降温的机理。模拟结果表明,实施碘化银催化后,地面降雨量增加、降雹量减少、地面温度降低。催化使得云中冰晶、霰和雨水的含量增加,造成云雨蒸发、霰的融化(蒸发)及冰晶升华量增加,从而使空气温度降低。     

分辨率对区域气候极端事件模拟的影响

利用NCAR MM5V3对1999年6月长江流域的极端异常降水事件进行了模拟,主要研究不同水平和垂直分辨率对极端区域气候事件模拟的影响。数值模拟试验表明:模式能够模拟出极端强降水的主要分布特征;水平分辨率的提高降低了模式模拟的强降水偏差,对逐日降水变化的模拟更加合理,而垂直分辨率的提高基本上也都减小了模拟的强降水过程的偏差,改善对强降水的模拟能力;模式水平、垂直分辨率的提高在一定程度上增强了对强降水过程的模拟能力。水平分辨率的提高能够改善模式对海平面气压的模拟,而垂直分辨率的提高可以改善模式模拟的地面气温和低层环流。分辨率对中层大气环流的影响不是很敏感。不同积云对流参数化方案模拟的对流降水比率随水平分辨率的变化是不同的,Grell方案对流降水比例随分辨率的提高而增加,而Kain-Fritsch方案的结果相反。     

华北盛夏暴雨过程的能量特征分析

本文利用实况资料和T213输出资料,对发生在2005年8月16日～17日的华北暴雨过程进行了能量学特征分析,结果表明:暴雨前24h～36h,对流层上下层有暖湿空气同时发展,是暴雨发生的重要动力机制;强的干、湿静力能锋区稳定维持是这次大范围暴雨产生的能量基础,中低层湿静力能差值大于0,且数值很大,说明对流不稳定层结很强,暴雨发生在高能、高不稳定度的条件下;水汽的充足补给作用,是这次暴雨过程能量转换的基础。