AMSU-A温度反演及其对南海热带低压暖心的分析

采用NOAA极轨卫星先进微波探测器AMSU-A L1B的观测资料,经过卫星天线模式校正和临边调整,然后采用线性回归及水平方向上逐步订正法得到水平分辨率0.5 °×0.5 °垂直方向1 000 ~ 0.1 hPa共40层的反演温度资料。通过个例分析发现,反演的温度资料可以揭示热带气旋暖心的细致结构。之后,选取了由中高层向下(“自上而下”)发展而来的发展和不发展的南海热带低压,对这些热带低压暖心结构的演变进行了对比分析。结果表明:由中高层向下发展而来的南海热带低压,都有暖心结构从中高层向下发展到近海面的过程;发展的南海热带低压在暖心向下发展到近海面时,高层暖心突然增强,使得热带低压的暖心在高层和低层同时出现显著增强;而不发展的南海热带低压的暖心结构并没有出现这种现象。因此,通过监测热带低压暖心结构的变化,可能对判别热带低压是否发展有一定的帮助。      

南海海气相互作用对热带气旋生成的影响——个例诊断

利用NCEP气候预报系统再分析资料（CFSR）和日本JRA-25再分析资料,对2010年西太平洋第5号热带风暴“蒲公英”的形成过程进行诊断,揭示南海海气相互作用对其初始涡旋生成的影响。分析表明,“蒲公英”的初始涡旋最早于8月20日00时出现在吕宋岛以东的对流层低层。西太平洋偏东风受到吕宋岛地形强迫可能是初始扰动形成的原因之一。在初始涡旋生成阶段,南海海域盛行较强的东南风,因风应力造成的Ekman输送导致南海中东部较暖海水流向吕宋岛附近,使得局地海温升高,自海洋向大气输送的感热通量增大。异常感热加热使其上空低层气压下降,近地层出现上升运动。但根据热力适应理论,感热加热随高度减小制造的低层负涡度不利于初始涡旋在异常感热加热区上空向高空进一步发展,可是在感热加热区东侧因涡度平流随高度增加,强迫出上升运动。该上升运动引起的凝结潜热释放,使得最大加热中心出现在对流层中层。于是,在对流层中低层非绝热加热随高度增加,制造正的相对涡度,使得初始涡旋在感热加热区的东侧生成,继而在对流层中低层增强。因此在初始涡旋增强阶段,凝结潜热加热及其对涡度的制造起主要作用。      

夏季热带北大西洋海温异常对西北太平洋热带气旋生成的影响

利用1979—2012年西北太平洋热带气旋最佳路径资料,Hadley中心的海温资料和NCEP/NCAR再分析资料等,研究了夏季(6—10月)热带北大西洋海温异常与西北太平洋热带气旋(Tropical Cyclone,TC)生成的关系及其可能机制。结果表明,夏季热带北大西洋海温异常与同期西北太平洋TC生成频次之间存在显著的负相关关系。热带北大西洋海温的异常增暖可产生一对东—西向分布的偶极型低层异常环流,其中气旋性异常环流位于北大西洋/东太平洋地区,反气旋异常环流位于西北太平洋地区。该反气旋环流异常使得TC主要生成区的对流活动受到抑制、低层涡度正异常、中低层相对湿度负异常、中层下沉气流异常,这些动力/热力条件均不利于TC生成。此外,西北太平洋地区低层涡旋动能负异常,同时来自大尺度环流的涡旋动能的正压转换也受到抑制,不能为TC的生成和发展提供额外能量源。反之亦然。     

南海地区发展型和不发展型中层涡旋的结构特征

对NCEP 1 °×1 °再分析资料进行分析表明,南海地区发展型和不发展型中层涡旋的结构特征有着明显的区别:发展型中层涡旋的中心涡度逐渐增大,并同时向高层和低层发展,其中低层辐合维持得较好;而不发展中层涡旋的中心涡度则逐渐减小,不再向高层发展,其中低层辐合维持得较差。对称性分析表明,发展型中层涡旋的水平风场和风速垂直剖面都具有良好的对称性,而不发展型的较差;发展型中层涡旋中心的风速垂直切变小,有利于对流层中上层暖心的建立与加强,而不发展型则相反。平衡性分析表明,发展型涡旋在对流层中低层的平衡性较好,涡旋维持着较好的风压平衡关系,有利于涡旋的发展;不发展型涡旋则较差。纬向剖面位涡异常分析表明,发展型中层涡旋的正位涡异常在增强的同时向上发展,涡旋处于持续发展的阶段;而不发展型位涡则相反,一方面强度在减小,另一方面没有向上发展,涡旋逐渐减弱。      

台风“凤凰”初始扰动生成过程及其结构演变特征分析

热带气旋的生成是一个由初始扰动转变成为暖心结构低压的过程,但已有的经典理论对初始流场的配置及初始扰动的触发都没有详细的描述。利用FY-2C的TBB资料和日本再分析资料,以2008年8号台风“凤凰”为例,跟踪其流场和涡度场演变,追溯该热带气旋从初始扰动生成到转为有暖心结构的热带低压的过程。分析结果显示,初始扰动在水平风垂直切变较强的鞍形场中触发。随着高层环流调整,水平风垂直切变迅速减弱,高空冷涡外围暖区云系为初始扰动的发展提供了有利动力和热力条件。在其发展为热带低压的过程中,结构上出现正涡度从对流层中层向低层加强并有中心下降的特点,低层涡度的快速增加主要受气压梯度增强导致的辐合增强的影响。由个例分析结果可以看到,高空冷涡和副热带高压对热带初始扰动的生成以及发展和结构演变都起到重要的作用。      

太平洋经向模态对西北太平洋热带气旋影响的数值模拟研究

在分析研究太平洋海气耦合经向模（Pacific Meridional Mode——PMM）和西北太平洋生成热带气旋频数变化关系的基础上,利用NCAR的大气环流模式CAM3模拟研究了太平洋海气耦合经向模态对西北太平洋生成热带气旋的影响。结果表明,海气耦合的经向模态通过影响热带气旋生成的大尺度环境从而影响热带气旋的频数和强度。在模式中当增加了PMM的海温强迫后,纬向风切变变小,对流层中低层相对湿度变大,热带西太平洋对流层低层出现西风异常,在西北太平洋地区形成一个异常的气旋性环流,并且匹配有较大的正涡度异常;对流层高层出现赤道东风异常和一个与低层气旋性环流相匹配的反气旋性环流,有利于对流活动的发展,从而有利于热带气旋的生成和发展。在增加了PMM的海温强迫的试验中,热带气旋中心的海平面最低气压降低,850 hPa中心附近最大切向风速增加,气旋中高层的暖心强度增强。热带气旋强度总体增加。数值模拟结果与资料分析相互映证,揭示了太平洋经向模态对西北太平洋热带气旋有重要影响。     

环境场对东海登陆热带气旋陆地路径的影响

采用合成分析的方法，对比分析了在华东南部沿海登陆，维持较短陆地路径和维持较长陆地路径的两类热带气旋的环境场变化，结果表明：环境场对登陆热带气旋的陆地路径的维持有重要影响。当西北太平洋副热带高压迅速加强西伸；中低层低压环流迅速减弱；中低层环境热力条件差，海洋水汽输送中断；高层水平流场呈纬向型，青藏高原反气旋环流相对较弱而西北太平洋副热带高压反气旋环流强大，中纬带西风急流强大时，环境场的配置使热带气旋周围的高层水平辐散弱并衰减，低层水平辐合弱并衰减，热带气旋的对流被抑制，仅能维持较短路径很快就减弱消亡。反之，当西北太平洋副热带高压减弱东退，西风槽南压；低层低压环流长时间维持；中低层环境热力条件好，海洋水汽输送通畅；高层水平流场呈经向型，青藏高原反气旋环流相对强大而西北太平洋副热带高压反气旋环流减弱东退，中纬带西风急流较弱，低空东南风急流强大时，环境场的配置使热带气旋周围高层水平辐散强并维持，低层水平辐合强并维持，热带气旋的对流通畅，能维持较长生命史和较长的路径。     

东北太平洋一次强爆发性气旋发展的机制分析

使用FNL Analysis全球格点资料,对东北太平洋一次强爆发性气旋的特殊性进行了分析,发现气旋对其西北部低压系统的吸收合并是其爆发性发展过程中的典型特征,斜压强迫对其快速发展的作用较弱,与西北太平洋爆发性气旋的发展过程存在显著差异。同时,使用Zwack-Okossi诊断方程,从影响爆发性发展的动力和热力因子方面,对其发展机制作了深入的探讨。研究表明,正涡度平流、暖平流和非绝热加热的共同作用使气旋开始爆发性发展,由潜热释放导致的非绝热加热的贡献最大,非绝热加热是其快速发展的主导因子,其中正涡度平流贡献主要来自于中高层,暖平流的贡献主要来自于中低层和高层,而非绝热加热主要发生在中低层,这为东北太平洋爆发性气旋的发展机制提供了一个新的认识。     

空间非均匀加热对副热带高压形成和变异的影响──Ⅲ:凝结潜热加热与南亚高压及西太平洋副高

通过理论分析和数值模拟,研究了降水所致凝结潜热影响副热带高压带断裂的物理机制。基于全型垂直涡度方程的尺度分析指出,强的对流凝结加热的垂直梯度的变化,导致副热带地区对流层高层和中低层的高低压分布呈现出反位相。数值试验证实凝结潜热是决定东半球夏季副热带高压位置和强度的关键因素：东亚季风降雨所致凝结潜热加热使高层南亚高压位于加热中心西侧,中层西太平洋副热带高压位于加热中心东侧。通过定常波的传播,副热带地区的凝结潜热加热对中高纬天气的形成和维持有一定影响。     

发展与不发展南海热带低压的数值研究

本文利用广东热带所的数值预报模式和ECMWF的格点资料,对两个南海低压进行了对比数值分析。其中一个低压在24小时内发展为台风,而另一个不发展。分析发现,二者初始场的温、湿、风、压结构都存在着显著差异。最为明显的是,发展者低层有明显的暖心结构和深厚的高湿层,使得条件不稳定层深厚;而不发展者暖心在高层明显,低层无暖心,而且,中低层湿度较小,所以条件不稳定层较浅薄。两个低压的不同发展趋势都得到了很好的数值模拟结果。利用模式输出结果的对比分析,讨论了南海热带气旋发展与不发展的最基本物理过程及其作用。表明对流层中低层的深厚高温层的存在及相应的温度结构和风场所导致的较强的对流加热的维持和加强,对低压的发展极为重要。     

Summer dryness in a warmer climate: a process study with a regional climate model

Earlier GCM studies have expressed the concern that an enhancement of greenhouse warming might increase the occurrence of summer droughts in mid-latitudes, especially in southern Europe and central North America. This could represent a severe threat for agriculture in the regions concerned, where summer is the main growing season. These predictions must however be considered as uncertain, since most studies featuring enhanced summer dryness in mid-latitudes use very simple representations of the land-surface processes ("bucket" models), despite their key importance for the issue considered. The current study uses a regional climate model including a land-surface scheme of intermediate complexity to investigate the sensitivity of the summer climate to enhanced greenhouse warming over the American Midwest. A surrogate climate change scenario is used for the simulation of a warmer climate. The control runs are driven at the lateral boundaries and the sea surface by reanalysis data and observations, respectively. The warmer climate experiments are forced by a modified set of initial and lateral boundary conditions. The modifications consist of a uniform 3 K temperature increase and an attendant increase of specific humidity (unchanged relative humidity). This strategy maintains a similar dynamical forcing in the warmer climate experiments, thus allowing to investigate thermodynamical impacts of climate change in comparative isolation. The atmospheric CO ₂ concentration of the sensitivity experiments is set to four times its pre-industrial value. The simulations are conducted from March 15 to October 1st, for 4 years corresponding to drought (1988), normal (1986, 1990) and flood (1993) conditions. The numerical experiments do not present any great enhancement of summer drying under warmer climatic conditions. First, the overall changes in the hydrological cycle (especially evapotranspiration) are of small magnitude despite the strong forcing applied. Second, precipitation increases in spring lead to higher soil water recharge during this season, compensating for the enhanced soil moisture depletion occurring later in the year. Additional simulations replacing the plant control on transpiration with a bucket-type formulation presented increased soil drying in 1988, the drought year. This suggests that vegetation control on transpiration might play an important part in counteracting an enhancement of summer drying when soil water gets limited. Though further aspects of this issue would need investigating, our results underline the importance of land-surface processes in climate integrations and suggest that the risk of enhanced summer dryness in the region studied might be less acute than previously assumed, provided the North American general circulation does not change markedly with global warming.     

Continental vegetation as a dynamic component of a global climate model: a preliminary assessment

A simplified vegetation distribution prediction scheme is used in combination with the Biosphere-Atmosphere Transfer Scheme (BATS) and coupled to a version of the NCAR Community Climate Model (CCM1) which includes a mixed-layer ocean. Employed in an off-line mode as a diagnostic tool, the scheme predicts a slightly darker and slightly rougher continental surface than when BATS' prescribed vegetation classes are used. The impact of tropical deforestation on regional climates, and hence on diagnosed vegetation, differs between South America and S.E. Asia. In the Amazon, the climatic effects of removing all the tropical forest are so marked that in only one of the 18 deforested grid elements could the new climate sustain tropical forest vegetation whereas in S.E. Asia in seven of the 9 deforested elements the climate could continue to support tropical forest. Following these off-line tests, the simple vegetation scheme has been coupled to the GCM as an interactive (or two-way) submodel for a test integration lasting 5.6 yr. It is found to be a stable component of the global climate system, producing only ~ 3% (absolute) interannual changes in the predicted percentages of continental vegetation, together with globally-averaged continental temperature increases of up to + 1.5 °C and evaporation increases of 0 to 5 W m^–2 and no discernible trends over the 67 months of integration. On the other hand, this interactive land biosphere causes regional-scale temperature differences of ± 10 °C and commensurate disturbances in other climatic parameters. Tuning, similar to the q-flux schemes used for ocean models, could improve the simulation of the present-day surface climate but, in the longer term, it will be important to focus on predicting the characteristics of the continental surface rather than simple vegetation classes. The coupling scheme will also have to allow for vegetation responses occurring over longer timescales so that the coupled system is buffered from sudden shocks.     

Numerical analysis of a thermotopographically-induced mesoscale circulation in a mountain basin using a non-hydrostatic model

Summary The boundary-layer wind field during weak synoptic conditions is largely controlled by the nature of the landscape. Mesoscale (sub-synoptic) circulations result from horizontal gradients of sensible heat flux due to variation in local topography, variation in surface-cover, and discontinuities such as land-sea contrasts. Such flows are usually referred to as thermally-driven circulations, and are diurnal in nature and often predictable. In this paper we use a state-of-the-art non-hydrostatic computer model to shed light on the physical mechanisms that drive a persistent easterly wind that develops in the afternoon in the Mackenzie Basin, New Zealand. The easterly – Canterbury Plains Breeze (CPB) – is observed early in the afternoon and is often intense, with mean wind speeds reaching up to 12 m s⁻¹. Although computer modelling in mountainous terrain is extremely challenging, the model is able to simulate this circulation satisfactorily. To further investigate the mechanisms that generate the Canterbury Plains Breeze, two additional idealized model experiments are performed. With each experiment, the effects of the synoptic scale wind and the ocean around the South Island, New Zealand were successively removed. The results show that contrary to previous suggestions, the Canterbury Plains Breeze is not an intrusion of the coastal sea breeze or the Canterbury north-easterly, but can be generated by heating of the basin alone. This conclusion highlights the importance of mountain basins and saddles in controlling near-surface wind regimes in complex terrain.     

A cold air lake formation in a basin — a simulation with a mesoscale numerical model

Summary A formation of a cold air lake in a basin is studied with a mesometeorological model.A dynamic Boussinesq hydrostatic mesoscale numerical model is developed in a staggered orthogonal grid with a horizontal resolution of 1 km and with a varying vertical grid. The topography is presented in a block shape so that computation levels are horizontal.The mesometeorological model is tested in three idealized topography cases (a valley, a single mountain, a basin) and test results are discussed.In an alpine basin surrounded by mountains and plateaus the air is supposed to be stagnant at the beginning of the night. Due to differences in radiation cooling an inversion layer is formed in the basin and local wind circulation is studied by model simulations.With 14 Figures     

一次台风前部龙卷的多普勒天气雷达分析

通过分析2004年8月25日发生在浙江省宁波市的一次台风前部龙卷发生发展的环境特征, 发现该龙卷发生在台风前部风切变区里, 尽管当时涡度、散度等物理量对于深对流发展不是非常有利, 但下湿中干、强的垂直风切变及地形条件等还是有利于局地弱龙卷的产生; 在宁波新一代天气雷达产品上表现为强的钩状回波, 速度场上有相邻的正负速度中心及强的组合切变值等。通过多个反射率产品、剖面产品等综合分析了该风暴的三维结构, 初步了解此类弱龙卷的发生机理, 为以后的预报提供一些经验。     

Effects of a Fence on Pollutant Dispersion in a Boundary Layer Exposed to a Rural-to-Urban Transition

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.     

Energy transfer in a fluid with a uniform density gradient

Energy transfer via resonance in a stratified fluid with a constant Brunt–Väisälä frequency is studied through the Manley–Rowe relation and direct numerical simulations. The objectives of this study are two-fold. One is to determine if there is a limitation on the lengthscale of small-scale waves to which primary energy can be effectively transferred. The other is to study factors affecting the growth of parametric subharmonic instability. Resonantly interacting modes are classified into three groups: local sum modes, quasi-subharmonic modes and remote parametric subharmonic instability modes (characterized by interaction with very small-scale waves). The latter two involve energy transfer from a primary wave to secondary waves with half the frequency. Most energy transfer is through local sum resonant modes and quasi-subharmonic modes. Energy cannot effectively transfer to higher wavenumber modes since dynamical systems are altered as wavenumbers of excited modes increase. In the remote modes, the solution is sinusoidal with high angular frequency and very small energy capacity. As a consequence, these modes are inactive in energy transfer despite their high energy growth rates. Effects of non-uniform white noise amplitude and primary mode propagation angle on the quasi-subharmonic modes are also investigated. Implications for energy transfer in the ocean are discussed.     

Sectoral approaches for a post-2012 climate regime: a taxonomy

Sectoral approaches have been gaining currency in the international climate debate as a possible remedy to the shortfalls of the Kyoto Protocol. Proponents argue that a sector-based architecture can more easily invite the participation of developing countries, address competitiveness issues, and enable immediate emissions reductions. However, given the numerous proposals, much confusion remains as to what sectoral approaches actually are. This article provides a simple, yet comprehensive, taxonomy of the various proposals for sectoral approaches. Based on the dual criteria of content and actors, three such types are identified and described: government targets and timetables; industry targets and timetables; and transnational technology cooperation. For each of these types, existing proposals and ongoing initiatives are discussed. In a second step, the article analyses the political landscape in which sectoral approaches are being debated, identifying the interests of their key advocates as well as the concerns of their critics. The Japanese government and energy-intensive manufacturing industries represent the main proponents of sectoral approaches to address the problems of carbon leakage and economic competitiveness. Developing countries, on the other hand, are wary of attempts to impose emissions reduction targets on their economies through sectoral target-setting. They, therefore, interpret sectoral approaches as sector-based forms of technology cooperation and technology transfer.     

Evaporation from a blanket bog in a foggy coastal environment

Frequent fog severely restricts evaporation from blanket bogs in Newfoundland because it more than halves the radiant energy input, and it eliminates the vapor pressure deficit, resulting in evaporation at the equilibrium rate (average = 0.99 during fog). During these periods, there is no surface resistance to evaporation because the bog has been wetted by fog drip, and although the latent heat flux dominates over sensible heat (average = 0.8), both are small. In contrast, the surface dries during clear periods, increasing the surface resistance to evaporation so that sensible heat becomes more important ( = 1.05). When the mosses are dry, evaporation is below the equilibrium rate ( = 0.87), although the higher available energy ensures that actual evaporation is higher. During clear periods, daily evaporation averaged 2.5 mm, compared to 1.1 and 0.7 mm for fog and rain, respectively. The suppressed evaporation at this site is important in maintaining appropriate hydrological conditions for blanket bog development.