基于傅立叶相位分析的卫星云图导风技术

导出云迹风(Cloud Motion Wind)的传统方法．是用最大相关系数法在间隔30min的卫星云图序列上计算示踪云的位移．以获得有关高度上风的估计值。但在用这种方法处理1min间隔云图时产生“亚像素尺度位移”问题。过去对傅立叶分析法的初步研究表明．对示踪云进行频域波谱分析,由谐波的相位变化计算波速。能够避免“亚像素尺度位移”问题。本文在此基础上,给出详细的理论推导和个例验证。     

The scavenging of nitrate by clouds and precipitation

A model with spectral microphysics was developed to describe the scavenging of nitrate aerosol particles and HNO₃ gas. This model was incorporated into the dynamic framework of an entraining air parcel model with which we computed the uptake of nitrate by cloud drops whose size distribution changes with time because of condensation, collision-coalescence and break-up. Significant differences were found between the scavenging behavior of nitrate and our former results on the scavenging behavior of sulfate. These reflect the following chemical and microphysical differences between the two systems:

1. nitrate particles occur in a larger size range than sulfate particles.
2. HNO₃ has a much greater solubility than SO₂ and is taken up irreversibly inside the drops in contrast to SO₂.
3. nitric acid in the cloud water is formed directly on uptake of HNO₃ gas whereas on uptake of SO₂ sulfuric acid is formed only after the reaction with oxidizing agents such as e.g., H₂O₂ or O₃.
4. nitrate resulting from uptake of HNO₃ is confined mainly to small drops, whereas sulfate resulting from uptake of SO₂ is most concentrated in the largest, oldest drops, which have had the greatest time for reaction.

Sensitivity studies showed that the nitrate concentration of small drops is significantly affected by the mass accommodation coefficient.     

不同云微物理方案对“7.21”特大暴雨模拟的对比试验

利用中尺度数值模式WRF v3.5.1中的17种不同云微物理过程参数化方案,对2012年7月21—22日北京特大暴雨过程进行了对比试验。模拟结果表明:不同云微物理方案对不同量级降水的模拟效果各有优势。NSSL 1-momlfo方案对中雨和大暴雨两个等级降水的模拟效果最好,降水中心值最接近实况;Eta(Ferrier)和Kessler方案分别对大雨和暴雨等级降水的模拟效果最好。总体上,能够较好地模拟出本次特大暴雨过程的方案依次为:NSSL 1-mom、NSSL 1-momlfo和Milbrandt 2-mom方案,而WDM6方案的模拟效果最差。云中水成物演变特征表明,模拟较好的方案中液态水、云冰和霰的含量较多,且随时间演变与地面降水强度的变化相一致。另外,模拟较好的方案中冰相粒子多,过冷水的范围大、含量高,有利于各相态粒子相互转化,促进冰相过程发展,致使降水量增多。     

基于CloudSat–CALIPSO资料的全球不同类型云的水平和垂直分布特征

基于Cloud Sat-CALIPSO(Cloud Sat–Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations)卫星观测资料,分析了全球总云量和8类云的云量、云底高、云顶高、云厚度的水平和垂直分布。分析结果表明,全球平均总云量为66.7%,其中卷云(Ci)和层积云(Sc)云量之和与其他6类云量总和相当,是全球云量最多的两类云。积状云云量呈现从赤道向极地递减的特征,层状云则相反,反映了二者不同的生成环境,同时下垫面地形和天气系统也严重影响云的分布。8类云的高度及厚度特征有显著差异。Ci的云底高度和云顶高度都较高,厚度则较薄;高层云(As)和高积云(Ac)的云底高度和云顶高度都位于大气中层,但As比Ac出现的高度高且厚度大;层云(St)、层积云和积云(Cu)的云底高度和云顶高度都很低,属于薄的低云;雨层云(Ns)和深对流云(DC)云底较低但云顶伸展很高,归属于厚云类。总体而言,海洋上云底高度较陆地低;赤道等大气不稳定地区,云底较高,云厚度较大;高原地区则表现出"高云不高,低云不低,云厚较薄"的特征。     

Changes in scavenging of particles by droplets due to weak electrification in clouds

Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds.Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (10³ in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities.The pressures and temperatures used in the models ranged from a representation of the lower troposphere at  1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at  30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m^− 3 to 2500 kg m^− 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%.For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed.When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces.In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m^− 3 particles up to the middle stratosphere.     

Evidence for cometary bombardment episodes

Evidence is found that large terrestrial impacts tend to cluster in discrete episodes, with characteristic separations 25–30 Myr and durations of about 1–2 Myr. The largest impactors are strongly concentrated within such events, and the Cretaceous–Tertiary extinctions occurred within one of them. The evidence also indicates the presence of a weak periodicity, which might be ∼24, ∼35 or ∼42 Myr depending on which peaks are taken as harmonics. The periodicity is most easily explained as a result of the action of the Galactic tide on the Oort comet cloud. The two longer period solutions are consistent with Galactic density estimates and with the current passage of the Solar system through the plane of the Galaxy. Other episodes may be a result of sporadic encounters with spiral arms, nebulae or stars.     

三江源地区秋季一次层积云飞机人工增雨催化试验的微物理响应

利用三江源地区一次在层积云顶部进行的飞机锯齿形催化作业及回穿探测的全球卫星定位系统(GPS)轨迹和机载云粒子测量系统(PMS)资料,在确定作业前后有效对比区间、区分云粒子相态的基础上,通过分析作业前、后液态云粒子及冰晶浓度变化、云粒子谱的演变和过冷水含量比率的变化,研究了催化的微物理响应。结果表明,作业区液态云粒子中值直径集中在3.5—18.5 μm,直径21.5—45.5 μm的云粒子基本上为冰晶,粒径大于50 μm的粒子相态为冰相;锯齿形作业后约2—23 min,在其航线下风方36 km范围内,前向散射粒子谱探头(FSSP-100)和二维灰度云粒子探头(OAP-2D-GA2,简称2DC)所测云粒子浓度、直径变化均未超出作业前云区内的自然起伏,但在过冷水含量大于0.01 g/m³的高过冷水区,液态云粒子浓度明显降低,前向散射粒子谱探头量程内的冰晶粒子浓度明显升高,冰相含水量增大,过冷水含量比率的平均值由作业前的 (68.3±23.1)% 减小至(34.2±12.4)% 。在过冷水含量越高的区域,催化效应越明显,而在低过冷水区和仅受原点催化影响的航迹交叉点处均未观测到催化响应。     

河北省降水性层状云宏微观物理特征

利用2009年河北省春秋季17架次飞机宏观观测资料和相应的机载PMS云微物理探测数据,经过回放处理、筛选和计算,统计分析近年河北省降水性层状云物理特征。结果表明：河北省降水性层状云是以Ns、Sc、As为主的多层云系,且多伴随有干层结构。云底平均高度、过冷层厚度、0℃层高度分别为2002、1106 m和3811 m。微物理方面,云滴数浓度平均为54.6 cm^-3,平均直径为8.64 μm。云粒子浓度和尺度均比20世纪90年代各特征值有所减少。统计计算的降水性层状云中液态水含量为0.13 g·m^-3,远大于20世纪90年代的值。     

改进的Holroyd云粒子形状识别方法及其应用

云降水粒子形状是影响云微物理过程的重要因素,准确的云粒子形状信息是诸多云微物理参量计算的前提。为获取机载云粒子成像仪（CIP）所测云粒子的形状信息,文中提出了一种改进的Holroyd云粒子形状识别方法,即先对云粒子形状进行预分类,然后针对预分类后的完整粒子和可识别的部分状粒子,分别选出合适的参数及其阈值再进行具体的分类,最终可将云粒子分为微小状、线形状、聚合状、霰、球形、板状、不规则和枝状。利用实测数据对原始的Holroyd方法和改进的Holroyd方法进行识别效果对比验证。结果表明改进的Holroyd方法在云粒子形状识别的准确度方面比原Holroyd方法有较大的提高。将所提方法应用于太原地区一次降水性层状云的云微物理飞机观测资料以分析不同的降水阶段云中冰晶粒子的形状分布、增长机制、冰晶粒子数浓度以及冰水含量的垂直分布特征,所获取的云中冰晶粒子属性表明新提出方法有助于云微物理分析。      

夏季白天中国中东部不同类型云分布特征及其对近地表气温的影响

本文利用2001～2017年ERA5再分析资料以及CERES卫星资料,探究夏季白天中国中东部不同类型云的云量及其光学厚度的时空变化特征,并利用一维辐射对流模式定量分析不同类型云对近地表气温的影响。观测结果表明:夏季白天中国中东部总云量及其光学厚度整体呈由南向北逐渐减小的分布特征,且中高云量占主导地位。总云量整体呈?0.3% a?1显著减少趋势,其中低云的贡献（?0.27% a?1）最大;总云光学厚度为0～0.1 a?1增加趋势,其中低云光学厚度（0.06 a?1）和中低云光学厚度（0.03 a?1）呈增加趋势,而中高云光学厚度（?0.08 a?1）和高云光学厚度（?0.03 a?1）呈减少趋势。模式结果表明:四种不同类型云的温度效应（Cloud Effect Temperature, CET）均为负值,表现为降温效应。低云、中低云、中高云和高云的年均CET值分别为?2.9°C、?2.7°C、?2.2°C和?1.7°C。其中,低云在华北平原降温可达?5°C;中低云和中高云在四川盆地和云贵高原降温可达?7.8°C。不同类型云温度效应与近地表气温的年际变化具有较好的一致性,具体表现为:2004年前（后）近地表气温呈现下降（上升）趋势,不同类型云的CET在此期间呈下降（上升）趋势,表现为云的降温效应增强（减弱）与近地表气温下降（上升）相对应,体现了夏季白天中国中东部4种不同类型云温度效应与近地表气温都呈正相关关系。特别地,夏季白天中国中东部中高云量占主导地位,其CET与近地表气温的相关系数高达0.63。综上,夏季白天中国中东部不同类型云温度效应对近地表气温的影响不同,但均呈正相关关系。定量分析不同类型云对近地表气温的影响可以为定量研究云反馈对区域增暖的作用以及合理预估未来区域增暖情景提供必要的科学参考。