高低层纬向风切变的年际变率及气候学意义

用1959～1998共40年全球格点风场资料计算了200 hPa与850 hPa的纬向风速差, 即对流层纬向风切变(简称TZWS),并在此基础上得到其距平值。为了全面考察对流层中环流异常的年际变率特征, 根据TZWS的标准差分布, 文中选出了7个TZWS标准差数值大于5 m/s的代表性区域。这7个区域分别位于赤道中太平洋、赤道东太平洋、北太平洋亚热带地区、南太平洋亚热带地区、赤道大西洋、亚洲西南部以及东北部。前5个分别位于赤道、亚热带太平洋和大西洋的区域TZWS指数, 其年际变率与ENSO循环有密切联系, 反映了热带海洋温度异常对低纬度地区对流层环流的影响; 后2个区域的TZWS指数反映的是亚洲西南部和东北部的气候统变率, 在年际时间尺度上与ENSO循环有着明显的区别。通过对全球陆地降水和温度场的分析, 比较了热带、副热带的TZWS指数以及北极涛动指数的异同, 发现后2个区域TZWS指数能很好且能独立反映出北半球中高纬度地区陆地降水及陆地温度的异常模态。     

风向变化特征在季风模拟评估中的应用

作者抓住季风风向具有季节性反转这一最本质的特征,引入了“有向转角”的新概念,它与传统盛行风夹角的概念不同,能够反映风向逐日变化的方向性和旋转角度的大小。用欧拉观点揭示了季风风矢量随时间演变的旋转特点, 体现了季风风向独具特色的季节演变过程。同时,发现不同地区风向的季节循环有6种基本类型:（I）先顺时针后逆时针旋转、（II）先逆时针后顺时针旋转、（III）完全顺时针旋转、（IV）完全逆时针旋转、（V）风向稳定型和（VI）风向变化不稳定型。且季风风向的季节性反转主要通过前四种旋转方式来实现。分析了这6种风向变化型的全球分布特征,研究了它们与大气环流系统演变的联系。并进一步将“有向转角” 概念用于IPCC第四次评估报告AMIP试验8个模式的模拟评估,结果表明,这一概念的引入能从逐日变化的角度凸显风向变化过程,不仅能够客观反映模式模拟季风风向逐日变化的动力过程以及风矢量旋转方式的全球分布,还能体现模式对大气环流系统季节演变的表征能力。研究还发现在这8个模式中,大多数模式基本能把风矢量旋转方式的全球分布形式模拟出来,但对于季风区风矢量旋转方式的模拟还有待于进一步提高。     

热带印度洋偶极子事件期间印度洋大气的变化——大气流函数场和势函数场的分析

大气环流的变异是热带印度洋偶极子（IOD）事件研究中的一个重要问题。本文从风场旋度分量和散度分量角度出发,利用观测资料和大气环流模式,对IOD事件发生时热带印度洋海区上空的大气环流变化进行了分析,揭示出风场不同分量在IOD事件期间的变化特征。研究结果表明,热带印度洋大气环流系统在IOD事件期间,旋度分量和散度分量在垂直方向上呈现明显的一阶斜压形式,而在水平方向上呈现明显的对称分布特征。对低空（850 hPa）来说,无辐散流函数距平场在IOD事件正位相期间表现为关于赤道对称的一对反气旋式环流;无旋度分量在IOD事件正位相期间的响应表现为东印度洋辐散、西印度洋辐合;大气环流的两种分量场均可以在赤道印度洋地区产生距平意义下的纬向东风,正是这种形式的距平东风使得IOD事件依靠海气系统正反馈机制得以维持和发展。而高空（200 hPa）大气环流形式刚好与850 hPa相反。     

集合预报风场扰动的物理结构及演变特征

初始扰动结构和振幅的合理性直接影响到集合预报的质量和整体性能,合理的初始扰动结构是确保集合预报扰动质量的关键.本文基于欧洲中期天气预报中心数据、我国T639全球集合预报系统以及GRAPES区域集合预报系统的预报场,针对模式初值不确定性,主要研究了风场初始扰动结构及其集合离散度、扰动能量等结构和演变特征,分析了集合预报风...     

The sodium tail of the Moon

During the few days centered about new Moon, the lunar surface is optically hidden from Earth-based observers. However, the Moon still offers an observable: an extended sodium tail. The lunar sodium tail is the escaping “hot” component of a coma-like exosphere of sodium generated by photon-stimulated desorption, solar wind sputtering and meteoroid impact. Neutral sodium atoms escaping lunar gravity experience solar radiation pressure that drives them into the anti-solar direction forming a comet-like tail. During new Moon time, the geometry of the Sun, Moon and Earth is such that the anti-sunward sodium flux is perturbed by the terrestrial gravitational field resulting in its focusing into a dense core that extends beyond the Earth. An all-sky camera situated at the El Leoncito Observatory (CASLEO) in Argentina has been successfully imaging this tail through a sodium filter at each lunation since April 2006. This paper reports on the results of the brightness of the lunar sodium tail spanning 31 lunations between April 2006 and September 2008. Brightness variability trends are compared with both sporadic and shower meteor activity, solar wind proton energy flux and solar near ultra violet (NUV) patterns for possible correlations. Results suggest minimal variability in the brightness of the observed lunar sodium tail, generally uncorrelated with any single source, yet consistent with a multi-year period of minimal solar activity and non-intense meteoric fluxes.     

Plasma environment of Jupiter family comets

As any comet nears the Sun, gas sublimes from the nucleus taking dust with it. Jupiter family comets are no exception. The neutral gas becomes ionized, and the interaction of a comet with the solar wind starts with ion pickup. This key process is also important in other solar system contexts wherever neutral particles become ionized and injected into a flowing plasma such as at Mars, Venus, Io, Titan and interstellar neutrals in the solar wind. At comets, ion pickup removes momentum and energy from the solar wind and puts it into cometary particles, which are then thermalised via plasma waves. Here we review what comets have shown us about how this process operates, and briefly look at how this can be applied in other contexts. We review the processes of pitch angle and energy scattering of the pickup ions, and the boundaries and regions in the comet-solar wind interaction. We use in-situ measurements from the four comets visited to date by spacecraft carrying plasma instrumentation: 21P/Giacobini-Zinner, 1P/Halley, 26P/Grigg-Skjellerup and 19P/Borrelly, to illustrate the process in action. While, of these, comet Halley is not a Jupiter class comet, it has told us the most about cometary plasma environments. The other comets, which are from the Jupiter family, give an interesting comparison as they have lower gas production rates and less-developed interactions. We examine the prospects for Rosetta at comet Churyumov-Gerasimenko, another Jupiter family comet where a wide range of gas production rates will be studied.     

Comparison of parallel group velocity of ECH waves with electron resonant velocity: Implication for electron diffusion by ECH waves

We have investigated the role of group velocity in the calculation of pitch-angle diffusion coefficients by electron cyclotron harmonic (ECH) waves in planetary magnetospheres. The assumption which is generally made that the parallel group velocity can be neglected in comparison with particle parallel velocity is examined in detail. It is found that for lowest harmonic band this assumption is quite good. It is found that in general it is not possible to ignore the parallel group velocity. However, for lowest harmonic band this assumption is quite good at low electron temperatures.     

Multispacecraft Observations of Magnetic Clouds and Their Solar Origins between 19 and 23 May 2007

We analyze a series of complex interplanetary events and their solar origins that occurred between 19 and 23 May 2007 using observations by the STEREO and Wind satellites. The analyses demonstrate the new opportunities offered by the STEREO multispacecraft configuration for diagnosing the structure of in situ events and relating them to their solar sources. The investigated period was characterized by two high-speed solar wind streams and magnetic clouds observed in the vicinity of the sector boundary. The observing satellites were separated by a longitudinal distance comparable to the typical radial extent of magnetic clouds at 1 AU (fraction of an AU), and, indeed, clear differences were evident in the records from these spacecraft. Two partial-halo coronal mass ejections (CMEs) were launched from the same active region less than a day apart, the first on 19 May and the second on 20 May 2007. The clear signatures of the magnetic cloud associated with the first CME were observed by STEREO B and Wind while only STEREO A recorded clear signatures of the magnetic cloud associated with the latter CME. Both magnetic clouds appeared to have interacted strongly with the ambient solar wind and the data showed evidence that they were a part of the coronal streamer belt. Wind and STEREO B also recorded a shocklike disturbance propagating inside a magnetic cloud that compressed the field and plasma at the cloud’s trailing portion. The results illustrate how distant multisatellite observations can reveal the complex structure of the extension of the coronal streamer into interplanetary space even during the solar activity minimum. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Solar Wind Effects Associated with a Recurrent,High-Latitude Coronal Brightness Enhancement

During the descent of Ulysses following the 2001 solar north pole passage, the SOHO LASCO C2 telescope recorded a particularly strong sequence of recurrent polarization brightness (pB) features at latitudes of around 55°. As Ulysses passed overhead, solar rotation swept the interplanetary extensions of these persistent coronal structures over the spacecraft. Comparison of solar remote sensing and Ulysses in situ observations through 2002 reveals the solar wind effects of very bright and recurrent K-coronal structures at high solar latitudes and of a steeply inclined heliospheric neutral sheet (HNS). Despite the high level of solar activity, the HNS at high latitude still organizes solar wind stream structure much as it did near the previous solar minimum. The recurrent coronal streamers originate slow solar wind and mark the northern extremity of a very tilted HNS whose passage at Ulysses is accompanied by slow, dense solar wind, enhanced temperature, depressed α abundance, enhanced magnetic fields, and magnetic field directional changes that evolve with spacecraft latitude.     

利用窄线宽激光测量锶原子束速率分布

利用多普勒频移效应,通过线宽2MHz窄线宽激光与锶原子束在大角度作用下的荧光信号分析,测量了锶原子束速率分布。比较和分析表明,理论结果与实验结果符合较好。