边界层参数化方案在“灰色区域”尺度下的适用性评估

随着数值预报模式分辨率的提高,当模式网格距与含能湍涡的长度尺度相当时,模式动力过程可解析一部分湍流运动,而剩余的湍流运动仍需参数化,此时便产生了湍流参数化的“灰色区域”问题。对传统的PBL（Planetary Boundary Layer）方案在“灰色区域”下的适用性评估,是改进PBL方案以使其能够适应分辨率变化的前提和基础。本研究基于干对流边界层的大涡模拟试验,比较了WRF（Weather Research and Forecast Model）模式中四种常用的边界层参数化方案[YSU（Yonsei University）、MYJ（Mellor-Yamada-Janjic）、MYNN2.5（Mellor-Yamada-Nakanishi-Niino Level 2.5）、MYNN3）]在“灰色区域”尺度下的表现。研究表明,混合层内总热通量对所使用的参数化方案和水平分辨率均不敏感。不同参数化方案中次网格与网格通量的比例表现出对水平网格距不同的依赖性。局地PBL方案（MYJ、MYNN2.5）在混合层内的平均位温随网格距减小而增大,次网格通量随网格距减小而减小,较参考湍流场对次网格通量有所低估。YSU方案的非局地项几乎不随水平格距改变而变化,对次网格通量的表征并未表现出较强的分辨率依赖性,且过强的非局地次网格输送使混合层内温度层结呈弱稳定,抑制了可分辨湍流输送,不易于激发次级环流。MYNN3方案的非局地次网格通量（负梯度输送项）随网格距减小而减小,使其对次网格通量的表征具有较好的分辨率依赖性。PBL方案在“灰色区域”尺度下的适用性与具体分辨率有关。以分辨率500 m为例,四种PBL方案中不存在一种最佳方案,能对边界层的热力结构和湍流统计特征均有准确的描述。     

磁场重联实验室模拟进展

磁场重联是空间等离子体和实验室等离子体中的常见现象,被认为是太阳耀斑和磁层亚暴的重要机制。实验室磁场重联的模拟研究已经有二十余年的历史,并且取得了一系列重要的结果。对几个主要的磁场重联实验装置进行了介绍,给出了各个装置的等离子体参数以及产生重联的方法,回顾了实验室研究中和太阳射电密切相关的几个问题。另外,以中国科技大学的线性磁化等离子体装置为基础,建立了国内首个磁场重联的实验装置,研制了实验中需要的诊断工具,并开展了初步的磁场重联实验。     

Rainfall effect on wind waves and the turbulence beneath air-sea interface

Rainfall effects on wind waves and turbulence are investigated through the laboratory experiments in a large wind-wave tank. It is found that the wind waves are damped as a whole at low wind speeds, but are enhanced at high wind speeds. This dual effect of rain on the wind waves increases with the increase of rain rate, while the influence of rainfall-area length is not observable. At the low wind speed, the corresponding turbulence in terms of the turbulent kinetic energy （TKE） dissipation rate is significantly enhanced by rain- fall as the waves are damped severely. At the high wind speed, the augment of the TKE dissipation rate is suppressed while the wind waves are enhanced simultaneously. In the field, however, rainfall usually hin- ders the development of waves. In order to explain this contradiction of rainfall effect on waves, a possibility about energy transfer from turbulence to waves in case of the spectral peak of waves overlapping the inertial subrange of turbulence is assumed. It can be applied to interpret the damping phenomenon of gas trans- fer velocity in the laboratory experiments, and the variation of the TKE dissipation rates near sea surface compared with the law of wall.     

The vertical structure of the atmospheric boundary layer over the central Arctic Ocean

The tropopause height and the atmospheric boundarylayer （PBL） height as well as the variation of inversion layer above the floating ice surface are presented using GPS （global position system ） radiosonde sounding data and relevant data obtained by Chinas fourth arctic scientific expedition team over the central Arctic Ocean （86°-88°N, 144°-170°W） during the summer of 2010. The tropopause height is from 9.8 to 10.5 km, with a temperature range between -52.2 and -54.10C in the central Arctic Ocean. Two zones of maximum wind （over 12 m/s） are found in the wind profile, namely, low- and upper-level jets, located in the middle troposphere and the tropopause, respectively. The wind direction has a marked variation point in the two jets from the southeast to the southwest. The average PBL height determined by two methods is 341 and 453 m respectively. These two methods can both be used when the inversion layer is very low, but the results vary significantly when the inversion layer is very high. A significant logarithmic relationship exists between the PBL height and the inversion intensity, with a correlation coefficient of 0.66, indicating that the more intense the temperature inversion is, the lower the boundary layer will be. The observation results obviously differ from those of the third arctic expedition zone （800-85° N）. The PBL height and the inversion layer thickness are much lower than those at 870-88° N, but the inversion temperature is more intense, meaning a strong ice- atmosphere interaction in the sea near the North Pole. The PBL structure is related to the weather system and the sea ice concentration, which affects the observation station.     

南海冬季一次海面大风天气的WRF模式预报检验

为寻找出适合南海冬季海面大风天气预报的边界层参数化方案,利用中尺度气象模式WRF中9种边界层方案(YSU、MYJ、QNSE-EDMF、MYNN2、MYNN3、ACM2、BouLac、UW、GBM),对2012年12月29-31日的大风过程进行预报,并用最终分析资料(FNL)检验10 m风场预报。结果表明:风速风向预报的整体平均偏差相当,风向预报的均方根误差较风速大;风速风向与实况的相关随着预报时间增加,整体呈现下降趋势;各方案对海陆交界风速预报普遍偏大2 m/s以上,而在远离陆地的海域偏差较小;YSU方案对北部湾、东沙群岛、西沙群岛、南沙群岛4个海区风场的变化趋势均能较好预报;整体而言,南海大部分海域的预报偏差较小,YSU、MYNN2、MYNN3方案对风速预报较好,ACM2方案对风向预报较好。     

高原东南缘大气近地层湍能特征与边界层动力、热力结构相关特征

基于云南省大理2008年3、5、7月GPS加密探空试验时段(14和02时)资料,结合边界层铁塔综合观测资料,采用温度梯度法、逆温强度法和涡动相关法分别计算高原东南缘对流边界层(convective boundary layer,CBL)及稳定边界层(stable boundary layer,SBL)顶高度,通过计算获取感热通量、潜热通量、湍流动能、切变项以及浮力项与大气动力、热力过程垂直相关特征综合分析,可发现湍能方程中浮力项、感热、潜热通量与NCEP再分析资料计算获取的大气视热源相关特征显著,这某种程度反映了高原东南缘近地层大气湍流动量、热量输送对低层大气视热源Q_1的重要贡献。低层视热源Q_1亦表现出与湍能方程分量类似的日变化周期,此特征反映了高原东南缘大气热源变化与下垫面水热过程及其湍流输送日变化密切相关;浮力项与湍能等项对大气低层热源与涡动特征、热力混合结构的形成有重要作用;低层大气视热源、水汽汇均与边界层高度有显著相关,综合分析结果某种程度描述了青藏高原东南缘近地层湍流动量、热量输送状况与低层大气热源,热力混合边界层结构的综合相关物理图像,初步探索了高原东南缘对流活跃区大气湍流运动与大气动力、热力过程相互作用特征。研究表明近地层湍流通量变化某种程度可反映未来局地大气视热源垂直结构变化的"强信号"特征。本文上述研究结论也可启发我们进一步关注近地层湍流通量异常变化特征及其对局地降水过程大气热源结构演变的影响问题。     

辐射雾局地爆发性增强原因探讨

利用常规气象资料、NCEP 1°×1°再分析资料、河北省高速公路专业气象自动监测资料以及天津市255 m气象铁塔梯度观测资料,对2010年12月21-22日发生在河北东北部沿海(以乐亭为代表站)和天津地区的一次历时短、局地性强的辐射雾爆发性增强原因及其边界层结构特征进行研究。结果表明,乐亭爆发性大雾的形成是由于低层暖湿气流受切变线的阻挡作用形成了弱水汽辐合,加上短波槽前弱冷空气和入夜后辐射降温共同影响所导致;副冷锋南下逼近天津境内时,渗透进低层的锋前弱冷空气是天津大雾爆发性发展的直接原因;同时,由于副冷锋南压过程中,锋后较强冷空气过境导致雾天气由北向南逐渐消散。雾过程中,始终维持着近地层湿冷、上层干暖的温湿结构。雾爆发性增强前,气温呈整体下降趋势的过程中存在小幅上升的现象,对局地浓雾的爆发有很好的指示意义。     

Spectrum Analysis of Wind Profiling Radar Measurements

Unlike previous studies on wind turbulence spectrum in the planetary boundary layer, this investigation focuses on high-altitude （1-5 km） wind energy spectrum and turbulence spectrum under various weather conditions. A fast Fourier transform （FFT） is used to calculate the wind energy and turbulence spectrum density at high altitudes （1-5 km） based on wind profiling radar （WPR） measurements. The turbulence spectrum under stable weather conditions at high altitudes is expressed in powers within a frequency range of 2 × 10-5-10-3 s-1, and the slope b is between -0.82 and -1.04, indicating that the turbulence is in the transition from the energetic area to the inertial sub-range. The features of strong weather are reflected less obviously in the wind energy spectrum than in the turbulence spectrum, with peaks showing up at different heights in the latter spectrum. Cold windy weather appears over a period of 1.5 days in the turbulence spectrum. Wide-range rainstorms exhibit two or three peaks in the spectrum over a period of 15-20 h, while in severe convective weather conditions, there are two peaks at 13 and 9 h. The results indicate that spectrum analysis of wind profiling radar measurements can be used as a supplemental and helpful method for weather analysis.     

非平稳风沙运动研究进展

风蚀输沙起动条件与输沙率预测是风蚀预报、沙尘数值预报与风沙地貌演化以及防沙治沙工程研究所需的关键参数,因而一直是风沙物理学研究的重点课题。早期的研究主要关注稳态输沙过程,对野外实际输沙过程的预测存在相当大的误差。近年高频的实时同步输沙、风速测量显示:大气边界层近地表风速脉动强烈,因而导致野外实际输沙具有高度的时空变化。研究发现:风沙输运的非平稳特性与边界层湍流结构密切相关,因而会对沙粒起动条件判断、输沙率预测等产生影响。回顾了近20年中非平稳输沙研究在实验、理论以及数值模拟方面的进展,并提出了目前存在的问题和今后发展的趋势。     

月球激光测距的新技术方法研究

月球激光测距是国内外所瞩目的宏伟目标，代表着单光子探测技术的高峰。本论文的目的是探索提高激光测月回波光子数的新方法，进而增加激光测月成功的几率。其思路是源于一个新的想法：在激光测月过程中引入大气波前倾斜量实时补偿的技术。首先介绍激光测月的现状和其难度所在：回波光子数太少，基本上属于亚光子探测范畴。在现有技术条件下，本文对影响激光测月回波光子数的因素逐一进行分析讨论，提出应该把激光束截面能量分布和大气湍流效应包括进去。为此分析讨论了大气湍流和大气中光场的基本统计性质、激光束在大气中传输时所受大气湍流的影响以及大气湍流对激光测距的影响，得出大气湍流特别对激光测月有着明显影响的结论。在此基础上对传统的激光测距方程进行了修正，使其应用到激光测月时更符合真实的情况，从而指导补偿的进行。涉及到在激光测月中对受大气湍流而畸变的激光束进行补偿，本文抓住重点，通过分析看出对大气倾斜量的实时补偿是提高激光测月回波光子数的重要因素。结合激光测月以及云南天文台现有测距系统的实际情况，本文独创性地提出利用激光测月目标近旁一定大小区域的扩展面源探测与计算大气倾斜量，然后对激光测月中的激光束进行大气倾斜量实时补偿的新技术方法。在分析比较各种从低对比度扩展源中计算大气倾斜量的算法后，本文认为采用绝对差分法最佳。最后是具体的激光测月的新技术方案，同时对此方法的理论与技术进行了分析讨论，推导出新技术方法的理论依据、补偿效果、可行性与实用性。