Similarity theory and calculation of turbulent fluxes at the surface for the stably stratified atmospheric boundary layer

In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved surface flux calculation technique for use in operational models.     

The influence of nonstationarity on the turbulent flux–gradient relationship for stable stratification

Extensive eddy-correlation datasets are analyzed to examine the influence of nonstationarity of the mean flow on the flux–gradient relationship near the surface. This nonstationarity is due to wavelike motions, meandering of the wind vector, and numerous unidentified small-scale mesoscale motions. While the data do not reveal an obvious critical gradient Richardson number, the maximum downward heat flux increases approximately linearly with increasing friction velocity for significant stability. The largest of our datasets is chosen to more closely examine the influence of stability, nonstationarity, distortion of the mean wind profile and self-correlation on the flux-gradient relationship. Stability is expressed in terms of z/L, the gradient Richardson number or the bulk Richardson number over the tower layer. The efficiency of the momentum transport systematically increases with increasing nonstationarity and attendant distortion of the mean wind profile. Enhancement of the turbulent momentum flux associated with nonstationarity is examined in terms of the nondimensional shear, Prandtl number and the eddy diffusivity.     

全国酸雨观测网未知水样考核结果的统计分析

应用客观统计方法,整理分析了1992年以来全国酸雨观测站网的11次pH未知水样和2次电导率未知水样的考核结果。分析显示,采用稀释和pH调制的标准缓冲溶液作为pH未知水样,其测量偏差随pH标准值的变化不显著,历年的测量标准差均小于0.2pH单位。pH未知水样测量偏差的波动状况指示出观测台站仪器和人员的技术状态变化,对仪器更新、人员培训以及新业务规范的执行等业务建设和质量管理活动有显著的正面响应。对pH未知水样测量偏差的统计还显示,2001年后pH平均测量偏差出现负偏态分布特征,该特征在总体偏差较大时尤为明显,说明部分技术状态不良的台站往往容易出现较大的系统性负偏差。根据考核结果,按照世界气象组织(WMO)推荐的方法,计算除1992年外各次考核的pH值测量的修正绝对中值偏差为0.03～0.10pH单位,1992年和2006年电导率测量的修正绝对中值偏差为4%～5%。修正绝对中值偏差的逐年波动情况反映了全国酸雨观测网pH测量的总体偏差也明显地受台站技术状态波动的影响。     

条件数在区域日降水量预报中的应用研究

针对前汛期广西北部暴雨洪涝灾害频发的问题,利用T213模式和日本细网格降水预报等数值预报产品,采用条件数计算选取回归因子的方法建立广西北部区域平均降水量的预报方程。进行了2004、2005年两年5-6月的前汛期业务预报应用试验。采用条件数方法建立的预报方程和传统的逐步回归预报方程的平均预报误差分别为6．3569mm和7．0096mm,同期T213模式的预报误差为7．9456mm。试验表明新方法比传统的逐步回归方程和T213模式具有更高的预报精度,有较好的业务应用前景。计算对比可知,采用条件数计算选择的预报因子间的复共线性较微弱,而逐步回归方法选择的因子间的复共线性很严重,影响了回归方程的预报效果。     

Relaxed Eddy Accumulation Simulations of Aerosol Number Fluxes and Potential Proxy Scalars

Direct eddy-covariance measurements of aerosol number fluxes obtained during the 2007 CHATS field experiment in Dixon, California, USA are compared with relaxed eddy accumulation simulations using temperature and water vapour concentration as proxy scalars. After a brief discussion of the limited time response of the aerosol measurement, the applicability of temperature and water vapour concentration as proxy scalars for aerosol number concentration is investigated by evaluating scalar and spectral correlation coefficients as simple measures of scalar similarity. In addition, the proportionality factor b, which compensates for the use of a constant sampling flow in relaxed eddy accumulation, is derived from the time series of aerosol number, temperature and water vapour, and its variability is analyzed. The reduction of the b factor due to application of a deadband, i.e. the rejection of data when the vertical wind speed is close to zero, is evaluated for all three studied scalars, and compared with published functional relationships. In this study, using temperature or water vapour as proxy scalars for aerosol number shows no advantage over the use of a constant b factor. Thus, it is suggested to apply a deadband H _REA  = w′/σ _w  = 0.6 to 0.8 (where w′ is the vertical velocity fluctuation and σ _w is its standard deviation), to use a theoretical b factor based on a parameterization that includes a stability dependence, and to calculate the deadband effect according to a derived relation for aerosol relaxed eddy accumulation.     

平流层重力占优气溶胶粒子的耦合碰并

研究大气平流层中的重力作用占优的气溶胶粒子之间的耦合碰并问题.平流层由于空气稀薄,必须引入高Knudsen数下分子体系介质近似,对于重力占优的气溶胶粒子,采用高Qij下的处理方法,在新的对分布方程的基础上,求得重力作用占优的气溶胶粒子的对分布函数的解析解.结果表明,对于重力占优的气溶胶粒子,平流层中碰并率要大于对流层中的碰并率.     

Self‐adjustment in rivers: Evidence for least action as the primary control of alluvial‐channel form and process

Least action principle (LAP) in rivers is demonstrated by maximum flow efficiency (MFE) and is the foundation of variational mechanics based on energy and work rather than Newtonian force and momentum. Empirical evidence shows it to be the primary control for the adjustment of alluvial channels. Because most rivers flow with imposed water and sediment loads down valley gradients they have largely inherited, they self‐regulate energy expenditure to match the work they are required to do to remain stable. Overpowered systems develop a variety of channel patterns to expend excess energy and remain stable. Australia offers an opportunity to study low‐energy rivers closely adjusted to very low continental gradients. The anabranching Marshall and single‐thread Plenty Rivers flow down nearly straight channels with average H numbers [ratio between excess bed shear and width/depth (W/D) ratio] close to the optimum of 0.3 for stationary equilibrium. Ridge‐form divisions of the original channel width create anabranches that radically alter W/D ratios relative to bed shear, the same being true for short‐wide islands on the large low‐gradient Yangtze River in China. In contrast, Mount Chambers Creek in Australia's tectonically more active Flinders Ranges is accreting an alluvial fan with unstable distributary channels exhibiting H numbers well below the optimum. LAP also explains profound biases in Earth's stratigraphic record. Because meandering is an energy‐shedding mechanism, sinuous rivers sequester relatively little sediment resulting in all sequences being just a few tens of metres thick. In contrast, low‐energy braided disequilibrium systems can sequester sediment piles over a kilometre in thickness and tens of kilometres wide. LAP provides a new paradigm for river research by identifying the attractor state controlling river channel evolution. It links advances in theoretical physics to fluvial geomorphology, stratigraphy and hydraulic engineering and opens opportunities for diverse investigations in Earth system science. Copyright © 2016 John Wiley & Sons, Ltd.     

云滴数浓度影响混合型层状云降水的数值模拟

使用耦合了Morrison双参数微物理方案的中尺度WRF模式V2．2,对2008年1月25-29日发生在我国南方的冰雪天气过程进行了数值试验。在模式准确再现了此次天气过程形势演变特点的基础上,对模式微物理方案中云滴数浓度影响累积降水量的情况进行了敏感性试验,发现云滴数浓度对降水量的影响是复杂和非线性的。对此次天气过程中的微物理量进行了详细的分析,并从各种水成物粒子的发展演变上,讨论了云滴数浓度的增加在暖云和冷云两种降水机制上对降水产生的不同影响。结果表明,云滴数浓度越大,云水混合比就越大,云滴的尺度越小。雨滴对不同云滴数浓度的响应与云滴的情况相反,随着云滴数浓度的增加,雨滴数浓度减小,雨水也减少,暖云降水过程受到了抑制;冰晶和雪晶的数浓度的演变过程没有明显变化,而冰晶和雪晶的混合比是相应增加的,冷云降水过程得到了一定程度的增强。从本文模拟的个例来看,设置不同云滴数浓度所得到的总累计降水量的差异在1％以内。总的来说,增加云滴数浓度,降水量会减少。从比例上来看,增加云滴数浓度对暖云降水过程的抑制作用比对冷云降水过程的增强作用更为显著,但是在本文模拟的个例中,冷云降水过程占主导地位,减少的降水和增加的降水的绝对值在同一个量级上并且数值相近,它们相互抵消后得到的结果是降水量变化的绝对值大大减小了,这解释了增加云滴数浓度后模拟的总累积降水量变化不明显的原因。     

三峡库区入汛早晚及暴雨特征分析

详细分析了三峡库区历年入汛日期早晚的气候特征，并与长江中下游地区的入梅日期作了比较，发现两者基本同步；另外，对库区各级、各类暴雨的多少及时空分布作了大量统计，结果表明，就区域平均而言，库区东部的蚶日数多于西部。     

感应电机矢量控制系统的仿真研究

根据2008年4—7月黄山大气气溶胶观测资料,研究了气溶胶粒子的数浓度、谱分布特征及其与气象因子的关系,探讨了雾天和非雾天气溶胶颗粒物时间和尺度分布特点。分析发现,黄山光明顶春、夏季大气气溶胶数浓度的平均值分别为3.14×103个/cm3和1.80×103个/cm3,其中超细粒子（粒径小于0.1μm的粒子）在春夏季分别约占总粒子数浓度的79%和68%;高数浓度值集中在粒径0.04～0.12μm;积聚模态气溶胶粒子（0.1～1.0μm）在体积浓度分布和表面积分布中占很大比例。结合气象资料比较了雾天与非雾天气溶胶分布的差异,发现细粒子浓度非雾天大于雾天,而气溶胶数浓度与温度呈正相关,与相对湿度成反相关。结果还发现,黄山在春季以西北风和偏南风为主,西北风时气溶胶数浓度较高,在夏季主要以偏南风,特别是西南风为主,但是气溶胶数浓度的高值多发生在偏东风的条件下。