A non-local closure model for vertical mixing in the convective boundary layer

A simple non-local closure model for vertical mixing in Convective Boundary Layers (CBL) has been developed specifically for application in regional or mesoscale atmospheric chemistry models. The model, named the Asymmetrical Convective Model (ACM), is based on the concept that vertical transport within the CBL is inherently asymmetrical. Upward transport by buoyant plumes originating in the surface layer is simulated by mixing from the lowest model layer directly to all other layers in the CBL. Downward transport, however, proceeds only to the next layer in order to emulate gradual compensatory subsidence. The ACM is similar to the model developed by Blackadar (1978, 4th Symp. on Atmospheric Turbulence, Diffusion and Air Quality, pp. 443–447, Reno, Am. Meteorol. Soc.) but differs in its treatment of downward transport. The realism of the ACM is tested through comparisons to large-eddy simulations of several idealized test cases. These tests show that while the ACM shares the Blackadar model's ability to simulate rapid transport upward from the surface layer to all levels in the CBL, it is clearly superior in its treatment of material emitted from elevated sources either within or above the CBL. The ACM is also tested in the context of the Regional Acid Deposition Model (RADM) both to determine sensitivity to different CBL mixing schemes and to compare to vertically resolved aircraft measurements. These tests demonstrate quicker upward transport of ground-level emissions by the ACM as compared to the eddy diffusion scheme currently used in RADM. The ACM also affects ozone photochemistry in the boundary layer resulting in lower ozone concentrations in areas of high NO_x emissions.     

Diurnal wind-structure variations and dispersion of pollutants in the boundary layer

A coupled boundary-layer model and Lagrangian particle model are used to investigate the role of boundary-layer shear especially that produced by inertial oscillations in affecting the horizontal dispersion of pollutants on time-scales of 24–36 h. The coupled models show that the amplitude and the effective periods of the inertial oscillations are the main cause of nocturnal accelerating dispersion. The effective width of the plume in the morning is determined by whether the morning daytime mixing coincides with the phase of the inertial oscillation being at a maximum or minimum value. The phase of the oscillation is determined by latitude. Thus, latitude is shown to be an extremely important parameter in determining horizontal dispersion. An analytical model is introduced to investigate the role of external parameters such as latitude in influencing the horizontal dispersion. The analytical model is based on a simple Ekman-type model for the daytime and nighttime boundary layer. The Ekman model is used to provide initial conditions to an inertial oscillation regime between the nighttime boundary layer and the old daytime boundary layer. The analytical model was able to reproduce the magnitude and phase of the inertial oscillations reasonably well. However, the Ekman model overestimates the shear in the boundary layer causing the inertial oscillation to be too large. A semi-empirical method was used to provide more reasonable estimates of the daytime boundary-layer structure. This semi-empirical approach gave rates of the horizontal dispersion which were in general agreement with the numerical results.     

Observed structure of the nocturnal urban boundary layer and its evolution into a convective mixed layer

The turbulence properties of the nocturnal urban boundary layer (UBL) and its transition to the convective mixed layer have been observed over the urban center of Sapporo, Japan. The nocturnal UBL was characterized by an elevated inversion layer and a near-neutral mixed layer below it. The inversion layer varied with time and showed a wavy pattern. Kelvin-Helmholtz instability (billow) was observed within the elevated inversion layer, the horizontal and vertical scales of this billow being evaluated to be of the order of the vertical scale of the elevated inversion layer. The transition from the nocturnal UBL to the convective mixed layer occured rapidly, within 1–1.5 h over the center of the urban area. At this time, the downward transport of sensible heat below the inversion base became important. The normalized turbulence intensities in the convective mixed layer were also examined.     

Large-eddy simulation of a turbulent reacting plume

The results of large-eddy simulations (LES) of a chemically reacting, dispersing plume in a turbulent boundary layer are presented. The detailed simulations of the dispersion mechanisms are used to investigate the effects of turbulent fluctuations on nonlinear chemical reactions. The idealized single reaction between ozone and nitrogen oxides, NO+O₃→NO₂+O₂, is used as a representative simple, irreversible binary reaction. Effects of different reaction rates are investigated by varying the source NO concentration, while maintaining a constant ambient ozone level. The simulations demonstrate that the rate of production of NO₂ is significantly affected by the turbulent nature of the dispersing plume. The effect of incomplete turbulent mixing of the two reactants on the reaction rate can persist for long distances downstream, particularly for the high NO levels typical of power plant stacks. The LES results also show that the important measure of the turbulent fluctuations, the turbulence segregation coefficient, may be reasonably approximated by a constant value over most of the plume cross-section. This result may prove useful for simplified plume modeling, but will depend on whether the range of constant segregation coefficient covers the whole reaction region since this takes place predominantly in the plumeedges.     

Observations and numerical simulation of the evolution of the tropical planetary boundary layer during total solar eclipses

Two convective boundary layer experiments were concluded in the tropics during total solar eclipses, one at Raichur, India on 16 February 1980 and the other at Tanjung Kodok, Java, Indonesia on 11 June 1983. Period of totality was about 3 min at Raichur and 5 min at Java. With the partial phase of the eclipse extending over a combined period of about 2-h before and after totality, there was sufficient time in these experiments for the atmospheric boundary layer to react to changes in solar radiation. Results from the Indian experiment indicated significant changes in the atmospheric stability during the eclipse with slightly stable conditions present after the second contact. Java observations showed similar results but with smaller effects. Changes in stability also caused changes in turbulence structure. In this paper, we present the observations made during these two experiments, as well as the results of numerical simulations using a one-dimensional, second-order closure PBL model. In terms of location, the Raichur site with approximately homogeneous conditions was better suited for the model simulations than Tanjung Kodok, which was located about 1 km downwind from the ocean.     

边界层方案对南京地区PM2.5浓度模拟的影响

基于WRF-Chem空气质量模式采用YSU、MYJ、MYNN2、ACM2,4种边界层方案和基于ACM2的敏感性实验方案ACM2R2（模式最低6层的湍流扩散系数阈值改为2m²/s）和ACM2R5（模式最低10层的湍流扩散系数阈值改为5m²/s）,对南京地区冬季出现的一次天气污染过程进行模拟,分析了不同边界层方案对南京地区PM_2.5浓度模拟的影响.结果表明,6种方案对地面气象要素和风温湿廓线的模拟显示了较为合理的日变化特征及高度变化趋势,不同边界层方案之间气象要素差异较小.各方案均较好地模拟出了边界层高度逐日变化和日变化特征,以及PM_2.5浓度随时间的变化趋势,但是YSU、MYJ、MYNN2和ACM2,4种边界层方案在夜间对PM_2.5浓度的模拟均存在较大程度的高估,而ACM2R2和ACM2R5在ACM2的基础上显著地降低了PM_2.5浓度,甚至转为低估.从整个时段的偏差上看,ACM2R2最接近观测值.这是由于ACM2R2夜间湍流扩散系数较高,更利于源于地表的污染物向上扩散,使得ACM2R2方案在夜间相比原ACM2方案模拟所得的地表PM_2.5浓度更低,从而改善了原方案高估的现象.ACM2R5夜间湍流扩散系数更高,后期扩散过强从而产生较大低估.这些结果表明湍流扩散系数的不同是PM_2.5浓度模拟差异的重要原因,准确地参数化边界层方案夜间湍流扩散系数对于提高PM_2.5浓度模拟的准确度十分必要.     

WRF模式对污染天气下边界层高度的模拟研究

大气边界层高度是影响大气污染物浓度的重要因素之一,但数值模式中选择不同边界层参数化方案模拟的边界层高度有很大差异.利用WRF模式中5种边界层参数化方案及2006~2007年春、秋、冬3季河北香河地区激光雷达观测资料,对比分析了污染天气下,不同边界层方案对边界层高度的模拟效果,并分析了误差产生的可能原因.结果表明：5种参数化方案均能模拟出3季污染天气下边界层高度的变化特征,但各方案模拟的边界层高度与观测之间均存在较大误差.模拟的最大边界层高度月变化特征显示,秋冬季的模拟结果与观测值匹配较好,春季偏差较大;模拟的边界层高度日变化显示,均方根误差：春季 > 秋季 > 冬季,且误差在午后（14：00~18：00）更加明显;对该地区而言,非局地YSU方案能较好地模拟污染天气下的边界层高度;各参数化方案中边界层高度计算方法的不同及对大气廓线、湍流动能的模拟差异,可能是造成模拟边界层高度产生误差的主要原因.     

Parallel computations of turbulent diffusion in convective boundary layers on shared-memory machines

Currently, time-splitting finite difference or finite element schemes are commonly used in air-pollution modeling. In light of the recent advances in parallel processing, it is crucial to evaluate the performance of these time-splitting schemes on different multiprocessors. In this paper, we use a K-theory diffusion model and a second-order closure model to simulate turbulent diffusion in convective boundary layers. The objective is to assess the “portability” of time-splitting finite element schemes for shared-memory multiprocessors, such as IBM 3090-600J, the Alliant FX/8, the Sequent Symmetry S81 and the Encore Multimax. It was found that significant speed-ups were obtained as a result of parallelization with small programming efforts. Furthermore, our two- and three-dimensional results also suggest that second-order closure models are now computationally affordable and feasible for realistic air pollution modeling.     

Simulation of boundary layer flow in Hong Kong

A three-dimensional hydrostatic numerical model using level-two turbulence closure was employed to simulate typical local wind phenomena in subtropical Hong Kong. Simulations for light wind, strong wind, sea breeze and stable flows were made and the model results compare favourably with observations.     

粉尘粒子运动扩散特性的大涡模拟研究

采用大涡模拟的方法(LES)模拟了气体运动,采用Lagrangian方法模拟了粉尘颗粒运动,针对平面尾迹流和湍流射流这2种基本流场类型对不同stokes(St)数粉尘颗粒随流场的空间分布特性进行了数值模拟．2种不同工况下的数值模拟结果均显示,小Stokes数颗粒在流场中受流体涡团的影响较大,一般分布于涡团核心区,并且小Stokes数颗粒可以和流场近似无滑移的同步发展;中等Stokes数颗粒由于受来流涡结构离心力与颗粒自身惯性力的影响大致相同,一般分布于涡核的边沿;而Stokes数远大于1的大颗粒,由于其在流场中跟随性减弱,更多的颗粒按其原有的方向运动,表现的是自身运动惯性的影响．     

风洞模拟湍流边界层的子波分析

应用子波分析对在环境风洞中模拟的平坦及城市近地面边界层湍流进行了研究,探讨了湍流的间歇性结构与湍流的串级输送.实验结果表明,近地面边界层的湍流行为具有强烈的间歇性特征,因而在用数值或物理模拟近地面层湍流时,这种间歇性时间尺度以及相应的几何尺度必须加以考虑.     

春季中国近海海表大气中六氟化硫的分布特征

基于2017年春季黄-东海航次,在我国南黄海和东海海域开展了海表大气中六氟化硫离散采样和测定工作。结果显示,观测期间,黄-东海海表大气中六氟化硫摩尔分数范围为（9.5~14.0）×10-12,平均值为（10.5±1.3）×10-12,总体呈近岸高,远岸低的分布特征。结合现场风速和风向数据及后向轨迹模拟计算分析可得,受短距离和低空陆源气团输送的主导影响,东海东北部和西南部六氟化硫摩尔分数较高。受长距离或来自太平洋地区气团输送的主导影响,南黄海和东海西北部及东南部海表大气六氟化硫摩尔分数较低且分布均匀。本研究所获数据和结论将有助于更深入准确地研究本海域其他大气成分的时空分布特征和调控机制。     

Thermodynamic features of the atmospheric boundary layer during the summer monsoon

Characteristic variations in the thermodynamic parameters of the boundary layer at Pune (18°32′N, 73°51′E, 559m a.s.l.) have been studied using the aerological observations collected during the summer monsoon seasons of 1980 and 1981, and the temperature observations from aircraft during the summer monsoons of 1976, 1979–1981. This study showed suppression of the mixed layer, absence of inversion/stable layers and decreased convective instability in the lower layers during the period of active monsoon conditions. The reverse was observed during the periods of weak/break monsoon conditions. Temperature stratification of sub-cloud layer has been classified into four different categories depending on the extent of the mixed layer and the gradient of potential temperature in the overlying stable layer. It was observed that these categories cover all types of weather, conditions which prevailed during the monsoon season. The results are discussed with the possible association of the weather conditions prevailed during the active and break monsoon periods.     

一种改进的边界层参数化模式

在能量平衡方法的基础上提出一套利用常规气象资料求取边界层参数的改进方案。利用中国科学院大气物理研究所铁塔观测资料,与廓线法进行对比分析,表明这种方案具有输入要求少,输出信息量大,结果合理的优点。此外,对此方案中的4个可变参数作了敏感性试验,结果表明湿度变化参数α的敏感性较强,在使用时应当合理选择其数值。该方案可以满足新一代空气质量模式对边界层参数化的需求。同时,由于仅需常规气象资料就能求取所有的边界层参数,因此特别适合于空气质量模式的法规应用。     

天津市郊区冬季大气边界层风、温场结构与特征

利用天津市郊区探空观测资料,对该地区冬季大气边界层内的风场、温度场结构与特征进行研究.结果表明,测试期间风向随着高度增加呈顺时针旋转, 由东风、东南风逐渐右转,转为西南风、西风,遵循Eckman螺线的规律;不同高度层风速日变化规律不同.一般来说,在较低高度风速白天变大,夜晚变小,而较高处则相反;接地逆温大约在20:00开始生成,随着时间的增加,逆温逐渐加强,平均逆温厚度在02:00达到最大值;混合层高度在早晨厚度较薄,午后混合层厚度较厚,有利于污染物的扩散;天津市市区10m和100m高度层风速的日变化规律与郊区相同,200m高度层风速日变化规律不同;市区与郊区相比,不同高度层风速随着时间起伏变化较小.     

青岛酸雨天气边界层气象特征

青岛地区酸雨成因研究表明,青岛市近代工业发展排放大量致酸物质、海洋上天然排放的二甲基硫是重要的原因;青岛地区边界层风场和温度场的特殊规律是形成酸雨的另一个重要原因;江淮气旋和黄淮气旋大型天气形势起了辅助作用。本文着重分析研究了1993年7月23日一次典型酸雨的实际例子,证明了上述论点。