积云中云滴群凝结增长的数值模拟

本文用一维非定常积云模式研究积云中云滴群的凝结增长。比较了几种盐核谱及不同湍流交换强度等情况下凝结形成的云滴谱特征。结果表明半径1.5—2.5微米的巨核在凝结过程形成大云滴中起主要作用,而较大的巨核对平衡态凝结谱影响不大,仅起到提早出现大云滴的作用。凝结核浓度过大不利于大云滴的形成,对半径 1.5—2.5微米的巨核浓度为 10~(-3)个/厘米~3比较有利。凝结平衡谱随湍流交换和夹卷作用的影响较大,在某些条件下可以模拟出双峰谱。     

海盐气溶胶和硫酸盐气溶胶在云微物理过程中的作用

利用大气气溶胶和云分档模式研究海盐气溶胶和硫酸盐气溶胶在云微物理过程中的作用, 计算结果表明:云中液态水含量随高度的分布并不随海盐、硫酸盐的数目以及云团上升速度的变化而变化; 随着云滴数目的增加, 云滴的有效半径会减小; 硫酸盐对云滴数目影响起主导作用, 海盐在水汽相对充足情况下增加了云滴数目, 在水汽相对不足的情况下减少了云滴数目; 硫酸盐粒子浓度特别强的情况下 (人类活动污染比较严重时), 如果水汽相对不足, 云滴数目会明显小于硫酸盐粒子浓度; 而海盐粒子的存在, 加剧了水汽的供应不足, 从而可以在很大程度上进一步降低云滴数目。也就是说, 在有些情况下, 如果不考虑海盐气溶胶的作用, 硫酸盐气溶胶对云特性的影响会被过高估计。     

三参数云微物理方案中气溶胶谱函数对云滴谱影响的数值模拟研究

观测和分档方案的数值模拟都证明气溶胶的谱分布特征对云滴谱的演变有直接影响继而作用于降水的发展。目前广泛使用的总体双参数云滴谱方案因为表征云滴谱的预报量不足,在凝结过程中云滴谱呈不正常的拓宽现象。因此在参数化方案中,气溶胶谱对云滴谱的影响未有明确结论。中科学院大气物理研究所(IAP)云降水物理与强风暴重点实验室(LACS)新研发的三参数方案(IAP-LACS)通过增加的预报量克服了云滴谱的拓宽问题,提高了云滴谱模拟的准确性。为了研究在参数化方案中气溶胶谱分布特征对云滴谱的影响,本文采用新方案进行WRF(Weather Research and Forecasting mode)大涡理想性试验,验证了新方案中气溶胶对数正态谱函数中数浓度、几何半径和标准差3个参量对云滴谱演变的影响。针对3个参量的敏感性试验表明新的气溶胶活化方案和三参数云滴凝结增长方案能够描述气溶胶谱对云滴谱演变的影响规律:气溶胶数浓度对云滴谱影响最显著,数浓度越高活化生成的云滴数量越多,云滴半径越小,云滴谱趋向窄谱,气溶胶数浓度低时,云滴数量少、半径大。较大的几何半径使气溶胶谱向大粒径移动,导致大云滴生成,标准差对云滴谱的影响最不显著。     

海洋层积云中的云滴谱宽度及其影响因子

云滴谱宽度对模式中云的光学厚度的参数化、气溶胶间接效应的评估以及降水形成过程的研究至关重要。本文利用美国POST（Physics of Stratocumulus Top）项目2008年7月19日的飞机观测资料，分析了微物理量和云滴谱的垂直分布及微物理过程。结果表明，该云系云滴谱宽度在云底附近较大，这是由低层核化过程导致的；中层凝结增长过程使得云滴谱宽度随高度增加逐渐减小；云顶附近夹卷混合过程导致云滴谱宽度增大。绝热云中垂直速度的增大会促进云凝结核的活化使云滴数浓度增大，促进凝结增长使云滴尺度增大、云滴谱宽度减小，云滴谱宽度与云滴数浓度、云滴尺度呈现负相关关系；云洞中受夹卷混合过程影响，垂直速度减小，云滴蒸发，云滴数浓度和云滴尺度减小、云滴谱宽度增大，且该效应随绝热程度减小而增强。建议云滴谱宽度的参数化将垂直速度、云滴数浓度、云滴尺度和绝热程度等考虑在内。     

山西省层状云微物理结构探测分析

针对山西省2009年6月18～19日的一次降水过程,利用机载DMT探测资料、Micaps常规天气资料和卫星云图等资料分析了这次降水的宏微观特征.结果表明:这次降水是锋面云系产生的,18日山西省处于冷锋锋前,19日为冷锋锋后;锋前云底较低,云层较厚,有夹层存在,云中上升气流很强,云滴数浓度最大为280 cm3、平均直径最大为15 μm、含水量最大值为0.35 g/m3,云滴谱呈双峰或多锋型;锋后云底较高,云层较薄,云滴数浓度最大值为170 cm-3、平均直径最大为10 μm、含水量最大值为0.05 g/m3,云滴谱呈双峰或多锋型;层状云在垂直方向和水平方向均存在不均匀性;垂直方向含水量变化与云滴尺度变化较为一致,水平方向含水量增加主要因为大云滴数密度的增加;T分布拟合云滴谱结果接近实际分布.     

华北中部夏季气溶胶垂直分布及其与云凝结核和云滴转化关系的飞机观测研究

气溶胶的时空分布及其核化成云的转化过程是云降水物理研究的重点,也是气候变化中气溶胶间接效应关注的热点问题。利用2013～2014年期间在华北中部山西地区开展的9架次夏季晴天和积云天气情况下的气溶胶、云凝结核（CCN）及云滴数浓度观测资料,分析研究了气溶胶的垂直分布、谱分布、来源特征及其与云凝结核、云滴数浓度的转化关系。研究结果表明,大气边界层逆温层结对气溶胶、CCN垂直分布有重要影响,不同天气条件下气溶胶谱型在低层差异较大而高层基本一致;垂直方向上CCN数浓度与气溶胶数浓度有较好的相关性,过饱和度0.3%条件下CCN比率（云凝结核/凝结核）与气溶胶有效直径呈线性关系;积云云下气溶胶与云滴的线性拟合方程为y=1.3x?616.3,拟合相关系数为0.96,气溶胶转化为云滴的比率可达到47%。在过饱和度0.3%条件下,云下CCN与云滴的线性拟合方程为y=1.6x?473.8,拟合相关系数也为0.96,CCN转化为云滴的比率可达到69%。     

一次降水性积层混合云系的微物理特征分析

利用山东省2007年10月27日1架次机载粒子测量系统（Particles Measuring System,PMS）积层混合云探测资料,分析了云中粒子浓度和尺度、液态含水量,以及小云滴和大云滴谱的垂直分布特征,比较了催化前后云微物理特征的变化。结果表明,催化前,云层中小云滴谱型为单峰,谱宽随高度增加先变窄后变宽,大云滴谱型在云低层为单峰,中高层为双峰谱,谱宽随高度增加先变宽后变窄,并且没有探测到降水粒子。催化后,小云滴尺度在低层减小、高层增加,整层液态水含量减小;大云滴浓度增加,尺度增大,出现降水粒子,固态粒子类型增多。在3 700~4 000 m高度层内小于10μm粒子明显增加,说明凝结过程比较明显,并且10~27.5μm粒子开始出现,启动了云滴的碰并机制。小云滴谱变化较小,基本为单峰谱,但在较大云滴处谱型略有起伏,在3 000m和3 300m高度的谱宽增宽。大云滴粒子谱有较大的变化,低层变成双峰谱,谱宽最宽可达650μm,中高层为双峰或多峰,峰值从小值向较大值移动。2D-P探头在催化云高层探测到降水粒子,谱型呈单调下降形态,谱宽最大为600μm。     

加强新时期气象部门财务管理与监督的思考

利用连续气流纵向热梯度云凝结核仪对华北地区空中云凝结核（CCN）进行观测研究.2009年飞机观测结果表明,同一块云内CCN数浓度变化范围和平均数浓度随高度增加逐渐减小,云的低层CCN数浓度变化趋势与云滴变化呈负相关,中高空CCN数浓度的水平分布规律与CN并不完全相关;比较三类不同天气条件下不同高度层的CCN谱分布特征发现,阴天和晴天谱型变化较小且较雨天的谱宽,雨天谱分布不连续.     

华东地区夏季云微物理结构的飞机观测分析

利用飞机搭载云粒子探头对2014年8月12-28日华东地区云的空间分布特征进行了探测,分析了云的垂直结构和水平分布特征,结合同时探测的气溶胶数据,探讨了云与气溶胶的相互作用关系。探测结果表明,安徽地区层状云云滴平均数浓度在24~297 cm^-3,液态含水量在0.04~0.13 g·m^-3,云滴数浓度随云底高度升高而减小,云滴粒径则随云底升高而增大。层积云(Sc)和雨层云(Ns)的云滴数浓度在云底最高,随高度上升浓度下降,液态含水量在云中部最高,云顶和云底处较低,高层云(As)云滴数浓度和液态含水量峰值均出现在云中上部。云的水平分布不均匀,云粒子双峰分布区域对应液态含水量高值区。Ns对气溶胶清除作用明显,清除方式以活化清除为主、碰并清除为辅。     

基于离子配对法的黄山地区云凝结核闭合研究

为了对黄山地区云凝结核（Cloud Condensation Nuclei,CCN）进行闭合研究,2014年6月30日至7月28日在黄山光明顶对大气气溶胶理化性质和CCN数浓度进行观测,分析了气溶胶化学组分、谱分布以及CCN数浓度随时间变化的特征,通过κ-Köhler理论并使用离子配对法计算得到CCN数浓度与观测得到的CCN数浓度进行对比。结果表明：计算与观测的CCN闭合结果较好,低过饱和度CCN闭合结果好于高过饱和度,过饱和度较低时低估了CCN数浓度,而过饱和度较高时则高估了CCN数浓度,由此说明气溶胶的化学组分数据对预测CCN数浓度至关重要,同时说明该方法可以实现CCN的闭合。考虑到40%水溶性有机碳（Water Soluble Organic Carbon,WSOC）对气溶胶粒子吸湿性影响,在较低过饱和度CCN闭合结果较好,但影响效果并不显著,尤其是在拟合结果相对较差的高过饱和度下基本没有影响。因此,气溶胶粒子中水溶性无机组分对CCN活化有重要影响,而含量较多、化学组分复杂并且吸湿性不确定的WSOC对CCN活化影响较为有限,这与一些研究得出无机组分对于气溶胶吸湿性的影响比具有复杂特征的有机组分更重要的结论相符合。     

Cloud condensation nuclei and cloud droplet measurements during ACE‐2

During the 2nd Aerosol Characterization Experiment (ACE‐2), relationships between stratocumulus cloud properties and aerosols were examined. Here, the relevant measurements including the cloud condensation nuclei (CCN) activation spectrum, updraft velocity, cloud microphysical and aerosol properties are presented. It is shown that calculations of droplet concentration based on updraft velocity and the CCN activation spectrum are consistent with direct observations. Also discussed is an apparent disparity among measurements of the CCN activation spectrum, the accumulation mode size distribution, and the composition of the submicrometric aerosol. The observed consistency between CCN, updraft and cloud droplets is a necessary refinement; however, extended analyses of the ACE‐2 data set are needed to guide improvements in model simulations of the interaction between aerosols and cloud microphysics. In particular, there is need for an examination of aerosol size spectra and chemical composition measurements with a view towards validating droplet activation schemes which relate the aerosol and cloud dynamical properties to cloud albedo.     

基于航测的云底气溶胶活化率与过饱和度估算

2016年11月13日在北京地区上空存在持续稳定的层状云天气背景下,利用飞机开展气溶胶粒径谱、化学组成、云滴谱等参量的垂直观测,研究该个例云底气溶胶的活化能力。结果表明:探测期间北京地区为轻度污染天气,地面气溶胶浓度(0.11~3 μm)达到4600 cm-3。云层高度为800~1200 m,云底气溶胶数浓度相对于近地面大幅度降低,有效粒径显著增大(0.3~0.6 μm)。同时,近地面气溶胶中疏水性的一次有机气溶胶贡献显著,而云底气溶胶中一次有机气溶胶的贡献大幅降低,无机组分和二次有机气溶胶的贡献明显增大,造成吸湿性参数κ由0.25(地面)增大至0.32(云底)。云中气溶胶和云滴的谱分布衔接较好,且两者的数浓度之和与云底气溶胶浓度一致,可分别代表未活化和已活化的粒子。基于云底气溶胶粒径谱和吸湿性参数计算得到不同过饱和比下云凝结核的活化率,通过与云中观测结果对比,反推得到云底过饱和度约为0.048%。     

Applying the WRF Double-Moment Six-Class Microphysics Scheme in the GRAPES_Meso Model: A Case Study

This study incorporated the Weather Research and Forecasting (WRF) model double-moment 6-class (WDM6) microphysics scheme into the mesoscale version of the Global/Regional Assimilation and PrEdiction System (GRAPES_Meso). A rainfall event that occurred during 3–5 June 2015 around Beijing was simulated by using the WDM6, the WRF single-moment 6-class scheme (WSM6), and the NCEP 5-class scheme, respectively. The results show that both the distribution and magnitude of the rainfall simulated with WDM6 were more consistent with the observation. Compared with WDM6, WSM6 simulated larger cloud liquid water content, which provided more water vapor for graupel growth, leading to increased precipitation in the cold-rain processes. For areas with the warmrain processes, the sensitivity experiments using WDM6 showed that an increase in cloud condensation nuclei (CCN) number concentration led to enhanced CCN activation ratio and larger cloud droplet number concentration (N_c) but decreased cloud droplet effective diameter. The formation of more small-size cloud droplets resulted in a decrease in raindrop number concentration (N_r), inhibiting the warm-rain processes, thus gradually decreasing the amount of precipitation. For areas mainly with the cold-rain processes, the overall amount of precipitation increased; however, it gradually decreased when the CCN number concentration reached a certain magnitude. Hence, the effect of CCN number concentration on precipitation exhibits significant differences in different rainfall areas of the same precipitation event.     

Kinetic limitations on cloud droplet formation and impact on cloud albedo

Under certain conditions mass transfer limitations on the growth of cloud condensation nuclei (CCN) may have a significant impact on the number of droplets that can form in a cloud. The assumption that particles remain in equilibrium until activated may therefore not always be appropriate for aerosol populations existing in the atmosphere. This work identifies three mechanisms that lead to kinetic limitations, the effect of which on activated cloud droplet number and cloud albedo is assessed using a one‐dimensional cloud parcel model with detailed microphysics for a variety of aerosol size distributions and updraft velocities. In assessing the effect of kinetic limitations, we have assumed as cloud droplets not only those that are strictly activated (as dictated by classical Köhler theory), but also unactivated drops large enough to have an impact on cloud optical properties. Aerosol number concentration is found to be the key parameter that controls the significance of kinetic effects. Simulations indicate that the equilibrium assumption leads to an overprediction of droplet number by less than 10% for marine aerosol; this overprediction can exceed 40% for urban type aerosol. Overall, the effect of kinetic limitations on cloud albedo can be considered important when equilibrium activation theory consistently overpredicts droplet number by more than 10%. The maximum change in cloud albedo as a result of kinetic limitations is less than 0.005 for cases such as marine aerosol; however albedo differences can exceed 0.1 under more polluted conditions. Kinetic limitations are thus not expected to be climatically significant on a global scale, but can regionally have a large impact on cloud albedo.     

The Role of Initial Cloud Condensation Nuclei Concentration in Hail Using the WRF NSSL 2-moment Microphysics Scheme

The effects of the initial cloud condensation nuclei(CCN) concentrations(100–3000 mg~(-1)) on hail properties were investigated in an idealized non-severe hail storm experiment using the Weather Research and Forecasting(WRF) model, with the National Severe Storms Laboratory 2-moment microphysics scheme. The initial CCN concentration(CCNC) had obvious non-monotonic effects on the mixing ratio, number concentrations, and radius of hail, both in clouds and at the surface, with a CCNC threshold between 300 and 500 mg~(-1). An increasing CCNC is conducive(suppressive) to the amount of surface hail precipitation below(above) the CCNC threshold. The non-monotonic effects were due to both the thermodynamics and microphysics. Below the CCNC threshold, the mixing ratios of cloud droplets and ice crystals increased dramatically with the increasing CCNC, resulting in more latent heat released from condensation and frozen between 4 and 8 km and intensified updraft volume. The extent of the riming process, which is the primary process for hail production, increased dramatically. Above the CCNC threshold, the mixing ratio of cloud droplets and ice crystals increased continuously, but the maximum updraft volume was weakened because of reduced frozen latent heating at low level. The smaller ice crystals reduced the formation of hail and smaller clouds, with decreased rain water reducing riming efficiency so that graupel and hail also decreased with increasing CCNC, which is unfavorable for hail growth.     

太原云凝结核的观测研究

本文利用连续气流纵向热梯度云凝结核仪对太原的云凝结核（CCN）进行了观测研究。结果表明,CCN数浓度具有明显的日变化特征。CCN数浓度一天中出现了两次峰值。降水、降雪对地面CCN具有明显的冲刷作用。本文根据关系式N=CS#拟合得到的地面CCN活化谱参数C值明显较大,k值较高,属于典型的大陆型核谱。