A two-moment cloud microphysics parameterization for mixed-phase clouds. Part 2: Maritime vs. continental deep convective storms

Summary A systematic modeling study investigates the effects of cloud condensation nuclei (CCNs) on the evolution of mixed-phase deep convective storms. Following previous studies the environmental conditions like buoyancy and vertical wind shear are varied to simulate different storm types like ordinary single cells, multicells and supercells. In addition, the CCN characteristics are changed from maritime to continental conditions. The results reveal very different effects of continentality on the cloud microphysics and dynamics of the different storms. While a negative feedback on total precipitation and maximum updraft velocity is found for ordinary single cells and supercell storms, a positive feedback exists for multicell cloud systems. The most important link between CCN properties, microphysics and dynamics is the release of latent heat of freezing.     

On the effect of the chemical composition of atmospheric aerosol particles on nucleation scavenging and the formation of a cloud interstitial aerosol

Nucleation scavenging and the formation of a cloud interstitial aerosol (CIA) were theoretically studied in terms of the chemical composition of atmospheric aerosol particles. For this study, we used our air-parcel cloud model, which includes the entrainment of air and detailed microphysics, for determining the growth and interaction of aerosol particles and drops. Maritime and remote continental aerosol particle spectrums were used whose size distributions were superpositions of three log-normal distributions, each of a prescribed chemical composition. Our results show (1) that the CIA exhibits a size distribution with a distinctive cut-off at a specific radius of the dry as well as of the wet particle size distribution. All particles above this limiting size become activated to cloud drops and, thus, are not present in the CIA spectrum. This limiting size was found to be independent of the chemical composition of the particles and only dependent on the prevailing supersaturation. Below this specific size, the CIA spectrum becomes depleted of dry aerosol particles in a manner which does depend on their chemical composition and on the supersaturation in the air. (2) The number of aerosol particles nucleated to cloud drops depends critically on the chemical composition of the particles and on the prevailing supersaturation.     

Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg

The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyser. The hygroscopic growth spectra were analysed in terms of diameter change with increasing RH from 20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990.Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04 and 1.02 for particle diameters of 50, 150 and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported.Hygroscopic growth spectra for total aerosol, interstitial aerosol and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.     

对流参数化与微物理过程的耦合及其对台风预报的影响研究

在SAS（Relaxed Arakawa-schubert Scheme）对流参数化方案中引入对流云和层状云的相互耦合机制,并通过一个台风个例对改进前后两种方案的预报效果进行了比较。试验结果表明：对于台风这种对流云和层状云相互作用非常强烈的天气系统,在对流参数化方案中引入对流云和层状云的耦合机制可以有效地提高模式对台风路径的预报水平,但是对于台风强度的预报效果不明显。考虑对流参数化和微物理过程耦合后模式的参数化降水变弱而格点降水增强,与NCEP再分析资料的对比发现,改进方案对于台风外围的大尺度温度场和湿度场的预报会有所改进,但仍然存在偏干偏冷的现象。对雨和雪的不同处理方式、不同云底条件以及是否考虑雨雪的卷入抬升三个方面进行了敏感性试验,发现72 h内模式预报结果对这些因素的差异不是很敏感。从多个个例的统计结果来看,新方案对台风路径预报的改进效果是比较稳定的。     

云微物理参数化方案对地面气温模拟性能的评估与改进

午后地面气温对对流的发生发展具有重要作用,对其准确预报一直是中尺度数值模式的基本要求。针对一次冷云过程,本文基于WRFv3.9.1模式评估了5种云微物理参数化方案对华东地区地面气温的模拟效果。结果表明,各方案模拟的午后及午夜地面气温都存在较大偏差。其中,WDM6方案对地面气温模拟的效果最佳,Thompson方案模拟的云冰含量过低,模拟效果最差。因此对WDM6方案进行进一步评估和改进,通过修改冰核浓度、初始云凝结核数、优化WDM6方案以及替换方案中云水向雨水自动转化过程的公式的方式设计了敏感性试验,以改进WDM6方案对地面气温的模拟。结果表明,使用Grabowski公式替换WDM6方案中的Berry公式,能提高云水含量,有效改善地面气温的模拟。并通过一次梅雨过程对改进方案进行了有效性验证。最后在此基础上将改进后的WDM6方案应用于江苏省精细化天气分析和预报系统PWAFS模式中,显著提高了PWAFS模式对午后地面气温的模拟效果,为模式的业务应用提供了技术支撑。     

Idealized simulation of the Colorado hailstorm case: comparison of bulk and detailed microphysics

One of the purposes of the Fourth Cloud Modeling Workshop was to compare different microphysical treatments. In this paper, the results of a widely used bulk treatment and five versions of a detailed microphysical model are presented. Sensitivity analysis was made to investigate the effect of bulk parametrization, ice initiation technique, CCN concentration and collision efficiency of rimed ice crystal–drop collision. The results show that: (i) The mixing ratios of different species of hydrometeors calculated by bulk and one of the detailed models show some similarity. However, the processes of hail/graupel formation are different in the bulk and the detailed models. (ii) Using different ice initiation in the detailed models' different processes became important in the hail and graupel formation. (iii) In the case of higher CCN concentration, the mixing ratio of liquid water, hail and graupel were more sensitive to the value of collision efficiency of rimed ice crystal–drop collision. (iv) The Bergeron–Findeisen process does not work in the updraft core of a convective cloud. The vapor content was always over water saturation; moreover, the supersaturation gradually increased after the appearance of precipitation ice particles.     

基于不同微物理过程的广西沿海南风型暖区暴雨的数值模拟研究

基于WRFV3.6.1,利用其8个云微物理参数化方案对2010—2016年华南汛期（4—9月）的6个南风型暖区暴雨个例进行数值模拟与多方案集成试验,并采用基于对象的诊断评估方法（MODE）对模拟结果进行评估。结果发现对于大多数个例,WRF模式都能较好地模拟出暖区暴雨的降水带,对暖区降水带模拟最好的参数化方案是WSM6方案,其次是Lin方案;模拟效果较差的参数化方案为CAM5.1与NSSL 2-mon方案。选取模拟结果较好的个例进行诊断分析,发现不同参数化方案得到的动力学特征以及云微物理特征相关变量存在较大差异,导致模拟降水的差异。在单方案模拟的基础上,开展多方案集成试验,发现多方案集成方法能够有效降低模式模拟的不确定性,产生更稳定的模拟结果。     

Kinematics, cloud microphysics and spatial structures of tropical cloud clusters: A two-dimensional cloud-resolving modeling study

Kinematics, cloud microphysics and spatial structures of tropical cloud clusters are investigated using hourly outputs from a two-dimensional cloud-resolving model simulation. The model is forced by the large-scale vertical velocity, zonal wind and horizontal advections obtained from Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). A period of 1600–2300 LST 21 December 1992 is selected for this study when the zonal-mean westerly winds in the lower troposphere intensify while the zonal-mean easterly winds above weaken. Under the vertical-shear environment, there are a westward-propagating cloud cluster, a newly-formed cloud cluster, and four eastward-moving cloud clusters. Two weak eastward-moving cloud clusters merge into strong westward-moving cloud clusters. Merged clouds display notable growth in the eastern edge, indicating that merging processes enhance convection. The development of the new cloud at the western edge of the existing cloud cluster before merging may account for the westward propagation of cloud cluster group, while the advection of the maximum total hydrometeor mixing ratio by the westerly winds after merging may cause the eastward propagation of individual cloud clusters.     

Simulation of the influence of aerosol particle characteristics on clouds and precipitation with LM-SPECS: Model description and first results

The novel model system LM-SPECS is presented combining a spectral bin microphysics scheme and the three-dimensional Lokalmodell (LM, today called COSMO) of the German Weather Service (“Deutscher Wetterdienst”). The model is designed to investigate in detail the interaction of atmospheric aerosol particles, clouds and precipitation. The microphysics scheme includes a combined spectrum of wetted aerosols, cloud droplets and rain drops. As a first application of the model, sensitivity studies on an artificial deep convective cloud were done. The results produced by LM-SPECS are satisfying. The studies show, e.g., that a diminished initial particle number leads to larger cloud droplets and thus to a higher efficiency of coalescence. This results in a larger amount of precipitation. Furthermore, studies on mixed phase clouds show the influence of varying ice nuclei, such as bacteria, kaolinite and soot, on cloud properties. Here, a more effective freezing leads to an increased number of ice particles with smaller radii. The results point to the importance of a detailed knowledge of the underlying microphysical processes in order to understand the formation of clouds and precipitation more accurately. Though to date the model was applied to artificial cases only, the use of the mesoscale weather model allows for more complex realistic cases which are subject to further studies.     

四川省冬季雾的数值模拟及能见度参数化

利用中尺度模式WRF对2009年冬季四川省入冬后最大的一次浓雾过程进行模拟,模拟结果表明,模式对雾区的模拟与实测结果基本一致,模拟的相对湿度也接近实况.利用成都双流机场的实测能见度和相对湿度资料分别用确定性和概率性方法获得能见度(Vis)和相对湿度(RH)的参数化方案,用模拟的相对湿度来估算能见度.研究结果表明,概率为5%的Vis-RH的参数化方案在浓雾过程中估算的能见度最为准确,可以为今后能见度的预报提供参考.     

A modelling study of aerosol processing by stratocumulus clouds and its impact on general circulation model parameterisations of cloud and aerosol

A model of the aqueous phase processing of an aerosol population undergoing multiple cycling through a stratocumulus (Sc) cloud layer is presented. Results indicate that a significant modification of the aerosol properties is achieved following the first cycle through cloud. In a polluted atmosphere, further modification in subsequent cycles is seen to be hydrogen peroxide limited unless there is a flux of ammonia entering the system through cloud base (CB). The modification of the aerosol population is seen to have little effect on the microphysics (specifically the cloud droplet concentration and effective radius) of the processing cloud. However, it enables processed aerosols to subsequently act as efficient cloud condensation nuclei (CCN) in less vigorous clouds (as a result of reducing the critical supersaturation required to activate them). The effects of variations in the internal mixture of soluble components of aerosols on the microphysics of clouds forming on them are also investigated using the cloud model. A (K2) parameterisation of the effects of variations in internally mixed nitrate loadings on the cloud droplet number concentration is presented. The effects of applying this K2 correction to the droplet number (derived from a parameterisation based on sulphate) for the presence of nitrate in aerosol have been investigated using the HadAM3 version of the Hadley Centre General Circulation Model (GCM). The effect on global annual mean simulations of the indirect forcing and effective radius is small, but more pronounced regionally. Suggestions (based on model results and observations) for parameterising the size distribution and in-cloud growth of aerosols for use in GCMs are presented.     

一次梅雨锋暴雨云物理特征的数值模拟研究

利用中尺度数值模式MM5（V3.6）,选用模式中不同的显式云物理方案,对2003年7月4—5日发生在江淮流域的梅雨锋暴雨过程进行了数值模拟,并根据模拟结果对造成此次暴雨过程的对流云团的微物理特征进行了分析。研究结果表明：(1)具有详细云物理过程的中尺度模式MM5对短时强降水过程具有较好的模拟能力,提高MM5模式的分辨率,可以更好地模拟短时梅雨锋暴雨过程,模式中的Goddard云物理方案的模拟结果要优于Reisner方案和Schultz方案。（2）梅雨锋对流云团是一种复杂的固、液、气三相混合体结构,在云体区域内的平均质量密度分布中,水汽的质量密度最大,其次是霰,而冰晶、雪、云水和雨水的质量密度较小且数值大小彼此接近,各种相态粒子质量密度峰值出现的高度随时间无明显变化。雨水、云冰和霰的质量密度随时间演变规律与地面降水强度的变化特征相一致,近地面层水汽密度随时间的演变规律比地面降水强度提前1—2个小时,水汽通量的辐合对暴雨时段内水汽的补充和维持起到了重要的作用。（3）除了最基本的云水向雨水转化的云微物理过程之外,此次降水过程还显示,在中层500—700 hPa范围内雪、冰晶等冰相粒子首先转化为霰粒子,而霰和云水的结合进一步加速（剧）云水向雨水的转换,成为短时特大暴雨形成不可或缺的动力机制,云物理过程中的相变潜热与对流运动的正反馈机制是促进暴雨维持和发展的最重要热力因子。     

Atmospheric variability and binary homogeneous nucleation: A parametrisation and conditions required for a significant effect

The binary water–sulphuric acid nucleation has long been considered an important path for new particle formation in the atmosphere. The formation rate depends nonlinearly on temperature and vapour concentrations. We aim to find out if atmospheric small scale and mesoscale spatial and temporal variability can increase the nucleation rate significantly. Simultaneous and separate effects of variation in temperature, humidity and sulphuric acid concentration on the mean nucleation rate have been modelled. We have assumed that the distributions of variables are Gaussian and compare the mean of simulated nucleation rates with the nucleation rate in mean conditions without variation. According to our studies the atmospheric variability can increase the mean nucleation rate by orders of magnitude but the significant effect is limited to specific conditions. The effect seems to be more important in the lower troposphere than in the upper troposphere, where temperature and sulphuric acid concentrations are lower. In addition, our model simulations indicate that the atmospheric variability can only explain part of the difference between measured and theoretically required sulphuric acid concentrations in conditions where particle formation has been observed. We have parametrised the effect of atmospheric variability as a simple correction factor for the effect of temperature and humidity variation.     

Model simulations of rainout and washout from a warm stratiform cloud

A one-dimensional, time-dependent cloud model with parameterized microphysics is used to investigate the processes which control the rainout and washout of soluble gases from warm, precipitating stratiform clouds. Calculations are presented simulating the distributions of soluble species within and below the cloud layer and in the precipitating raindrops as a function of time and species' solubility. Our calculations indicate that for species with low solubility, wet removal processes are relatively slow and thus do not significantly affect the species' gas-phase abundance. As a result, the removal of low-solubility species by rainout and washout is controlled by thermodynamic processes with the concentration of the species in cloud and rainwater largely determined by the species' solubility. For highly soluble species on the other hand, dissolution into cloud droplets and removal in rain is quite rapid and the abundance of highly soluble species within and below the cloud falls rapidly as soon as the precipitation begins. Because of this rapid decrease in concentration, we find that for highly soluble species: concentrations in cloud droplets near the cloud base can exceed that of raindrops by factors of 2 to 10; washout can dominate over rainout as a removal mechanism; and that, after an extended period of rainfall, the rate of removal becomes independent of the microphysical properties and rainfall rate of the cloud and is controlled by the rate of transport of material into the precipitating column by horizontal advection.     

一个用于气候模式的简单冻土过程参数化方案的建立和检验

在NCAR／LSM的基础上，发展了一个简单的冻土过程参数化方案，并使用苏联6个站的水气象观测资料考察了耦合了新方案模式的气候效应。在新方案中，加入了对含冰量的求解和在相变过程中的能量变化;并使用Johanson的方案替代了模式中原有的土壤导热率的参数化方案，考虑了含冰量对土壤水热性质的影响。原模式和改进后模式的模拟结果的比较得到，冻土过程方案能够合理的模拟土壤列中的能量收支及水热性质随含冰量的变化。随着入渗的减少和径流的增加，春季的土壤湿度减小。因此，热通量的分配和土壤温度也产生了相应的变化。     

Evaluation of cloud and precipitation parameterization using a single-column model: A TWP-ICE case study

Cloud and precipitation parameterization schemes are evaluated, and their sensitivity to the method and/or parameters used to determine cloud physical processes is examined using a singlecolumn version of the Unified Model (SCUM). In the experiment for TWP-ICE, cloud fraction is overestimated (underestimated) in the upper (lower) troposphere due to the wet (dry) bias. The precipitation rate is well simulated during the active monsoon period, but overestimated during the suppressed monsoon and clear skies periods. In the moist convection scheme, trigger condition and entrainment process affect the lower tropospheric humidity through the impact on convective occurrence frequency and intensity, respectively. Strengthening the trigger condition and using the adaptive entrainment method alleviate the low-level dry bias. In the microphysics scheme, more large-scale precipitation is produced with prognostic rain, due to rain sedimentation considering vertical velocity of rain drop, than with diagnostic rain. Less ice/snow deposition with the prognostic two-ice category results in lower ice water content and upper-level cloud fraction than with the diagnostic splitting method for the twoice category. In the cloud macrophysics scheme, the prognostic cloud fraction and cloud/ice water content scheme produces a larger cloud fraction and more cloud/ice water content than the diagnostic scheme, mainly due to detrainment from moist convection (cloud source) that surpasses the effect of convective heating and drying (cloud sink). This affects temperature by influencing the radiative, convective, and microphysical processes. The experiment with combined modifications in cloud and precipitation schemes shows that interaction between modified moist convection and cloud macrophysics schemes results in more alleviation of the cold bias not only at the lower levels but also at the upper levels.     

华北暴雪的云微物理参数化方案的比较模拟

用MM5模式模拟2004年12月20-23日发生在华北地区的一次暴雪天气的发生发展及其演变过程.在四个嵌套网格区域内分别采用Goddard方案(试验G)和Reisner 2方案(试验R)的两个纯显式冰相云微物理参数化方案进行试验.模拟结果表明:试验G和试验R均可以较好的模拟暴雪过程中的环流形势演变、降雪分布和强度,并且两个试验的结果差别不大,但是云中的微物理过程有很大的不同.试验G的主要云微物理过程包括云水的凝结增长、云冰的凝华增长、云冰初始化、云冰被雪碰并、云水被雪碰并、雪的凝华增长、云冰的Bergeron过程等;而试验R的主要云微物理过程包括云冰的凝华增长、云冰转化成雪、雪的凝华增长和霰的凝华增长等.     

Numerical quantification of sources and phase partitioning of chemical species in cloud: application to wintertime anthropogenic air masses at the Puy de Dôme station

The Model of Multiphase Cloud Chemistry M2C2 has recently been extended to account for nucleation scavenging of aerosol particles in the cloud water chemical composition. This extended version has been applied to multiphase measurements available at the Puy de Dôme station for typical wintertime anthropogenic air masses. The simulated ion concentrations in cloud water are in reasonable agreement with the experimental data. The analysis of the sources of the chemical species in cloud water shows an important contribution from nucleation scavenging of particles which prevails for nitrate, sulphate and ammonium. Moreover, the simulation shows that iron, which comes only from the dissolution of aerosol particles in cloud water, has a significant contribution in the hydroxyl radical production. Finally, the simulated phase partitioning of chemical species in cloud are compared with measurements. Numerical results show an underestimation of interstitial particulate phase fraction with respect to the measurements, which could be due to an overestimation of activated mass by the model. However, the simulated number scavenging efficiency of particles agrees well with the measured value of 40% of total number of aerosol particles activated in cloud droplets. Concerning the origin of chemical species in cloud water, the model reproduces quite well the contribution of gas and aerosol scavenging estimated from measurements. In addition, the simulation provides the contribution of in-cloud chemical reactivity to cloud water concentrations.