Vol.32, NO.6, 2021

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Reviews
Advances in Aircraft Measurements of Clouds and Precipitation in China
Guo Xueliang, Fu Danhong, Guo Xin, Fang Chungang
2021, 32(6): 641-652. DOI: 10.11898/1001-7313.20210601
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Abstract:
Aircraft measurement is an important way in observing the phase, distribution and conversion of clouds and precipitation particles in clouds. The data of aircraft measurements are the foundation for clarifying cloud microphysical structure and precipitation formation mechanism, as well as the parameterization of cloud physical processes in numerical model. The main achievements in technology, instrument and research of aircraft measurements in China are summarized.The aircraft measurements of clouds and precipitation started in the 1960s with an airborne aluminum foil sampler in China, and then size distribution, number concentration and images were acquired with a microscope. Early in the 1980s, the particle measuring system (PMS) was firstly imported and used. After 60 years of development, China has made important advances and achievements in aircraft measurement platform, airborne measurement technology and research in cloud microphysical processes and precipitation formation mechanisms. Some important research results are summarized as follows. First, the properties of atmospheric aerosols and conversion into cloud condensation nuclei (CCN) are found to be closely related with atmospheric stratification, aerosol origins and secondary aerosol formation. The atmospheric inversion plays a critical role in the accumulation of aerosols in low levels. The secondary aerosols formed in air pollution events have low conversion rate as CCN since the aerosols are fine particles and need higher supersatuation for nucleation, and the high-level large-size dust aerosols have higher conversion rate. Second, the microphysical properties of stratiform clouds and stratiform clouds with embedded convection are closely associated not only with cloud-top temperature, water vapor content and cloud thickness, but also with the location in the high-level trough and cold/warm frontal system. The embedded convection region has more supercooled water and the riming process is critical, and the precipitation formation follows the seeder-feeder mechanism. In the thinner stratiform region, the supercooled water content is less, deposition and aggregation are dominant, and therefore the precipitation formation cannot follow the seeder-feeder mechanism. However, when clouds grow moister and thicker, the deposition, aggregation, and riming processes are dominant. Third, the microphysical formation mechanism in winter snow events in northern China are mainly due to deposition and aggregation processes. Only under the condition with plentiful water vapor and deeper cloud, the riming process is important. Aircraft measurements are important in verifying the microphysical processes in numerical model, however, the current studies are only limited in direct comparisons. The improvements of microphysical processes in a model through parameterization of aircraft measurements are very limited. In addition, aircraft measurements in convective clouds, warm-rain microphysics and applications in cloud seeding effectiveness evaluation and verification in remote sensing observational data are still insufficient.
Articles
Estimation of Aerosol Activation Ratio and Water Vapor Supersaturation at Cloud Base Using Aircraft Measurement
Gao Qian, Liu Quan, Bi Kai, Wang Fei, Sheng Jiujiang, He Hui, Liu Xiang
2021, 32(6): 653-664. DOI: 10.11898/1001-7313.20210602
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Abstract:
The vertical evolution of aerosol physiochemical properties significantly influence the capacity of particle water uptake at different atmospheric levels, which are important for the estimation of direct and indirect radiative impacts of aerosol. However, as one of the most important environmental parameters during the formation of cloud or fog, water vapor supersaturation cannot be directly measured. An aircraft observation was carried out on 13 November 2016 with stable stratiform clouds covering Beijing area. The results of aircraft in-situ measured aerosol particle size distribution, chemical composition, cloud droplet spectrum, and vertical distribution of aerosol physicochemical properties and activation ability near the cloud base are analyzed. The results show that Beijing area is under polluted conditions during the flight detection with the surface aerosol concentration (0.11-3 μm) of 4600 cm-3. The height range of cloud layer is 800-1200 m. The number concentration of aerosols at the cloud base is greatly lower compared with the surface, and the effective diameter significantly increases from 0.3 μm to 0.6 μm. Aerosol composition varies drastically with altitudes. The hydrophobic primary organic aerosol (POA) has a significant contribution at surface, but sharply decreases at the cloud base. Meanwhile, the fraction of inorganic species and secondary organic aerosol (SOA) increase significantly from surface to cloud base, resulting in hygroscopic parameter (κ) increasing from 0.25 (ground) to 0.32 (cloud base). The number concentration size spectrum of cloud droplets and aerosols in the cloud can be well connected, which could be deemed as activated and un-activated particles respectively. Meanwhile, the sum of their number concentrations is approximately equal to the total aerosol concentration below the cloud, indicating aerosol particles below the cloud base has a dominant contribution to the cloud droplet formation near the cloud base by activation. Thereby, the actual activation ratio of aerosol particles serves as cloud condensation nuclei (CCN) in the measured cloud can be obtained. Combining with in-situ measured aerosol size distribution, chemical composition, and calculated hygroscopic parameter below the cloud base, the aerosol activation ratio under different supersaturation ratio can be derived. Estimated through the comparison of calculated activation rate and the measurement, the mean supersaturation near this stratiform cloud base is about 0.048%. It implies that the aerosol characteristics at surface may not represent that at upper levels, where the evolution in vertical direction should be considered in evaluating the contribution of surface emissions to cloud particle nucleation and their atmospheric lifetime. This supersaturation estimating method is mainly based on conventional measurement of aerosol and droplets, which has a potential value for further application on cloud analysis.
Characteristics of Aerosol and Cloud over the Central Plain of North China in Summer
Li Yiyu, Sun Hongping, Yang Junmei, Ren Gang, Zhao Delong, Zhou Wei, Liu Zhichao
2021, 32(6): 665-676. DOI: 10.11898/1001-7313.20210603
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Abstract:
The vertical distribution of aerosols and clouds is a key issue in the indirect climate effects of aerosols. Based on the aerosol and cloud droplet number concentration data observed by 6 aircrafts over the central plain of North China from July to August in 2018, the vertical distribution and spectral characteristics of aerosols and clouds are studied. The results show that the mean value of aerosol number concentration ( Na ) of this area is higher than that in Central China, the in-cloud aerosol number concentration (Nacc) accounts for over 80% of the total concentration, and the mean particle diameter ( Dm ) is between 0.12 and 0.52 μm, which means fine particles contribute the largest proportion. The aerosol number concentration decreases with the height. The total aerosol number concentration is higher with middle and high stratus comparing to low-level stratus. The vertical distribution of Na is significantly affected by atmospheric stratification, as inversion temperature layer block the vertical transport of aerosols. The vertical distribution of mean diameter below 3500 m is relatively stable, with an average of 0.23 μm, while increases with the height above 3500 m. The vertical distribution of aerosol particle mean diameter at high altitude (above 2000 m) is greatly affected by relative humidity. Compared with the vertical distribution of aerosol number concentration and mean diameter, the horizontal variety is smaller. In low-level stratus, the cloud droplet number concentration (Nc) is large, and the liquid water content (L) is small. In middle and high stratus, the cloud droplet number concentration is small, and the liquid water content is large. The probability distribution function of cloud droplet number concentration and liquid water content are bimodal distribution, and the probability distribution function of cloud droplet effective radius (Re) show a single mode. The distribution of aerosol number concentration spectrum shows multi-peak distribution, and the average aerosol spectrum show three-peak distribution. The distribution of cloud droplet number concentration spectrum shows a single peak, which appears at the scale of 9-16 μm, and the spectral width of cloud droplets is 50 μm. The parameter characteristics of aerosol spectral distribution obtained by fitting aerosol spectral distribution can provide an important in situ measurement basis for improving the parameterized schemes such as regional climate model and aerosol-cloud model.
Aircraft Measurement of the Vertical Structure of a Weak Stratiform Cloud in Early Winter
Wang Shuo, Zhang Dianguo, Wang Wenqing, Liu Quan, Wu Juxiu, Liu Chang
2021, 32(6): 677-690. DOI: 10.11898/1001-7313.20210604
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Abstract:
In order to obtain the vertical microphysical structure of the stratiform cloud and characteristics of the radar parameters and reveal the precipitation mechanism, the airborne Ka-band cloud radar and DMT particle measurement system are used to target the stable precipitation layer of a cold front in Shandong Province on 17 November 2019. The results show that the observed cloud layer consists of two parts: Altostratus (As, 3100-4500 m) and nimbostratus (Ns, 800-2600 m). The content of As supercooled water is low, with an average value of 0.0026 g·m-3 and the maximum value of 0.008 g·m-3. The average ice crystal content in the cloud is 8.2 L-1. In the vertical space, the ice crystal size and spectral are different. Ice crystals grow through deposition, with a maximum diameter of 900 μm. In the state of equilibrium spectrum, the ice concentration has a good correlation with radar reflectivity, and the maximum correlation coefficient is 0.84. The movement of particles in the cloud is different. The speed of small particles varies greatly and is easily affected by updrafts. The falling speed of large-scale ice crystals is stable. The central part of the Ns (1750-2150 m) is rich in supercooled water, with the maximum content of 0.354 g·m-3. The average radar reflectivity of the supercooled water region is 7.48 dBZ, the Doppler velocity is -2.3 m·s-1, and the velocity spectral width is 0.7 m·s-1. The height of the supercooled water layer in the cloud can be comprehensively judged by combining a variety of detection data and parameters. The upper part of the Ns is dominated by ice crystals and the lower part is filled by melted particles in the warm zone. The average concentration of ice crystals is 208 L-1, which increases through the riming process, and the maximum diameter is 450 μm. The radar reflectivity profile increases as the height decreases from 2200 m to 1500 m, remains unchanged from 1500 m to 1200 m, and decreases below 1200 m. There is no obvious bright band at 0℃ level, and the velocity spectral width profile increases as the height decreases. The supercooled water in the stratiform cloud in early winter is abundant, and the concentration of ice crystals meets the standard of seeding area, which has a certain potential for rainfall enhancement.
Aircraft Measurement of Microphysical Characteristics of a Topographic Cloud Precipitation in Qilian Mountains
Cheng Peng, Luo Han, Chang Yi, Gan Zewen, Zhang Fengwei, Liu Weicheng, Chen Qi, Mao Lixin
2021, 32(6): 691-705. DOI: 10.11898/1001-7313.20210605
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Abstract:
Qilian Mountains are an important ecological barrier in Northwest China. The precipitation in Qilian Mountains is mainly caused by topographical cloud system. Aircraft detection in Qilian Mountains is of great significance for deepening the understanding of cloud microphysical processes, and for scientifically and effectively carrying out artificial precipitation operations to improve the ecological environment. Using the airborne observations of a topographic cloud precipitation process in Qilian Mountains on 29 August 2020, the microphysical characteristics of the summer cloud precipitation process in Qilian Mountains are studied. The cloud system presents an obvious layered structure. The height of the cloud base is 4000 m, and the water content of the whole layer is relatively rich. The liquid water content (L) is between 0.65 and 1.1 g·m-3, and the cloud water large value area appears at 4500-5300 m altitude, which has a high concentration of cloud droplets. The water content of cloud water is mainly contributed by cloud droplets between 15 and 20 μm. The average concentrations of small cloud particles and large cloud particles are 7.54 cm-3 and 0.86 cm-3. The average effective diameters of small cloud particles and large cloud particles are 11.02 μm and 198.11 μm. The cloud particles in Qilian Mountains have the characteristics of small concentration and large diameter. There are obvious differences in cloud microphysical characteristics between the north and south slopes of Qilian Mountains. Affected by the topography, the concentration and diameter of cloud droplets on the northern slope are larger than those on the southern slope, and L on the northern slope are significantly larger than those on the southern slope too. The spectra of cloud droplets at different heights in Qilian Mountains are respectively unimodal distribution. The spectrum of cloud droplets with a diameter less than 50 μm can be fitted by Gamma distribution, while the spectrum of cloud droplets with a diameter greater than 50 μm shows a power exponent distribution. The ice crystals in the ice layer are mainly grown through the process of sublimation and coalescence. The growth mechanism of the ice crystals in the mixed layer is mainly the Bergeron process, and accompanied by attachment and aggregation growth.
Aircraft Measurements on Properties of Aerosols over the Central and Eastern Qinghai-Tibet Plateau
Ma Xueqian, Guo Xueliang, Liu Na, Zhang Yuxin, Han Huibang, Kang Xiaoyan
2021, 32(6): 706-719. DOI: 10.11898/1001-7313.20210606
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Abstract:
The central and eastern Qinghai-Tibet Plateau is the birthplace of the Yellow River, the Yangtze River and the Lancang River. It is also a climate change sensitive area and a key ecological protection area. Based on observations of 11 sorties conducted in summer and autumn of 2011 and 2013, the vertical distribution of aerosol number concentration and number spectrum at Golmud, and the transport characteristics as well as the horizontal distribution and nucleation characteristics of aerosol number concentration and cloud condensation nuclei (CCN) number concentration at mid-altitude of the central and eastern Plateau are analyzed. The results indicate that the vertical and horizontal distributions of aerosol number concentration (Na) and volume diameter (Dv) in the central and eastern Qinghai-Tibet Plateau are significantly different due to the influence of weather system, topography, and surface characteristics. The aerosol number concentration is high in the northwest and low in the southeast. The volume diameter is large in the lower layer, small in the upper layer, and there are dust layers in the middle and upper layers. Precipitation has clear effects on low-level aerosols when east wind prevails at Golmud, and the aerosol number concentration and volume diameter are significantly reduced. At the same time, due to plateau gale or convection, sand and dust layers also form at the middle and upper altitudes of 6.2 km and 7.2-7.4 km. The aerosol volume diameter of the lower layer is 1.8 μm mainly when westerly wind prevails, the number concentration increases with the increase of height and wind speed, the variation of aerosol volume diameter is small, and the sand and dust layer also appears at 6.2 km altitude. Under the influence of different weather systems over 6.5 km altitude, submicron particles are imported, and the number concentration even reaches 5×103 cm-3. Moreover, the aerosol number concentration and particle size imported at about 8.0 km altitude when east wind prevails are denser and smaller than those imported when west wind prevails, and the vertical distribution of spectrum also shows the same characteristics. The measurement of cloud condensation nuclei number concentration (Nccn) with different supersaturation shows that the mean nucleation rate is generally 1%-16%, except that the nucleation rate below 6 km altitude at Golmud is 21%-47%, and the nucleation rate at 6.0-8.5 km altitude is generally low. When the aerosol number concentration increases, the nucleation rate decreases significantly, and its value is relatively high with the supersaturation being 1%-2%, in the layer between -15 and -5℃ or the particle size of 1-3 μm.
Microphysical Characteristics and Precipitation Formation Mechanisms of Convective Clouds over the Tibetan Plateau in Summer
Chang Yi, Guo Xueliang, Tang Jie, Lu Guangxian, Qi Peng
2021, 32(6): 720-734. DOI: 10.11898/1001-7313.20210607
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Abstract:
Tibetan Plateau (TP) has high impact on weather, climate, and water cycle of China, and it also affects the flood and drought in south China by modulating the onset and retreat of the Asian monsoon. However, owning to the lack of direct observations, the knowledge of microphysical characteristics and mechanisms inside the clouds over TP is still seriously lacking. During the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-Ⅲ), field observations is carried out in the summer of 2014, which employed ground-based and airborne instruments. By using the aircraft measurements collected during the TIPEX-Ⅲ, the microphysical characteristics and precipitation formation mechanisms of summertime clouds are studied. The results show that clouds detected by the aircraft are mainly newly born or developing mixed-phase convective clouds, as well as some residual clouds. The maximum and average concentrations of cloud drops are 1.1×105 L-1 and (9±10)×103 L-1, respectively, and the order of magnitude is 104 L-1, which is lower than clouds of plain and maritime regions by 1-2 orders. The maximum concentration for larger cloud particles is 28.82 L-1, and the order of magnitude is 100-101 L-1, which is also lower than other regions. The maximum liquid and total water content are 0.25 g·m-3 and 1.33 g·m-3, respectively, and the order of magnitude is 10-1-100 g·m-3, with abundant supercooled liquid water content in the clouds. The uplifting velocity distributes mainly in the range of 1-4 m·s-1 with a maximum of 4.3 m·s-1, indicating the convective clouds over the TP are weaker than other regions. The cloud drop size distributions (DSD) are mostly bimodal with different second peaks at the larger end, and some of the DSDs are unimodal, which are mainly found in newly borne clouds. There are more large cloud drops and drizzles in the clouds over the TP, which is the result of active warm rain processes. And the ice particles mainly consist of opaque and dense graupels as well as some needles and plates, indicating active rimming processes. The warm rain processes do not generate rain directly, but contribute to the subsequent glaciation and rimming processes, leading to the quick formation of precipitation over the TP. The residual clouds show similar ice characteristics with convective clouds, but much drier and weaker, and they also maintain small amount of supercooled liquid water.
Shape Recognition of DMT Airborne Cloud Particle Images and Its Application
Zhang Rong, Li Hongyu, Zhou Xu, Li Hao, Hu Xiangfeng, Xia Qiang
2021, 32(6): 735-747. DOI: 10.11898/1001-7313.20210608
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Abstract:
Currently, the most direct and effective way to obtain cloud precipitation microphysical characteristics is from in-situ measurements acquired by airborne imaging probes. There are many studies based on CIP (cloud imaging probe) and PIP (precipitation imaging probe) detection data, which are mostly based on the limited output from the software PADS (Particle Analysis and Display System) provided by DMT (Droplet Measurement Technologies). Since PADS only outputs the second-by-second statistical results rather than the detailed particle-by-particle information, it greatly limits the deep mining and analyzing of cloud particle image data. Besides, particle shapes in previous studies are mainly classified through naked-eye observations, which is time consuming, subjective, and unreliable to conduct statistical analysis on thousands of cloud particle images. Therefore, it is impossible to calculate the hydrometeor content based on mass-dimension relationships for particles of different shapes in ice or mixed cloud observations.The operation principle of airborne two-dimensional optical array probes is introduced. Then techniques of recognition and elimination of shattering particles and fake particles are illustrated in detail. Particle shapes are divided into 8 types (tiny, linear, aggregated, graupel, spherical, plate, dendritic and irregular) based on geometric characteristics of particle shapes. Statistical characteristics of different cloud particle shapes and their areas are analyzed by using gray CIP data detected in three wintertime stratiform clouds in Henan Province. The recognition of particle shapes are basically consistent with results through naked-eye observations, and also consistent with dominant particle shapes in each temperature range obtained by previous studies. The hydrometeor content obtained using the mass-dimension relationship for particles of different shapes is compared with that from treating all particles as spherical liquid particles (i.e., the algorithm used by PADS). It is found that when all particles are treated as spherical liquid particles, the hydrometeor content is roughly one magnitude higher than that from considering different particle shapes, indicating the technique of particle shape classification can improve the accuracy of hydrometeor content in ice or mixed clouds. In addition, some matters needing attention in the use of two-dimensional particle image data are pointed out to ensure proper use of two-dimensional particle image data.
Calibration for Data Observed by Airborne Hot-wire Liquid Water Content Sensor
Liu Xiaolu, Zhang Yuan, Liu Dongsheng
2021, 32(6): 748-758. DOI: 10.11898/1001-7313.20210609
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Abstract:
Based on the cloud microphysical detection data of 10-sortie aircraft over southern Sichuan Basin in 2015 and 2017, the liquid water content measured by DMT (Droplet Measurement Technologies) hot-wire liquid water content sensor is examined, and abnormal values in maximum, minimum and negative values are found.There are 4 possible causes for the abnormal maximum, minimum and negative values of liquid water content. First, the errors are caused by multiple parameters such as temperature, air pressure and vacuum velocity, which may lead to the error superposition of calculated values. Second, the on-board operators didn't calibrate the zero before entering the cloud. Third, the on-board operators only calibrate the zero once before entering the cloud during the whole flight. Fourth, the interval between cloud entry and exit is too short, so that the manual zero calibration is inaccurate.Using cloud particle spectrum data from cloud droplet probe (CDP), cloud imaging probe (CIP) and precipitation imaging probe (PIP), three solutions are proposed for calibrating hot-wire liquid water content sensor. Solution 1 is to set the criteria for entering cloud as the concentration of particle above a certain size from CDP probe greater than 0. Solution 2 is to set the criteria for entering cloud as the number concentration of cloud particles greater than 10 cm-3 from CDP probe. Solution 3 is to set the criteria for entering cloud as the number concentration from CDP, CIP and PIP probe greater than 0. The results show that when the number concentration is 0 from CDP, CIP and PIP probe, the original non-zero liquid water content problems are corrected by these solutions.To avoid the influence of ice phase particles on CDP number concentration, the verification is carried out in the positive temperature zone. All the test results show that the negative proportion of liquid water content is also significantly reduced compared with the original data. Solution 1 reduces the negative proportion of liquid water content, and make the minimum and maximum more reasonable than other scales. The liquid water content measured by Solution 1 are more reasonable than Solution 2 and 3.
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