Vol.32, NO.3, 2021

Display Method:
A Review on the Effect of Sound Waves upon the Coalescence of Aerosol and Cloud and Fog Particles
Xiao Hui, Shu Weixi, Fu Danhong, Feng Qiang, Sun Yue, Yang Huiling
2021, 32(3): 257-271. DOI: 10.11898/1001-7313.20210301
The effect of sound waves on the coalescence of aerosol particles and cloud and fog droplets is a frontier scientific problem in the field of cloud physics and weather modification. The technology of acoustic coalescence has attracted much attention due to its relatively simple experimental device, strong adaptability, and short operation time. These advantages make it a potential new technology of aerosol coalescence. The research progress of acoustic coalescence of aerosol particles and cloud and fog droplets is reviewed from the aspects of theory, experiment, and numerical simulation. The mechanisms of acoustic coalescence mainly include orthokinetic interaction, hydrodynamic interaction (including acoustic wake effect, mutual radiation pressure effect, and mutual scattering effect), and acoustic-induced turbulence effect. The coalescence of aerosol particles in the sound field appears under the combined action. The low-frequency strong sound wave can increase the relative motion between cloud and fog droplets and promote the process of collision and coalescence, which has a significant impact on cloud and fog growth and precipitation. Finally, the existing problems and improvement direction of the research on the theory, experimental observation, and numerical simulation of acoustic coalescence are discussed. The complexity of the acoustic coalescence process, the diversity of experimental conditions, and the limitations of the theory, the optimal experimental conditions and parameter configuration for high efficiency of acoustic coalescence are still imperfect, which requires further experimental studies and numerical simulations. In addition, the research on the coalescence effect of sound waves on fog and cloud particles is not deep enough, and the similarities and differences of acoustic coalescence mechanisms between cloud and fog particles and ordinary aerosols are not clear enough. It is emphasized that the cloud chamber and numerical simulation research on the effect of acoustic coalescence on cloud and fog particles should be strengthened, and a large number of field comprehensive observation experiments should be carried out, which is of great scientific significance for the development of new weather modification technologies.
Circulation Characteristics and Thermal and Dynamic Conditions 48 Hours Before Typhoon Formation in the South China Sea
Gao Shuanzhu, Zhang Shengjun, Lü Xinyan, Wei Fengying
2021, 32(3): 272-288. DOI: 10.11898/1001-7313.20210302
Based on the tropical cyclone best track data from Shanghai Typhoon Institute of China Meteorological Administration and the geostationary infrared satellite cloud image from April to November during 1979-2019, 189 typhoons formed in the South China Sea are selected as target cases. The circulation characteristics, dynamic and thermal conditions from 48 hours before typhoon formation to the time of typhoon generation are analyzed using the reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF) with 1°×1° grid. The results indicate that the typhoon in the South China Sea is formed in a large range of tropical ocean with high surface temperature, high water vapor content and unstable stratification. The development of deep convection and its distance to the tropical disturbance center can be used as an observation criterion for whether the tropical disturbance can develop into typhoon in the next 48 hours. Intertropical convergence is the dominant background circulation of typhoon formation in the South China Sea, and south-west monsoon or easterly wave are also main large-scale circulation. The center of typhoon disturbance is often in the transition zone of the vertical shear, where the vertical shear of the north side of the disturbance becomes smaller and the vertical shear of the south side becomes larger. Sometimes the disturbance center is slightly inclined to the weak vertical shear center. As a whole, there is no significant correlation between wind vertical shear and typhoon disturbance development. The center of tropical disturbance generally coincides with the center of vertical vorticity, and the center of vertical vorticity could be considered as a physical quantity representing the strength of disturbance itself. Furthermore, it is difficult to define the development of the vertical vorticity as an indicator to characterize the development trend of disturbance, while the Okubo-Weiss (OW) index is a good indicator for the development of disturbance and the determination of the disturbance location. In the process of typhoon disturbance development, there is a potential vortex column running through the whole troposphere near the disturbance center. Within the potential vortex column near the disturbance center, the interaction between lower disturbance and the potential vortex in middle and low layer is beneficial to the development of typhoon disturbance.
Characteristics and Mechanisms of Long-lived Concentric Eyewalls in Typhoon Lekima in 2019
Liu Tao, Duan Yihong, Feng Jianing, Wang Hui
2021, 32(3): 289-301. DOI: 10.11898/1001-7313.20210303
The structure change of typhoon eyewall has important influences on tropical cyclone (TC) intensity, and it brings great difficulties to TC intensity forecast. Compared to the TC with only one eyewall, the dynamic and physical processes controlling the intensity change of concentric eyewalls are much more complex. Strengthening the research on this type of typhoons is conducive to improve the understanding of the structure and intensity change of TC. Recently, the process of eyewall replacement cycle (ERC) is well understood, but the mechanism of concentric eyewalls maintenance (CEM) remains unclear. Therefore, it is necessary to use a variety of observation data, numerical model, and data assimilation methods to analyze real typhoons to further explore the mechanism of CEM.The evolution of concentric eyewalls in Typhoon Lekima in 2019 is analyzed with CIMSS microwave satellite images, Wenzhou Doppler radar in China and Ishigaki radar in Japan. Observational analysis indicates that secondary eyewall formation (SEF) happens at about 0600 UTC 8 August 2019. Unlike most concentric-eyewall typhoons, Typhoon Lekima has not undergone ERC, and the concentric eyewalls in Typhoon Lekima maintains for 35 hours.Meanwhile, a numerical experiment of Typhoon Lekima is performed using a WRF-based ensemble Kalman filtering (EnKF) data assimilation system. The evolution process of Typhoon Lekima is reproduced by the results of the analysis after assimilating the Ishigaki radar radial wind, and the simulated track, intensity, and structure are basically consistent with observational analysis. Therefore, based on the EnKF analysis fields, evolution characteristics of three-dimensional eyewalls structure in Typhoon Lekima are further analyzed. The results show that, in the initial stage of SEF, the inertial stability of the middle and lower layers of typhoon is high, and the thermal conditions of the environmental are conducive to the development of Typhoon Lekima. However, due to the strong vertical wind shear (VWS) and dry air intrusion in the mid-upper level, the outer eyewall has a weakening and asymmetric process. In addition, the maintenance mechanism of persistent concentric eyewalls is studied from the perspective of transverse circulation. The Sawyer-Eliassen diagnose is performed to investigate the transverse circulation and to reveal that the interference between the convection/subsidence couplet of the inner and outer eyewalls transverse circulation is not obvious so that the convection of the inner eyewall is not inhibited by the outer eyewall, and the inner eyewall maintains. Under the condition of strong VWS, the asymmetric outer eyewall cannot continuously enhance or contract and replace the inner eyewall, the structure of concentric eyewalls can be maintained for a long time. The structure of the outer eyewall and the distribution of transverse circulation in Typhoon Lekima play an important role in the maintenance of concentric eyewalls.
Climatic Characteristics of Regional Persistent Heat Event in the Eastern China During Recent 60 Years
Lin Ailan, Gu Dejun, Peng Dongdong, Zheng Bin, Li Chunhui
2021, 32(3): 302-314. DOI: 10.11898/1001-7313.20210304
Regional persistent heat event is extensively studied and its intensity is normally investigated from the view of regional average. Climatic characteristics of persistent heat event in 4 regions in the eastern China are analyzed on the basis of distinguishing historical persistent heat event in the South China, the Yangtze River, the Huanghuai and the North China, using the daily maximum temperature data of 2407 stations in China during 1961-2019. The definition index of regional persistent heat event is established, in which the proximity of high temperature stations, the spatial range, the temporal consistency, the regional climate characteristics and the universality of methods are synthesized. Average times of regional persistent heat event in the South China, the Yangtze River, the Huanghuai and the North China are 3.3, 2.8, 2.2 and 0.8 per year respectively, with an average duration of 5.1 d, 6.4 d, 5.0 d and 3.9 d for per event, and the average annual cumulative days in climatology are 16.8 d, 17.8 d, 11.0 d, 3.1 d. Both the duration and the average annual cumulative days of persistent heat event are the longest in the Yangtze River, indicating that the persistent heat event in the Yangtze River is the most serious in climatology among these regions. The average annual cumulative days of persistent heat event in the North China are the least, and it usually ends at the end of July or before. There is a significant linear growing trend in the persistent heat event index in the South China, the North China, and the Yangtze River. There are only 4 non-summer persistent heat events in the Huanghuai in recent 20 years. The difference in strength of the persistent heat event index between the Yangtze River and the South China has significant inter-decadal trends. The index in the Yangtze River is obviously stronger than that in the South China during the first period of 1961-1978, while the index in the South China is slightly stronger than that in the Yangtze River during the second period of 1979-2019. The difference of persistent heat event between these two regions changes by nearly 10 d over these years.
A Calibration Method of Wind Profile Radar Echo Intensity with Doppler Velocity Spectrum
Li Feng, Ruan Zheng, Wang Hongyan, Ge Runsheng
2021, 32(3): 315-331. DOI: 10.11898/1001-7313.20210305
The L-band wind profile radar(WPR) detects the Bragg scattering processes from back scattered energy of changes in refractive index, meanwhile it is high sensitive to Rayleigh scattering processes from back scattered energy of hydrometeors in the precipitating clouds. A method named data calibration with noise power (DCNP) is established for calibrating WPR return signal, in which the Doppler velocity spectrum is processed with FFT. The power of unit amplitude in return signal power spectrum is calculated based on radar noise power. Using calibrated power spectrum, echo intensity spectral density, echo intensity and structure parameter of refractive index are derived, and can be used to study vertical structure of precipitating clouds, microphysical properties, and clear air turbulences. The errors derived from noise temperature and noise amplitude are discussed. When the range of actual noise temperature is from 280 to 320 K, the error range caused by using 300 K to calculate noise power is from -0.28 to 0.3 dB. For each observation mode, the fluctuation of monthly average noise amplitude at the last gate is stable, nearly in normal distribution. The error caused by noise amplitude is between -0.3 and 0.3 dB. The method is estimated with data from Beijing (54399) in 2017, Nanjing (58235) in 2016 and Meizhou (59303) in 2018. These WPR types are different, and they are the main types in operation. Three precipitation cases from different stations are used to estimate the calibration method. It shows that the magnitudes between echo intensities calculated with DCNP and weather radars are similar. The evolutions of the two sorts of echo intensity products are also simultaneous. Estimations show that consistence between different observation mode is good. The difference between the high and low mode from Meizhou (59303) is the smallest. The differences between modes from Beijing (54399) are larger than the other two stations. It is consistent with the range of noise amplitude from the farthest gate in each observation mode. Compared with nearby weather radars, the consistence between WPRs and weather radars is also good considering different observation modes. The calibration method is proved stable and reliable. Radar echo intensity calculated with DCNP is compared with that derived from SNR. In most cases, values from the two methods are well consistent. When noise amplitude is large, the echo intensities identified by the method with SNR are usually lower than the values derived from the method using DCNP. The error from turbulence is analyzed with two-peak spectrum from Meizhou (59303). It indictes that the return signal from turbulence can be ignored for the cases.
Observation Analysis and Application Evaluation of Wind Profile Radar to Diagnosing the Boundary Layer of Landing Typhoon
Yan Jiaming, Zhao Bingke, Zhang Shuai, Lin Limin, Tang Jie
2021, 32(3): 332-346. DOI: 10.11898/1001-7313.20210306
The feasibility of wind profile radar in typhoon observation is investigated with 6 cases including Feng-wong(1422), Chan-hom(1509), Nepartak(1601), Meranti(1614), Megi(1617), and Lekima(1909) during 2014 to 2019. Thirty-four groups datasets, including the Airda 3000 boundary layer wind profile radar, GPS balloon sounding and PARSIVEL laser precipitation data are analyzed.Preliminary analysis show that 30 out of 34 datasets satisfy the prerequisite condition of greater than 80% data completion. The average wind speed standard deviation of these 30 datasets is about 3.64 m·s-1 and the average difference is 4.67 m·s-1. Furthermore, 19 out of the 34 datasets achieve good results (standard deviation less than 4 m·s-1) when the observation by wind profile radar and sounding agree well above 250 m altitude, though they overlap less below that altitude. The sounding wind speed is observed to be much smaller than the wind profile radar data for altitudes below 250 m, which is possibly caused by the fact that sounding accelerates from stationary to consistent with environmental winds below 250 m altitude in typhoon environment, or by the disturbance in lower atmosphere of wind profile radar. Thus, ignoring the lowest 250 m altitude, the standard deviation of wind profile radar and balloon sounding decrease remarkably. This may imply that wind profile radar has high feasibility in boundary layer from 250 m altitude to the layer top under typhoon environment. In addition, data with lower validity are always located in the region which is about more than 200 km away from typhoon center, while the distribution of high validity data observation show no obvious pattern and locates from typhoon center to the outer region. There is also no significant relationship found between data validity and precipitation intensity. These may imply that wind profile radar have great potential under the condition of heavy precipitation and severe wind. The analysis of data with lower validity indicates that the distribution of humidity in typhoon and local disturbance cause uneven wind in the radar detection beam. In addition, because the coastal areas of Zhejiang and Fujian are mostly hilly terrain, the low-level circulation structure of typhoon is destroyed by terrain, which may also be one cause for the poor match of horizontal wind speed.Despite the limited data, wind profile radar shows a very hopeful potential and high validity in the observation and diagnosis of boundary layer even in severe convective weather environment such as typhoon inner core region.
Quality Control and Evaluation on Non-cloud Echo of Ka-band Cloud Radar
Zeng Zhengmao, Zheng Jiafeng, Yang Hui, Zheng Min, Zeng Yingting
2021, 32(3): 347-357. DOI: 10.11898/1001-7313.20210307
Aiming at non-cloud echoes in Ka-band millimeter wave cloud radar observation, an improved data quality control method is proposed. Using the observation at Pinghe of Fujian from September 2018 to August 2020, the quality of the radar is quantitatively evaluated to study the actual impact of data quality control on cloud-precipitation detection. The non-cloud echoes show the characteristics of weak radar reflectivity factor (Z) and strong depolarization ratio (R) at Pinghe. But statistics show that there is difference from those of Qinghai-Tibet Plateau or Guangdong. Therefore, using the radar reflectivity factor (Z) less than -5 dBZ and linear depolarization ratio (R) greater than -22 dB as the judgment condition, and with the aid of filtering, non-cloud echoes can be effectively filtered. At the same time, a typical example is used to verify the effectiveness of the algorithm. Non-cloud echoes have a significant effect on the detection of cloud below 3 km, especially weak echoes. Non-cloud echoes account for 9.20% of all radar reflectivity factor samples, and 34.05% of all radar linear depolarization ratio samples. For the weak echo below -5 dBZ, the impact of non-cloud echoes is more significant, which accounts for 67.20% of all radar reflectivity factor sample. The detection rate of non-cloud echoes matter is closely related to the radar sensitivity, and the overall decrease with the height increaseing. The detection rate of non-cloud echoes decreases with the height increaseing. Meanwhile, non-cloud echoes have a certain relationship with the boundary layer, with an obvious diurnal change trend. From afternoon to midnight, due to strong turbulent activity, the detection rate of non-cloud echo matter is also higher, and the peak occurs at 1700 BT. From midnight to sunrise, due to the weakening of turbulent motion, the detection rate of non-cloud echo gradually decreases, and the lowest value occurs at 0400 BT. Non-cloud echoes have a significant effect on the vertical distribution of cloud precipitation. After quality control, the number of samples at the height of 0.12-2.5 km for radar reflectivity factor decrease by 17.68%, and the number of samples at the height of 0.12-4 km for radar linear depolarization ratio decrease by 14.29%.
Thunderstorm Feature Dataset and Characteristics of Thunderstorm Activities in China
Ma Ruiyang, Zheng Dong, Yao Wen, Zhang Wenjuan
2021, 32(3): 358-369. DOI: 10.11898/1001-7313.20210308
A thunderstorm feature dataset (TFD) is built up based on the black body temperature (TBB) product and cloud classification (CLC) product of FY-2E meteorological satellite as well as the lightning data of the World-Wide Lightning Location Network (WWLLN). In the TFD, thunderstorm cloud is determined when there is WWLLN lightning in the area with TBB not higher than -32℃ or its fitted ellipse. The characteristic parameters of thunderstorms including time, location, morphology, structure, and lightning activities are obtained to establish the TFD. Based on the dataset, thunderstorms in the land of China and the adjacent seas are analyzed after the quality control.The results show that South China, Southwest China, Eastern and Central of Tibetan Plateau and South China Sea are the areas with most frequent thunderstorm activities. North China and Northeast China are two areas with relatively frequent thunderstorm activities in the north part of China. Meanwhile, thunderstorm activity is the weakest in Northwest China.The seasonal variation of thunderstorm activity shows obvious differences between land and sea. The active stage of thunderstorms on land is from June to August. In high latitudes, the peak appears earlier. There is a peak of thunderstorm activity in the South China Sea around May, and another peak after August. The lower the latitude is, the later the second peak appears. The peak time of thunderstorm activity in diurnal variation in most parts of the land is from 1400 BT to 2000 BT and the peak of thunderstorm activity in adjacent sea areas mainly occurs in the morning. In the Sichuan Basin, thunderstorms are more frequent in the early morning. The diurnal variation of thunderstorm activity in the South China Sea is relatively weak.The area of thunderstorm cloud with TBB not higher than -32℃ follows a log-normal distribution, with the peak interval being 1×103-1×104 km2, and the average area is 3.0×104 km2. The area of thunderstorm cloud over the sea is obviously larger than that of land, and the South China Sea has the largest area of thunderstorm clouds. On the land, the area of thunderstorm clouds in the east is larger than that in the west, and the average area of thunderstorm clouds greater than 1.2×105 km2 can be predominantly found in the first step of Chinese topography. Meanwhile, there is a local center with an average area of thunderstorm clouds greater than 1.2×105 km2 in the Qaidam Basin.
Hydrometeor Particle Characteristics During a Late Summer Hailstorm in Northern Shandong
Wang Jun, Wang Wenqing, Wang Hong, Zhang Qiuchen, Gong Dianli, Yang Xuebin
2021, 32(3): 370-384. DOI: 10.11898/1001-7313.20210309
Based on the data of PARSIVEL disdrometer, particle phase identification and reflectivity factor of CINRAD/SA-D dual polarization weather radar, a hailstorm process occurred in northern Shandong Province on 16 Aug 2019 is analyzed. The rain and hail particles are distinguished, and the evolution of the raindrop size distributions is analyzed. PPI scans at 0.5°elevation of dual polarization weather radar show that there are raindrops at Dezhou and Lingxian observational stations, while it rains heavily and hails at Linyi. As for PARSIVEL disdrometer, a small number of ice particles is identified at all three sites, and the distribution of ice particles varies dramatically. Raindrops are classified into rain or ice particles according to the particle size and falling speed. However, the observational results still need artificial verification. When the hailstorm passes over the three observational stations, ice particles are identified by PARSIVEL disdrometer. At Dezhou station, 5 ice particles are identified, including 2 large hailstones (diameter > 8 mm), 2 small hailstones (diameter between 5-8 mm) and 1 graupel particle (diameter between 2-5 mm). There are 29 ice particles in Lingxian station, including 2 large hailstones, 19 small hailstones and 8 graupel particles. At Linyi station, 17 ice particles are observed, including 10 large hailstones, 3 small hailstones and 4 graupel particles. The Z-R relation retrieved by PARSIVEL disdrometer data is Z=1523R1.21, which has a larger coefficient, but a smaller index compared with the Z-R relation of convective precipitation of the new generation Doppler radar. In the stage of increasing rain intensity in front of hailstorm, the raindrop size distributions feature low number density of small raindrops and more large raindrops, therefore the total raindrop concentration is low, and the radar reflectivity is high. Meanwhile, in the stage of rain intensity weakening, the concentration of small raindrops with a diameter less than 3.0 mm increase significantly, while that of large raindrops is relatively small. Consequently, the total raindrop concentration increases significantly with low radar reflectivity. Furthermore, there are fewer small raindrops and more large raindrops in the vicinity of the main updraft region of hailstorm, the raindrop concentration is low, and the mass weighted diameter Dm and reflectivity factor Z are high. In the downdraft region of the hailstorm, more small raindrops lead to higher total raindrop concentration and small Dm and Z.