Vol.30, NO.3, 2019

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Articles
Optical and Electric Field Observations of Two Concurrent Upward Flashes Triggered by a Positive Cloud-to-ground Flash
Wu Bin, Lü Weitao, Qi Qi, Ma Ying, Chen Lüwen, Su Zhiguo, Wu Shanshan
2019, 30(3): 257-266. DOI: 10.11898/1001-7313.20190301
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Abstract:
Tall objects are easy to trigger upward flashes. They also provide more opportunities for observation of upward flashes and a suitable platform for analyzing the intiation and development of upward flashes. Upward flashes are usually divided into two types, one is self-initiated and the other is triggered by environment, and the latter type is often related to the activity of positive cloud-to-ground (CG) flash. At present, development characteristics and triggering mechanism of the upward lightning channel are mainly studied based on the image of the extracloud lighting channel, the ground electric field change data and radar echo data. Few examples of mutiple upward flashes triggered by the same positive CG flash on different tall objects have been observed, and detailed characteristics of upward flashes are not well understood.On 16 June 2017, a positive CG flash (single stroke, peak current +141 kA) is recorded by the lightning photoelectric synchronous observation system of Tall Object Lightning Observatory in Guangzhou (TOLOG), triggering upward flashes of two nearby super-high-rise objects. Results show that within about 0.8 ms after the return stroke of the positive CG flash, two upward negative flashes intiate from the Canton Tower (600 m high) and the East Tower (530 m high), which are about 3.9 km and 4.1 km away from the positive CG flash, respectively. The initiation of two upward flashes could be caused by combined effects of the return stroke of positive CG flash, its associated continuing current, and the negative leader in the cloud approching to towers. A total of 7 strokes occur in about 353 ms, 6 strokes on the Canton Tower and 1 stroke on the East Tower, and the leader/return stroke sequence on the Canton Tower occur after the leader/return stroke sequence and the attempt leader on the East Tower, i.e., there is no overlapping between strokes of two upward flashes. The average peak current (-21.4 kA) of return strokes of upward flash initiating from the Canton Tower is about 3 times of the peak current (-7.3 kA) from the East Tower. It's supposed that the negative charge region in the upper cloud of the Canton Tower is wider than that in the upper cloud of the East Tower, and the charge amount is larger. The 2D velocity range of the positive leader of two upward flashes is 9.4×104 to 1.8×106 m·s-1, with an average of 6.9×105 m·s-1.
Lightning Location Algorithm Based on DBSCAN and Grid Search
Liang Li, Lei Yong, Zhang Shuaichi, Li Tao, Pang Wenjing, Wang Zhichao
2019, 30(3): 267-278. DOI: 10.11898/1001-7313.20190302
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Abstract:
Lightning location system can monitor the time and location of lightning in real time, supporting disaster early warning and post-disaster treatment in meteorology, power, aerospace, forest fire prevention and other fields. The location algorithm directly affects the accuracy of lightning detection results. Traditional location algorithms may often fall into the local optimum and needs a large amount of calculation. The practical application is limited by the computer capability and the anti-error interference ability is poor. A new lightning location algorithm DG-LLA (DBSCAN and Grid-Search Lighting Location Algorithm) is proposed. The algorithm is verified by a lightning accident example and regional simulation, and then by locating historical data detected in the national lightning monitoring network. The performance of the new algorithm is compared and further analyzed from three aspects:Lightning frequency temporal distribution, lightning spatial distribution and regional spatial distribution.Simulation results of lightning example show that the location error of TDOA (time difference of arrival) method is the largest, reaching 1314 m. Taylor series expansion method is a classical iterative algorithm with an error of 881 m. The error of proposed DG-LLA algorithm is significantly reduced to 84 m. Examples of artificial lightning initiation show that the new algorithm DG-LLA is more accurate than the national lightning monitoring network, and the average location error is 32.2% lower than operational network. Lightning location algorithm based on adaptive DBSCAN and grid-search optimization can effectively identify noise data and enhance the ability of anti-error interference. Regional simulation result shows that TDOA method and Taylor series expansion method have large positioning errors, with the RMSE (root mean square error) of 982 m and 668 m, respectively. When DBSCAN is added to location, the RMSE is significantly reduced to 406 m. After DBSCAN and grid search are added, the RMSE is further reduced to 349 m. Lightning location algorithm based on adaptive DBSCAN and grid search optimization improves the local searching ability and global searching ability of space and overcomes shortcomings of traditional iterative algorithm, such as easy divergence and local optimum of optimization algorithm, and solve the lightning strike point stably and accurately, and it performances better than operational network. The utilization rate of return data increases from 43.4% to 51.5%. The radar echo around new locations has stronger characteristics and higher locating accuracy. It provides a new method for lightning location.
The Scavenging Process and Physical Removing Mechanism of Pollutant Aerosols by Different Precipitation Intensities
Luan Tian, Guo Xueliang, Zhang Tianhang, Guo Lijun
2019, 30(3): 279-291. DOI: 10.11898/1001-7313.20190303
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Abstract:

The aerosol scavenging process of precipitation is an important mechanism for cleaning polluted aerosols in atmosphere. But there are many uncertainties due to complexities of precipitation processes and atmospheric pollutant particulate matter. PM2.5 scavenging rates by different intensities of precipitation are investigated based on aerosol and precipitation measurements in Beijing from March 2014 to July 2016. Effects of raindrop size distribution, wind speed and rain duration on PM2.5 scavenging rate are studied. Results show that stronger precipitation is more efficient in removing polluted aerosols in atmosphere. The mean PM2.5 scavenging rate is 5.1%, 38.5% and 50.6% for light, moderate and heavy rain, respectively. However, PM2.5 scavenging rate by light rain has large difference. In about 50% light rain cases, PM2.5 mass concentration decreases, while in the other 50% light rain cases, PM2.5 mass concentration increases. In all moderate and heavy rain cases, PM2.5 concentration apparently decreases. Scavenging rates exceed 40% for 10% of light rain cases, 50% of moderate rain cases, and 78% of heavy rain cases. Since light precipitation has generally narrower size distribution and more smaller drops, PM2.5 scavenging rate by light rain is much lower, while moderate and heavy rain usually have wider size distribution and more larger drops, so that PM2.5 scavenging rates by these precipitation are much higher. In addition, further investigations indicate that PM2.5 scavenging rate for light rain is strongly influenced by precipitation duration and wind speed. The longer precipitation duration and higher the wind speed is, the higher the scavenging rate for light rain becomes. In some light rain cases, these factors enhance scavenging rates, but influences of precipitation duration and wind speed on PM2.5 scavenging rates are relatively smaller for moderate and heavy rain. This is because that the moderate and heavy rain can scavenge most of PM2.5 in a short time. The size distribution of raindrops is not an important factor to cause the different PM2.5 scavenging rate for the same rain intensity.
Environmental Parameter Characteristics of Severe Wind with Extreme Thunderstorm
Ma Shuping, Wang Xiuming, Yu Xiaoding
2019, 30(3): 292-301. DOI: 10.11898/1001-7313.20190304
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Abstract:
Cases of severe wind with extreme thunderstorm and ordinary thunderstorm without strong convection in various regions of China are analyzed to study characteristics of environmental element of the severe wind, and 95 cases are selected for each type from 2002 to 2017. The comparison of key environmental parameters of severe wind with extreme thunderstorm and ordinary thunderstorm reveal the key environmental parameter characteristics. Results show that severe wind occurs in a relatively dry environment in the middle troposphere. The single-layered maximum depression of dew point of severe wind is 25.7℃ and the average depression of dew point is 13.6℃ when those of ordinary thunderstorm are 16.2℃ and 6.5℃, respectively. Differences between their ground dew point temperature are not significant, when the mean ground dew point temperature of severe wind is 20.2℃, and the mean of ordinary thunderstorm is 21℃. However, the average precipitable water of the former is 37 mm, significantly lower than that of the latter which is 51 mm due to the discrepancy in moisture layer. The moisture layer thickness of the former is below 2 km in most of cases, obviously shallower than the average thickness of ordinary thunderstorm moisture layer, which is 3.6 km. The mean vertical temperature lapse rate in the middle and lower troposphere of severe wind is larger than that of ordinary thunderstorm. Its average temperature difference between 850 hPa and 500 hPa is 28.2℃, obviously larger than that of ordinary events, which is 23.3℃. At the same time, as the ground dew point temperature is not much different, the mean convective available potential energy of severe wind is 1820 J·kg-1, larger than the average of ordinary thunderstorm which is only 470 J·kg-1. The convective inhibition for two types of thunderstorms are not significantly different, the average convective inhibition of severe wind is 79 J·kg-1, comparing to 55 J·kg-1 of ordinary thunderstorms. 0-6 km vertical wind shear of severe wind is 18.1 m·s-1 and 0-3 km vertical wind shear is 13.2 m·s-1 comparing to 14.3 m·s-1 and 10.5 m·s-1, respectively. The convective available potential energy of downdraft of severe wind is larger whose average value is 1110 J·kg-1 while the mean convective available potential energy of downdraft of ordinary thunderstorm is 620 J·kg-1. And the median entrainment zone mean wind speed of severe wind is 14 m·s-1. It is slightly larger than that of ordinary thunderstorm, which is 12 m·s-1. In addition, there is also discrepancy in the height of characteristic layer of severe wind with extreme thunderstorm and ordinary thunderstorm, such as 0℃ layer, -20℃ layer, and the lifting condensation level.
Characteristics of PM2.5 in Heavy Pollution Events in Beijing and Surrounding Areas from November to December in 2016
Jia Xiaofang, Yan Peng, Meng Zhaoyang, Tang Jie, Zhang Yong, Yan Xu
2019, 30(3): 302-315. DOI: 10.11898/1001-7313.20190305
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Abstract:
PM2.5 and PM10 mass as well as meteorological data at six stations in Beijing and Hebei Province are analyzed to investigate characteristics of heavy pollution processes from November to December of 2016. Results show that PM2.5 concentrations are 73.1, 130.8 μg·m-3 and 226.0 μg·m-3 at Shangdianzi, Shunyi and Chaoyang stations in Beijing during the heavy pollution, which are lower than those measured at Baoding and Shijiazhuang stations in Hebei (357.8 μg·m-3 and 346.9 μg·m-3, respectively). The average concentration of PM2.5 for the heavy haze days is 3-4 times of that in clean days at all six stations, with the haze accompanied by calm wind, high humidity, and other adverse weather conditions. Observations indicate most pollution cases last longer in Hebei than those in Beijing, which is probably caused by intensified emissions from industry in Shijiazhuang. In addition, the sulfur dioxide, nitrogen oxides and particulate matter discharged from Shijiazhuang and Baoding are blocked by the Taihang Mountains, which make pollutants easy to accumulate in this area.The daily average air quality index (AQI) during heavy pollution events has a complex relationship with the type, strength, duration and thickness of the inversion layer. Meanwhile, it has good consistency with the duration of the inversion both before and after the heating period in Beijing. The analysis of sounding data indicates that the atmospheric boundary layer also plays an important role in the accumulation of pollutants. Comparing with inversion at higher level, the inversion near the ground has significantly greater suppression effects on the diffusion. The pollution case from 17 December to 21 December lasts 5 days and PM2.5 mass concentrations are higher than the case from 3 November to 5 November in 2016. It suggests that the vertical diffusion of pollutants is suppressed for longer time and contaminants accumulate on the ground with the temperature inversion. On the other hand, the horizontal wind speed is lower, and pollutants cannot spread horizontally which aggravate pollution. Concentrations of OC and EC in PM10 at Gucheng in Hebei in two cases are also significantly different. Much higher OC, EC and OC/EC concentrations on 22 December are observed than on 3 November 2016, which may indicate more automobile exhaust and coal combustion in this heavy pollution event. The continuous appearance of the inversion layer, lower horizontal wind speed and more coal combustion and vehicle exhaust emissions are the main causes for this heavy pollution process.
ARTICLES
Characteristics and Application of Background Errors in GRAPES_Meso
Zhuang Zhaorong, Wang Ruichun, Wang Jincheng, Gong Jiandong
2019, 30(3): 316-331. DOI: 10.11898/1001-7313.20190306
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Abstract:
The statistic structure of background covariance is studied by NMC method of USA based on GRAPES regional model forecast data spanning one year from June 2015 to May 2016. The horizontal correlation length scale is estimated with Gauss function linear fitting method. Characteristics of background error and horizontal correlation length scale with the latitude, height and season are investigated. Results show that background error and horizontal correlation characteristic scale obviously change with height and latitude, and the unbalanced non-dimensional pressure and humidity are closely related to the season. Background errors of four control variables are nonhomogeneous, among which background errors of stream function and unbalanced velocity potential mainly change with latitude and height, background errors of unbalanced non-dimensional pressure and humidity show local and seasonal characteristics. The biggest background errors of the unbalanced non-dimensional pressure occur in the Tibetan Plateau, and they are larger in winter while smaller in summer. The biggest background errors of humidity happen in low latitude of tropical monsoon region, and they are larger in summer while smaller in winter. The horizontal correlation length scales of four control variables with Gauss function fitting are reasonable except that correlation coefficients of the unbalanced non-dimensional pressure are overestimated in close distance and underestimated in far distance. Horizontal correlation length scales of steam, unbalanced velocity potential and humidity obviously change with height are largest in tropopause. The length scale of unbalanced non-dimensional pressure obviously changes with latitude and is larger in low latitude of middle tropospheric. The horizontal correlation length scale of the unbalanced non-dimensional pressure and humidity both are larger in winter and smaller in summer. The horizontal correlation length scales changing with height are used in GRAPES-3DVar system instead of single parameter, and then the analysis and forecast experiment results of one month indicate that, qualities of 6-hour geopotential height forecast in troposphere are improved. Analysis and 12-hour forecast of wind in stratosphere are greatly improved; all levels of 24-hour accumulated precipitation forecast are obviously improved; the false prediction of 24-hour accumulated precipitation of light rain, moderate rain and heavy rain are improved; 12-24-hour accumulated precipitation of extra torrential rain in control test fails to be reported, but the experiment with changing horizontal correlation length scales improves forecasts of positions and values for extra torrential rain.
Assessment on Unsystematic Errors of GRAPES_GFS 2.0
Zhang Meng, Yu Haipeng, Huang Jianping, Shen Xueshun, Su Yong, Xue Haile, Dou Baocheng
2019, 30(3): 332-344. DOI: 10.11898/1001-7313.20190307
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Abstract:
Unsystematic error is one of main sources of model simulation error, which is mainly induced by initial field error and model defect. Global and Regional Assimilation and Prediction System (GRAPES) global model forecast data and final analysis data made by National Centers for Environmental Prediction (NCEP) during January, April, July, October in 2014 are chosen to be compared and analyzed. In terms of temporal variation, conclusion could be made that peaks of unsystematic errors in both north and south hemispheres occur in their respective winters, and errors show periodical changes. With the increase of forecasting time, the model unsystematic mean square error along with geopotential height field increases over time, first in an exponential function way, and then linear growth. In addition, linear growth in temperature field and zonal wind field are discovered. It shows in the consequence that the large value of model unsystematic mean square error center in mid-latitude and distribute like zonal banded. Large value regions basically do not change along with forecast time. In zonal average geopotential height field and zonal wind field, large value regions are found in tropopause, whereas it is found in boundary layer in temperature field. The cause is that the parameterization scheme does not fully describe differences of these two stratifications in physical process and dynamic framework. It is worth mentioning that the error of the increase in the height of the temperature field at middle and upper levels of the troposphere decreasing. After fitting the error-time line, the error of the initial field, the upper limit of the prediction and the proportion error taken up by the initial field in the south hemisphere are all higher than that in the north hemisphere. Besides, it is found that the initial field gradually decreases in proportion with height increasing. It shows in the above results that the precision of GRAPES_GFS 2.0 for the simulation of mid-latitudes of the south hemisphere and the entire troposphere should be further improved. In order to discover internal physical mechanism of model forecast's defect, and correct error target, it is necessary to research the feature of unsystematic error of GRAPES global model, and analyze the spatial-temporal evolution of the error.
Recognition Method of the Tibetan Plateau Vortex Based on Meteorological Satellite Data
Ren Suling, Fang Xiang, Lu Naimeng, Liu Qinghua, Li Yun
2019, 30(3): 345-359. DOI: 10.11898/1001-7313.20190308
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Abstract:
Based on long-term meteorological satellite data and multi-source observation and reanalysis datasets, the recognition method of the Tibetan Plateau vortex is studied. Based on the method, the Plateau weather analysis software is developed and the vortex dataset of almost 30 years is established. The location, track and distribution of low vortexes based on yearbooks and satellite are compared and the origin region, track and seasonal distribution of low vortexes are studied. Results show that the height and wind fields over the Tibetan Plateau of NCEP/NCAR reanalysis dataset are the most consistent with sounding data which can be used to identify the Tibetan Plateau vortex. Climate vortexes from satellite show there are two vortex activity centers located in the east and the west of the Plateau, respectively. In the eastern part of the Plateau with several sounding stations, high value vortex activity centers are coincided with which from yearbooks(east of 90°E).In winter, the frequency of vortex activity from satellite data is obviously higher than that from yearbooks caused by the activity of vortex in the western part of the Plateau. The analysis of annual vortex tracks also show that vortexes from the satellite recognition are in good agreement with that from yearbooks except for the central, western and southern parts of the Plateau without sounding stations, which indicates that vortex data from the satellite recognition is feasible in the eastern part of the Plateau. After three new sounding stations are built in the central and western part of the Plateau in 2015, vortexes in yearbook show there are several vortexes to the west of 90°E near new stations which account for about 22% of the total number in 2015. The distribution of vortex from satellite and yearbook is accordant near three new stations which indicates the credibility of vortex data from the satellite recognition in the central and western part of the Plateau. Therefore, vortexes from satellite recognition are consistent with vortexes from yearbooks when there are sounding stations and it also can be used to track the origin of the vortex. At the same time, it also can identify vortexes occurring in western part of the Plateau, especially in winter. It is an effective supplement to the low vortex yearbook datasets.
The Relationship Between East-west Movement of Subtropical High over Northwestern Pacific and Precipitation in Southwestern China
Yan Hongming, Wang Ling
2019, 30(3): 360-375. DOI: 10.11898/1001-7313.20190309
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Abstract:
The Northwest Pacific subtropical high (abbreviated as subtropical high) is one of the large-scale circulation systems that affect the climate in East Asia. The seasonal north-south movement of the subtropical high has a very important effect on the position of summer rain belt in eastern China. Most of the related researches focus on the movement of the subtropical high in the north-south direction, while less attention is paid to the east-west displacement, especially the influence on the climate in Southwest China. Southwest China is located in the eastward part of the Qinghai-Xizang Plateau, with high altitude and low latitude. Though the influence of the subtropical high on the climate in Southwest China is not as direct as that in the eastern part of China, it is found that the change of subtropical high also plays a very important role by affecting the configuration of atmospheric circulation system. Therefore, it is of great significance to study the anomalous movement of the east-west position of the subtropical high and its influence on the climate in southwestern China in order to understand causes of the climate anomaly.To further understand the influence of the change of the subtropical high, a new index to measure the east-west position of the summer subtropical high is defined by the regionally averaged relative vorticity at 700 hPa in different regions based on the climatology circulation. Results show that the index can characterize the east-west displacement of the anticyclonic circulation of the subtropical high more objectively and qualitatively, and it can also reflect the linkage between the subtropical high and meridional circulation over East Asia. When the subtropical high is east (west), there is a negative-positive-negative (positive-negative-positive) meridional anomalous wave train over East Asia. Comparing with other indices, the new index can better reflect the seasonal movement of rain belt position in eastern China in summer and has a significant correlation with precipitation in Southwest China in June and July. When the subtropical high is west, there is less precipitation in western and southern Sichuan, and most in Guizhou in June, and there is more precipitation in northern and eastern Sichuan, northeast Guizhou, and less precipitation in central and northwest Yunnan in July. When the subtropical high is east, the precipitation distribution is almost the contrary. Further analysis also shows that the relationship between the subtropical high and SST is related to the active position of the subtropical high. The farther north the subtropical high is, the weaker the relationship between the subtropical high and the SST is.
Application of Variational Method to Correction of ASCAT Wind Field in the Bohai Sea
Guo Yudi, Liu Binxian, Liang Dongpo
2019, 30(3): 376-384. DOI: 10.11898/1001-7313.20190310
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Abstract:
It is found that the wind velocity and wind direction of ASCAT have large errors comparing with observations of the Bohai Sea stations. Error analysis results show that the mean wind velocity deviations of ASCAT of the Bohai Bay, Bohai Strait, Laizhou Bay, Central Bohai Sea and Liaodong Bay areas are from the largest to the smallest. The mean wind direction deviation of ASCAT of the Laizhou Bay, Bohai Bay, Liaodong Bay, Bohai Strait and Central Bohai Sea area are from the largest to the smallest. The wind speed deviation of the Bohai Bay is the largest, and the wind direction deviation of the Laizhou Bay is the largest. According to error analysis results, the reliability of ASCAT wind field in the Bohai Sea needs to be improved. Therefore, the wind field of ASCAT is corrected using observations of the Bohai Sea stations in terms of variational method, which can obtain the corrected wind field with spatial resolution of 12.5 km×12.5 km and temporal resolution of 12 hours. In addition, to test the applicability of variational method in the sea area, the corrected wind field in five sea areas including the Liaodong Bay, Bohai Bay, Laizhou Bay, Central Bohai Sea and Bohai Strait are verified. Verification results show that the accuracy of ASCAT wind field is improved significantly after correction, the average deviation of wind velocity is reduced from 4 m·s-1 to 1 m·s-1, and the average deviation of wind direction is reduced from-30 °-30 ° to -7 °-4 °. The variational method is proved effective on ASCAT wind field correction. The ASCAT wind field revised by variational method from September 2017 to February 2018 shows that the distribution of the wind field is consistent with the wind field observed on the sea surface. The corrected wind speed of the Bohai Sea is proportional to the offshore distance and contour depth, which means areas further offshore, with deeper depth contour and wider surface are likely less influenced by submarine topography and the friction coefficient, and therefore the wind speed value is greater, and the wind direction is from the north to the northwest. The analysis of a gale process on 18 December 2017 shows that the corrected wind field can reflect coastal wind field information and the extreme value area of wind velocity in the process of wind, and it can monitor the process of gale changes dynamically. The variational method solves the problem of low spatial resolution of stations and low accuracy of ASCAT. It also has a good guiding significance for gale forecasting and can provide more accurate initial field for ocean models.
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